Hydrocarbons as refrigerants in small heat pump and refrigeration systems – A review

international journal of refrigeration 31 (2008) 552–563

available at www.sciencedirect.com

w w w . i i fi i r . o r g

journal homepage: www.elsevier.com/locate/ijrefrig

Review

Hydrocarbons as refrigerants in small heat pump and refrigeration systems – A review Bjo¨rn Palm* Royal Institute of Technology, Department of Energy Technology, Division of Applied Thermodynamics and Refrigeration, Brinellva¨gen 68, 100 44 Stockholm, Sweden

article info

abstract

Article history:

Due to the concern for the effects of the release of HFC refrigerants on the global environ-

Received 16 July 2007

ment caused by the high global warming potential of these substances, there is a large

Received in revised form

interest in Europe and elsewhere for the use of hydrocarbons as refrigerants. This article

25 November 2007

presents a comparison of the properties and performance of hydrocarbons as refrigerants

Accepted 26 November 2007

in small-size heat pump and refrigeration systems (<20 kW cooling). A listing of several

Published online 8 December 2007

commercially available systems is also presented. The designs, safety precautions and performances of some of these systems are described.

Keywords:

As a general conclusion, it is shown that using hydrocarbons will result in COPs equal

Refrigeration system

to, or higher than, those of similar HFC systems. It is also shown that components suitable

Heat pump

for hydrocarbon systems are available on the market, even though the number of large-

Survey

size hermetic compressors is limited. A major concern, which should not be taken lightly,

Hydrocarbon

is the safety issue. Reduced charge through indirect systems and compact heat exchangers,

Thermal property

outdoor placing of the unit, hydrocarbon sensors and alarms and forced ventilation are all

Compatibility

steps which may be applied to reduce the risks under normal operation.

Material

ª 2007 Elsevier Ltd and IIR. All rights reserved.

Flammability Manufacturer

Les hydrocarbures en tant que frigorige`nes dans les syste`mes frigorifiques et a` pompe a` chaleur de petite taille - l’e´tat de l’art Mots cle´s : Syste`me frigorifique ; Pompe a` chaleur ; Enqueˆte ; Hydrocarbure ; Proprie´te´ thermique ; Compatibilite´ ; Mate´riau ; Inflammabilite´ ; Fabricant

* Tel.: þ46 8 790 74 53; fax: þ46 8 20 41 61. E-mail address: [email protected] 0140-7007/$ – see front matter ª 2007 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2007.11.016

international journal of refrigeration 31 (2008) 552–563

Nomenclature A COP1 COP2 d f1 FOM hfg L m M p1 p2 Dp Q qv Re T1c,is Tcr Tnbp

1.

cross-section area (m2) coefficient of performance, heating coefficient of performance, cooling diameter (m) friction factor figure of merit heat of vaporization (kJ kg1) length (m) mass flow (kg s1) molar mass (kg kmol1) condensing pressure (bar) evaporation pressure (bar) pressure drop (bar) heat flow (W) volumetric refrigerating effect (kJ m3) Reynolds number hot gas temperature after isentropic compression ( C) critical temperature ( C) normal boiling point ( C)

Introduction

When the CFC refrigerants were introduced at the end of the 1920s this was in response to a deeply felt need for a safe alternative to the flammable and poisonous substances used as refrigerants. At that time, sulphur dioxide, methyl chloride and ammonia were the common refrigerants. Ammonia and sulphur dioxide both have a very strong smell, which must have made even the smallest leaks noticeable. It is easy to understand that the industry sought for better alternatives for the dormant domestic market for refrigerators. Specifically, as the hermetic compressors were not yet introduced, leakage through the shaft seals must have been a constant problem. Hydrocarbons such as propane and isobutane were also used as refrigerants at that time (but to a lesser extent). These do not have such a pungent smell as SO2 and NH3, but are much more flammable. When, after a directed effort to find a substitute, the CFC refrigerants had been presented by Thomas Midgley in 1930, they soon took over the complete domestic refrigerant market which immediately started to grow. Sixty-five years later, society had accepted the alarms of Rowland and Molina and the CFCs were banned in many countries while the two researchers the same year were awarded the Nobel Prize for their findings of the ozone depleting potential of these substances. The fact that these ‘‘harmless’’ substances were found to have a very unexpected and harmful influence on the global environment raised doubts also about the use of other manmade substances not present in the natural environment. Even though the shift from the CFCs first went to the HCFCs and in a second stage to the HFCs with no ozone depleting potential, voices were raised for moving over to ‘‘natural’’ refrigerants, i.e., to the use of substances naturally occurring in the environment. The substances included in this group are

Ttr V w 3is 3v h m n rl rv

553

triple point temperature ( C) volume flow (m3 s1) velocity (m s1) isentropic work of compression (kJ kg1) volumetric compression work (kJ m3) efficiency dynamic viscosity (Ns m2) kinematic viscosity (m2 s1) liquid density (kg m3) vapour density (kg m3)

Subscripts 1 high pressure side 2 low pressure side Carnot carnot cycle c compressor is isentropic ideal ideal l liquid Dp pressure drop v vapour

ammonia, carbon dioxide, hydrocarbons, and sometimes also water and air. The doubts about the manmade substances have later been confirmed by the realization that emissions of these refrigerants contributed by more than 20% of the global release of CO2-equivalents during some years before the ban of the CFCs (IPCC/TEAP, 2005). The drawback of hydrocarbons, being in focus in the present text, as refrigerants, is of course the same today as 80 years ago. They are flammable and special precautions need to be exercised in their use. The main advantages compared to the other natural refrigerants are that they are in many respects very similar to the halogenated hydrocarbons the industry is accustomed to. No changes in the system or component designs are necessary from a technical point of view. Also, the hydrocarbons can be expected to give similar performance as the CFCs, HCFCs and HFCs. In this article, the properties of some of the most frequently used hydrocarbon refrigerants will be discussed and compared to those of R22, R134a and NH3. In the second part of the article, the availability of components for hydrocarbons is discussed, and a listing of commercial systems with hydrocarbons is presented. For some of these systems, the design and the solutions to the safety issue are presented. Finally, some conclusions are drawn concerning the possible future use of hydrocarbon refrigerants.

2.

Refrigerants under consideration

Several hydrocarbons have been used as refrigerants through the years. As already noted, propane and isobutane were amongst those used prior to 1930. During the last 15 years several hydrocarbons and hydrocarbon blends have been used commercially as refrigerants.

554

international journal of refrigeration 31 (2008) 552–563

Table 1 – Refrigerant properties at 40 8C (where applicable) Propane

Propene

R134a

R22

Ammonia

R600a CH–(CH3)3 58.12 11.67 311.4 530.0 13.667 159.59 134.67

R290 CH3–CH2–CH3 44.096 42.09 306.51 467.07 30.202 187.67 96.675

R1270 CH2]CH–CH3 42.08 47.69 303.14 476.66 35.708 185.2 92.42

R134a CF3–CH2F 102.03 26.074 163.02 1146.7 50.085 103.3 101.06

R22 CHClF2 86.468 40.81 166.6 1128.5 66.193 157.42 96.145

R717 NH3 17.03 33.327 1099.31 579.44 12.034 77.655 132.25

Isobutane (R600a) is the most frequently used hydrocarbon refrigerant. In Europe it is the totally dominating refrigerant in household refrigerators, with a market share of more than 95% in many countries. In 2004, 33% of the world production of domestic refrigerators and freezers used pure isobutane, or isobutane blends. The same year, about 200 million cabinets had been released on the market from the introduction in 1993 (RTOC, 2006). Propane (R290) and propene (propylene, R1270) are used by many heat pump manufacturers, and have also been used in air conditioners and in commercial refrigeration systems. The British company Calor Gas’ refrigerants business CARE Refrigerants (now owned by BOC refrigerants, part of the Linde-group) distributes a series of hydrocarbon based fluids as refrigerants. Apart from pure isobutane, propane and propene they also have blends of isobutane/propane and propane/ ethane to match the vapour pressure curves of R12/R134a and R22/R407C, respectively. Butane (R600) has also been under discussion, but has not been used commercially to our knowledge. Its properties are quite similar to those of isobutane further discussed below. Pentane and isopentane have been considered for use in centrifugal systems to replace R11 (Tadros et al., 2006). Table 1 gives a general overview of several important properties of most of the mentioned hydrocarbons, as well as of ammonia, R134a and R22 for comparison. As seen from the table, the molar masses of the hydrocarbons are much lower than those of R134a and R22. This difference is also manifested in the densities. Comparing propane, propene and R22, all having about the same normal boiling point, it is seen that the vapour densities of the HCs are about half of that of R22. More important for the charge, the liquid densities of the HCs are less than half of R134a and R22. On the other hand, the heat of evaporation of the HCs is about twice that of the two H(C)FCs. Ammonia has extremely high heat of evaporation, but has also a very low vapour density for the fairly low normal boiling point. Looking at the triple point temperature and the critical temperature, there is no obvious difference between the groups of fluids. From this initial comparison it is clear that the main difference between the groups is in the molecular weight, resulting in density differences, and in the heat of evaporation, which is important for any refrigerant fluid. In the following section a more detailed comparison between the fluids will be made.

3.

Properties of hydrocarbons as refrigerants

When selecting a fluid as a refrigerant, the vapour pressure curves of possible alternatives are first investigated. Traditionally, the fluid has been selected so that the pressure in all parts of the system is in the range 1–25 bar. The lower limit is selected so as to avoid ambient air being drawn into the system, while the upper limit is chosen to avoid excessive pressure levels in the system. In Fig. 1 the vapour pressure curves of the HC-refrigerants and of the alternatives are shown. The curves for propane and propene are quite similar to those of R22 and ammonia, indicating that the application areas would be the same. The slope of the curve gives an indication of the differences in pressure ratio between the fluids. This is important as the volumetric and isentropic efficiencies can be expected to decrease with increasing pressure ratio. Comparing the fluids at the assumed condensing and evaporation temperatures of þ40/20  C gives the pressure ratios shown in Table 2. Generally speaking, when operating between two specific temperatures, fluids with low vapour pressures (high normal boiling point) will have larger pressure ratios than fluids with high vapour pressures. However, there are exceptions to this general rule and this is clearly seen in the comparison. No general differences are found between the HC-refrigerants and the other fluids in this respect. A method of visualizing the difference between refrigerants is to compare their enthalpy–temperature diagrams. In Fig. 2a the diagrams of propane, R134a and ammonia are shown. In Fig. 2b propene and isobutane are included as

30

Saturation pressure, (bar)

Refrigerant no Chemical composition M (kg/kmol) Tnbp ( C) hfg (kJ/kg) rl (kg/m3) rv (kg/m3) Ttr ( C) Tcr ( C)

Isobutane

25

Isobutane R134a

Propane R22

Propene NH3

20 15 10 5 0 -40

-30

-20

-10

0

10

20

30

40

50

Temperature, (°C) Fig. 1 – Vapour pressure curves of HC-refrigerants and some alternatives.

60

555

international journal of refrigeration 31 (2008) 552–563

Table 2 – Cycle performance properties at D40/L20 8C

Refrigerant p1/p2 qv (kJ/m3) 3is (kJ/kg) COP2ideal hCarnot y1 (%/ C) y2 (%/ C) y3 (%/ C) T1c,is ( C)

Isobutane

Propane

Propene

R134a

R22

Ammonia

R600a 7.36 477 74.15 3.12 0.74 1.099 0.207 0.189 40.0

R290 5.60 1345 72.23 3.01 0.71 1.186 0.157 0.129 48.2

R1270 5.39 1683 67.24 3.03 0.72 1.117 0.082 0.035 56.0

R134a 7.66 883 37.71 3.06 0.73 1.138 0.140 0.109 48.6

R22 6.25 1590 31.42 3.15 0.75 0.892 0.047 0.051 70.3

R717 8.18 1679 322.3 3.28 0.78 0.469 0.235 0.227 135.5

well. The diagrams clearly show the difference in heat of evaporation between the types of fluids. High heat of evaporation is beneficial, as less refrigerant then has to be circulated to achieve a certain cooling (or heating) capacity. However, it is important to note that the mass flow is determined by the swept volume flow of the compressor and by the density of the refrigerant. It is therefore of interest to compare the product of the heat of vaporization and the density of the refrigerant vapour as it enters into the compressor, as this product gives an indication of the capacity achieved with a specific compressor (i.e., a specific volume flow). This product is shown as a function of the pressure level for the discussed refrigerants in Fig. 3. From the figure it is quite clear that the

pressure level more or less determines the capacity per unit swept volume. This is a general conclusion, valid for all fluids, not only for the compared refrigerants. A more exact comparison of the refrigeration capacity for a given compressor swept volume flow with different refrigerants is achieved by comparing the volumetric refrigerating effect qv for specified evaporation and condensing temperatures. Fig. 4 shows this effect, as a function of the evaporation temperature, assuming a condensing temperature of þ40  C and no superheat or subcooling. As understood from the previous figure, the fluids with high pressure have higher volumetric refrigerating effect. Comparing the fluids we see that ammonia will give the highest capacity with a given swept volume flow, except at the very lowest evaporation temperatures shown, but the difference is not as dramatic as expected from Fig. 2a. Propene and R22 have almost identical capacities at all temperatures. At 0  C evaporation temperature, propene has over 20% higher capacity than propane, due to the difference in pressure. As expected, isobutane has low volumetric refrigerating effect due to its low vapour pressure. The volumetric refrigerating effects at 20/þ40  C are also given in Table 2. Hydrocarbons do not show any unexpected behaviour in this respect. For the efficiency of the process, not only the capacity per unit mass or unit swept volume is of interest, but also the compressor work. To compare the fluids, in Table 2 the specific work of isentropic compression from the temperature 20 to þ40  C is given for the different fluids. As may be expected, ammonia has by far the highest value, while R22 14000 Isobutane Propane Propene R134a R22 Ammonia

hfg *ρv (kJ/m3)

12000 10000 8000 6000 4000 2000 0

Fig. 2 – h–T Diagrams of (a) propane, R134a, and ammonia and (b) isobutane, propane, propene, R134a, R22 and ammonia.

0

2

4

6

8

10

12

14

16

Pressure (bar) Fig. 3 – Product of heat of vaporization and vapour density vs pressure.

18

Volumetric refrigerating effect, (kJ/m3)

556

international journal of refrigeration 31 (2008) 552–563

y2 percentage increases in volumetric refrigerating effect qv per degree of internal superheating and y3 percentage increases in COP2 per degree of internal superheating.

8000 Propane Isobutane Propene R22 R134a Ammonia Butane

7000 6000 5000 4000 3000 2000 1000 0 -20

-15

-10

-5

0

5

10

15

20

Evaporation temp (°C) Fig. 4 – Volumetric refrigerating effect vs evaporation temperature, assuming a condensing temperature of D40 8C. No superheating or subcooling.

and R134a have the lowest. However, these values should be related to the heat of vaporization, or more correctly, to the heat absorbed or rejected by the cycle. To get a clear picture, the coefficient of performance for cooling (COP2) or for heating (COP1) may be calculated. Table 2 gives the values of COP2 for the fluids under the following conditions: condensing temperature: þ40  C, evaporation temperature: 20  C, no superheat at the compressor inlet and no subcooling at the condenser outlet, isentropic compression, no heat losses to the ambient and no pressure drop in the lines or heat exchangers, i.e., for an ideal process without subcooling or superheating. Under these conditions, COP1 is equal to COP2 þ 1. As seen from the table, the differences between the fluids are rather small with R22 and R134a being in between the hydrocarbon refrigerants. Ammonia is topping the group. The efficiency of the cycle compared to the Carnot process may be expressed in terms of the Carnot efficiency, hCarnot defined as hCarnot ¼

COP2ideal COP2Carnot

(1)

The values are again given in Table 2 and the order of the refrigerants is of course the same as for COP2. It should be noted that the values of COP2 depend highly on the assumed temperatures, and that the order of the fluids will be slightly different if they are compared at other conditions. As shown in Fig. 2 the shapes of the vapour domes are different for the different types of fluids. An important implication of this is that the performance may be more or less influenced by superheating and subcooling of the fluid. This influence may be expressed as a percentage increase (or decrease) in, e.g., the cooling capacity per degree of subcooling or superheating. These percentages, based purely on the properties of the fluids, are sometimes referred to as y-factors (Ekroth, 1979). In Table 2, three y-factors are given for the case of 40  C condensing temperature and 20  C evaporation temperature. The y-factors have the following meaning: y1 percentage increases in volumetric refrigerating effect qv and in COP2 per degree of subcooling,

By internal superheating it is understood that the heat absorbed during the superheating is useful, i.e., taken from the refrigerated space. The use of an internal heat exchanger between the suction line and the liquid line has the same influence on the system as internal superheating. As shown by the y1-factor, the influence of subcooling is about the same for the different fluids, except for ammonia for which the gain of subcooling is smaller than for the others. While subcooling is always positive for the cooling capacity and the COP2, superheating may be either positive or negative. The hydrocarbons benefit, both in terms of cooling capacity and in COP2, from superheating at the compressor inlet. For R134a the benefit is less and for R22 the effect is very small. For ammonia on the other hand, superheating has a negative effect both on the capacity and the COP2. In conclusion, the hydrocarbons benefit more from the use of an internal heat exchanger between the suction line and the liquid line than other common refrigerants. As an example, if isobutane is superheated by 25  C in an internal heat exchanger in the case of 20/þ40  C, the COP2 would be identical to that of ammonia under the same conditions but without internal heat exchanger. Likewise, propane, with an internal heat exchanger and 25  C superheating, has only slightly lower COP2 than R22 with no superheat or heat exchanger. A factor which may determine whether or not two stage compression is needed is the hot gas temperature. This temperature is also connected to the shape of the vapour dome. For the case of 20/þ40  C and isentropic compression the hot gas temperatures T1c,is are given in Table 2. For isobutane, for which the vapour dome is considerably ‘‘bent’’ in the h–T (and h  log( p)) diagram, the line of constant entropy starting at the vapour saturation curve at 20  C goes directly into the two-phase region (see Fig. 2b). An isentropic compression would then end in the two-phase region, and the outlet temperature would be equal to the saturation temperature, 40  C. For propane and R134a, the vapour is only slightly superheated after the isentropic compression. For R22 with a less bent vapour dome, the hot gas temperature would be considerably higher, about 70  C. Ammonia deviates considerably from the rest with a hot gas temperature, after isentropic compression from 20 to þ40  C of 135.5  C, much higher than any of the other fluids. In the real case, the compression is not isentropic, and the temperatures will be substantially higher than shown here. For isobutane, the non-isentropic compression also means that the state of the fluid after compression will be in vapour phase. In conclusion, the hydrocarbons considered to be used as refrigerants will give equal or lower hot gas temperatures compared to other commonly used refrigerants. The transport properties, viscosity and thermal conductivity, influence the pressure drop and the heat transfer in the heat exchangers of the system. In Table 3 these properties are given for the liquid and vapour phase at 40  C. As

557

international journal of refrigeration 31 (2008) 552–563

Table 3 – Transport properties at 40 8C

Refrigerant no kl (mW/m K) kv (mW/m K) ml (mNs/m2) mv (mNs/m2) FOMDp,l FOMDp,v FOMSPHT,l FOMSPHT,v FOMpb FOMCond/104

Isobutane

Propane

Propene

R134a

R22

Ammonia

R600a 83.7 18.5 128.2 7.91 2.75 53.2 1.20 1.33 0.11 3.4

R290 86.8 21.5 82.6 8.89 2.87 25.5 1.56 1.50 0.19 3.8

R1270 102.9 22.0 81.6 9.77 2.86 22.5 1.74 1.45 0.21 4.5

R134a 74.7 15.4 161.5 12.37 4.18 50.4 1.38 1.35 0.11 3.8

R22 76.6 12.8 138.7 13.3 3.94 37.3 1.40 1.09 0.14 4.1

R717 443.5 28.4 114.0 10.33 0.27 7.10 1.62 0.71 0.21 30.5

shown, the thermal conductivities of the hydrocarbons are substantially higher than those of R22 and R134a, indicating favourable heat transfer performances. Ammonia’s thermal conductivity is much higher than those of any of the other fluids. For the viscosity, the hydrocarbons have notably lower values than R22 and R134a. Ammonia in this case is in between the two groups. A method of visualizing what these differences will mean in terms of pressure drop and heat transfer coefficients is to use figures of merit derived from correlations for heat transfer and pressure drop. As an example, pressure drop in turbulent single-phase flow is according to Blasius calculated as Dp ¼ f1

rw2 L wd where f1 ¼ 0:316Re1=4 and Re ¼ n 2 d

(2)

In a refrigeration system, the mass flow is connected to the desired cooling capacity through the heat of evaporation, so that the single-phase flow velocity in a circular tube at a given cooling capacity can be expressed as w¼

_ fg _ Q=h 4Q_ V_ m ¼ z pd2 ¼ 2 h A rpd4 2 r4 fg rpd

(3)

Nu ¼ 0:023Re0:8 Pr0:4

Combining Eqs. (2) and (3), the pressure drop can be expressed as LQ_ 7=4 m1=4 Dp ¼ 0:241 19=4 d rh7=4 fg

(4)

The pressure drop of a certain tube with length L and diameter d at a cooling capacity Q_ is thus directly proportional to the ratio of the fluid properties in the expression, which is then a figure of merit for pressure drop in single-phase turbulent flow: FOMDp ¼

m1=4 rh7=4 fg

while R22 and R134a have at least 35% higher values. Ammonia on the other hand has extremely low FOMDp, showing that, in the same heat exchanger, with the same heat flow, the pressure drop would be very much lower for ammonia than for the other fluids. For the vapour phase, the saturation pressure has large influence. This is obvious from the high value of isobutane. For the fluids having approximately equal pressures, the trend is the same as for the liquid, with the values for R22 and R134a being substantially higher than for propane and propene, while ammonia has by far the lowest value. A figure of merit for heat transfer in laminar flow is easily found from the fact that in this case the Nusselt number is constant. This means that the heat transfer coefficient for a specific diameter tube is directly proportional to the thermal conductivity, which is then the FOM for laminar flow heat transfer. As already pointed out, the thermal conductivities of the hydrocarbons are larger than for R22/R134a. Similarly, we may define a figure of merit for turbulent single-phase heat transfer based on, e.g., the Dittus–Boelter equation.

(5)

(As follows from Eq. (4), the value should preferably be as low as possible for a low pressure drop.) From this figure we see that not only low viscosity is important for a low pressure drop, but even more important is high heat of vaporization, which determines the necessary mass flow to achieve a certain cooling capacity, and the density of the fluid. This figure is derived for single-phase flow, but may also be used as an indication of the two-phase pressure drop. In Table 3 FOMDp is given for liquid and vapour flow at 40  C. Looking first at the liquid phase, the three hydrocarbons have very similar values,

(6)

or  0:8 hd rwd ¼ 0:023 Pr0:4 k m

(7)

Solving for the heat transfer coefficient gives !0:8  _ 0:8 _ k rVd k md 0:4 Pr ¼ 0:023 pd2 h ¼ 0:023 d Am d 4m _ Qd k Pr z0:023 d hfg pd4 2 m 0:4

!0:8 Pr0:4 ¼

 0:8 Q_ 0:8 kPr0:4 4 0:023 1:8  0:8 p d hfg m

(8)

The last ratio is the figure of merit for single-phase heat transfer in turbulent flow. FOMSPHT ¼ 

kPr0:4 0:8 hfg m

(9)

A high value of this parameter indicates that high heat transfer coefficients should be expected. The parameter has been calculated for both the liquid and the vapour phase for each of the refrigerants, and the values are found in Table 3. For the liquid phase, isobutane has considerably lower values

558

international journal of refrigeration 31 (2008) 552–563

than propane and propene. R22 and R134a fall in between these. Ammonia in this case is close to propene. Looking at the individual properties, it seems that higher viscosity of isobutane is mainly responsible for the lower value of this fluid. For the vapour phase, some of the trends are similar, with isobutane having lower values than propane and propene, while R134a is in between. However, R22 and, particularly, ammonia in this case have lower values. From these calculations, a general conclusion about the hydrocarbons cannot be drawn and it is difficult to pinpoint any property in particular as responsible for the differences. It is clear, though, that propane and propene can be expected to give higher heat transfer coefficients than R22 and R134a if the mass flow is determined by the required cooling (or heating) capacity. In refrigeration systems and heat pumps, heat transfer in boiling and condensation is more important than single-phase heat transfer. For certain types of two-phase flow the liquid single-phase heat transfer coefficient (and figure of merit) may be indicative of the heat transfer performance. We may, however, also use correlations for pool boiling and for film condensation to determine the figures of merit. Using Cooper’s (1984) pool boiling correlation, the following figure of merit is derived. 0:120:2 log10 Rp 

hpb ¼ 55pr

 log10 pr

0:55

M0:5 q_ 0:67

(10)

Assuming a surface roughness Rp of 1 mm, we get  0:55 0:5 FOMpb ¼ pr0:12  log10 pr M

(11)

(A high value indicates high pool boiling heat transfer coefficients.) As follows from Eq. (11), high reduced pressure and low molecular weight will promote high heat transfer in boiling. From the values presented in Table 3 it is clear that propane, propene and ammonia have similar values, considerably higher than the other fluids. The values are calculated for the temperature þ40  C, but the relation between the values is the same at 20  C. For condensation, we may use Nusselt’s film theory as a basis for the figure of merit. This will give  1=3 hfg r2l (12) FOMCond ¼ k ml As for the FOMpb, a high value indicates high heat transfer coefficients. The difference between the fluids in this case is not so large, and there are no obvious differences between the hydrocarbons and R22/R134a. The exception is ammonia, which gives about eight times higher value than the other fluids. This figure of merit for condensation is relevant for heat exchangers where the Reynolds number is low, i.e., where the liquid film thickness is determined by gravity rather than by shear forces. In conclusion, the comparisons of expected heat transfer and pressure drop performance using figures of merit indicate that the hydrocarbons have equal or better heat transfer performance and lower pressure drop compared to R22 and R134a. Of the three hydrocarbons discussed, propane and propene give the best performance. Finally, the comparison has also shown the superior properties of ammonia. The results above are also supported by tests reported in the literature, e.g., by Chang et al. (2000), Pelletier and Palm (1996) and Granryd (2001).

4. Compatibility with construction materials, choice of oil Hydrocarbons are compatible with construction materials as metal alloys and polymers typically used in HFC systems. This includes neoprene, viton, nitrile rubber, HNBR, PTFE and nylon. Materials to be avoided are EPDM, natural rubbers and silicone rubbers (Colbourne and Ritter, 2000). Listings of material compatibility are also found in Pelletier (1998) and in Corberan and Seguardo (2006). Being hydrocarbons, these refrigerants are highly soluble in mineral oil. They are also compatible with most common synthetic oils as alkyl benzene (AB), polyolester (POE) and polyalkylene glycol (PAG). The high solubility ensures oil return to the compressor, but may also pose a problem due to reduced viscosity of the diluted oil and excessive foaming upon start-up. The supplier of CARE hydrocarbon refrigerants recommends using a higher viscosity grade of mineral oil and POE oil for hydrocarbons to reduce the risk of compressor failures (BOC-CARE refrigerants, 2007). Some system manufacturers have experienced intolerably high failure rates of compressors with hydrocarbon refrigerants, and this has been explained by the high solubility of refrigerant in the oil. Other manufacturers have, with proper oil selection and use of crankcase heater and/or internal liquid line/suction line heat exchanger seen less failures with HCs than with H(C)FCs. The solubility of propane in oil differs considerably, as has been shown by Fernando et al. (2003). In this work it was shown that propane was much less soluble in a PAG oil than in two compared POE oils. No experience from tests in an actual system with the PAG oil was reported. Navarro et al. (2004, 2005) compared the performance of a piston compressor working with propane using two different oils, a mineral oil and a POE oil. They found that the volumetric efficiencies were identical. The (isentropic) compressor efficiencies were also similar, but the maximum in the efficiency vs pressure ratio plot was shifted slightly to lower pressure ratios with the POE oil. It was also found that the solubility was higher in mineral oil, that foaming was more excessive in POE and that the oil circulation rate was almost twice as high with POE.

5.

Flammability

The hydrocarbons are all highly flammable and this fact must not be taken lightly when designing for these refrigerants. Table 4 gives some important figures. The flammability limits are from about 2% to 9 or 10% refrigerant in air. The auto-ignition temperature is around 460  C, the energy of combustion is approximately 50 000 kJ/kg, the laminar burning velocity is about 0.4 m/s and the minimum ignition energy is about 0.25 mJ for the listed hydrocarbons. These values could be compared to the corresponding values of ammonia, which is also considered flammable. Ammonia has quite different numbers, indicating the lower risks due to the flammability with this substance.

international journal of refrigeration 31 (2008) 552–563

559

Table 4 – Flammability properties

Refrigerant no LFL/UFL (vol.%) Auto-ignition temperature ( C) Flash point ( C) Laminar flame speed (m/s) Minimum ignition energy (mJ) Energy of combustion, (kJ/kg) a b c d

Isobutane

Propane

Propene

Ammonia

R600a 1.8/8.5a 460a 83a 0.37d 0.25a 49 500a

R290 2.1/9.5a 470a 104a 0.43d 0.25a 50 500a

R1270 2.5/10.1a 455a 108a 0.48d 0.28a 49 000a

R717 16/25b 850b –b 0.07d 680b 18 600c

BOC-CARE refrigerants (2007). CCOHS (2007). ARTA (2007). Woodward (1998).

Risk assessment of hydrocarbon installations has been presented, e.g., by Colbourne and Suen (2004) and by Colbourne et al. (2003). The regulations concerning the use of hydrocarbons differ from country to country, but for Europe, the new norm EN378 expected to be coming out late 2007 should give sufficient support for the designer. Rules and regulations for hydrocarbons are treated by Corberan et al. (in this issue). In general, decreasing the amount of refrigerant in the system is highly important for reducing the risks. This fact is reflected in the regulations. For this reason, there have been several attempts to reduce the charge of systems by use of indirect systems, compact heat exchangers and compact designs. Fernando et al. (2004) has reported on the design of a heating only, water to water heat pump with a heating capacity of 5 kW using less than 200 g of propane. More than one-third of this amount was found to be solved in the compressor oil during operation. At University of Valencia, a reversible water to water heat pump is supplying cooling and heating (17 kW) with a propane charge of about 450 g (Corberan, 2006). There is also a European Union sponsored research project called MINIREF focusing on charge reduction running since 2005 (Cordis, 2005). This project, however, is not limited to hydrocarbon refrigerants. It is to be expected that charge reduction will come more in focus, both for safety reasons, when flammable or high toxicity refrigerants are used, and for environmental reasons, when using high GWP synthetic refrigerants. As already mentioned, since 1993 more than 200 million refrigerators and freezers charged with isobutane have reached the market. According to Wennerstro¨m (2007) there have been no accidents during normal operation of these cabinets. On the other hand, there have been accidents reported taking place during manufacturing and servicing, and in equipment retrofitted with hydrocarbons. A few such cases are reported by Colbourne et al. (2003).

6.

Components for hydrocarbon systems

As already noted, most of the materials used in HFC systems can also be used for hydrocarbons. This indicates that components for hydrocarbon systems should not be different from those of HFC systems and therefore should be widely available on the market. With one important exception this conclusion

is valid: The choice of compressors is much more limited while designing for hydrocarbon refrigerants. As the market for hydrocarbon systems, with the exception of refrigerators and freezers, is much smaller than for HFC refrigerants, the compressor manufacturers have not developed special models for hydrocarbons. Because of the flammability, the European Pressure Equipment Directive demands additional certification requirements and approvals for compressors being used for hydrocarbons. Due to the small market, it has not been in the interest of the compressor manufacturers to carry out the more onerous procedures. For this reason, the number of available larger-capacity hermetic compressors is quite limited. US compressor manufacturers often refer to the risk of liability claims as a reason to deny the use of their compressors with hydrocarbons, even if no special safety measures are required by the PED. The producer of CARE hydrocarbon refrigerants has on their web page a listing of manufacturers of compressors and other components which are designed for hydrocarbons (BOC-CARE refrigerants, 2007). The list includes 23 compressor manufacturers, including Americold, Bitzer, Bock, Danfoss, Dorin, Electrolux, Embraco, Mitsubishi, Sabroe, Samsung, Tecumseh, York and Zanussi. Looking closer at the listing, it is clear that the compressors are either small, up to 1600 W input (hermetic, rotary or reciprocating) or large (semi-hermetic or open, reciprocating or centrifugal compressors), demonstrating a notable absence of medium-sized hermetic reciprocating, scroll and rotary types. For large industrial systems, screw compressors have been used with hydrocarbons by, e.g., Aerzener, Howden, Johnson ¨ hman, 2007). Controls, Frick and Kobelco (O Danfoss is one of the manufacturers with extensive experience of the use of hydrocarbons in small refrigeration systems. The company has also produced an interesting document on the use of hydrocarbons which is available on the web (Danfoss, 2000). Based on an internal evaluation of the safety aspects Danfoss decided to supply HC equipment only for restricted use in hermetic systems with a charge of 150 g or less. Several sources have stated that scroll compressors tend to break down when used with hydrocarbons. Still, there are some manufacturers using this technology stating that they have no problems. From our experience at KTH of a few scroll

560

international journal of refrigeration 31 (2008) 552–563

compressors operating in total 8–10 000 h we have seen no reason to support this statement. Experiences from the University of Valencia are similar (Corberan, 2006). However, no durability tests have been run at either of these two institutions.

7.

Systems with hydrocarbon refrigerants

Several manufacturers of small factory built heat pump and refrigeration systems offer products with hydrocarbon refrigerants. As the companies change their products, it is difficult to give a complete and correctly updated list. Table 5 contains a listing of products which are, or have recently been, on the market. Much of the information originates from the Swiss Wa¨rmepumpen Testzentrum, WPZ (WPZ Bulletin, 2007) which on their web site display test results for all types of heat pumps. Other information comes from direct contact with the manufacturers. It seems that the number of producers using hydrocarbons as refrigerants has decreased during the last few years. One important reason for this seems to be the limited supply of compressors, which is at least partly a result of the new Pressure Equipment Directive. Another significant factor is the increasingly strict requirements that are specified within European and International safety standards that make it difficult to design systems with a competitive market price. Below are given brief descriptions of a few hydrocarbon systems available on the market today, mainly with the purpose to show how manufacturers have solved the problems of safety and reliability of the systems. The Italian manufacturer De’ Longhi produces small AC systems using propane as refrigerant. The cooling capacity ranges from 500 to 3200 W and the refrigerant charge is 100–500 g. At least in some of the products they are using Rechi rotary compressors with mineral oil. To ensure safe operation, the systems are designed for minimum charge of refrigerant, with sealed relays and thermal protectors and extra low voltage controls. A minimum room area of 15 m2 is prescribed for a unit with 300 g charge. During production, extra care is taken for leak detection. The systems are found to have 5–10% higher efficiency than with HFC (Aloisi, 2007). The Austrian heat pump manufacturer Neura has a system for direct expansion of propane in copper tubes buried in the ground. This increases the charge, but the whole unit is placed outside with only the insulated water circuits of the heating system attaching the heat pump to the building. The reason for using direct expansion is probably the strict regulations concerning the use of anti-freeze in ground collectors in Austria. The German company Dimplex has manufactured heat pumps with propane for over 20 years. The systems are designed to be placed outside and use ambient air as heat source. The capacities go up to about 19 kW and the charge of propane is from 1 to 2.5 kg depending on the size. Piston compressors from Maneurop are used, and the company claims that the reliability of the compressors is very good. The British heat pump manufacturer Kensa is developing a water to water heat pump for location outside. It will use about 600 g of propane for a capacity of around 10 kW. The company is planning to use Mitsubishi scroll compressors

originally designed for R407C. The oil will be exchanged for a higher viscosity grade mineral oil. The system will be built to follow the new EN378 standard. The Swedish company Nibe manufactures exhaust air heat pumps with 300–490 g of propane. As the system is located in the air duct with very good ventilation, it is considered safe. Apart from the systems listed, there are both smaller and larger systems which should be mentioned here. Refrigerators and freezers with isobutane were introduced in response to a market demand on the European market already in 1993. HC refrigerators were first put on the market by the small German manufacturer Foron, urged by Greenpeace. This created a demand, primarily in Germany, as at that time the ozone depleting potential of CFC refrigerants was much discussed in the media. Due to previous accidents caused by leaking spray bottles in refrigerators, the industry was at first reluctant, but also prepared for the introduction of flammable refrigerants (Wennerstro¨m, 2007). The main concern was to prevent leakage into the cabinet and to avoid any source of ignition inside. The risks of leaks have been reduced by eliminating tube brazings inside, and at least in some cases by using double metal layers in the evaporator. As the charges in these systems are usually in the range 30–50 g, a leakage to the outside was not considered as dangerous. The reasons for choosing isobutane as refrigerant were primarily two, both related to the lower pressure of isobutane compared to propane. First, the lower pressure resulted in lower noise levels, which is extremely important for domestic products. Second, the small displacement compressors necessary for using propane were not as efficient as the larger ones required for isobutane. The efficiencies of the first compressors for isobutane were not as high as for R12 compressors but higher than for the R134a-compressors introduced the same year. Later, the efficiency has increased, increasing the COP2 of the system from less than 1 to about 1.5 under ‘‘refrigerator’’ conditions (Wennerstro¨m, 2007). Slightly larger but similar systems are used as ice cream freezers used in shops and kiosks. Unilever has introduced such freezers using propane as refrigerant, and presently about 2 00 000 units are in operation, mainly in Europe (Gerrard, 2007). The cooling capacity is typically 250 W, the refrigerant used is propane and the charge is about 90 g. Piston compressors of different brands are used, with mineral oil of higher viscosity than for H(C)FC. To reduce risks, spark-proof components are used, the number of joints is minimized and the compressor compartment is well ventilated. The experience is that there is no increase in the number of failures or need for service. The efficiency is shown to be 9% higher than for the corresponding HFC freezer (Elefsen et al., 2004). A group of large international companies have joined in an initiative called Refrigerants Naturally. The participating companies are Mc Donald’s, Coca Cola, Unilever, Carlsberg, IKEA and PepsiCo. ‘‘The goal of the initiative is to promote a shift in the point-of-sale cooling technology in the food and drink, food service and retail sectors towards alternative HFC-free refrigeration technologies.’’ (Refrigerants Naturally, 2007). It seems likely that this initiative will result in wider use of hydrocarbon refrigerants in this sector, even though CO2 systems are also being promoted as another alternative.

561

international journal of refrigeration 31 (2008) 552–563

Table 5 – Factory built, commercial products with hydrocarbons as refrigerants Manufacturer Alpha-InnoTec, Germany Alpha-InnoTec, Germany Calmotherm AG, Switzerland Calmotherm AG, Switzerland CTA AG, Switzerland CTC Wa¨rme, Switzerland De’ Longhi, Italy Dimplex, Germany Dimplex, Germany Dimplex, Germany Dimplex, Germany Dimplex, Germany Earthcare Elcotherm AG, Switzerland Foster Hautec, Germany Hautec, Germany Hautec, Germany Hautec, Germany Hautec, Germany Heim, Switzerland KKW, Germany KVS, Germany KVS, Germany KVS, Germany Kylma, Sweden Lexeta, Switzerland Neura, Austria Neura, Austria Neura, Austria Nibe, Sweden Nibe, Sweden Nibe, Sweden Novelan, Switzerland Novelan, Switzerland Octopus Energi, Sweden Octopus Energi, Sweden Octopus Energi, Sweden Octopus Energi, Sweden Oekotherm, Austria Oekotherm, Austria Oekotherm, Austria RLM Benelux, The Netherlands SIXMADUN AG, Switzerland Solar- und Wa¨rmepumpentechnik, Switzerland Solar- und Wa¨rmepumpentechnik, Switzerland Terrawatt, Sweden Terrawatt, Sweden Terrawatt, Sweden Terrawatt, Sweden Terrawatt, Sweden Terrawatt, Sweden Terrawatt, Sweden

Model

Type

LW 80N-I LW 110H-I LW 80N-I LW 110H-I Aeroheat 10l Aerotec 3 Several models LA 9 PS LA 12 PS LA 18 PS LA 22 PS LA 26 PS Several models Aerotop 10l Several HWS serie E HWS serie E HWS serie E HWS serie E HWS serie E LI10P LI 10P HLP90a HLP120a HLP180a Compacta CVKP LW 110H-I Europa Pro-D 5/10 Wi Pro-D 9/18 Wi Fighter 100P Fighter 200P Fighter 360P Siemens LI8H Siemens LI11H IS22 IS48 IS61 IS81 SuPRO Therma, 7DS SuPRO Therma, 14DS SuPRO Therma, 23DS RLM

A/W HP A/W HP A/W HP A/W HP A/W HP A/W HP A/A AC A/W HP A/W HP A/W HP A/W HP A/W HP A/A AC A/W AC A/W HP Prof. food refrig. G/W HP G/W HP G/W HP G/W HP G/W HP A/W HP A/W HP A/W HP A/W HP A/W HP W/W R A/W HP G/W HP G/W HP G/W HP EA/A HP EA/A HP EA/A HP A/W HP A/W HP A/W HP A/W HP A/W HP A/W HP G/W HP W/W HP

LI10P Futura HSWP 40EVU Futura HSWP 81EVU Tw 2 1⁄2 Tw 3 Tw 31⁄2 Tw 4 Tw 41⁄2 Tw 5 Tw 61⁄2

Inside/outside Installation

Refrigerant

Charge (kg)

Capacity (kW) (H/C)

Source

1.4 1.9 1.4 1.9 1.4 1.7 0.1–0.5 1.0 1.4 2.0 2.2 2.5

7.8 H 11.7 H 7.8 H 11.7 H 8.5 H 6.3 H 0.5–3.2 C 7.1 9.4 14.1 16.7 18.8

1.4

8.5 H

0.3 0.48 0.80 1.25 2.1 1.4 1.4 1.0 1.4 1.6

3.9 7.2 12.6 18.2 27.0 8.5 H 8.5 H 7.1 9.4 14.1

Inside Inside Outside Outside Outside Inside Inside Inside Inside Inside Outside Outside Outside Outside Inside

R290 R290 R290 R290 R290 R290 R290 R290 R290 R290 R290 R290 HC R290 HC R290 R290 R290 R290 R290 R290 R290 R290 R290 R290 R290 R290 R290 R290 R290 R290 R290 R290 R290 R290 R290 R290 R290 R290 R290

1.9

11.7 H

3.0 4.5 0.30 0.42 0.49 1.4 1.9

2.25

11.4 15.6 0.75 1.42 1.93 7.8 11.7 4.5 9 11.5 15 7.98

1 1 1 1 1 1 5 6 6 6 6 6 9 1 10 6 6 6 6 6 1 1 6 6 6 4 1 2 6 6 6 6 6 1 1 7 7 7 7 6

G/W HP W/W HP

Inside

R290

3.25

14.1

6

G/W HP W/W HP

Inside

R290

5.5

23.1

6

W/W HP

Inside

R290

6–45

6

A/W HP G/W HP W/W HP

Inside

R290 R290

1.4 1.6

8.5 H 8.6 H

1 1

R290

2.3

17.2 H

1

R290 R290 R290 R290 R290 R290 R290

0.35 0.40 0.50 0.65 0.75 0.85 1.0

5.3 H 6.7 H 7.5 H 9.0 H 10.5 H 12 H 15 H

8 8 8 8 8 8 8

Inside Inside Inside Inside Inside Inside Outside Outside Outside Outside Outside Split, outside Inside Inside Inside Inside Inside Inside Inside Inside Inside

G/W HP W/W HP W/W W/W W/W W/W W/W W/W W/W

HP HP HP HP HP HP HP

Inside Inside Inside Inside Inside Inside Inside

Sources 1, WPZ Bulletin (2007); 2, http://www.neura.at/index_en.php; 3, http://www.oekotherm.com/; 4, Kylma brochure, 1.0026.13; 5, Aloisi (2007); 6, Sherhpa (2006); 7, http://www.octopus.tm/varmepumpar.htm; 8, Christer Ra˚sba¨ck, Terrawatt private communication, and http:// www.terrawatt.se/; 9, Earthcare: http://www.earthcareproducts.co.uk/ and 10, Foster: http://www.foster-uk.com/default.asp?p¼5. A/A, air–air; A/W, air– water; AC, air conditioner; EA/A, exhaust air–air; HP, heat pump; G/W, ground–water and R, refrigeration unit.

562

8.

international journal of refrigeration 31 (2008) 552–563

Conclusions

This paper has investigated the properties of the hydrocarbons propane, propene and isobutane and compared them to R22, R134a and ammonia. It is shown that the properties of the hydrocarbons make them suitable as refrigerants and that the system efficiencies should be expected to be equal to, or higher than, those of R22 and R134a systems. The risks caused by the flammability of the hydrocarbons must be taken seriously. The risks can be reduced by designing the systems for minimum charge of refrigerant, careful leak detection during production, hermetic design with minimum number of connections, use of spark-proof electric components and ventilation of confined spaces. Most components for designing and building AC, heat pump or refrigeration systems with hydrocarbons are available. For the key component, the compressor, the supply is quite limited for cooling capacities between about 1 and 20 kW. Many companies are marketing products with hydrocarbon refrigerants. However, the number has decreased during the last years, probably at least partly due to lack of suitable compressors on the market. The prime reason seems to be the European Pressure Equipment Directive, according to which many of the compressors designed for H(C)FC refrigerants, but previously used for hydrocarbons, were no longer allowed to be used. The experiences from the field of hydrocarbon refrigerants are mostly positive. The increase in efficiency predicted based on the thermodynamic and transport properties is experienced also in real applications. Some manufacturers have reported high failure rates of the compressors. This is thought to be caused by decreased viscosity of the compressor oil, resulting from the high solubility of hydrocarbons in most types of oils. This problem has been solved by changing to a higher viscosity oil, use of crankcase heater and use of an internal heat exchanger in between the liquid line and the suction line. In conclusion, it seems likely that the hydrocarbons will continue to be used as refrigerants in small- and mediumsized refrigeration-, AC- and heat pump system. However, working with these fluids requires careful design and skilled personnel for manufacturing and servicing.

references

Aloisi, A., 2007. De’ Longhi, private communication. ARTA, 2007. Available from: http://www.nh3tech.org/ones.html. BOC-CARE refrigerants. Available from: http://www.carerefrigerants.co.uk (accessed 08.07.07). CCOHS, 2007. http://www.worksafesask.ca/files/ccohs/cheminfo/ cie48.htm/files/ccohs/cheminfo/cie48.html citing Compressed Gas Association, 1990. Handbook of Compressed Gases, third ed., Van Nostrand Reinhold Company, pp. 231–252. Chang, Y.S., Kim, M.S., Ro, S.T., 2000. Performance and heat transfer characteristics of hydrocarbon refrigerants in a heat pump system. International Journal of Refrigeration 23 (3), 232–242. Colbourne, D., Ritter, T.J., 2000. Compatibility of non-metallic materials with hydrocarbon refrigerant and lubricant

mixtures. In: Proceedings of the Fourth IIR Gustav Lorentzen Conference on Natural Working Fluids, Purdue, USA, 25–28 July. Colbourne, D., Suen, K.O., Ritter, T.J., 2003. A risk assessment model for hydrocarbon refrigerants. In: Proc. Inst. Refrigeration (UK) 2003–04, pp. 1–14. Colbourne, D., Suen, K.O., 2004. Appraising the flammability hazards of hydrocarbon refrigerants using quantitative risk assessment model. Part I: modelling approach. International Journal of Refrigeration 27 (7), 774–783. Cooper, M.G., 1984. Saturated nucleate pool boiling – a simple correlation. In: First UK National Heat Transfer Conference. IchemE Symposium Series 86, no. 2, pp. 785–793. Corberan, J.M., 2006. Private communication. Corberan, J.M, Seguardo, J., 2006. Characteristics of hydrocarbon refrigerants. Comparison with R22. Report D10, for Sherhpa, EU project COLL-CT-2004-500229. Available from: http:// sherhpa.fiz-karlsruhe.de/script/tool/forg/doc364/SherhpaD10.pdf. Corberan, J.M., Segurado, J., Colbourne, D., Gonzalvez, J. Review of standards for the use of hydrocarbon refrigerants in A/C, heat pump and refrigeration equipment. International Journal of Refrigeration, in this issue. Cordis, 2005. Available from: http://cordis.europa.eu/search/ index.cfm?fuseaction¼proj.simpledocumentlucene&HD_ID¼ 7897664&CFID¼1811126&CFTOKEN¼57953899 Danfoss, 2000. Practical application of refrigerant R290 propane in small hermetic systems. Available from: http://rc.danfoss. com/TechnicalInfo/literature/manuals/06/application_r290_ 11-2000_cn60f102.pdf Ekroth, I., 1979. Termodynamiska egenskaper fo¨r ko¨ldmedier. Ph.D. thesis, Royal Institute of Technology, Stockholm, Sweden. Elefsen, F., Nyvad, J., Gerrard, A., 2004. Field test of 75 R404A and R290 ice cream freezers in Australia. EcoLibrium AU 2 (10), 24–27. Fernando, P., Han, H., Palm, B., Granryd, E., Lundqvist, P., 2003. The solubility of propane (R290) with commonly used compressor lubrication oils, Proceedings of the International Conference on Compressors and their Systems. IMechE Conference Transactions 2003–4. ISSN: 1356-1448 (Professional Engineering Publishing). Fernando, P., Palm, B., Lundqvist, P., Granryd, E., 2004. Propane heat pump with low refrigerant charge: design and laboratory tests. International Journal of Refrigeration 27 (7), 761–773. Gerrard, A., 2007. Private communication. Granryd, E., 2001. Hydrocarbons as refrigerants – an overview. International Journal of Refrigeration 24 (1), 15–24. IPCC/TEAP, 2005. Safeguarding the ozone layer and the global climate system: issues related to hydrofluorocarbons and perfluorocarbons. Available from: http://www.ipcc.ch/press/ SPM.pdf Navarro, E., Urchueguı´a, J.F., Gonza´lvez, J., Corbera´n, J.M., 2004. Comparative experimental investigation of oil behaviour in a hermetic piston compressor using propane (R290) as refrigerant. In: Proceedings of the Sixth IIR-Gustav Lorentzen Conference on Natural Working Fluids, Glasgow, pp. 45–49. Navarro, E., Urchueguia, J., Gonzalvez, J., Corberan, J.M., 2005. Test results of performance and oil circulation rate of commercial reciprocating compressors of different capacities working with propane (R290) as refrigerant. International Journal of Refrigeration 28 (6), 881–888. ¨ hman, H., 2007. SRM, private communication. O Pelletier, O., Palm, B., 1996. Performance of plate heat exchangers and compressor in a domestic heat pump using propane. In: Proceedings of the IIF/IIR Conference on Applications for Natural Refrigerants, Aarhus, Denmark, pp. 497–505.

international journal of refrigeration 31 (2008) 552–563

Pelletier, O., 1998. Propane as Refrigerant in Residential Heat Pumps. Licentiate thesis. Royal Institute of Technology, Stockholm, Sweden, ISSN 1102-0245, ISRN KTH/REFR/R-98/24-SE. Refrigerants Naturally, 2007. Available from: http://www. refrigerantsnaturally.com/news.php?ref¼200701246212 RTOC, 2006. UNEP Report of the Refrigeration, Air Conditioning and Heat Pumps Technical Options Committee, 2006 Assessment. Available from: http://ozone.unep.org/teap/ Reports/RTOC/rtoc_assessment_report06.pdf. Tadros, A., Clark, J.W., Maclaine-Cross, I.L., Leonardi, E., 2006. Replacing fluorocarbons with hydrocarbon mixtures in centrifugal water chillers. In: Proceedings of the Seventh

563

IIR-Gustav Lorentzen Conference on Natural Working Fluids, Trondheim. Wennerstro¨m, P., May 2007. HC in domestic refrigeration. In: Proceedings of the SHERHPA Course: Natural Refrigerant Heat Pumps. Royal Inst. of Technology (KTH), Department of Energy Technology, Stockholm, Sweden. Woodward, J.L., 1998–1998. Estimating the Flammable Mass of a Vapour Cloud. Centre for Chemical Process Safety, American Institute of Chemical Engineers, New York, USA. WPZ Bulletin 01-2007. Available from: http://www.pac.ch/ dateien/WPZ-Bulletin%2001-2007%20F.pdf, 2007.