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Marine refrigeration and container ships
Marine refrigeration and container ships G. R. Scrine Key words: refrigeration, ship
Le froid & bord des navires et les porte-conteneurs
types de navires, les conteneurs, leur construction, leur isolation et les syst#mes frigorifiques pour diffarentes cargaisons. On examine les transferts de chaleur.
On passe en revue les aquipements de froid ~ bord des porte-conteneurs en donnant des data/Is sur les
A review of refrigeration in container ships, with details of the types of ship, the containers, their construction, insulation and
systems of refrigeration for different cargoes. Heat transfer is discussed.
It is now some twenty-five years since container ships operating in regular liner services first carried perishable produce, principally from the West Coast of North America to Hawaii and also to Alaska. Some of the early container ships were converted tankers and were capable of carrying both dry cargo containers and insulated containers with a refrigeration unit mounted in the front bulkhead.
Following the containerization of this service, there was a rapid growth in container services from the principal exporting country of primary productAustralia, to Japan and South East Asia and to the east and west coasts of North America. More recently New Zealand has containerized the majority of its exports of perishable foodstuffs and although still heavily dependent on the European market for exports of butter and lamb, is pursuing additional markets with some vigour.
The Matson Line operated a number of insulated containers which were fitted with a Thermo King SL5 unit; this tended to influence the design of integral containers for a number of years. Another operator of refrigerated containers of the same type was Sea Land which was engaged in a number of services from the United States to Europe, Central America and the Far East. This company was particularly conspicuous in using containers 35 ft (10668 m) in length, many of which were subsequently lengthened by welding ~,1 an additional 5 ft (1.524 m) to bring them into line with current International Standards. During the 1960s numerous patents were taken out principally in the United States 1~,1b,but also in the United Kingdom ld.le, which covered a variety of methods of refrigerating containers on board ship, many of which are in use today with little or no modification. Some patents covering container equipment such as couplings, closures and temperature control are given in references. 2~-2e
The Caribbean to Europe (CAROL) service was the first to use ships specially designed for the carriage of bananas, and pioneered the use of 40 ft (1 2.1 92 m) porthole containers which are cooled by ducting air to the interior via two 350 mm diameter holes in the front bulkhead. However both United Brands and Sea Land operate 40 ft (1 2.1 92 m) integral containers for the carriage of bananas from Central America to the United States. Within the last two years additional services have commenced between Europe and South Africa and more recently between Australia and the Gulf of Mexico ports of the United States of America. With new ships being constructed for services between South America and Europe it would appear for the time being that the majority of the principal exporting countries of refrigerated produce can benefit from a liner container service.
In the mid 1960s it became apparent that traditional methods of carrying both refrigerated and general cargo by sea were becoming increasingly expensive and proposals were put in hand to containerize the service between Europe and Australia involving the British, Continental and Australian lines.
A number of methods have been proposed for carrying containers under refrigeration at sea. and in general these have been developed to suit particular operational circumstances in both exporting and importing countries. As each of these concepts has several noteworthy features, it is proposed to consider these features in some detail.
The author is Technical Director, Shipowners Refrigerated Cargo Research Association, Newmarket Road. Cambridge. UK Paper received April 1981, being an edited version of that originally presented at 53rd Thomas Lowe Gray Lecture
All container ships whether carrying refrigerated, insulated or dry cargo containers are designed to carry a quantity of each type and the systems which have been evolved have had a profound effect on
Volume 5 Number 1 January 1982
0140-7007/82/010009-1253.00 @1982 Butterworth & Co (Publishers) Ltd and IIR
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the design of the container ship and to a lesser extent on the refrigeration plant.
T h e c o n t a i n e r ship The majority of the container ships in refrigerated liner ('reefer') trades are fully cellular, ie all containers are located within guides and there are no conventional cargo decks loaded by cranes, with the possible exception of No. 1 hatch. There are however some vessels which are part container and part conventional cargo. The refrigeration systems for the various container services have been influenced by the type of cargo carried, although there have been variations within each trade depending upon an individual operator's particular solution. For example, in the Australia to Europe service the ships designed by OCL (Overseas Containers Limited) which had a vertical duct system with a maximum of six containers per cooler in one duct giving a certain amount of flexibility: at the ship's side above wing tanks, it was only possible to accommodate three containers. The solution adopted by A C T A / A N L (Associated
Container Transportation (Australia)/Australian National Line) in the same trade was to have one cooler with up to 24 containers connected to a number of parallel horizontal ducts. Ships operating between Australia and North America were more varied with the PACE (PacificAmerica Container Express) Line operating designs similar to A C T A / A N L and Columbus Line operating ships with air ducts and couplings similar to those of OCL but without hold insulation. Farrell Lines" ships had no air ducts whatsoever but containers were located in guides in a fully insulated and refrigerated hold. Some excellent descriptions of the early ACT Class vessels were given by Jenks s and of the OCL ships by Meek. 4 It may be of interest to consider some of the details and dimensions of ships built for various trades as given in Tables 1-4 and two examples are given in Figs 1 and 2. Of the less conventional type of container ship, the MV Ariake built for the Australia to Japan container service has the details given in Table 3.
Table 1. Comparison of various container ships belonging to British and Australian lines Tableau 1. Comparaison de divers porte-conteneurs des/ignes britanniques et austra/iennes Encounter Bay
ACTI
Mairangi Bay
Australian Venture
Table Bay
Loa, m
227.1
217.3
248.6
248.6
2585
dwt
29 500
26 376
38 757
32 800
47 200
grt
26 750
24 820
44000
44 345
53 780
Containers total insulated
1 572 356
1 334 326
1812 1223
1822 913
2436 980
Speed, knots
23
22
21 5
23
21
Machinery
2 x Stal turbines
2 x Stai turbines
2 x MAN diesel
2 x MAN diesel
2 x MAN diesel
Power, kW
24 170
22 380
38 747
39 187
38 270
Table 2. Comparison of various container ships belonging to Continental and American lines Tableau 2. Comparaison de divers porte-conteneurs des/idnes continenta/es et amdricaines NedLIoyd Houtman
Farrell C6
Farrell C8
Columbus Queensland
Caribia Express
Loa, m
258.5
204.0
248.0
1 84.9
204.0
dwt
48 000
19 553
28 672
24 400
18 500
21 489
31 487
19193
27 936
1186 288
1708 768
1t74 596
1456 120 × 40 ft
grt Containers total insulated below deck Speed, knots
2314 352 21
226
22'
!9 5
,'b5
Machinery
2 x Sulzer
t~lrbine
turbine
Sulzer
(;ZI ,, Sulzer
Power, kW
39 500
71 634
2 ~ 63,4
!()950
2i b?,'i
10
Revue Internationale dLJ Frond
Table 3. Cellular container ship carrying integral containers Tableau 3. Porte-conteneur compartiment~ transportant des conteneurs int#gr#s Loa, m
237.8
dwt
28 295
grt
38 000
Containers total reefer
1748 x TEU 606*
Speed, knots
26
Machinery
2 x MAN
Power, kW
39 687
*
These are integral containers carried below deck.
Table 4. Parameters of ACT 7 shown in Fig. 2
since proved unfounded, as has been demonstrated on the vessels built for the Australia-Japan Container Line as exemplified by the Ariake. In 1965, the Technical Committee of the SRCRA (Shipowners Refrigerated Cargo Research Association) investigated various alternative means of refrigerating containers at sea, the British lines being in favour of the two porthole container, a design which has become popular in services where there is a large volume of refrigerated cargo carried at the same temperature, such as frozen meat and butter. Continental lines in the Australian Conference, Hapag Lloyd AG, NedLIoyd, Messageries Maritimes (now Compagnie G6n6rale Maritime) together with the Australian National Line also use this particular design of container and its associated refrigeration system.
Tableau 4. Param~tres de ACT 7 pr#sent~ ~ /a Fig. 2 Dimensions
Capacity
Length o.a.
248.60 m
Length b.p.
236.20 m
Beam
32.29 m
Depth
21.50 m
Draught
12.025 m
Displacement
61 428 tonne
933 reefer containers 1069 general containers 2002 standard ISO container equivalents
Container dimensions External The overall weight of container and cargo has been restricted by regulation in the United Kingdom and other countries and is effectively limited to a gross of about 20 tonnes. Current designs of dry freight container defined the width of the container at 2.44 m (8 ft). The weight restrictions virtually fixed the overall length at 20 ft (6,097 m) for containers loaded with butter and certain meat cuts, this conforming with the standard container of the day.
As a comment on the size of container ships which were designed to carry dry cargo containers together with some integral containers on deck, the Sea Land Galloway Class and Liverpool Bay must rank as the largest and the most powerful in terms of installed power (89 520 kW and 6 0 4 2 6 kW respectively) both being originally engined with steam turbines. These are now being converted to diesel power, which is a direct result of escalating fuel costs, and sets the trend for perhaps another decade or more.
Containers Before considering the types of container ship which were proposed in the 1960s for the Europe to Australia service, it is pertinent to consider some of the factors leading to the possible choices of a given container type and the reasons for that selection.
Fig. 1 deck
Mairangi Bay 1223 insulated containers carried below
Fig. 1 Les 1223 conteneurs iso/ds de Mairangi Bay transport#s au-dessous du pont !
The original services, such as those operated by the Matson Line, had a relatively small number of containers with inbuilt refrigeration units which are commonly referred to as integral units. When alternative designs of container were considered for the Australian service some doubts were expressed concerning the reliability of integral units, and the possibility of failure from 400 containers during a 28 day voyage could not be ruled out. Such fears have
Volume 5 Num~ro 1 Janvier 1982
/
Brine room
/ Con,ro, room IA,r~=~ / J / Engine room
~=~1~1~11~1 " / J ~ l t ~ l ~ l ~ l ~ l ~ l ~ l l ~ l l ~ Bollosf and fuel tanks Bow thruster Fig. 2
Act 7
Fig. 2
ACT 7
-/
J /
11
When loaded with frozen lamb in carcase, chilled meat in cuts or fruit, container loads vary from about 9 to 13 tonnes, thereby giving some support to the case for using 40 ft (1 2.19 m) containers, although the same overall weight restrictions apply when travelling on the road. The two porthole container was originally conceived as a module having external dimensions of 20 x 8 x 8 ft (6.095 x 2.438 x 2.438 m), although these have subsequently been increased to 20 x 8 x 8.5 ft (6.095 x 2.438 x 2.591 m), for South Africa to Europe and more recently 40 x 8 x 8.5 ft (1 2.19 x 2.438 x 2.591 m) in services between the Caribbean and North West Europe. /nterna/. Some typical dimensions for insulated two porthole containers are 5.666 x 2.235 x 2.083 m for containers manufactured for the Australian service. More recently containers for the South Africa to Europe service have internal dimensions of 5.77 x 2.3 x 2.285 m which were specified in order to accommodate ten 1200 x 1000 mm pallets of fruit. This specification largely resulted in containers with a higher overall heat transfer coefficient of about 30 kcal h -1 °C -1 (34.9 W K -1) although one particular design had a figure of 23 kcal h -1 °C -r (26.74 W K -1) albeit by improving the insulation under the floor girders. This does little to improve the temperature range of air and cargo and such containers have a poor performance compared with the more highly insulated types when off refrigeration.
Polyurethone foam insulation
f 75mm
Permanent dunnage battens
Air outlet =
250mm diameter
/~
Air inlet
P
/ Spring loaded closure valve
J
J
50mm T section
Fig. 3
Section of insulated container
Fig. 3
Coupe d'un conteneur isol#
compression. Floors are usually supported by steel cross members with wood grounds to provide some insulation between steel and the aluminium floor. High density polyurethane or PVC foam is sometimes used in place of wood. The usual wall thickness of 20 ft insulated containers is about 75 mm, of which 16 mm is plywood and GRP, and that of the roof and floor about 100 mm.
Container insulation and structure Although the original containers in the Australian service were designed for six-high stacking, the bottom container having to support 100 tonnes between its four corner posts, later designs of container ship can accommodate nine-high stacking under the hatches which gives a working load of t 60 tonnes. Including the appropriate safety factors, the present design load of each corner post is 84 tonnes, which is achieved with a relatively small cross-section fabricated from high quality steel. The floor, side and end walls together with the doors are designed to meet the requirements of ISO 1496 I and II and of the Container Safety Convention at present being ratified by a number of countries. There are however individual company ,-equirements for deflections of end walls and side walls under uniformly distributed loads. The structural strength of side walls is accomplished in several ways, the most successful being of sandwich panel construction, the outer skin generally being plywood faced with glass reinforced polyester resin (GRP) and the inner face GRP entirely. Glass reinforced polyester or resin inserts are used in some designs to provide structural rigidity in bending and to a lesser extent in
1;~
The r e f r i g e r a t i o n of c o n t a i n e r s The cooling and maintenance of the temperature inside a container is by circulation of air either from a duct system through ports in the front wall of the container (Fig. 3) or from a fan and cooler unit located in the front wall of the container with its associated compressor and condenser. The system of placing a pre-cooled container in a cold well is not very flexible and relies on an efficient transport and holding system prior to loading on board, as well as cargo being pre-cooled to or below the required carrying temperature. It is proposed to consider the various types of container system in some detail as follows: Vertica/ ducts, ho/ds insu/ated. This design was originally used in the first generation container ships for Overseas Containers Ltd and although proposed by the Consortia's engineers was eventually patented by the German Shipyard and their subcontractors and is now widely known under the trade name 'ConAir'. lc A British designed version of the same system is known as 'Sea Rod" (Fig. 4) and similar equipment has been designed by various refrigeration
International Journal of Refrigeration
machinery contractors in other European Countries, but having no known trade-mark. The principal advantage of this type of system is the flexibility it offers in terms of individual temperatures. For example it is nominally possible to carry two varieties of fruit, eg plums at 8°C and grapes at O°C on adjacent coolers with the same inlet brine temperature to the cooler. The choice of an insulated hold provides an environment around the outside of the container which minimizes sensible heat inflow to the container air. reduces the air circulation within the container to maintain a prescribed temperature difference and also reduces the refrigeration power necessary for a given number of containers. The more recent designs for the South Africa to Europe service have increased the air circulation from 1 200 m 3 h -1 (40 air changes) per container with a net volume of 27 m 3, to approximately 1 870 m 3 h -1 (60 air changes). This has largely come about to meet the requirements of the Perishable Products Export Control Board (PPECB) which prescribed a 1°C range of temperature within the container. An additional requirement of this trade was that of relative humidity which was defined by the temperature difference between the air leaving the container and the surface temperature of the coil. This is essentially a vapour pressure difference with its lower limit depending on the surface area of the cooler and its upper limit largely governed by water conditions at the surface of the particular fruit at the prescribed storage temperature.
The insulated hold was not w i t h o u t its problems, as w i t h o u t an air heating system around the outside of the containers, hold air temperatures of - 8 ° C to - 1 0 ° C were envisaged, leading to the requirement of E quality steel which would not be susceptible to failure from brittle fracture. Other solutions to the problem were to heat the air to maintain temperatures of +1°C to +2°C around the containers or to leave the boundaries of the hold uninsulated although this would need some heating at tow ambient temperatures. This enabled designers to use D quality steel with some obvious cost savings. One further advantage of the insulated hold and its ability to keep temperature gradients to a minimum has been the acceptance by the Plant Quarantine Service of the United States Department of Agriculture 5 for imported fruit from regions where Mediterranean or Queensland Fruit Fly are endemic. For example, apples and pears from the Austalian mainland have to be kept below +0.6°C for a period of 14 days to ensure 100% mortality of the fruit fly larvae.
Vertica/ ducts with uninsulated holds. This system was developed in the Columbus Line fleet principally for the service between Australia and the east coast of North America. Of similar capacity to the PACE Line vessels, the containers had a somewhat lower overall heat transfer coefficient but the principal difference was a 50% increase in the rate of air circulation within the container. This figure was increased from 1 200 m 3 h -1 (40 air changes) to 1 800 m 3 h -~ (60 air changes) per container with a consequent increase in fan Watts and refrigeration capacity. Although little was known about heat transfer from the steel boundary of uninsulated holds to tall stacks of containers the design was successful and apparently had few problems with low hold air temperatures except in the North American winter. The combination of greater air flow and lower heat transfer of the containers enabled the ships to comply with the United States Department of Agriculture Plant Quarantine regulations, although some time elapsed before this approval was finally granted. Although some theoretical assessment had been made of heat transfer from uninsulated vertical steel surfaces, it was not until 1977 that a serious attempt was made to measure this in a hold filled with containers. 6 One additional feature common to all container ships carrying containers is the heat balance between the outside of the hold and the cargo within the containers.
Fig. 4
'Sea Rod' vertical air duct
Fig. 4
Gaine d'air vertica/e de Sea Rod
Volume 5 Number 1 January 1982
One of the principal disadvantages is perhaps the greater range of air temperature in the air space around the outside of the container. Some measurements have shown temperatures under the hatch cover to be at or near ambient which may
13
give rise to a greater range of air temperatures wihin the container. This will probably be of little consequence when carrying frozen cargo, but may be embarrassing under Plant Quarantine conditions. (It was noticeable in the PACE ships with insulated holds that when hatches were lifted container temperatures in the top tier rose significantly and on occasions jeopardized the Plant Quarantine treatment.)
I f
H o r i z o n t a l f i n g e r d u c t s , i n s u l a t e d h o l d . This has
been one of the more popular concepts amongst the British lines and their associates in the Australia and New Zealand trade (Fig. 5).
/
/
In the early ACT vessels up to 24 containers were cooled from a single cooler and fan. The more recent designs however have increased this figure to a maximum of 48 containers. Whereas this is very economic for containers carrying large quantities of frozen produce such as meat and butter, it can result in a certain loss of flexibility as far as carrying temperatures are concerned.
/
t
/ n s u l a t e d h o l d , n o air d u c t s . This system has been
commonly described as the 'cold well' but has found few followers outside the United States. Again the subject of an early patent TM, the original concept was to use containers with apertures in the front wall and located within vertical guide systems but with fans blowing air into the interior of the container. The early Farrell Line container ships were the first to use the cold well, but this was followed by a modified system with air ducts 7 and nozzles designed for the Swedish Johnson Line container ships (Fig. 6). A similar version of the Stal system was used in two multi-purpose reefer-container ships which were converted in the mid 1970s. The Farrell Line's vessels were designed to fulfil the concept that refrigeration would only be required for frozen and those chill cargoes not requiring fresh air ventilation. By accepting this concept it was possible to design a simple air circulation system, as cargoes could be
/
Fig. 6 Nozzlesystem for ducting air to containers Fig. 6
Syst~rne d'a/utages pour amener /'aJr aux conteneurs
cooled by conduction from cold air circulating around the outside of the container. This necessitated adequate pre-cooling of the cargo to the prescribed carrying temperature as the centre of a container could take a considerable time to cool by conduction alone. The absence of any ducts, couplings and venting systems allowed for some economy in construction, albeit at the cost of perhaps a greater thickness of insulation on the boundaries together with the use of high quality steel for lower temperatures. One operational difficulty feared in the early days, was the possible icing up of containers when discharged from the ship and difficulties in handling. In practice these fears do not appear to have been justified and the system, when modified as on the Swedish Johnson Line vessels, does offer scope for more flexibility in terms of cargo, though at a somewhat higher cost.
The heat balance
The principal means by which heat enters the refrigerated hold is by conduction through the various boundary surfaces, and also by infiltration of fresh air either accidentally through leakage in hatches and access doors or deliberately by fresh air ventilation of fruit cargoes. The energy from air circulating fans is dissipated within the air duct and container systems and has to be removed by the refrigeration plant. Fig 5 Horizontal air ducts Act 7 Fi,q 5
14
Games d'air horlzontales de ACT 7
During early investigations into the design of insulated container holds some thought was given
Revue Internationale du Froid
to the heat transfer mechanism between heat entering the boundary surfaces of the hold and then transferred to the air circulating inside the container. It was thought at the time that air w o u l d either convect naturally between the vertical stacks of containers or circulate deliberately by hold air conditioning systems. The top surface of the top containers and the bottom surface of the bottom container were considered as major heat transfer surfaces together with the doors, front and side walls. For the early ships designed for the British lines this meant an effective container surface area of about 67% of the total available. Some recent investigations 6 have shown that the effective heat transfer coefficients for the system were within 10% of those originally estimated for the vessels concerned. It is virtually impossible to use a mean heat transfer coefficient for the hold as a whole and it is necessary to consider each boundary on its own merits and calculate the heat transfer in terms of the temperature difference. As the insulation of a modern container ship is rather simpler than a conventional reefer ship, ie a general absence of deep frames buried within the insulation, estimation of heat transfer is somewhat easier. With the uninsulated hold, however, conditions are different and it is largely a question of estimating the surface air velocity between adjacent surfaces and the resulting heat transfer coefficients. There is also the possibility of heat transfer by radiation although this is unlikely to be significant from wing tanks and double bottoms in insulated holds. The problem of container surface area transferring heat from the hold to the internal air has also been investigated by the same author w h o has concluded that between 91 and 95% of the external surface area conducts heat from the hold to the inside of the container. This factor depends on the type of air circulation, whether forced or natural and the height of the container stack. This has removed any argument about the behaviour of air trapped beneath the top of one container and under the floor of the adjacent container. There appears to be a movement of air between adjacent vertical columns because of convection arising from pressure and temperature differences. Some measurements made recently on one of the South African container ships by SRCRA have confirmed that there is little difference between the air temperature in the vertical slots between adjacent rows of containers and that under the floors of containers.
gradient in the vertical slots under these conditions was between 2 and 4°C. Measurements by Stera 6 have suggested little difference between the two systems. The calculation of the heat balance for individual holds is straightforward and is related to the heat removed by the brine in the case of a system using a secondary refrigerant or the change in enthalpy multiplied by the mass flow in the case of a primary refrigerant using, say, R 22.
Effect of container deterioration on refrigeration requirements The majority of the design calculations for sizing refrigeration plant were based upon a container heat leak of 26.7 W K -1 (23 kcal h -1 °C-1). Many of the original designs were at least 10% below this figure leaving some margin for deterioration of containers in service. It has been found following some intensive retesting by the SRCRA 8 of in-service containers that whereas the majority of containers deteriorate at a reasonable rate, some designs were worse than others. Some typical curves of container deterioration with age for two containers, one of loose panel construction, the other sandwich panel and both insulated with polyurethane foam, are given in Fig. 7. Should a container ship hold be loaded with a large number of old containers, the general result will be to lower the hold temperature and increase the heat load on the refrigeration plant. The range of air temperatures within the container could increase but this is more likely to be a problem on ships with uninsulated holds particularly under Plant Quarantine conditions as laid down by the United States Department of Agriculture. 5 The behaviour of the container on land is, however, more likely to be of concern particularly if such equipment as clip-on units, whether mechanical or 80 Manufacturer 'A' (loose panel)
40
'B' (sandwich panel)
I 20 Data corrected to o mean wall temperature of 283K 0
It is also interesting to note that temperature gradients from top to bottom of a six-high stack under natural convection were somewhat less at 1.0°C than the corresponding temperature difference in a ship where hold air temperatures had to be maintained above 2°C by forced air circulation. The
Volume 5 Num6ro 1 Janvier 1982
O
I 2
I 4
I 6
I 8
I I0
Years
Fig, 7 Deterioration curves for containers insulated w i t h p o l y u r e t h a n e foam
F i g 7 Courbes de d#t#rioration des conteneurs iso/#s avec de/a mousse de polyur#thane
15
liquid nitrogen, may not have the capacity to cope with a container whose heat leak has increased by 80%.
R e f r i g e r a t i o n systems It is almost universal to use both primary and secondary refrigerants to cool air which in turn maintains cargoes at acceptable temperatures. The major exceptions have been vessels engaged on the North American service from Australia, which use direct expansion. Those from Australia and New Zealand to Japan, the North Atlantic services and a number of small refrigerated container operations use integral containers. These units are mostly electric drive plugged in to appropriate power points on shore or onboard ship. A number of units nowadays are compact enough to allow for a removable diesel alternator set to be fitted when the container is travelling over the road, or sited in areas where a suitable three-phase supply is not available. By far the most interesting new development in refrigeration machinery has been the introduction of the single rotor screw compressor. A version of this is manufactured and marketed in this country, and an alternative version in Holland. Possibly one of the long term advantages of this type of compressor is a trend towards a reduction in lubrication and oil separation problems, and the ability to scale the machine up or down. It also has the ability to pump liquid refrigerant provided the rotor and planet wheels are manufactured with compatible materials.
Integral c o n t a i n e r s Whilst most refrigerated containers are of the twoporthole type there are significant numbers of 20
and 40 ft containers which have an inbuilt refrigeration system to carry cargoes in the range + 2 0 to -25°C. These units are mostly electric drive plugged into appropriate power points on shore or on board ship. A typical specification for a 20 ft unit for Far East service is shown in Table 5. Some services require the use of water cooled condensers below deck which are in series with the air cooled condenser and act as liquid receivers when operating in the air cooled mode. The condenser fan is stopped by a switch activated by water pressure. It is usual to recirculate fresh water through these systems, the water being maintained at a constant inlet temperature through a water to water heat exchanger. These condensers have not been without their faults particularly in areas where the winter temperatures fall below O°C and the water does not completely drain from the system. More recently there has been a tendency to abandon the water cooled system in favour of air cooled condensers operating below deck but with a hold ventilation system capable of removing the surplus heat. Capacity control is normally achieved by use of dual compresso[s or cylinder unloading but temperature control is by hot gas injection operated from a multi-step controller, or continuously modulating electromagnetic valve. This effectively balances the refrigeration output with the load over a wide range of ambient temperatures. Many units are still in daily use with an on-off control of the compressors combined with a suction pressure regulator or fixed hot gas injection. The operation of a container system offers scope for microprocessor control particularly if one requires full system monitoring at the same time. There is no doubt that in the short term these systems will be more expensive than those they replace but could offer significant advantages as manpower becomes more expensive. The mechanical controls are
Table 5. Specification for a 20 ft unit for Far East service Tableau 5. Prescriptions pour un conteneur de 20 ft ~ uti/iser pour/'Extreme-Orient Electrical supply
380-460 V, 50 or 60 Hz
Power consumption
5-6 kW
Ambient temperature
+ 40°C
Temperature control
+0.25°C in range -4°C to +10°C +0.5°C in remainder of range
Internal air circulation
60 changes per hour at an internal load resistance of about 10 mm water gauge
Temperature difference, evaporator to air delivery
8°C at a container temperature of - 18°C
Fresh air changes
At least 1 per 4 h
CO 2 sampling
Yes
Condensers
Finned copper or coated alloy if air cooled
Electric defrosting
3 kW plus tray and drain heaters
Compressor
3.7-5.6 kW semi-hermetic 1 500 or 1750 rev min frequency
Evaporator and condenser fans
Propeller 1800
16
m 3 h -1,
-1
depending on
consuming about 1 kW each
International Journal of Refrigeration
generally robust and inexpensive but in the event of failure of the system as a whole it may require some special skills to diagnose and rectify the fault. The SRCRA have undertaken a development programme to produce a prototype control and monitoring system which is microprocessor based. With a high capability for detecting various conditions of temperature and pressure within the refrigeration system together with power measurement, it is hoped to give a series of alarm conditions which will help to pinpoint the possible cause of failure.
C a r r i a g e of c a r g o It w o u l d be possible to dwell at some considerable length on the various factors which govern the successful carriage of cargo in containers. The container ship in terms of carriage of cargo, was initially a sideways step from conventional carriage in the refrigerated ship of the day. Some of the early design requirements for the container were based on the then current designs of refrigerated liner vessels (not specialist banana carriers) and that they have been eminently successful has been proved beyond doubt.
Rates of air circulation and fan design The lowest acceptable rate of air circulation for both deciduous fruit and frozen cargo is approximately 30-40 air changes per hour of the empty volume of the container when carried in an insulated hold. For special requirements such as those of the Perishable Products Export Control Board of South Africa, this rate of air circulation is increased to 60 changes per hour when carrying fruit and 30-40 changes per hour when carrying frozen cargo. The container ship with uninsulated holds will also have similar rates of air circulation. By far the highest is that for the specialized container ship carrying bananas as exemplified in the CAROL (Caribbean Overseas Line) service where a rate of 70-80 per hour is required. This is reduced to 35-40 per hour when the containers are carrying frozen cargo. Fans have varied from simple axial flow in the early Encounter Bay Class to the mixed flow for the ACT Class and on the later vessels built for the ANZECS service (Australia New Zealand Europe Container Service). The centrifugal fan with two speed motors is used in some vertical duct systems where pressure loss in the ductwork can be considerable. However, where two speed operation is required it is usual to have two axial fans in operation for maximum air circulation reducing to one for frozen cargo or when a reduced number of containers is carried on a stack. The ships designed to carry the early 40 ft porthole containers, had an unusual fault in that the closure device slammed shut when the number of containers on a cooler duct was at a critical level. 9 Some details are given in Table 6.
Volume 5 Number 1 January 1982
Table 6. Conditions for slam closing of container closures Tableau 6. Conditions de la fermeture des conteneurs par c/aquement Air changes Plate Initial spring to slam diameter, force/area of closing mm plate, g cm -2 40 ft (12.195 m) container 20 ft (6.097 m) container (South African service) 20 ft (6.097 m) container (Australian service)
120
415
1.39
153
336
2.23
200
285
4.17
As cargoes can be broadly divided into two major categories, ie chilled and frozen, the former embracing both deciduous, sub-tropical and tropical produce, the latter any commodity which requires storage below - 2 ° C , it is proposed to consider each briefly and draw attention to some of the more interesting and perhaps difficult conditions of carriage.
Chill cargoes. These require in general a greater degree of temperature control to avoid freezing, in the case of some deciduous fruit, and chilling injury with the subtropical and tropical fruits. Of the latter some of the more important are the banana, the avocado pear and the pineapple. The carrying temperatures for these fruits may vary in certain countries of the world and it should not be taken for granted that a fruit grown in one hemisphere will behave in exactly the same way as the same variety grown in another. All fruit is living and produces heat and carbon dioxide when converting starch in the tissue to sugar. The refrigeration requirements to remove such heat together with cooling fresh air which is used to remove unwanted volatiles can be considerable. As the range of temperatures is said to be critical for some produce it has become fashionable to circulate air at a very high rate when carrying bananas, for example, and the penalty for the resulting fan power can be considerable. To quote an example for a container ship carrying about 350 porthole containers and with a maximum rate of air circulation of about 70 changes with fans at full speed and 40 changes with fans at low speed, the difference in power consumption is considerable, and is shown in Table 7. Deciduous fruit may be no less demanding, particularly when carried under conditions to satisfy the Plant Quarantine requirements for the United States Department of Agriculture. 5 The Mediterranean fruit fly, Ceratitis capitota, which is endemic in most Mediterranean countries has not so far penetrated the United States. 13 By far the most
17
Table 7. P o w e r consumption figures for a small container ship w i t h insulated holds Tableau 7. Consommation d'#nergie pour un petit porte-conteneur avec ca/es iso/#es
Cooling Holding
Compressors
Pumps F a n s
Total, kW
808 448
88 64
1037 552
137 40
troublesome species of fruit fly comes from Queensland and requires a somewhat longer time at a given temperature than some of its relations from Mexico, South Africa or Israel. A maximum fruit temperature of 0.6°C for 14 days is specified for the Queensland fly although some recent work in Australia has suggested that a period of 25 days or thereabouts at + 1.8°C is equally effective. In order to achieve a maximum fruit temperature of 0.6°C it has been found necessary not only to take particular care to load a container with fruit pre-cooled to below +2°C and preferably at carrying temperature, but to maintain such conditions up to the time of loading in the ship. The design of container ships favoured by the British Consortium operating from Australia to East Coast North America (PACE Line) with insulated and temperature controlled holds was particularly suitable for this operation. With a temperature range in the container of - 1 . 5 ° C to a maximum of +0.6°C and a controlled hold air temperature of between 2°C and 5°C, it was nevertheless necessary to ensure that the stowage of the container did not give rise to 'hot spots' at an air circulation rate of 40 air changes per hour. It was necessary not only to use vertical dunnage between the cartons but also to ensure that the dimensions of the carton were compatible with the specified dunnage and the internal length of the container. A simple calculation with deciduous fruit such as pears carried at - 0 . 5 ° C indicates that even with these conditions the range of air temperature in the container is such that temperature control must be exemplary as maximum fruit temperatures are usually above the return air temperature from the container: the sensible heat flow into container is 147 W, heat of respiration (maximum) is 176 W, difference in delivery and return air temperature based on 40 air changes per hour is -1.0°C.
temperatures and maximum and minimum air temperatures. Although having a maximum temperature difference of 5°C between cooler surface and container return air, under certain loading conditions of cargo such as citrus fruit, for which a relatively high rate of fresh air is required, the cooler is running wet. The relatively narrow trunking of some designs of container ship has resulted in water carry over from the cooler space to the air ducts and a high relative humidity in the containers. It is true to say that a large quantity of fruit is carried in both porthole and integral containers quite successfully, but it is important to recognize that container design, packaging, wrapping, fungal treatment as well as temperature control cannot in themselves improve the condition of fruit which begins to die the moment it leaves its parent. Only with correct harvesting, prompt pre-cooling and removal of unsound fruit can one hope to have an exemplary outcome, The use of pallets has become widespread particularly in the carriage of South African deciduous fruit and the container service between that country and North West Europe was specifically designed around the concept of the 1200 x 1000 mm pallet. Frozen cargo. Although in principle little has changed since the advent of the container service, nevertheless the carriage of some refrigerated cargo in containers is more likely to come into the sphere of national and international regulations for the carriage of foodstuffs.
It is now several years since the gradual downward trend of carrying temperatures, particularly for fish and frozen meat, reduced such temperatures from say - 1 0 ° C to - 1 8 ° C for meat and - 2 3 ° C for quick frozen foods, cream, juice concentrates etc and fish, This has come about not necessarily as a result of commercial pressures or food quality but from the enormous refrigerating capacity of the multitemperature ship which is essentially designed around the cargo with the largest heat load bananas.
Nevertheless, container ships w i t h o u t hold insulation and temperature control have managed to comply with Plant Quarantine regulations although the rate of air circulation has been 50% higher at 60 per hour and the heat leak into the containers some 20% less.
The container operator has largely followed these downward trends and the temperatures given above are commonplace. Nevertheless the competing International and European requirements for carriage of frozen and quick frozen food cause concern to the deepsea container operator w h o is caught up in regulations which were formulated some two decades ago but have only recently begun to be implemented.
Some stringent requirements for carriage of both deciduous and sub-tropical fruit are in evidence from the Perishable Products Export Control Board of South Africa where the surface temperature of the cooler is specified together with carrying
Although containers which have been in transit by sea for more than 150 km are normally exempt from such regulations as the Agreement for the Transport of Perishables (ATP) 1° the fact remains that individual governments can exercise control over the
18
Revue Internationate du Froid
movement of containers regardless of international agreements.
some cost, as the stowage factor is poor and the tare weight of the container high at about 10 tonnes.
The traditional cargoes of frozen meat and butter are still much in evidence, although fish and fish products are now an important part of container operations particularly in trade routes to Japan. It is perhaps the partiality of the Japanese to certain varieties of tuna which controls the storage temperature of such fish in order to preserve its colour, and temperatures of - 4 0 ° C to - 5 0 ° C have been quoted, but in practice have not been available.
After several unsuccessful attempts, a recent trial from Australia to the United States has demonstrated that chilled lamb can be transported in this way for about six weeks and with a low weight loss of about 0.5%. It is felt however that there may be further obstacles in the distribution and retail chain where good temperature control and hygiene are essential.
There are signs that carcase meat and jointed frozen meat in cartons may eventually be replaced by jointed meat frozen in a plate freezer. This will enable the exporter to use pallets more efficiently and, for example, increase the weight of lamb which can be shipped in a 20 ft container.
Low pressure and modified atmosphere storage No paper on container ships and their containers would be complete without some reference to modified atmospheres whether by packaging or engineering. The use of film wraps to modify atmospheres has been well known for fruit cargoes for some time and some significant advances have been made, more particularly to control such variables as relative humidity, but also to improve the micro climate around individual fruits. However it is with the carriage of chilled meat that more significant advances have been made, and joints of lamb or beef are placed in a particular type of polyethylene bag and all the air evacuated (Cryovac). The bag is then sealed and the meat stored at about - 1 ° C . With the oxygen removed from the bag, spoilage is restricted to bacteria which grow in the absence of oxygen (anaerobes) which are less destructive than their aerobic counterparts, ie those w h o require oxygen. It is possible to store chilled lamb for about six to seven weeks in this way and beef somewhat longer. The hypobaric system 11, on the other hand attempts to produce a similar condition by reducing the pressure around the meat to a few millimetres of absolute pressure. In this way the oxygen is reduced to a very low level thus creating similar conditions to Cryovac. There is however one important difference - with Cryovac there is no need to control relative humidity. With the hypobaric system however, it is necessary to maintain a very high relative humidity to avoid weight loss. This is accomplished by removing water from the air, which is removed by the vacuum pump, and returning it to the container. One major advantage of this type of container is that one can transport whole carcases of lamb, albeit at
Volume 5 Num~ro 1 Janvier 1982
Conclusions In a review of this nature it is difficult to draw any conclusion as to the most satisfactory type of refrigerated container ship as each type has its own advantages and disadvantages. It is perhaps true to say that in marine refrigeration generally there has been a swing back to the brine cooled air battery and, apart from the Columbus and Farrell vessels, this trend has continued with container ships. Other major differences are between the insulated and uninsulated hold, and this is unlikely to be resolved either way until there are clear economic conclusions. At present these tend to favour the insulated hold as lower machinery costs and running costs offset the cost of insulating the hold. This was reported some years ago 12 and although there was some discussion between the British and Continental lines when the container ships were ordered for the South Africa to Europe service, the argument was largely settled by Perishable Products Export Control Board in favour of the insulated hold. In terms of investment in capital equipment for refrigeration purposes there would be some difference between the cold well and the ducted air system but the former has operational disadvantages. The later trends in the ANZECS vessels to have a mixture of horizontal and vertical ducts appear to offer the best compromise in terms of flexibility of carrying temperature. The screw compressor, whether twin rotor or single screw, is likely to dominate the market for machinery in the foreseeable future, although smaller decentralized systems will still have their adherents. My thanks are due to the Shipowners Refrigerated Cargo Research Association for permission to publish this paper, to my colleagues at Cambridge w h o have been involved with much of the experimental work with containers during the past fifteen years, to Hall Thermotank International and Hall Thermotank Products for their help with photographs and to my many friends in the marine and refrigeration world w h o have contributed so much to this very absorbing and often forgotten subject. Last but not least to Mrs V. Potter w h o typed the manuscript.
19
References 1
2
3 4
Patents shipboard systems: a - BP 1 0 8 8 0 7 0 Improved containerised cargo refrigeration system (1967): b - BP 1 126 270 Refrigerated containerised cargo transport system and container (1968); c - BP 1 267 760 Improvement in or relating to transport ships for refrigerated containers (1972); d - BP 1 160391 Improvements in and relating to marine refrigeration (1969); e - BP 1 129859 Improvement in and relating to marine refrigeration (1968); f - BP 1 453555 Improvements in or relating to refrigeration of container cargo (1976) Patents containers and ancillary systems: a - BP 1 188240 A cargo container and closure therefor; b - BP 1 155993 Flexible couplings for providing fluid communication between two members; c - BP 1 318421 (Patent of Addition to BP 1 155993) Improvements relating to couplings or connectors for use with refrigerated cargo containers; d - BP 1 454 508 Improvements in and relating to refrigeration control systems; e - BP 1 579662 (Patent of addition to BP 1 454 508) Improvements in and relating to refrigeration control systems Jenks, W. S. C. Some factors in the design of cellular container ships with particular reference to refrigerated cargo, Trans/nst Mar Engrs 83 1 (1971 ) 1-20 M e e k , M. The first OCL container ships Trans R Inst Nav Arch 112 (1970)
5
6
7 8
9 10
11
12 13
USDA Animal and Plant Health Inspection Service P/ant protect~on and quarantine programs, Title 7, Chapter 3, Part 319, Subpart Fruits and Vegetables. Cold treatments of certain imported fruits (Jan 1. 1973) S t e r a , A. C., W i l s o n , J. J. The determination of an overall heat transfer coefficient for uninsulated holds of a refrigerated container ship. /n[/ns Refrig Bu//, Annexe Hamburg (1977-2) 59-72 B j o r k l u n d , J. S. An open air circulation system for refrigerated c o n t a i n e r s / n t Inst Refr/~7 Buff Annexe Hamburg (1977-2) 93-46 Hales, K. C., Mansfield, J. E., Scrine, G. R. Thermal testing of insulated vehicles. Proc Inst Refn~7 75 (197879) 55-64 B o w y e r , C. J., Scrine, G. R. Factors affecting the design of containers and the carriage of refrigerated cargo. Paper presented to Inst Refrig 3rd April 1980 Agreement on the international carriage of perishable foodstuffs and on the special equipment to be used for such carriage. Command Paper 8272 (1981) HMSO M a r m e l s t e i n , N. H. Hypobaric transport and storage of fresh meats and produce earns 1979 IFT Food Technology Industrial Achievement Award. Food Techno/ogy 33 (July 1979) 32-40 Hales, K. C., Stott, J. R., Wilson, J. J. The sea transport of refrigerated cargoes in containers Translnst MarEngrs 84 8 (1972) 251-263 Since the presentation of this paper there has been an outbreak of Mediterranean Fruit Fly in California, USA
A monthly journal to keep you in touch with developments in the low temperature field. Cryogenics is the international journal of low temperature research and engineering. The subject matter covers the complete field of science below --100°C. This includes low temperature plant technology, cryosurgery, train suspensions and superconducting motors. Each issue contains specially commissioned review articles and original papers. Comphmentary to these are research and technical notes describing preliminary results and details of work in progress, and letters to the editor on areas of discussion and controversy. Other features include reports on conferences, book reviews and a lively news section covering the cryogenics industry. The quality of the editorial is ensured by a distinguished board of international editors. For further details and sample copy contact= Geraldine Hills, IPC Science and Technology Press Ltd, PO Box 63, Westbury House, Bury Street, Guildford, Surrey GU2 5BH. Tel: 0483 31261
20
International
Journal
of Refrigeration
Le froid & bord des navires et les porte-conteneurs
types de navires, les conteneurs, leur construction, leur isolation et les syst#mes frigorifiques pour diffarentes cargaisons. On examine les transferts de chaleur.
On passe en revue les aquipements de froid ~ bord des porte-conteneurs en donnant des data/Is sur les
A review of refrigeration in container ships, with details of the types of ship, the containers, their construction, insulation and
systems of refrigeration for different cargoes. Heat transfer is discussed.
It is now some twenty-five years since container ships operating in regular liner services first carried perishable produce, principally from the West Coast of North America to Hawaii and also to Alaska. Some of the early container ships were converted tankers and were capable of carrying both dry cargo containers and insulated containers with a refrigeration unit mounted in the front bulkhead.
Following the containerization of this service, there was a rapid growth in container services from the principal exporting country of primary productAustralia, to Japan and South East Asia and to the east and west coasts of North America. More recently New Zealand has containerized the majority of its exports of perishable foodstuffs and although still heavily dependent on the European market for exports of butter and lamb, is pursuing additional markets with some vigour.
The Matson Line operated a number of insulated containers which were fitted with a Thermo King SL5 unit; this tended to influence the design of integral containers for a number of years. Another operator of refrigerated containers of the same type was Sea Land which was engaged in a number of services from the United States to Europe, Central America and the Far East. This company was particularly conspicuous in using containers 35 ft (10668 m) in length, many of which were subsequently lengthened by welding ~,1 an additional 5 ft (1.524 m) to bring them into line with current International Standards. During the 1960s numerous patents were taken out principally in the United States 1~,1b,but also in the United Kingdom ld.le, which covered a variety of methods of refrigerating containers on board ship, many of which are in use today with little or no modification. Some patents covering container equipment such as couplings, closures and temperature control are given in references. 2~-2e
The Caribbean to Europe (CAROL) service was the first to use ships specially designed for the carriage of bananas, and pioneered the use of 40 ft (1 2.1 92 m) porthole containers which are cooled by ducting air to the interior via two 350 mm diameter holes in the front bulkhead. However both United Brands and Sea Land operate 40 ft (1 2.1 92 m) integral containers for the carriage of bananas from Central America to the United States. Within the last two years additional services have commenced between Europe and South Africa and more recently between Australia and the Gulf of Mexico ports of the United States of America. With new ships being constructed for services between South America and Europe it would appear for the time being that the majority of the principal exporting countries of refrigerated produce can benefit from a liner container service.
In the mid 1960s it became apparent that traditional methods of carrying both refrigerated and general cargo by sea were becoming increasingly expensive and proposals were put in hand to containerize the service between Europe and Australia involving the British, Continental and Australian lines.
A number of methods have been proposed for carrying containers under refrigeration at sea. and in general these have been developed to suit particular operational circumstances in both exporting and importing countries. As each of these concepts has several noteworthy features, it is proposed to consider these features in some detail.
The author is Technical Director, Shipowners Refrigerated Cargo Research Association, Newmarket Road. Cambridge. UK Paper received April 1981, being an edited version of that originally presented at 53rd Thomas Lowe Gray Lecture
All container ships whether carrying refrigerated, insulated or dry cargo containers are designed to carry a quantity of each type and the systems which have been evolved have had a profound effect on
Volume 5 Number 1 January 1982
0140-7007/82/010009-1253.00 @1982 Butterworth & Co (Publishers) Ltd and IIR
9
the design of the container ship and to a lesser extent on the refrigeration plant.
T h e c o n t a i n e r ship The majority of the container ships in refrigerated liner ('reefer') trades are fully cellular, ie all containers are located within guides and there are no conventional cargo decks loaded by cranes, with the possible exception of No. 1 hatch. There are however some vessels which are part container and part conventional cargo. The refrigeration systems for the various container services have been influenced by the type of cargo carried, although there have been variations within each trade depending upon an individual operator's particular solution. For example, in the Australia to Europe service the ships designed by OCL (Overseas Containers Limited) which had a vertical duct system with a maximum of six containers per cooler in one duct giving a certain amount of flexibility: at the ship's side above wing tanks, it was only possible to accommodate three containers. The solution adopted by A C T A / A N L (Associated
Container Transportation (Australia)/Australian National Line) in the same trade was to have one cooler with up to 24 containers connected to a number of parallel horizontal ducts. Ships operating between Australia and North America were more varied with the PACE (PacificAmerica Container Express) Line operating designs similar to A C T A / A N L and Columbus Line operating ships with air ducts and couplings similar to those of OCL but without hold insulation. Farrell Lines" ships had no air ducts whatsoever but containers were located in guides in a fully insulated and refrigerated hold. Some excellent descriptions of the early ACT Class vessels were given by Jenks s and of the OCL ships by Meek. 4 It may be of interest to consider some of the details and dimensions of ships built for various trades as given in Tables 1-4 and two examples are given in Figs 1 and 2. Of the less conventional type of container ship, the MV Ariake built for the Australia to Japan container service has the details given in Table 3.
Table 1. Comparison of various container ships belonging to British and Australian lines Tableau 1. Comparaison de divers porte-conteneurs des/ignes britanniques et austra/iennes Encounter Bay
ACTI
Mairangi Bay
Australian Venture
Table Bay
Loa, m
227.1
217.3
248.6
248.6
2585
dwt
29 500
26 376
38 757
32 800
47 200
grt
26 750
24 820
44000
44 345
53 780
Containers total insulated
1 572 356
1 334 326
1812 1223
1822 913
2436 980
Speed, knots
23
22
21 5
23
21
Machinery
2 x Stal turbines
2 x Stai turbines
2 x MAN diesel
2 x MAN diesel
2 x MAN diesel
Power, kW
24 170
22 380
38 747
39 187
38 270
Table 2. Comparison of various container ships belonging to Continental and American lines Tableau 2. Comparaison de divers porte-conteneurs des/idnes continenta/es et amdricaines NedLIoyd Houtman
Farrell C6
Farrell C8
Columbus Queensland
Caribia Express
Loa, m
258.5
204.0
248.0
1 84.9
204.0
dwt
48 000
19 553
28 672
24 400
18 500
21 489
31 487
19193
27 936
1186 288
1708 768
1t74 596
1456 120 × 40 ft
grt Containers total insulated below deck Speed, knots
2314 352 21
226
22'
!9 5
,'b5
Machinery
2 x Sulzer
t~lrbine
turbine
Sulzer
(;ZI ,, Sulzer
Power, kW
39 500
71 634
2 ~ 63,4
!()950
2i b?,'i
10
Revue Internationale dLJ Frond
Table 3. Cellular container ship carrying integral containers Tableau 3. Porte-conteneur compartiment~ transportant des conteneurs int#gr#s Loa, m
237.8
dwt
28 295
grt
38 000
Containers total reefer
1748 x TEU 606*
Speed, knots
26
Machinery
2 x MAN
Power, kW
39 687
*
These are integral containers carried below deck.
Table 4. Parameters of ACT 7 shown in Fig. 2
since proved unfounded, as has been demonstrated on the vessels built for the Australia-Japan Container Line as exemplified by the Ariake. In 1965, the Technical Committee of the SRCRA (Shipowners Refrigerated Cargo Research Association) investigated various alternative means of refrigerating containers at sea, the British lines being in favour of the two porthole container, a design which has become popular in services where there is a large volume of refrigerated cargo carried at the same temperature, such as frozen meat and butter. Continental lines in the Australian Conference, Hapag Lloyd AG, NedLIoyd, Messageries Maritimes (now Compagnie G6n6rale Maritime) together with the Australian National Line also use this particular design of container and its associated refrigeration system.
Tableau 4. Param~tres de ACT 7 pr#sent~ ~ /a Fig. 2 Dimensions
Capacity
Length o.a.
248.60 m
Length b.p.
236.20 m
Beam
32.29 m
Depth
21.50 m
Draught
12.025 m
Displacement
61 428 tonne
933 reefer containers 1069 general containers 2002 standard ISO container equivalents
Container dimensions External The overall weight of container and cargo has been restricted by regulation in the United Kingdom and other countries and is effectively limited to a gross of about 20 tonnes. Current designs of dry freight container defined the width of the container at 2.44 m (8 ft). The weight restrictions virtually fixed the overall length at 20 ft (6,097 m) for containers loaded with butter and certain meat cuts, this conforming with the standard container of the day.
As a comment on the size of container ships which were designed to carry dry cargo containers together with some integral containers on deck, the Sea Land Galloway Class and Liverpool Bay must rank as the largest and the most powerful in terms of installed power (89 520 kW and 6 0 4 2 6 kW respectively) both being originally engined with steam turbines. These are now being converted to diesel power, which is a direct result of escalating fuel costs, and sets the trend for perhaps another decade or more.
Containers Before considering the types of container ship which were proposed in the 1960s for the Europe to Australia service, it is pertinent to consider some of the factors leading to the possible choices of a given container type and the reasons for that selection.
Fig. 1 deck
Mairangi Bay 1223 insulated containers carried below
Fig. 1 Les 1223 conteneurs iso/ds de Mairangi Bay transport#s au-dessous du pont !
The original services, such as those operated by the Matson Line, had a relatively small number of containers with inbuilt refrigeration units which are commonly referred to as integral units. When alternative designs of container were considered for the Australian service some doubts were expressed concerning the reliability of integral units, and the possibility of failure from 400 containers during a 28 day voyage could not be ruled out. Such fears have
Volume 5 Num~ro 1 Janvier 1982
/
Brine room
/ Con,ro, room IA,r~=~ / J / Engine room
~=~1~1~11~1 " / J ~ l t ~ l ~ l ~ l ~ l ~ l ~ l l ~ l l ~ Bollosf and fuel tanks Bow thruster Fig. 2
Act 7
Fig. 2
ACT 7
-/
J /
11
When loaded with frozen lamb in carcase, chilled meat in cuts or fruit, container loads vary from about 9 to 13 tonnes, thereby giving some support to the case for using 40 ft (1 2.19 m) containers, although the same overall weight restrictions apply when travelling on the road. The two porthole container was originally conceived as a module having external dimensions of 20 x 8 x 8 ft (6.095 x 2.438 x 2.438 m), although these have subsequently been increased to 20 x 8 x 8.5 ft (6.095 x 2.438 x 2.591 m), for South Africa to Europe and more recently 40 x 8 x 8.5 ft (1 2.19 x 2.438 x 2.591 m) in services between the Caribbean and North West Europe. /nterna/. Some typical dimensions for insulated two porthole containers are 5.666 x 2.235 x 2.083 m for containers manufactured for the Australian service. More recently containers for the South Africa to Europe service have internal dimensions of 5.77 x 2.3 x 2.285 m which were specified in order to accommodate ten 1200 x 1000 mm pallets of fruit. This specification largely resulted in containers with a higher overall heat transfer coefficient of about 30 kcal h -1 °C -1 (34.9 W K -1) although one particular design had a figure of 23 kcal h -1 °C -r (26.74 W K -1) albeit by improving the insulation under the floor girders. This does little to improve the temperature range of air and cargo and such containers have a poor performance compared with the more highly insulated types when off refrigeration.
Polyurethone foam insulation
f 75mm
Permanent dunnage battens
Air outlet =
250mm diameter
/~
Air inlet
P
/ Spring loaded closure valve
J
J
50mm T section
Fig. 3
Section of insulated container
Fig. 3
Coupe d'un conteneur isol#
compression. Floors are usually supported by steel cross members with wood grounds to provide some insulation between steel and the aluminium floor. High density polyurethane or PVC foam is sometimes used in place of wood. The usual wall thickness of 20 ft insulated containers is about 75 mm, of which 16 mm is plywood and GRP, and that of the roof and floor about 100 mm.
Container insulation and structure Although the original containers in the Australian service were designed for six-high stacking, the bottom container having to support 100 tonnes between its four corner posts, later designs of container ship can accommodate nine-high stacking under the hatches which gives a working load of t 60 tonnes. Including the appropriate safety factors, the present design load of each corner post is 84 tonnes, which is achieved with a relatively small cross-section fabricated from high quality steel. The floor, side and end walls together with the doors are designed to meet the requirements of ISO 1496 I and II and of the Container Safety Convention at present being ratified by a number of countries. There are however individual company ,-equirements for deflections of end walls and side walls under uniformly distributed loads. The structural strength of side walls is accomplished in several ways, the most successful being of sandwich panel construction, the outer skin generally being plywood faced with glass reinforced polyester resin (GRP) and the inner face GRP entirely. Glass reinforced polyester or resin inserts are used in some designs to provide structural rigidity in bending and to a lesser extent in
1;~
The r e f r i g e r a t i o n of c o n t a i n e r s The cooling and maintenance of the temperature inside a container is by circulation of air either from a duct system through ports in the front wall of the container (Fig. 3) or from a fan and cooler unit located in the front wall of the container with its associated compressor and condenser. The system of placing a pre-cooled container in a cold well is not very flexible and relies on an efficient transport and holding system prior to loading on board, as well as cargo being pre-cooled to or below the required carrying temperature. It is proposed to consider the various types of container system in some detail as follows: Vertica/ ducts, ho/ds insu/ated. This design was originally used in the first generation container ships for Overseas Containers Ltd and although proposed by the Consortia's engineers was eventually patented by the German Shipyard and their subcontractors and is now widely known under the trade name 'ConAir'. lc A British designed version of the same system is known as 'Sea Rod" (Fig. 4) and similar equipment has been designed by various refrigeration
International Journal of Refrigeration
machinery contractors in other European Countries, but having no known trade-mark. The principal advantage of this type of system is the flexibility it offers in terms of individual temperatures. For example it is nominally possible to carry two varieties of fruit, eg plums at 8°C and grapes at O°C on adjacent coolers with the same inlet brine temperature to the cooler. The choice of an insulated hold provides an environment around the outside of the container which minimizes sensible heat inflow to the container air. reduces the air circulation within the container to maintain a prescribed temperature difference and also reduces the refrigeration power necessary for a given number of containers. The more recent designs for the South Africa to Europe service have increased the air circulation from 1 200 m 3 h -1 (40 air changes) per container with a net volume of 27 m 3, to approximately 1 870 m 3 h -1 (60 air changes). This has largely come about to meet the requirements of the Perishable Products Export Control Board (PPECB) which prescribed a 1°C range of temperature within the container. An additional requirement of this trade was that of relative humidity which was defined by the temperature difference between the air leaving the container and the surface temperature of the coil. This is essentially a vapour pressure difference with its lower limit depending on the surface area of the cooler and its upper limit largely governed by water conditions at the surface of the particular fruit at the prescribed storage temperature.
The insulated hold was not w i t h o u t its problems, as w i t h o u t an air heating system around the outside of the containers, hold air temperatures of - 8 ° C to - 1 0 ° C were envisaged, leading to the requirement of E quality steel which would not be susceptible to failure from brittle fracture. Other solutions to the problem were to heat the air to maintain temperatures of +1°C to +2°C around the containers or to leave the boundaries of the hold uninsulated although this would need some heating at tow ambient temperatures. This enabled designers to use D quality steel with some obvious cost savings. One further advantage of the insulated hold and its ability to keep temperature gradients to a minimum has been the acceptance by the Plant Quarantine Service of the United States Department of Agriculture 5 for imported fruit from regions where Mediterranean or Queensland Fruit Fly are endemic. For example, apples and pears from the Austalian mainland have to be kept below +0.6°C for a period of 14 days to ensure 100% mortality of the fruit fly larvae.
Vertica/ ducts with uninsulated holds. This system was developed in the Columbus Line fleet principally for the service between Australia and the east coast of North America. Of similar capacity to the PACE Line vessels, the containers had a somewhat lower overall heat transfer coefficient but the principal difference was a 50% increase in the rate of air circulation within the container. This figure was increased from 1 200 m 3 h -1 (40 air changes) to 1 800 m 3 h -~ (60 air changes) per container with a consequent increase in fan Watts and refrigeration capacity. Although little was known about heat transfer from the steel boundary of uninsulated holds to tall stacks of containers the design was successful and apparently had few problems with low hold air temperatures except in the North American winter. The combination of greater air flow and lower heat transfer of the containers enabled the ships to comply with the United States Department of Agriculture Plant Quarantine regulations, although some time elapsed before this approval was finally granted. Although some theoretical assessment had been made of heat transfer from uninsulated vertical steel surfaces, it was not until 1977 that a serious attempt was made to measure this in a hold filled with containers. 6 One additional feature common to all container ships carrying containers is the heat balance between the outside of the hold and the cargo within the containers.
Fig. 4
'Sea Rod' vertical air duct
Fig. 4
Gaine d'air vertica/e de Sea Rod
Volume 5 Number 1 January 1982
One of the principal disadvantages is perhaps the greater range of air temperature in the air space around the outside of the container. Some measurements have shown temperatures under the hatch cover to be at or near ambient which may
13
give rise to a greater range of air temperatures wihin the container. This will probably be of little consequence when carrying frozen cargo, but may be embarrassing under Plant Quarantine conditions. (It was noticeable in the PACE ships with insulated holds that when hatches were lifted container temperatures in the top tier rose significantly and on occasions jeopardized the Plant Quarantine treatment.)
I f
H o r i z o n t a l f i n g e r d u c t s , i n s u l a t e d h o l d . This has
been one of the more popular concepts amongst the British lines and their associates in the Australia and New Zealand trade (Fig. 5).
/
/
In the early ACT vessels up to 24 containers were cooled from a single cooler and fan. The more recent designs however have increased this figure to a maximum of 48 containers. Whereas this is very economic for containers carrying large quantities of frozen produce such as meat and butter, it can result in a certain loss of flexibility as far as carrying temperatures are concerned.
/
t
/ n s u l a t e d h o l d , n o air d u c t s . This system has been
commonly described as the 'cold well' but has found few followers outside the United States. Again the subject of an early patent TM, the original concept was to use containers with apertures in the front wall and located within vertical guide systems but with fans blowing air into the interior of the container. The early Farrell Line container ships were the first to use the cold well, but this was followed by a modified system with air ducts 7 and nozzles designed for the Swedish Johnson Line container ships (Fig. 6). A similar version of the Stal system was used in two multi-purpose reefer-container ships which were converted in the mid 1970s. The Farrell Line's vessels were designed to fulfil the concept that refrigeration would only be required for frozen and those chill cargoes not requiring fresh air ventilation. By accepting this concept it was possible to design a simple air circulation system, as cargoes could be
/
Fig. 6 Nozzlesystem for ducting air to containers Fig. 6
Syst~rne d'a/utages pour amener /'aJr aux conteneurs
cooled by conduction from cold air circulating around the outside of the container. This necessitated adequate pre-cooling of the cargo to the prescribed carrying temperature as the centre of a container could take a considerable time to cool by conduction alone. The absence of any ducts, couplings and venting systems allowed for some economy in construction, albeit at the cost of perhaps a greater thickness of insulation on the boundaries together with the use of high quality steel for lower temperatures. One operational difficulty feared in the early days, was the possible icing up of containers when discharged from the ship and difficulties in handling. In practice these fears do not appear to have been justified and the system, when modified as on the Swedish Johnson Line vessels, does offer scope for more flexibility in terms of cargo, though at a somewhat higher cost.
The heat balance
The principal means by which heat enters the refrigerated hold is by conduction through the various boundary surfaces, and also by infiltration of fresh air either accidentally through leakage in hatches and access doors or deliberately by fresh air ventilation of fruit cargoes. The energy from air circulating fans is dissipated within the air duct and container systems and has to be removed by the refrigeration plant. Fig 5 Horizontal air ducts Act 7 Fi,q 5
14
Games d'air horlzontales de ACT 7
During early investigations into the design of insulated container holds some thought was given
Revue Internationale du Froid
to the heat transfer mechanism between heat entering the boundary surfaces of the hold and then transferred to the air circulating inside the container. It was thought at the time that air w o u l d either convect naturally between the vertical stacks of containers or circulate deliberately by hold air conditioning systems. The top surface of the top containers and the bottom surface of the bottom container were considered as major heat transfer surfaces together with the doors, front and side walls. For the early ships designed for the British lines this meant an effective container surface area of about 67% of the total available. Some recent investigations 6 have shown that the effective heat transfer coefficients for the system were within 10% of those originally estimated for the vessels concerned. It is virtually impossible to use a mean heat transfer coefficient for the hold as a whole and it is necessary to consider each boundary on its own merits and calculate the heat transfer in terms of the temperature difference. As the insulation of a modern container ship is rather simpler than a conventional reefer ship, ie a general absence of deep frames buried within the insulation, estimation of heat transfer is somewhat easier. With the uninsulated hold, however, conditions are different and it is largely a question of estimating the surface air velocity between adjacent surfaces and the resulting heat transfer coefficients. There is also the possibility of heat transfer by radiation although this is unlikely to be significant from wing tanks and double bottoms in insulated holds. The problem of container surface area transferring heat from the hold to the internal air has also been investigated by the same author w h o has concluded that between 91 and 95% of the external surface area conducts heat from the hold to the inside of the container. This factor depends on the type of air circulation, whether forced or natural and the height of the container stack. This has removed any argument about the behaviour of air trapped beneath the top of one container and under the floor of the adjacent container. There appears to be a movement of air between adjacent vertical columns because of convection arising from pressure and temperature differences. Some measurements made recently on one of the South African container ships by SRCRA have confirmed that there is little difference between the air temperature in the vertical slots between adjacent rows of containers and that under the floors of containers.
gradient in the vertical slots under these conditions was between 2 and 4°C. Measurements by Stera 6 have suggested little difference between the two systems. The calculation of the heat balance for individual holds is straightforward and is related to the heat removed by the brine in the case of a system using a secondary refrigerant or the change in enthalpy multiplied by the mass flow in the case of a primary refrigerant using, say, R 22.
Effect of container deterioration on refrigeration requirements The majority of the design calculations for sizing refrigeration plant were based upon a container heat leak of 26.7 W K -1 (23 kcal h -1 °C-1). Many of the original designs were at least 10% below this figure leaving some margin for deterioration of containers in service. It has been found following some intensive retesting by the SRCRA 8 of in-service containers that whereas the majority of containers deteriorate at a reasonable rate, some designs were worse than others. Some typical curves of container deterioration with age for two containers, one of loose panel construction, the other sandwich panel and both insulated with polyurethane foam, are given in Fig. 7. Should a container ship hold be loaded with a large number of old containers, the general result will be to lower the hold temperature and increase the heat load on the refrigeration plant. The range of air temperatures within the container could increase but this is more likely to be a problem on ships with uninsulated holds particularly under Plant Quarantine conditions as laid down by the United States Department of Agriculture. 5 The behaviour of the container on land is, however, more likely to be of concern particularly if such equipment as clip-on units, whether mechanical or 80 Manufacturer 'A' (loose panel)
40
'B' (sandwich panel)
I 20 Data corrected to o mean wall temperature of 283K 0
It is also interesting to note that temperature gradients from top to bottom of a six-high stack under natural convection were somewhat less at 1.0°C than the corresponding temperature difference in a ship where hold air temperatures had to be maintained above 2°C by forced air circulation. The
Volume 5 Num6ro 1 Janvier 1982
O
I 2
I 4
I 6
I 8
I I0
Years
Fig, 7 Deterioration curves for containers insulated w i t h p o l y u r e t h a n e foam
F i g 7 Courbes de d#t#rioration des conteneurs iso/#s avec de/a mousse de polyur#thane
15
liquid nitrogen, may not have the capacity to cope with a container whose heat leak has increased by 80%.
R e f r i g e r a t i o n systems It is almost universal to use both primary and secondary refrigerants to cool air which in turn maintains cargoes at acceptable temperatures. The major exceptions have been vessels engaged on the North American service from Australia, which use direct expansion. Those from Australia and New Zealand to Japan, the North Atlantic services and a number of small refrigerated container operations use integral containers. These units are mostly electric drive plugged in to appropriate power points on shore or onboard ship. A number of units nowadays are compact enough to allow for a removable diesel alternator set to be fitted when the container is travelling over the road, or sited in areas where a suitable three-phase supply is not available. By far the most interesting new development in refrigeration machinery has been the introduction of the single rotor screw compressor. A version of this is manufactured and marketed in this country, and an alternative version in Holland. Possibly one of the long term advantages of this type of compressor is a trend towards a reduction in lubrication and oil separation problems, and the ability to scale the machine up or down. It also has the ability to pump liquid refrigerant provided the rotor and planet wheels are manufactured with compatible materials.
Integral c o n t a i n e r s Whilst most refrigerated containers are of the twoporthole type there are significant numbers of 20
and 40 ft containers which have an inbuilt refrigeration system to carry cargoes in the range + 2 0 to -25°C. These units are mostly electric drive plugged into appropriate power points on shore or on board ship. A typical specification for a 20 ft unit for Far East service is shown in Table 5. Some services require the use of water cooled condensers below deck which are in series with the air cooled condenser and act as liquid receivers when operating in the air cooled mode. The condenser fan is stopped by a switch activated by water pressure. It is usual to recirculate fresh water through these systems, the water being maintained at a constant inlet temperature through a water to water heat exchanger. These condensers have not been without their faults particularly in areas where the winter temperatures fall below O°C and the water does not completely drain from the system. More recently there has been a tendency to abandon the water cooled system in favour of air cooled condensers operating below deck but with a hold ventilation system capable of removing the surplus heat. Capacity control is normally achieved by use of dual compresso[s or cylinder unloading but temperature control is by hot gas injection operated from a multi-step controller, or continuously modulating electromagnetic valve. This effectively balances the refrigeration output with the load over a wide range of ambient temperatures. Many units are still in daily use with an on-off control of the compressors combined with a suction pressure regulator or fixed hot gas injection. The operation of a container system offers scope for microprocessor control particularly if one requires full system monitoring at the same time. There is no doubt that in the short term these systems will be more expensive than those they replace but could offer significant advantages as manpower becomes more expensive. The mechanical controls are
Table 5. Specification for a 20 ft unit for Far East service Tableau 5. Prescriptions pour un conteneur de 20 ft ~ uti/iser pour/'Extreme-Orient Electrical supply
380-460 V, 50 or 60 Hz
Power consumption
5-6 kW
Ambient temperature
+ 40°C
Temperature control
+0.25°C in range -4°C to +10°C +0.5°C in remainder of range
Internal air circulation
60 changes per hour at an internal load resistance of about 10 mm water gauge
Temperature difference, evaporator to air delivery
8°C at a container temperature of - 18°C
Fresh air changes
At least 1 per 4 h
CO 2 sampling
Yes
Condensers
Finned copper or coated alloy if air cooled
Electric defrosting
3 kW plus tray and drain heaters
Compressor
3.7-5.6 kW semi-hermetic 1 500 or 1750 rev min frequency
Evaporator and condenser fans
Propeller 1800
16
m 3 h -1,
-1
depending on
consuming about 1 kW each
International Journal of Refrigeration
generally robust and inexpensive but in the event of failure of the system as a whole it may require some special skills to diagnose and rectify the fault. The SRCRA have undertaken a development programme to produce a prototype control and monitoring system which is microprocessor based. With a high capability for detecting various conditions of temperature and pressure within the refrigeration system together with power measurement, it is hoped to give a series of alarm conditions which will help to pinpoint the possible cause of failure.
C a r r i a g e of c a r g o It w o u l d be possible to dwell at some considerable length on the various factors which govern the successful carriage of cargo in containers. The container ship in terms of carriage of cargo, was initially a sideways step from conventional carriage in the refrigerated ship of the day. Some of the early design requirements for the container were based on the then current designs of refrigerated liner vessels (not specialist banana carriers) and that they have been eminently successful has been proved beyond doubt.
Rates of air circulation and fan design The lowest acceptable rate of air circulation for both deciduous fruit and frozen cargo is approximately 30-40 air changes per hour of the empty volume of the container when carried in an insulated hold. For special requirements such as those of the Perishable Products Export Control Board of South Africa, this rate of air circulation is increased to 60 changes per hour when carrying fruit and 30-40 changes per hour when carrying frozen cargo. The container ship with uninsulated holds will also have similar rates of air circulation. By far the highest is that for the specialized container ship carrying bananas as exemplified in the CAROL (Caribbean Overseas Line) service where a rate of 70-80 per hour is required. This is reduced to 35-40 per hour when the containers are carrying frozen cargo. Fans have varied from simple axial flow in the early Encounter Bay Class to the mixed flow for the ACT Class and on the later vessels built for the ANZECS service (Australia New Zealand Europe Container Service). The centrifugal fan with two speed motors is used in some vertical duct systems where pressure loss in the ductwork can be considerable. However, where two speed operation is required it is usual to have two axial fans in operation for maximum air circulation reducing to one for frozen cargo or when a reduced number of containers is carried on a stack. The ships designed to carry the early 40 ft porthole containers, had an unusual fault in that the closure device slammed shut when the number of containers on a cooler duct was at a critical level. 9 Some details are given in Table 6.
Volume 5 Number 1 January 1982
Table 6. Conditions for slam closing of container closures Tableau 6. Conditions de la fermeture des conteneurs par c/aquement Air changes Plate Initial spring to slam diameter, force/area of closing mm plate, g cm -2 40 ft (12.195 m) container 20 ft (6.097 m) container (South African service) 20 ft (6.097 m) container (Australian service)
120
415
1.39
153
336
2.23
200
285
4.17
As cargoes can be broadly divided into two major categories, ie chilled and frozen, the former embracing both deciduous, sub-tropical and tropical produce, the latter any commodity which requires storage below - 2 ° C , it is proposed to consider each briefly and draw attention to some of the more interesting and perhaps difficult conditions of carriage.
Chill cargoes. These require in general a greater degree of temperature control to avoid freezing, in the case of some deciduous fruit, and chilling injury with the subtropical and tropical fruits. Of the latter some of the more important are the banana, the avocado pear and the pineapple. The carrying temperatures for these fruits may vary in certain countries of the world and it should not be taken for granted that a fruit grown in one hemisphere will behave in exactly the same way as the same variety grown in another. All fruit is living and produces heat and carbon dioxide when converting starch in the tissue to sugar. The refrigeration requirements to remove such heat together with cooling fresh air which is used to remove unwanted volatiles can be considerable. As the range of temperatures is said to be critical for some produce it has become fashionable to circulate air at a very high rate when carrying bananas, for example, and the penalty for the resulting fan power can be considerable. To quote an example for a container ship carrying about 350 porthole containers and with a maximum rate of air circulation of about 70 changes with fans at full speed and 40 changes with fans at low speed, the difference in power consumption is considerable, and is shown in Table 7. Deciduous fruit may be no less demanding, particularly when carried under conditions to satisfy the Plant Quarantine requirements for the United States Department of Agriculture. 5 The Mediterranean fruit fly, Ceratitis capitota, which is endemic in most Mediterranean countries has not so far penetrated the United States. 13 By far the most
17
Table 7. P o w e r consumption figures for a small container ship w i t h insulated holds Tableau 7. Consommation d'#nergie pour un petit porte-conteneur avec ca/es iso/#es
Cooling Holding
Compressors
Pumps F a n s
Total, kW
808 448
88 64
1037 552
137 40
troublesome species of fruit fly comes from Queensland and requires a somewhat longer time at a given temperature than some of its relations from Mexico, South Africa or Israel. A maximum fruit temperature of 0.6°C for 14 days is specified for the Queensland fly although some recent work in Australia has suggested that a period of 25 days or thereabouts at + 1.8°C is equally effective. In order to achieve a maximum fruit temperature of 0.6°C it has been found necessary not only to take particular care to load a container with fruit pre-cooled to below +2°C and preferably at carrying temperature, but to maintain such conditions up to the time of loading in the ship. The design of container ships favoured by the British Consortium operating from Australia to East Coast North America (PACE Line) with insulated and temperature controlled holds was particularly suitable for this operation. With a temperature range in the container of - 1 . 5 ° C to a maximum of +0.6°C and a controlled hold air temperature of between 2°C and 5°C, it was nevertheless necessary to ensure that the stowage of the container did not give rise to 'hot spots' at an air circulation rate of 40 air changes per hour. It was necessary not only to use vertical dunnage between the cartons but also to ensure that the dimensions of the carton were compatible with the specified dunnage and the internal length of the container. A simple calculation with deciduous fruit such as pears carried at - 0 . 5 ° C indicates that even with these conditions the range of air temperature in the container is such that temperature control must be exemplary as maximum fruit temperatures are usually above the return air temperature from the container: the sensible heat flow into container is 147 W, heat of respiration (maximum) is 176 W, difference in delivery and return air temperature based on 40 air changes per hour is -1.0°C.
temperatures and maximum and minimum air temperatures. Although having a maximum temperature difference of 5°C between cooler surface and container return air, under certain loading conditions of cargo such as citrus fruit, for which a relatively high rate of fresh air is required, the cooler is running wet. The relatively narrow trunking of some designs of container ship has resulted in water carry over from the cooler space to the air ducts and a high relative humidity in the containers. It is true to say that a large quantity of fruit is carried in both porthole and integral containers quite successfully, but it is important to recognize that container design, packaging, wrapping, fungal treatment as well as temperature control cannot in themselves improve the condition of fruit which begins to die the moment it leaves its parent. Only with correct harvesting, prompt pre-cooling and removal of unsound fruit can one hope to have an exemplary outcome, The use of pallets has become widespread particularly in the carriage of South African deciduous fruit and the container service between that country and North West Europe was specifically designed around the concept of the 1200 x 1000 mm pallet. Frozen cargo. Although in principle little has changed since the advent of the container service, nevertheless the carriage of some refrigerated cargo in containers is more likely to come into the sphere of national and international regulations for the carriage of foodstuffs.
It is now several years since the gradual downward trend of carrying temperatures, particularly for fish and frozen meat, reduced such temperatures from say - 1 0 ° C to - 1 8 ° C for meat and - 2 3 ° C for quick frozen foods, cream, juice concentrates etc and fish, This has come about not necessarily as a result of commercial pressures or food quality but from the enormous refrigerating capacity of the multitemperature ship which is essentially designed around the cargo with the largest heat load bananas.
Nevertheless, container ships w i t h o u t hold insulation and temperature control have managed to comply with Plant Quarantine regulations although the rate of air circulation has been 50% higher at 60 per hour and the heat leak into the containers some 20% less.
The container operator has largely followed these downward trends and the temperatures given above are commonplace. Nevertheless the competing International and European requirements for carriage of frozen and quick frozen food cause concern to the deepsea container operator w h o is caught up in regulations which were formulated some two decades ago but have only recently begun to be implemented.
Some stringent requirements for carriage of both deciduous and sub-tropical fruit are in evidence from the Perishable Products Export Control Board of South Africa where the surface temperature of the cooler is specified together with carrying
Although containers which have been in transit by sea for more than 150 km are normally exempt from such regulations as the Agreement for the Transport of Perishables (ATP) 1° the fact remains that individual governments can exercise control over the
18
Revue Internationate du Froid
movement of containers regardless of international agreements.
some cost, as the stowage factor is poor and the tare weight of the container high at about 10 tonnes.
The traditional cargoes of frozen meat and butter are still much in evidence, although fish and fish products are now an important part of container operations particularly in trade routes to Japan. It is perhaps the partiality of the Japanese to certain varieties of tuna which controls the storage temperature of such fish in order to preserve its colour, and temperatures of - 4 0 ° C to - 5 0 ° C have been quoted, but in practice have not been available.
After several unsuccessful attempts, a recent trial from Australia to the United States has demonstrated that chilled lamb can be transported in this way for about six weeks and with a low weight loss of about 0.5%. It is felt however that there may be further obstacles in the distribution and retail chain where good temperature control and hygiene are essential.
There are signs that carcase meat and jointed frozen meat in cartons may eventually be replaced by jointed meat frozen in a plate freezer. This will enable the exporter to use pallets more efficiently and, for example, increase the weight of lamb which can be shipped in a 20 ft container.
Low pressure and modified atmosphere storage No paper on container ships and their containers would be complete without some reference to modified atmospheres whether by packaging or engineering. The use of film wraps to modify atmospheres has been well known for fruit cargoes for some time and some significant advances have been made, more particularly to control such variables as relative humidity, but also to improve the micro climate around individual fruits. However it is with the carriage of chilled meat that more significant advances have been made, and joints of lamb or beef are placed in a particular type of polyethylene bag and all the air evacuated (Cryovac). The bag is then sealed and the meat stored at about - 1 ° C . With the oxygen removed from the bag, spoilage is restricted to bacteria which grow in the absence of oxygen (anaerobes) which are less destructive than their aerobic counterparts, ie those w h o require oxygen. It is possible to store chilled lamb for about six to seven weeks in this way and beef somewhat longer. The hypobaric system 11, on the other hand attempts to produce a similar condition by reducing the pressure around the meat to a few millimetres of absolute pressure. In this way the oxygen is reduced to a very low level thus creating similar conditions to Cryovac. There is however one important difference - with Cryovac there is no need to control relative humidity. With the hypobaric system however, it is necessary to maintain a very high relative humidity to avoid weight loss. This is accomplished by removing water from the air, which is removed by the vacuum pump, and returning it to the container. One major advantage of this type of container is that one can transport whole carcases of lamb, albeit at
Volume 5 Num~ro 1 Janvier 1982
Conclusions In a review of this nature it is difficult to draw any conclusion as to the most satisfactory type of refrigerated container ship as each type has its own advantages and disadvantages. It is perhaps true to say that in marine refrigeration generally there has been a swing back to the brine cooled air battery and, apart from the Columbus and Farrell vessels, this trend has continued with container ships. Other major differences are between the insulated and uninsulated hold, and this is unlikely to be resolved either way until there are clear economic conclusions. At present these tend to favour the insulated hold as lower machinery costs and running costs offset the cost of insulating the hold. This was reported some years ago 12 and although there was some discussion between the British and Continental lines when the container ships were ordered for the South Africa to Europe service, the argument was largely settled by Perishable Products Export Control Board in favour of the insulated hold. In terms of investment in capital equipment for refrigeration purposes there would be some difference between the cold well and the ducted air system but the former has operational disadvantages. The later trends in the ANZECS vessels to have a mixture of horizontal and vertical ducts appear to offer the best compromise in terms of flexibility of carrying temperature. The screw compressor, whether twin rotor or single screw, is likely to dominate the market for machinery in the foreseeable future, although smaller decentralized systems will still have their adherents. My thanks are due to the Shipowners Refrigerated Cargo Research Association for permission to publish this paper, to my colleagues at Cambridge w h o have been involved with much of the experimental work with containers during the past fifteen years, to Hall Thermotank International and Hall Thermotank Products for their help with photographs and to my many friends in the marine and refrigeration world w h o have contributed so much to this very absorbing and often forgotten subject. Last but not least to Mrs V. Potter w h o typed the manuscript.
19
References 1
2
3 4
Patents shipboard systems: a - BP 1 0 8 8 0 7 0 Improved containerised cargo refrigeration system (1967): b - BP 1 126 270 Refrigerated containerised cargo transport system and container (1968); c - BP 1 267 760 Improvement in or relating to transport ships for refrigerated containers (1972); d - BP 1 160391 Improvements in and relating to marine refrigeration (1969); e - BP 1 129859 Improvement in and relating to marine refrigeration (1968); f - BP 1 453555 Improvements in or relating to refrigeration of container cargo (1976) Patents containers and ancillary systems: a - BP 1 188240 A cargo container and closure therefor; b - BP 1 155993 Flexible couplings for providing fluid communication between two members; c - BP 1 318421 (Patent of Addition to BP 1 155993) Improvements relating to couplings or connectors for use with refrigerated cargo containers; d - BP 1 454 508 Improvements in and relating to refrigeration control systems; e - BP 1 579662 (Patent of addition to BP 1 454 508) Improvements in and relating to refrigeration control systems Jenks, W. S. C. Some factors in the design of cellular container ships with particular reference to refrigerated cargo, Trans/nst Mar Engrs 83 1 (1971 ) 1-20 M e e k , M. The first OCL container ships Trans R Inst Nav Arch 112 (1970)
5
6
7 8
9 10
11
12 13
USDA Animal and Plant Health Inspection Service P/ant protect~on and quarantine programs, Title 7, Chapter 3, Part 319, Subpart Fruits and Vegetables. Cold treatments of certain imported fruits (Jan 1. 1973) S t e r a , A. C., W i l s o n , J. J. The determination of an overall heat transfer coefficient for uninsulated holds of a refrigerated container ship. /n[/ns Refrig Bu//, Annexe Hamburg (1977-2) 59-72 B j o r k l u n d , J. S. An open air circulation system for refrigerated c o n t a i n e r s / n t Inst Refr/~7 Buff Annexe Hamburg (1977-2) 93-46 Hales, K. C., Mansfield, J. E., Scrine, G. R. Thermal testing of insulated vehicles. Proc Inst Refn~7 75 (197879) 55-64 B o w y e r , C. J., Scrine, G. R. Factors affecting the design of containers and the carriage of refrigerated cargo. Paper presented to Inst Refrig 3rd April 1980 Agreement on the international carriage of perishable foodstuffs and on the special equipment to be used for such carriage. Command Paper 8272 (1981) HMSO M a r m e l s t e i n , N. H. Hypobaric transport and storage of fresh meats and produce earns 1979 IFT Food Technology Industrial Achievement Award. Food Techno/ogy 33 (July 1979) 32-40 Hales, K. C., Stott, J. R., Wilson, J. J. The sea transport of refrigerated cargoes in containers Translnst MarEngrs 84 8 (1972) 251-263 Since the presentation of this paper there has been an outbreak of Mediterranean Fruit Fly in California, USA
A monthly journal to keep you in touch with developments in the low temperature field. Cryogenics is the international journal of low temperature research and engineering. The subject matter covers the complete field of science below --100°C. This includes low temperature plant technology, cryosurgery, train suspensions and superconducting motors. Each issue contains specially commissioned review articles and original papers. Comphmentary to these are research and technical notes describing preliminary results and details of work in progress, and letters to the editor on areas of discussion and controversy. Other features include reports on conferences, book reviews and a lively news section covering the cryogenics industry. The quality of the editorial is ensured by a distinguished board of international editors. For further details and sample copy contact= Geraldine Hills, IPC Science and Technology Press Ltd, PO Box 63, Westbury House, Bury Street, Guildford, Surrey GU2 5BH. Tel: 0483 31261
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International
Journal
of Refrigeration