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Cryogenics: a technological tool for space scientists
CRYOGENICS: A TECHNOLOGICAL TOOL FOR SPACE SCIENTISTS S. M E R C E R Cryosystems Ltd, 40 Broadway, London, S.W.1, UK Received 6 February 1968
THE NUMBER of objects launched into space is now measured in hundreds and very few of these have failed in their purpose; malfunctioning in space often leads to total loss and must be avoided if at all possible. One of the prime reasons for the high success rate can be attributed to carefully conducted check-out procedures, including tests carried out by the simulation of conditions in space as far as this is practicable. Within the solar system, the principal characteristics of space are high vacuum, an infinite very cold heat sink, and intense solar radiation alternating with total darkness. Other properties, such as radiation from the earth's surface, loss of gravity, etc., may also have to be taken into account. Two principal types of facility are available to conduct a satellite through an adequate series of tests: those with solar simulation and those without. The space simulator with solar simulation can be very complex since, in addition to providing the environmental characteristics of space, it is necessary to provide attitude and spin conditions for the space-craft under test. The facility without solar simulation is known as a thermal vacuum chamber and is used for testing satellites under the alternating conditions of the effects of solar radiation and total darkness. Environmental chambers used for simulating the characteristics of space depend on cryogenics in varying degrees for their successful operation. In many instances it is necessary to provide only an isothermal cryogenic system while in others it is necessary to vary the temperature over a wide range. In addition, cryogenics is sometimes used for producing the required high vacuum. Communication satellites, typical of those orbiting the earth, are used for relaying high frequency radio and television signals. A station of this type requires power for its operation and this is derived from batteries which are kept charged by light sensitive solar cells. Thus a communication satellite is a complex unit consisting of aerials, receiver, transmitter, and power source, all of which must be capable of withstanding the wide changes in temperature experienced during each orbit. In order to provide the means for testing satellites and space probes under simulated environmental conditions, suitable centres have been set up throughout the world
and these have industrial, national, or international sponsors. Characteristics of space and thermal balance The three principal characteristics of space affecting the thermal balance of a satellite are solar radiation, pressure, and the heat sink of space. Solar radiation. Solar radiation is very intense and represents a heat flux of 1 400 W/m 2 of projected area for a satellite in an earth orbit. The absorption of all this energy would lead to an intensely high temperature within the satellite but for the fact that the emissivity of its outer surface approaches zero; thus only a fraction of the heat is absorbed. Pressure. The pressure exerted on a satellite depends on its altitude, as is shown in graphical form on Figure 1. The effect of the environmental pressure is important because when the pressure is below 1 x 10-6 torr the molecular mean free path becomes very large and intermolecular collisions are rare. This results in the mechanism of heat transfer by conduction and convection in 500
40C
30O ~b
E 200 <( 100
0
I 1000 100 10 1 1()1 103 105 Pressure.mm.Hg
10?
109
Figure 1. Environmental pressure at various altitudes CRYOGENICS • A P R I L 1968
the gas being virtually eliminated. Consequently heat transfer through the environment is reduced to the radiaation effect only.
The heat sink of space. Space itself has no temperature but represents a black body with an emissivity of unity. Moreover, it has an infinite area, which means that the heat rejected by an object to space depends only on its own temperature, surface area, and emissivity• Therma'l balance• The heat transfer between the heat source, satellite, and heat sink is entirely radiative, as discussed previously• The solar intensity at a satellite in an earth orbit is known to be 1 400 W/m 2 of projected area and from this value suitable solar simulators can be provided; these are generally specified to provide 2 000 W/m 2. In the case of the heat sink of space it is usual to restrict the simulated temperature to 100°K. The reason for this can be understood readily by an analysis of the StefanBoltzman law. aA,(T~- T~) •
where
Q = total heat flow O ' = Stefan's constant Al= area of satellite a l = emissivity of satellite T I = temperature of satellite .42 =- area of space a 2 = emissivity of space T2= temperature of space
Since
~r = 0.533 x 10.8 W/°K.ft 2
. .
(1)
%
...
Satellite
(2)
Az,a2-1)]
Q = 0"533a1A1(l~0) 4
In the case of a satellite receiving solar radiation, it is only necessary t o provide a heat sink at 100°K because the temperature of the satellite will be relatively high; consequently the error in heat balance determination will be small• For this reason it is normal practice for an environmental chamber with solar simulation to have an isothermal shroud operating at the normal temperature of 100°K. Nevertheless, it must be recognized that the satellite will indergo wide changes in temperature during its orbit, which necessitates thermal cycling tests to complete the series of events experienced. For this reason the solar simulation chamber is supplemented by thermal vacuum chambers which are designed with the ability of testing the satellite through a number of cycles between 400 and 200°K.
"'-7",
~,Y6-6! - \l-N! j
T2 approaches
The maintenance of a heat sink at 100°K or less Thermal cycling systems operating over a wide range of temperature, namely 400 to 100°K The cooling of optical windows and the provision of cold traps and cryopumps Complex control functions and accurate temperature distribution•
shroud
Q = 0.533A, [ 1 + A 1 [ I
and since A2 is infinite and
Thus if T 2 = 100°K, we have, for T~ = 400°K, X = 0-39~o, and for T1 = 200°K, I; = 6.25~. Therefore the percentage error by simulating the heat sink of space at 100°K is only significant at low temperatures. This by itself will not materially affect simulated testing, and the advantages of providing a heat sink approaching 0°K would not justify the resultant escalation in cost. The cryogenic content of space simulation systems is both interesting and widespread as it can involve some or all or the following:
i
I I
0°K
!
t z
I I I
•
.
.
(3)
Consequently the heat transferred is only a function of area, temperature, and emissivity of the satellite• Rewriting equation (3) Q =J|7-h-AJ \lUU/
...
(4)
where f is a constant• The percentage error caused by T2 ~ 0 i s
X = 100
Lamps
/'T, ~4 /'T2 ~' (T, '~41 ~,i-6-6! + \T6-O! - ~,ib-6! / i~!
x
CRYOGENICS
=
100(T2~ 4
~!
• APRIL
1968
•..
(5)
Figure 2• Space chamber with solar simulator consisting of carbon arc or x e n o n lamps, reflectors, lenses, and windows 8g
Space chamber systems -
Solar simulators. In this type of system (see Figure 2), tests are undertaken to determine the heat balance and thermal distribution characteristics of a satellite under solar exposure. For reasons stated previously it is necessary for the shroud temperature to be at or below 100°K under all conditions of test. The heat absorbed by the shroud and rejected to the refrigerant is derived from two principal sources, namely, the solar simulator and the chamber wall.
-
i
The two principal types of space simulation chamber are those with solar simulation and those without, or solar simulators and thermal vacuum chambers.
I1
:-©': I. . . . . .
Liquid N2 from storage
4
Gaseous N2 to recovery
Figure 4. Basic m e t h o d s of refrigerating c h a m b e r s
T A B L E 1. H E A T RECEIVED B Y S H R O U D
Refrigerants and refrigeration
Symbol
Flux, W/m s
%
Os OR OK
240 70 30
70 21 g
QT
340
100
The values given in Table 1 relate to a 6 metre chamber but are representative of any similar system. The symbols are as follows: Qs is the mean flux due to solar simulation, Qn is the heat radiated from chamber wall, QK is the heat transfer by solid conduction, and Q~, is the total heat transferred per unit area. A very good indication of the cryogenic requirements can be derived from these figures, but only on an overall basis because in practice the unit load is not constant; part of the shroud will receive heat by reflection from the satellite, part will be in the direct path of the beam of energy, and part will be covered by the shadow of the satellite. Consequently, consideration must be given to the distribution of the refrigerant through the shroud. As the satellite shown in Figure 3 orbits the earth it passes through three main zones: solar radiation zone, twilight zone, and zone of total darkness. During the course of each orbit the temperature of the satellite will vary from a maximum to a minimum.
Orbiting D
Solar radiation
----"
o o 'O'tJ
. )
\ x
\
Figure 3. Satellite in earth orbit
70
/ - - N~o
Refrigerants. The principal factor governing the choice of refrigerant is the thermal duty expected of it, in particular, the lowest temperature required. As 100°K has been established as an acceptable lower limit, and higher temperatures are often satisfactory, a reasonably wide range of fluids are available: e.g. nitrogen, acetone, pentane, etc. The most common refrigerant in use today is nitrogen, which is available in liquid form from the many large tonnage air separation plants scattered throughout the world. Considerable benefit can be derived from its use because it avoids the need for large installed refrigeration units which may be completely uneconomical because they are not fully utilized. Such units would need to be part of, or connected to, a substantially large air separation plant if complete independence is required. Even though a particular test facility may operate at a significantly higher temperature than the normal boiling point of nitrogen, there is no reason why nitrogen cannot be used as the primary refrigerant. The phase in which the secondary refrigerant operates can be liquid, vapour, or liquid and vapour; the choice of refrigerant will be the determining factor. If a facility is to operate at the lower temperature of 100°K, liquid nitrogen is a feasible choice. On the other hand, when the lowest temperature is 200°K, liquid nitrogen cannot be used as the secondary refrigerant, although nitrogen can be used, but in the vapour phase. In this regime other secondary refrigerants such as acetone may be used with the added advantage that the high critical temperature makes it possible to operate the facility over its entire temperature range in the liquid phase. Refrigeration. A fully independent facility would probably consist of an air separation plant with liquid nitrogen production to provide the primary refrigerant, a secondary refrigeration cycle to cool the shroud, and a third refrigeration cycle to recover the primary refrigerant. Such a system would be used only where several large space chambers were in use simultaneously. There are various methods of refrigerating chambers and the three principal ones are shown in Figure 4. Chambers 1 and 2 are maintained at a single constant temperature, whereas chamber 3 operates over a variable temperature range. CRYOGENICS
• APRIL
1968
Mechanism of heat transfer
GQSeOUS N2
When using a low boiling fluid to cool a shroud, two methods of heat transfer are possible: latent heat of evaporation and sensible heat transfer. In the former case the liquid is allowed to evaporate isothermally in absorbing heat and in the latter there is no phase change but there is a change in temperature.
Evaporatit:e. In this case the refrigerant boils isothermally and the temperature is varied by altering the vapour pressure. When the system to be cooled is large, isothermal conditions are not met exactly because of the hydrostatic head between the top and bottom of the shroud. However, this does not constitute a penalty providing the temperature difference is small or in the case of a solar simulator, providing the highest temperature does not exceed 100°K. A major difficulty of the evaporative system is in ensuring good distribution and the prevention of gas pockets causing local overheating. Sensible. A sensible heat system can operate in either the liquid or gaseous phase. If a liquid system is used it is first necessary to ensure that the liquid is sub-cooled. This is realized by raising the pressure without raising the temperature to effectively increase the boiling point. The most suitable method of achieving this state is by means of a cryogenic pump. Under these circumstances, isothermal conditions cannot be met although by circulating sufficiently large quantities it is possible to limit the temperature rise to a few degrees. Such a system has the advantage that good distribution can be effected. Cryogenic cycles
Single temperature--two phase. Liquid nitrogen is drawn from a storage vessel and fed into a level control vessel where disengaging of vapour and liquid fractions takes place (see Figure 5). The liquid fraction passes into the shroud where evaporation takes place in absorbing heat from the shroud.
os o ,m,
....
I Level control vessel
Liquid N 2 Figure 5. Evaporator system C R Y O G E N I C S • A P R I L 1968
Shroud
q
Separator vessel,
hroud
Liquid N2
Cryogenic pump Figure 6. Liquid cooled system
The controller L maintains a constant liquid level and the temperature is fixed by the pressure controller P which maintains a constant vapour pressure, hence boiling point. In the case of a system using liquid nitrogen as the refrigerant a pressure of 2 atm (absolute) will correspond to a constant boiling point of 84"1{. The primary and secondary refrigeration function are both realized with the same media.
Single temperature--single phase. Liquid nitrogen is drawn from the storage tank and fed into a separator vessel where the vapour content, if any, is removed (see Figure 6). The liquid then enters a cryogenic pump where its pressure is raised to about 6 atm (absolute). Assuming the liquid is stored at 1.2 atm, its boiling point will be 79°K whereas at 6 atm its boiling point is 96°K. The heat entering the nitrogen in the pump will correspond to a temperature rise of about l°C, consequently the liquid nitrogen leaving the pump will be 16°C below its boiling point--very conveniently subcooled. As the liquid transverses the shroud its temperature will rise according to the quantity circulated. A comparison of the latent and sensible heats shows that for a I°C temperature rise it would be necessary to circulate about 100 times more than in the evaporative case. However, such a small temperature rise is not generally necessary, particularly if the refrigerant is always colder than 100°K. In the example discussed above it is satisfactory to allow a temperature rise in the neighbourhood of 10°C and have sufficient margin in hand to ensure that there is no risk of local boiling occurring. A further few degrees temperature rise could be tolerated in regions of high flux or in the case of short term transients caused by malfunctioning of the solar simulation unit. When the sub-cooled liquid leaves the shroud it is expanded into the separator vessel where the liquid and saturated vapour fraction are separated. The liquid is then recycled together with make-up from the storage tank and the vapour is rejected or recovered. Different degrees of sub-cooling can be achieved by varying the system pressure. Multi-temperature--single phase. The variable temperature cycle is suitable for systems operating over a wide range of temperature and where any temperature may be selected in the range specified. A typical example is in the case of a thermal vacuum chamber where the temperature range may be from 200 to 400°K. Such a system is not "/1
Gaseous N2 i
N2 evaporator > > >
Liquid N2
Shroud > >
L. . . . . . .
nitrogen affords an excellent primary refrigerant. Also, the secondary refrigerant may or may not be nitrogen depending on the thermal specification for the system under review. In all these examples the evaporated nitrogen has not been given attention as it does not in itself affect the functioning of the cooling of the shroud. However, in all cases it can be rejected or recovered depending on the overall economics of the system.
>
I
I.
L
I
Heat exchanger Electricct[ heater irc ator Figure 7, Variable temperature cycle--gas
.... -~~
=
c
!
>
,~Shroud > > >
~
exchan;r Electrical, hea e "--
Figure 8, Variable temperature cycle--liquid
entirely cryogenic as the achievement of temperatures above ambient is met with a suitable heating system. In the cycle shown in Figure 7 primary refrigeration is provided by the evaporation of liquid nitrogen which is heat exchanged with the circulating fluid. Gaseous nitrogen can be used as the secondary refrigerant and circulated in a closed cycle as shown. For temperatures below ambient, the gas is circulated through the cold evaporator and a suitable temperature control loop to ensure the maintenance of the temperature selected. Above ambient temperature, the nitrogen evaporator is rendered inactive and an electrical heater is suitably energized and de-energized as appropriate. As the cycle temperature can be both hot and cold it is necessary to provide thermal isolation for the gas circulator and this is achieved by means of the heat exchanger installed between the booster and the remainder of the system. In order to circulate a sufficiently large mass flow it is necessary to operate the system at an elevated pressure. Liquids can be used as the secondary refrigerant (see Figure 8), but their temperature range is more restrictive and it is often necessary to design to a relaxed specification. Such a design depends on some of the physical properties of the circulating fluid such as critical temperature and pressure, freezing point, and viscosity. Summarizing the above, it can be seen that in all cases 72
Rejection and recovery of primary refrigerant In every case it is a simple matter to vent the evaporated refrigerant to atmosphere and replace it with liquid bought in or made on site. When the facility is of such magnitude or so remote from an industrial centre that an air separation plant is associated with it, it is quite easy to route the saturated nitrogen gas back into an appropriate part of the plant so that recondensation and recycling can be effected. However, if the facility is near an industrial centre, site storage can be provided with easy means of supplying make-up. Nevertheless when large quantities of liquid nitrogen are required it may be more economical to recondense on site and limit the supply of make-up to overcome plant losses only. This can be achieved using cold gas engines based on the Stirling cycle. A recondensation unit is shown in Figure 9.
Shroud The shroud constitutes an important and integral part of the space chamber and cryogenic system. Its function is to provide a cold heat sink or a mechanism for simulating the effects of solar radiation and total darkness in a cyclic manner. To achieve this purpose it must operate isothermally, be optically tight, and envelop completely the satellite under test. In order to provide the required radiant properties it must be an effective black body with an emissivity of that part seen by the satellite approaching unity; in practice emissivity is usually about 0.95 and reflection is virtually eliminated. Since the shroud is contained inside a high vacuum environment it is important that the correct type and quality of material is Chosen, otherwise outgassing may constitute an undesirable difficulty. The outer surface of the shroud facing the chamber wall presents a very different problem as heat transfer between the shroud and wall is undesirable. Because this is reduced to the mechanism of radiation only it is best minimized by surface preparation leading to low values of emissivity. A good practical emissivity to aim for is 0.1, and this can be obtained by electro-polishing. It is of equal importance that the inner wall of the chamber
Make up L - ~ Liquid ~nitrogen /~.:------~'~ st orage ~ ( .~...~.~_.~..~_. . ) t an k I \x~::-." ~-'~,~/ " h . " ~ ' - " ~f" I r"'-l'~| .J==L. ~J-
~
~ ~ \
~ _,t ~ Transfer pump
Figure 9. Recondensation
Space chamber complex Recondensationl unit |
unit
CRYOGENICS • APRIL 1968
F7
t
~
•
J
I
.~f-
f-~
I TI
T2
i It
~
Tz I "T~
Figure 10.
should have a similar emissivity, since the radiant heat transfer is a function of this also. As stated previously, the temperature distribution over the shroud can be fixed by providing an adequate quantity of refrigerant. This statement is only true for the ideal case where the entire inner surface of the shroud is covered by a film of refrigerant. In the majority of cases expanded tube in sheet construction is used, and this introduces a thermal gradient between adjacent pairs of tubes. If the temperature variation is to be small it is essential that the fin temperature be determined and this may be done in the following manner (see Figure 10):
advantage that the cold surface can be placed inside the chamber resulting in a high effective pumping speed. The panel is cooled usually with liquid helium and pressures below 1 x 10 -15 torr are possible when the condensable gas is nitrogen. For pressures below 1 x 10 7 torr it is necessary to provide the initial high vacuum (1 x 10 4 torr) with another high vacuum pump. Such a fore pump may be cryogenic relying on low temperature adsorption on surface active material. The inclusion of cryopumps in the space chamber complex introduces the need for compact helium refrigeration equipment which may rely on 'bought in' liquid helium or a closed cycle helium refrigerator. If standard oil vapour diffusion pump equipment is used there is always the danger of contamination by back diffusion of the oil. This can be prevented by placing liquid nitrogen cooled, optically tight, baffles between the chamber and pump. The penalty in installing these baffles is that the effective pumping speed may be reduced by as much as 50~.
Costs nw 2 (T2 - T~) = 2 k t
where
T1 = refrigerant temperature at tube wall T2 = temperature at mid point between tubes or tip of fin w = fin width H = radiant heat flux t = fin thickness k = thermal conductivity of fin material.
Miscellaneous Cryogenics may be used for purposes other than provision of the simulated heat sink of space. In a space simulator system using a solar simulator it is necessary to fit windows in the chamber wall for transmission of the beam of light. Although these windows are transparent to light, they are opaque to heat, and consequently get hot during service. If the window is small it will conduct sufficiently to the wall of the chamber so that its equilibrium temperature is not very high. But, in the case of large windows, it is impossible to conduct sufficient heat to the chamber wall with the result that the window temperature will be excessively high; cooling of the window is therefore necessary. One method of achieving this is by constructing the window in two halves with a space between them through which a cooling fluid can be circulated. During the period when the solar unit is operating it would be normal to cool with low pressure nitrogen gas at around 0°C. When the solar unit is not operating, this temperature would be raised to around 20°C to prevent condensation occurring on the face of the window. Nitrogen gas required for this purpose can be conveniently provided from the shroud boil-off. The normal method for producing the high vacuum is by means of oil vapour diffusion pump systems, but for ultra-high vacua and prevention of contamination in the high vacuum environment it is possible to use cryogenic pumps. The cryogenic pump utilizes the mechanism of condensation on a low temperature surface and has the CRYOGENICS • A P R I L 1968
Consideration must be given to a large number of factors when determining the cost of a space chamber testing facility. In arriving at the budget costs given in Figure I 1 the systems for the primary refrigerant have not been
._ "~
Thermal, vacuum c h ~ [
','41xi05 "-79x104 ~Bx104 c) 7xi04 6xI04 5xll 0
--
I
2
Solar s i m u t a t o r / chamber ~
3 4 5 6 ? 8 Chamber diameter~ m
~lxlOS~ _19xi05 .
9
10
-jSx1OS~ _j7x10s ~6xI0 s J5x105
Figure II. Approximate cost of complete space chamber facility
accounted for as these depend on geographical location and existing facilities. The main purpose of including these figures is to give the reader an indication or order of magnitude of the costs involved in providing satellite testing equipment. Two curves are given, one for solar simulation chambers and one for thermal vacuum chambers; both representing turn-key prices for essentially automatic systems. An approximate breakdown of these is as follows. Cost, °/o Solar simulation chamber
Chamber Motion gear Cryogenic system Instrumentation Solar simulator Vacuum system Erection
22 2 12 5 45 7 7 100
Thermal vacuum chamber
12 40 20 15 13 I00
"/3
The economics of recovering nitrogen depends on the annual usage, capital charges, and cost of utilities. The following approximate example gives an indication of whether recovery or rejection should be adopted and relates to a single thermal vacuum chamber unit.
Charges and utilities Cost of recondensation facility Liquid nitrogen consumption Operating period Power consumed by cold gas engine Electricity Liquid nitrogen
£15 000 200 litres/h 2 000 h/year 80 kWh 1d./kW 7d./litre
Rejection Cost of liquid nitrogen -
200x7 2~ x 2000
=
£11 680p.a.
Recovery Cost of recondensate equipment Cost of electricity -
£15 000
80x 1 2 ~ x 2 000 =
666 £15 666 p.a.
The foregoing gives a brief indication of the part played by cryogenics in space technology. The facts presented relate to the industry in general and in support of this the following relates to an installation provided to the European Space Technology Centre, Noordwijk, Holland. Twin thermal vacuum chamber facility The system was built to an exacting specification which introduced the need to develop a complex control and monitoring system so that the required environment could be achieved from a remote station. The principal requirements are listed below. 1. Working volume of chamber: 1.5 m diameter × 2 m long 2. Chamber diameter: 2 m. 3. Chamber operating pressure--with shroud hot: 1 x 10-s torr; with shroud cold: 1 x 10-btorr 4. Shroud temperature--mode 1 : below 100°K; mode 2: 150-300°K; mode 3: 300-400°K 5. Temperature distribution over shroud: + 2.5°C max. from mean.
Vacuum chambers. Each of the identical twin chambers is constructed with the longitudinal axis in the vertical plane and comprises four main sections. The major body section has external brackets to accommodate support legs and is surmounted by a narrow ring section with feed-through ports and provision internally for mounting the satellite or component to be tested. To complete the chamber two covers are provided for the top and bottom of the vessel. Each section is connected with flange joints utilizing double O-ring seals. The vessels are made from austenitic grade stainless steel, type EN58J, and designed in accordance with "/4
BS1500 class I pressure vessel code with all welds fully radiographed. To minimize heat transfer between the chamber and shroud the inner surface of the chamber wall is mechanically polished to give a near mirror finish (about 10 micro-inch surface finish), and emissivity not greater than 0.1. On completion of the fabrication the leak rate was below 1 x 10-4torr litres/s measured with a helium mass spectrometer. Provision of the narrow ring section offers an easy means of satellite loading; the ring section is placed on a separate supporting frame with unrestricted access for mounting the satellite and wiring it up. During this period the top cover is replaced on the main body section so that the internal cleanliness of the chamber is preserved.
Vacuum system. Two identical pumping units are attached to each chamber and these consist of 610 mm oil vapour diffusion pumps, high vacuum baffle valves, refrigerated chevron baffles, single stage, air ballasted, rotary mechanical roughing pump, and single stage, air ballasted, rotary mechanical backing pumps. The overall pumping speed is greater than 4 000 litres/s at 1 × 10-4 torr and the ultimate pressure when pumping a clean uncooled chamber is less than 5 x 10-6 torr. With the shroud cold, a chamber pressure below 1 x 10-7 torr can be achieved. A simplified flow diagram of the vacuum complex is shown in Figure 12. The sequence of events from atmospheric pressure to the high vacuum state is followed entirely automatically once initiation has been made. A pirani gauge monitors the roughing and backing pressure and controls the opening of the high vacuum valve at 0.1 torr. The high vacuum is measured with a Penning gauge over the range 1 x 10-3 torr to less than 1 x 10-6 torr while a record is made using an ionization gauge over the range 5 x 10-3 torr and 5 x 10-9 torr. Cryogenic or heating and cooling system Because of the wide range of temperatures required, a combined variable temperature gas cycle and single temperature liquid cycle is used. The primary refrigerant for all low temperature conditions is liquid nitrogen and
Pirani gauge Penning gauge
Ionization gauge I / J _ 6 ~ " ~ I I
Diffusion pump
I [
I I
Penning
gauge7
High vacuum/ valve /
Pirani J_~ gauge ~1 Holding I pump j Roughing pump"
I~,~
/
Figure 12. Schematic flow diagram of vacuum system CRYOGENICS • APRIL 1968
Liquid
Vent N;
,. . . .
7
:® ®
'--o
Nitrogen gas is circulated by the booster 6 through the exchanger 5 and part through and part around t.he evaporator 4 in order to cool to the appropriate temperature. It then traverses the shroud, where heat is absorbed and returns by way of the exchanger 5 to the booster 6 for recycling. Mode 3 (300-400°K). During this mode of operation heat is not absorbed by the system but is rejected. Consequently refrigeration is not used. Heat is provided electrically by the heater 7. The gas is circulated by the booster 6 from which it passes through the exchanger 5, heater 7, shroud 3, and exchanger 5 during each cycle. In this case the shroud cools down the gas.
(5 ® 3ct,3b,3 ¢: s h r o u d
Figure 13, S c h e m a t i c f l o w d i a g r a m of c r y o g e n i c s y s t e m
the secondary refrigerant is nitrogen for all duties; liquid nitrogen is used in mode 1 while gaseous nitrogen is used in modes 2 and 3. The major difficulty confronting the designer is in ensuring that the temperature difference between A and B (Figure 13) is never greater than 5°C and that the required mean temperature is midway between A and B. The former is a function of accurate determination of the heat absorbed by the system while the latter is governed by the control engineering and instrumentation accuracy. These two forces are of equal importance as the purpose of the cryogenic system is to reach, and maintain accurately, selected environmental conditions. Figure 13 shows the cryogenic system adopted which operates in the following manner. Mode 1 (below 100°K). Liquid nitrogen passes from a storage installation into the separator vessel 1 where any nitrogen evaporated in transit is removed. The liquid then fills the system completely, i.e. pump 2, shroud 3, and evaporator 4, and is pressurized to 4 atm. With the nitrogen feed isolated, the pump circulates the liquid through the cycle where it is sub-cooled in the evaporator 4 and heated in the shroud 3. In this mode of operation it is of no consequence to maintain accurate temperature distribution but rather to ensure that the maximum temperature nowhere exceeds 100°K. Mode 2 (150-300°K). The liquid nitrogen pump is isolated as gaseous nitrogen is used for the secondary refrigerant. Since a small temperature rise is allowed in the shroud it is necessary to circulate a large mass flow, and since the specific heat of nitrogen gas is low, it is necessary to circulate the refrigerant at a generally elevated press u r e - i n this particular case 7 atm. In theory it is quite feasible to circulate at low pressure, but this would be unsatisfactory because very large flow passages would be required. This would lead to increased capital charges because the shroud would be larger and consequently the chamber would be larger; moreoever, the cryogenic cycle would obviously be much more bulky and require increased building space. CRYOGENICS
• APRIL
1968
Control
The entire control system is very complex and includes electronic, electro-pneumatic, and pneumatic instruments of the indicating, recording, and control types with mimic diagrams showing the state of the facility at all times. Extensive interlocks and fail safe mechanisms prevent maloperation, hazards, or damage to the expensive equipment under test. Conventional control techniques are used in the liquid cycle for the mode 1 requirement whereas less conventional instrumentation practice is utilized for the thermal stability required during modes 2 and 3. During operation over the temperature range 150 to 400°K a number of interesting conditions can arise. If the temperature is raised, the specific volume of the circulating gas will increase, and consequently the pressure in the system will also increase. Conversely, when the temperature is lowered the pressure will fall. Furthermore, at low temperatures the amount of gas passing through the refrigerator (nitrogen evaporator) may be small and there will be tendency for it to liquefy; this can occur because the nitrogen boils at about 79°K and the condensing temperature of the circulating stream is 100°K. Consequently, there will be a rapid reduction in the system pressure causing instability. Moreover, the frictional resistance in the cryogenic system is directly related to the temperature and for a constant circulating pressure there is a larger differential pressure across the booster at higher temperatures. Thus, to ensure a constant booster pressure, it is necessary to vary the flow conditions, otherwise over-pressurization and overheating of the booster would take place. The control system provided ensures that stability is automatically preserved at any selected temperature in the operating range. A split range pressure control loop maintains the pressure constant. This is a pneumatic control device which operates with a conventional instrument air supply over the range 3 to 15 lb/in 2. The charging and venting valves have opposite actions and one is open at 3 lb/in z and closed at 8 lb/in 2, whereas the other is closed at 10 lb/in 2 and open at 15 lb/in 2. Thus, automatic charging and venting is achieved and, because of the 8-10 lb/in 2 inactive band, hunting of the valves is prevented. The booster delivery pressure is maintained constant by a differential control unit fitted across the compressor; this actuates a control valve fitted in the booster bypass. During low temperature operation the temperature of 76
Cooling
FsWOter &
Local start
|
E
the gas entering the shroud is fixed by the quantities of gas passing through the evaporator and its bypass respectively. The control valves in the evaporator feed and bypass line have opposite actions and are positioned so as to ensure a mixed gas temperature at the particular value selected. To prevent condensation of the circulating nitrogen, a temperature controller in cascade with the level control of the evaporating liquid resets the level of nitrogen on the boiling side of the heat exchanger. The temperature setting on the cascade unit is about 5°C above the condensing temperature of the high pressure
Liquid N2 pump interlock
./___,
. . . . Interlock - " . . . . over Sd"e
r~
for test purposes
=-rr-,
gas. Interlocks for motorized i s o l a t i n g valves
C = Motor control coil
Figure 14. Circuit diagram of booster motor starter ---;Command flashing suppl Fault flashing supply
,upp,y .Mode
Remotepush -buttons
[,Go 'Nogo'
interiocks~;:~ lamp:green~j['~ lamp:am ber v
v
Figure 15. ' G o - n o go' circuit diagram for booster
Distribution of gas through each section of the shroud is fixed by temperature control loops situated in the lines leaving the shroud; these are set 5°C above the temperature of the feed gas for the low temperature duties. For high temperature duties heat is introduced automatically by energizing and de-energizing the electrical heater with a mercury switch system built into the standard electro-pneumatic temperature controller. In this instance the temperature of the gas leaving the shroud is set 5°C below the inlet. The entire plant can be operated from a remote control centre or from local control stations. Interlocks prevent maloperation: the gas booster cannot function unless the liquid nitrogen pump is isolated and vice-versa; refrigerant cannot enter the shroud unless the pressure within the chamber is at or below 1 x 10-4 torr; the pressure inside the chamber cannot be raised unless the shroud is at or above ambient temperature; refrigeration and heat cannot be introduced concurrently. A foolproof
Figure 16. Machinery floor showing switchgear and cold box 76
CRYOGENICS
• A P R I L 1968
Figure 17. T e s t floor and gallery showing twin thermal vacuum chambers and part of control panel
"go-no go' system incorporates these functions together with many other features including the state of the system at all times. All important functions and parameters are displayed on the mimic diagram with amber and green signal lights which either flash or are steady. The sequence is as follows: Amber steady - - initiation has not been made Amber flashing - - a fault has occurred Green flashing - - initiation has taken place and the system is moving towards the stationary state Green s t e a d y - the system is operating satisfactorily in the stationary state. The electronic system for controlling and monitoring these functions is very complex but follows the principles described below for an individual module. The module described concerns one of the gas boosters used for circulating nitrogen during modes 2 and 3. As can be seen from Figures 14 and 15 it is not possible to energize the booster motor unless the following conditions are met: 1. 2. 3. 4.
The correct mode has been selected Cooling water is circulating through the booster The liquid nitrogen pump is inoperative The motorized valves isolating the liquid and gas cycles are in the correct position 5. The pressure in the chamber is below 1 x 10-4 torr 6. The overload contact is closed 7. Power to the 'go-no go' system is on.
CRYOGENICS
• A P R I L 1968
After all these conditions have been satisfied the 'go-no go' system will indicate amber steady; if the conditions are not met an amber flashing signal will be indicated. When initiation has been given, the amber signal will be replaced by a green steady signal indicating that the booster is operating correctly. If a fault develops after start-up the green light will be extinguished and the amber light will flash; at the same time the booster will shut down automatically and cannot be restarted until the fault has been corrected. The command stage utilizing the green flashing signal is not built into the booster circuit since the time to run up to speed is very short. On completion of all command signals when the stationary condition has been achieved, the lights on the mimic diagram will show steady green with the exception of those showing steady amber by intent. For example, when the booster is in operation, the liquid nitrogen pump will be inactive and indicated as such by a steady amber signal.
Operation The plant operation is simple and requires very little attention from the operator. In principle it is necessary only to select the required mode by means of a three position switch on the panel, open or close a few hand valves as appropriate, set the required temperature on the control instruments, and start-up the plant. Apart from adjusting the manual valves (which is necessary only when changing from one mode to another) all the operations can be carried out at the remote control centre. 7"/
400
103 • 760torr o
102
~
d 300
101
10( L.. I~I!
~ 4
Roughing pump
op of shroud
~
X 200
~ottom
E
of shroud 100
l
0
I
1
~ 1
20
*_2.5oc
~
t
~-I
I
40
I
1
60
I
I
80
1O0
1
I
I
120
I
I
140
!
160
High vacuum pump
o
• ~ lg 2
Time,rain
~63
Figure 20. Temperature distribution over shroud during cool down to 150°K
.___.......Shroudat 400°K
L.
a. 104
,-'" "'. Shroud (:atambient Zone of "~ temperature . ~ N,,%,' degassmg . \
/
s ~'
1~5 106 ~ r 1~7 i
o I 2
u I
d i 4
o.
at 200°K I I I 6 8
I
400
or
I;~'s
I
I 10
I
I 12
300
Time,h Figure 18. Pumping down curve for high vacuum system s h o w i n g effect of temperature on vacuum
40(3
E 200
1OOol "10
I
3d
0
~- 3oo
~,, *-2'5°C/~
2Q) 200
--
Shroud inlet temperature . . . . Shroud outlet temperqture
n
E
~ loo 0
I
0
I
5
I
\_J_/" Gase I
10 Time,h
,
I 2
I
3
I 4
I 5
I 6
Maximum t e m p e r a t u r e d i f f e r e n c e over s h r o u d , °C
I',
Figure 21. Mean temperature of shroud versus maximum temperature difference over shroud
ogen L uidn~tmgen 1
I
15
I
I
20
Figure 19. Typical heating and cooling curves for thermal vacuum chamber without satellite
Naturally, the vacuum system must be operational before the cryogenic system can function and while the vacuum is being achieved, the operator is free to carry out the setting up procedure for the cryogenic system. The plant is installed in a building with three levels: a basement for machinery (see Figure 16), a test floor through which the upper section of each chamber protrudes, and a gallery containing the remote control units. This form of plant layout facilitates a clear and unhindered test floor allowing complete freedom when
"/8
1
mounting and wiring up a satellite inside the chambers (see Figure 17). Figure 18 shows a typical pump-down curve for the chambers and shows the effect on the vacuum of the shroud temperature. A typical heating and cooling curve for the shroud is shown in Figure 19 and this covers the cycle of events from ambient temperature to the extremities of each operational mode. The thermal distribution over the shroud during the cool-down to 150°K is shown in Figure 20, while Figure 21 shows the temperature difference over the shroud through the range 150 to 400°K. Since the two chambers were handed over in March 1967, tests have been conducted continuously and these have included thermal cycling tests for the ESRO 1, ESRO II, and HEOS satellites.
CRYOGENICS
• APRIL
1968
THE NUMBER of objects launched into space is now measured in hundreds and very few of these have failed in their purpose; malfunctioning in space often leads to total loss and must be avoided if at all possible. One of the prime reasons for the high success rate can be attributed to carefully conducted check-out procedures, including tests carried out by the simulation of conditions in space as far as this is practicable. Within the solar system, the principal characteristics of space are high vacuum, an infinite very cold heat sink, and intense solar radiation alternating with total darkness. Other properties, such as radiation from the earth's surface, loss of gravity, etc., may also have to be taken into account. Two principal types of facility are available to conduct a satellite through an adequate series of tests: those with solar simulation and those without. The space simulator with solar simulation can be very complex since, in addition to providing the environmental characteristics of space, it is necessary to provide attitude and spin conditions for the space-craft under test. The facility without solar simulation is known as a thermal vacuum chamber and is used for testing satellites under the alternating conditions of the effects of solar radiation and total darkness. Environmental chambers used for simulating the characteristics of space depend on cryogenics in varying degrees for their successful operation. In many instances it is necessary to provide only an isothermal cryogenic system while in others it is necessary to vary the temperature over a wide range. In addition, cryogenics is sometimes used for producing the required high vacuum. Communication satellites, typical of those orbiting the earth, are used for relaying high frequency radio and television signals. A station of this type requires power for its operation and this is derived from batteries which are kept charged by light sensitive solar cells. Thus a communication satellite is a complex unit consisting of aerials, receiver, transmitter, and power source, all of which must be capable of withstanding the wide changes in temperature experienced during each orbit. In order to provide the means for testing satellites and space probes under simulated environmental conditions, suitable centres have been set up throughout the world
and these have industrial, national, or international sponsors. Characteristics of space and thermal balance The three principal characteristics of space affecting the thermal balance of a satellite are solar radiation, pressure, and the heat sink of space. Solar radiation. Solar radiation is very intense and represents a heat flux of 1 400 W/m 2 of projected area for a satellite in an earth orbit. The absorption of all this energy would lead to an intensely high temperature within the satellite but for the fact that the emissivity of its outer surface approaches zero; thus only a fraction of the heat is absorbed. Pressure. The pressure exerted on a satellite depends on its altitude, as is shown in graphical form on Figure 1. The effect of the environmental pressure is important because when the pressure is below 1 x 10-6 torr the molecular mean free path becomes very large and intermolecular collisions are rare. This results in the mechanism of heat transfer by conduction and convection in 500
40C
30O ~b
E 200 <( 100
0
I 1000 100 10 1 1()1 103 105 Pressure.mm.Hg
10?
109
Figure 1. Environmental pressure at various altitudes CRYOGENICS • A P R I L 1968
the gas being virtually eliminated. Consequently heat transfer through the environment is reduced to the radiaation effect only.
The heat sink of space. Space itself has no temperature but represents a black body with an emissivity of unity. Moreover, it has an infinite area, which means that the heat rejected by an object to space depends only on its own temperature, surface area, and emissivity• Therma'l balance• The heat transfer between the heat source, satellite, and heat sink is entirely radiative, as discussed previously• The solar intensity at a satellite in an earth orbit is known to be 1 400 W/m 2 of projected area and from this value suitable solar simulators can be provided; these are generally specified to provide 2 000 W/m 2. In the case of the heat sink of space it is usual to restrict the simulated temperature to 100°K. The reason for this can be understood readily by an analysis of the StefanBoltzman law. aA,(T~- T~) •
where
Q = total heat flow O ' = Stefan's constant Al= area of satellite a l = emissivity of satellite T I = temperature of satellite .42 =- area of space a 2 = emissivity of space T2= temperature of space
Since
~r = 0.533 x 10.8 W/°K.ft 2
. .
(1)
%
...
Satellite
(2)
Az,a2-1)]
Q = 0"533a1A1(l~0) 4
In the case of a satellite receiving solar radiation, it is only necessary t o provide a heat sink at 100°K because the temperature of the satellite will be relatively high; consequently the error in heat balance determination will be small• For this reason it is normal practice for an environmental chamber with solar simulation to have an isothermal shroud operating at the normal temperature of 100°K. Nevertheless, it must be recognized that the satellite will indergo wide changes in temperature during its orbit, which necessitates thermal cycling tests to complete the series of events experienced. For this reason the solar simulation chamber is supplemented by thermal vacuum chambers which are designed with the ability of testing the satellite through a number of cycles between 400 and 200°K.
"'-7",
~,Y6-6! - \l-N! j
T2 approaches
The maintenance of a heat sink at 100°K or less Thermal cycling systems operating over a wide range of temperature, namely 400 to 100°K The cooling of optical windows and the provision of cold traps and cryopumps Complex control functions and accurate temperature distribution•
shroud
Q = 0.533A, [ 1 + A 1 [ I
and since A2 is infinite and
Thus if T 2 = 100°K, we have, for T~ = 400°K, X = 0-39~o, and for T1 = 200°K, I; = 6.25~. Therefore the percentage error by simulating the heat sink of space at 100°K is only significant at low temperatures. This by itself will not materially affect simulated testing, and the advantages of providing a heat sink approaching 0°K would not justify the resultant escalation in cost. The cryogenic content of space simulation systems is both interesting and widespread as it can involve some or all or the following:
i
I I
0°K
!
t z
I I I
•
.
.
(3)
Consequently the heat transferred is only a function of area, temperature, and emissivity of the satellite• Rewriting equation (3) Q =J|7-h-AJ \lUU/
...
(4)
where f is a constant• The percentage error caused by T2 ~ 0 i s
X = 100
Lamps
/'T, ~4 /'T2 ~' (T, '~41 ~,i-6-6! + \T6-O! - ~,ib-6! / i~!
x
CRYOGENICS
=
100(T2~ 4
~!
• APRIL
1968
•..
(5)
Figure 2• Space chamber with solar simulator consisting of carbon arc or x e n o n lamps, reflectors, lenses, and windows 8g
Space chamber systems -
Solar simulators. In this type of system (see Figure 2), tests are undertaken to determine the heat balance and thermal distribution characteristics of a satellite under solar exposure. For reasons stated previously it is necessary for the shroud temperature to be at or below 100°K under all conditions of test. The heat absorbed by the shroud and rejected to the refrigerant is derived from two principal sources, namely, the solar simulator and the chamber wall.
-
i
The two principal types of space simulation chamber are those with solar simulation and those without, or solar simulators and thermal vacuum chambers.
I1
:-©': I. . . . . .
Liquid N2 from storage
4
Gaseous N2 to recovery
Figure 4. Basic m e t h o d s of refrigerating c h a m b e r s
T A B L E 1. H E A T RECEIVED B Y S H R O U D
Refrigerants and refrigeration
Symbol
Flux, W/m s
%
Os OR OK
240 70 30
70 21 g
QT
340
100
The values given in Table 1 relate to a 6 metre chamber but are representative of any similar system. The symbols are as follows: Qs is the mean flux due to solar simulation, Qn is the heat radiated from chamber wall, QK is the heat transfer by solid conduction, and Q~, is the total heat transferred per unit area. A very good indication of the cryogenic requirements can be derived from these figures, but only on an overall basis because in practice the unit load is not constant; part of the shroud will receive heat by reflection from the satellite, part will be in the direct path of the beam of energy, and part will be covered by the shadow of the satellite. Consequently, consideration must be given to the distribution of the refrigerant through the shroud. As the satellite shown in Figure 3 orbits the earth it passes through three main zones: solar radiation zone, twilight zone, and zone of total darkness. During the course of each orbit the temperature of the satellite will vary from a maximum to a minimum.
Orbiting D
Solar radiation
----"
o o 'O'tJ
. )
\ x
\
Figure 3. Satellite in earth orbit
70
/ - - N~o
Refrigerants. The principal factor governing the choice of refrigerant is the thermal duty expected of it, in particular, the lowest temperature required. As 100°K has been established as an acceptable lower limit, and higher temperatures are often satisfactory, a reasonably wide range of fluids are available: e.g. nitrogen, acetone, pentane, etc. The most common refrigerant in use today is nitrogen, which is available in liquid form from the many large tonnage air separation plants scattered throughout the world. Considerable benefit can be derived from its use because it avoids the need for large installed refrigeration units which may be completely uneconomical because they are not fully utilized. Such units would need to be part of, or connected to, a substantially large air separation plant if complete independence is required. Even though a particular test facility may operate at a significantly higher temperature than the normal boiling point of nitrogen, there is no reason why nitrogen cannot be used as the primary refrigerant. The phase in which the secondary refrigerant operates can be liquid, vapour, or liquid and vapour; the choice of refrigerant will be the determining factor. If a facility is to operate at the lower temperature of 100°K, liquid nitrogen is a feasible choice. On the other hand, when the lowest temperature is 200°K, liquid nitrogen cannot be used as the secondary refrigerant, although nitrogen can be used, but in the vapour phase. In this regime other secondary refrigerants such as acetone may be used with the added advantage that the high critical temperature makes it possible to operate the facility over its entire temperature range in the liquid phase. Refrigeration. A fully independent facility would probably consist of an air separation plant with liquid nitrogen production to provide the primary refrigerant, a secondary refrigeration cycle to cool the shroud, and a third refrigeration cycle to recover the primary refrigerant. Such a system would be used only where several large space chambers were in use simultaneously. There are various methods of refrigerating chambers and the three principal ones are shown in Figure 4. Chambers 1 and 2 are maintained at a single constant temperature, whereas chamber 3 operates over a variable temperature range. CRYOGENICS
• APRIL
1968
Mechanism of heat transfer
GQSeOUS N2
When using a low boiling fluid to cool a shroud, two methods of heat transfer are possible: latent heat of evaporation and sensible heat transfer. In the former case the liquid is allowed to evaporate isothermally in absorbing heat and in the latter there is no phase change but there is a change in temperature.
Evaporatit:e. In this case the refrigerant boils isothermally and the temperature is varied by altering the vapour pressure. When the system to be cooled is large, isothermal conditions are not met exactly because of the hydrostatic head between the top and bottom of the shroud. However, this does not constitute a penalty providing the temperature difference is small or in the case of a solar simulator, providing the highest temperature does not exceed 100°K. A major difficulty of the evaporative system is in ensuring good distribution and the prevention of gas pockets causing local overheating. Sensible. A sensible heat system can operate in either the liquid or gaseous phase. If a liquid system is used it is first necessary to ensure that the liquid is sub-cooled. This is realized by raising the pressure without raising the temperature to effectively increase the boiling point. The most suitable method of achieving this state is by means of a cryogenic pump. Under these circumstances, isothermal conditions cannot be met although by circulating sufficiently large quantities it is possible to limit the temperature rise to a few degrees. Such a system has the advantage that good distribution can be effected. Cryogenic cycles
Single temperature--two phase. Liquid nitrogen is drawn from a storage vessel and fed into a level control vessel where disengaging of vapour and liquid fractions takes place (see Figure 5). The liquid fraction passes into the shroud where evaporation takes place in absorbing heat from the shroud.
os o ,m,
....
I Level control vessel
Liquid N 2 Figure 5. Evaporator system C R Y O G E N I C S • A P R I L 1968
Shroud
q
Separator vessel,
hroud
Liquid N2
Cryogenic pump Figure 6. Liquid cooled system
The controller L maintains a constant liquid level and the temperature is fixed by the pressure controller P which maintains a constant vapour pressure, hence boiling point. In the case of a system using liquid nitrogen as the refrigerant a pressure of 2 atm (absolute) will correspond to a constant boiling point of 84"1{. The primary and secondary refrigeration function are both realized with the same media.
Single temperature--single phase. Liquid nitrogen is drawn from the storage tank and fed into a separator vessel where the vapour content, if any, is removed (see Figure 6). The liquid then enters a cryogenic pump where its pressure is raised to about 6 atm (absolute). Assuming the liquid is stored at 1.2 atm, its boiling point will be 79°K whereas at 6 atm its boiling point is 96°K. The heat entering the nitrogen in the pump will correspond to a temperature rise of about l°C, consequently the liquid nitrogen leaving the pump will be 16°C below its boiling point--very conveniently subcooled. As the liquid transverses the shroud its temperature will rise according to the quantity circulated. A comparison of the latent and sensible heats shows that for a I°C temperature rise it would be necessary to circulate about 100 times more than in the evaporative case. However, such a small temperature rise is not generally necessary, particularly if the refrigerant is always colder than 100°K. In the example discussed above it is satisfactory to allow a temperature rise in the neighbourhood of 10°C and have sufficient margin in hand to ensure that there is no risk of local boiling occurring. A further few degrees temperature rise could be tolerated in regions of high flux or in the case of short term transients caused by malfunctioning of the solar simulation unit. When the sub-cooled liquid leaves the shroud it is expanded into the separator vessel where the liquid and saturated vapour fraction are separated. The liquid is then recycled together with make-up from the storage tank and the vapour is rejected or recovered. Different degrees of sub-cooling can be achieved by varying the system pressure. Multi-temperature--single phase. The variable temperature cycle is suitable for systems operating over a wide range of temperature and where any temperature may be selected in the range specified. A typical example is in the case of a thermal vacuum chamber where the temperature range may be from 200 to 400°K. Such a system is not "/1
Gaseous N2 i
N2 evaporator > > >
Liquid N2
Shroud > >
L. . . . . . .
nitrogen affords an excellent primary refrigerant. Also, the secondary refrigerant may or may not be nitrogen depending on the thermal specification for the system under review. In all these examples the evaporated nitrogen has not been given attention as it does not in itself affect the functioning of the cooling of the shroud. However, in all cases it can be rejected or recovered depending on the overall economics of the system.
>
I
I.
L
I
Heat exchanger Electricct[ heater irc ator Figure 7, Variable temperature cycle--gas
.... -~~
=
c
!
>
,~Shroud > > >
~
exchan;r Electrical, hea e "--
Figure 8, Variable temperature cycle--liquid
entirely cryogenic as the achievement of temperatures above ambient is met with a suitable heating system. In the cycle shown in Figure 7 primary refrigeration is provided by the evaporation of liquid nitrogen which is heat exchanged with the circulating fluid. Gaseous nitrogen can be used as the secondary refrigerant and circulated in a closed cycle as shown. For temperatures below ambient, the gas is circulated through the cold evaporator and a suitable temperature control loop to ensure the maintenance of the temperature selected. Above ambient temperature, the nitrogen evaporator is rendered inactive and an electrical heater is suitably energized and de-energized as appropriate. As the cycle temperature can be both hot and cold it is necessary to provide thermal isolation for the gas circulator and this is achieved by means of the heat exchanger installed between the booster and the remainder of the system. In order to circulate a sufficiently large mass flow it is necessary to operate the system at an elevated pressure. Liquids can be used as the secondary refrigerant (see Figure 8), but their temperature range is more restrictive and it is often necessary to design to a relaxed specification. Such a design depends on some of the physical properties of the circulating fluid such as critical temperature and pressure, freezing point, and viscosity. Summarizing the above, it can be seen that in all cases 72
Rejection and recovery of primary refrigerant In every case it is a simple matter to vent the evaporated refrigerant to atmosphere and replace it with liquid bought in or made on site. When the facility is of such magnitude or so remote from an industrial centre that an air separation plant is associated with it, it is quite easy to route the saturated nitrogen gas back into an appropriate part of the plant so that recondensation and recycling can be effected. However, if the facility is near an industrial centre, site storage can be provided with easy means of supplying make-up. Nevertheless when large quantities of liquid nitrogen are required it may be more economical to recondense on site and limit the supply of make-up to overcome plant losses only. This can be achieved using cold gas engines based on the Stirling cycle. A recondensation unit is shown in Figure 9.
Shroud The shroud constitutes an important and integral part of the space chamber and cryogenic system. Its function is to provide a cold heat sink or a mechanism for simulating the effects of solar radiation and total darkness in a cyclic manner. To achieve this purpose it must operate isothermally, be optically tight, and envelop completely the satellite under test. In order to provide the required radiant properties it must be an effective black body with an emissivity of that part seen by the satellite approaching unity; in practice emissivity is usually about 0.95 and reflection is virtually eliminated. Since the shroud is contained inside a high vacuum environment it is important that the correct type and quality of material is Chosen, otherwise outgassing may constitute an undesirable difficulty. The outer surface of the shroud facing the chamber wall presents a very different problem as heat transfer between the shroud and wall is undesirable. Because this is reduced to the mechanism of radiation only it is best minimized by surface preparation leading to low values of emissivity. A good practical emissivity to aim for is 0.1, and this can be obtained by electro-polishing. It is of equal importance that the inner wall of the chamber
Make up L - ~ Liquid ~nitrogen /~.:------~'~ st orage ~ ( .~...~.~_.~..~_. . ) t an k I \x~::-." ~-'~,~/ " h . " ~ ' - " ~f" I r"'-l'~| .J==L. ~J-
~
~ ~ \
~ _,t ~ Transfer pump
Figure 9. Recondensation
Space chamber complex Recondensationl unit |
unit
CRYOGENICS • APRIL 1968
F7
t
~
•
J
I
.~f-
f-~
I TI
T2
i It
~
Tz I "T~
Figure 10.
should have a similar emissivity, since the radiant heat transfer is a function of this also. As stated previously, the temperature distribution over the shroud can be fixed by providing an adequate quantity of refrigerant. This statement is only true for the ideal case where the entire inner surface of the shroud is covered by a film of refrigerant. In the majority of cases expanded tube in sheet construction is used, and this introduces a thermal gradient between adjacent pairs of tubes. If the temperature variation is to be small it is essential that the fin temperature be determined and this may be done in the following manner (see Figure 10):
advantage that the cold surface can be placed inside the chamber resulting in a high effective pumping speed. The panel is cooled usually with liquid helium and pressures below 1 x 10 -15 torr are possible when the condensable gas is nitrogen. For pressures below 1 x 10 7 torr it is necessary to provide the initial high vacuum (1 x 10 4 torr) with another high vacuum pump. Such a fore pump may be cryogenic relying on low temperature adsorption on surface active material. The inclusion of cryopumps in the space chamber complex introduces the need for compact helium refrigeration equipment which may rely on 'bought in' liquid helium or a closed cycle helium refrigerator. If standard oil vapour diffusion pump equipment is used there is always the danger of contamination by back diffusion of the oil. This can be prevented by placing liquid nitrogen cooled, optically tight, baffles between the chamber and pump. The penalty in installing these baffles is that the effective pumping speed may be reduced by as much as 50~.
Costs nw 2 (T2 - T~) = 2 k t
where
T1 = refrigerant temperature at tube wall T2 = temperature at mid point between tubes or tip of fin w = fin width H = radiant heat flux t = fin thickness k = thermal conductivity of fin material.
Miscellaneous Cryogenics may be used for purposes other than provision of the simulated heat sink of space. In a space simulator system using a solar simulator it is necessary to fit windows in the chamber wall for transmission of the beam of light. Although these windows are transparent to light, they are opaque to heat, and consequently get hot during service. If the window is small it will conduct sufficiently to the wall of the chamber so that its equilibrium temperature is not very high. But, in the case of large windows, it is impossible to conduct sufficient heat to the chamber wall with the result that the window temperature will be excessively high; cooling of the window is therefore necessary. One method of achieving this is by constructing the window in two halves with a space between them through which a cooling fluid can be circulated. During the period when the solar unit is operating it would be normal to cool with low pressure nitrogen gas at around 0°C. When the solar unit is not operating, this temperature would be raised to around 20°C to prevent condensation occurring on the face of the window. Nitrogen gas required for this purpose can be conveniently provided from the shroud boil-off. The normal method for producing the high vacuum is by means of oil vapour diffusion pump systems, but for ultra-high vacua and prevention of contamination in the high vacuum environment it is possible to use cryogenic pumps. The cryogenic pump utilizes the mechanism of condensation on a low temperature surface and has the CRYOGENICS • A P R I L 1968
Consideration must be given to a large number of factors when determining the cost of a space chamber testing facility. In arriving at the budget costs given in Figure I 1 the systems for the primary refrigerant have not been
._ "~
Thermal, vacuum c h ~ [
','41xi05 "-79x104 ~Bx104 c) 7xi04 6xI04 5xll 0
--
I
2
Solar s i m u t a t o r / chamber ~
3 4 5 6 ? 8 Chamber diameter~ m
~lxlOS~ _19xi05 .
9
10
-jSx1OS~ _j7x10s ~6xI0 s J5x105
Figure II. Approximate cost of complete space chamber facility
accounted for as these depend on geographical location and existing facilities. The main purpose of including these figures is to give the reader an indication or order of magnitude of the costs involved in providing satellite testing equipment. Two curves are given, one for solar simulation chambers and one for thermal vacuum chambers; both representing turn-key prices for essentially automatic systems. An approximate breakdown of these is as follows. Cost, °/o Solar simulation chamber
Chamber Motion gear Cryogenic system Instrumentation Solar simulator Vacuum system Erection
22 2 12 5 45 7 7 100
Thermal vacuum chamber
12 40 20 15 13 I00
"/3
The economics of recovering nitrogen depends on the annual usage, capital charges, and cost of utilities. The following approximate example gives an indication of whether recovery or rejection should be adopted and relates to a single thermal vacuum chamber unit.
Charges and utilities Cost of recondensation facility Liquid nitrogen consumption Operating period Power consumed by cold gas engine Electricity Liquid nitrogen
£15 000 200 litres/h 2 000 h/year 80 kWh 1d./kW 7d./litre
Rejection Cost of liquid nitrogen -
200x7 2~ x 2000
=
£11 680p.a.
Recovery Cost of recondensate equipment Cost of electricity -
£15 000
80x 1 2 ~ x 2 000 =
666 £15 666 p.a.
The foregoing gives a brief indication of the part played by cryogenics in space technology. The facts presented relate to the industry in general and in support of this the following relates to an installation provided to the European Space Technology Centre, Noordwijk, Holland. Twin thermal vacuum chamber facility The system was built to an exacting specification which introduced the need to develop a complex control and monitoring system so that the required environment could be achieved from a remote station. The principal requirements are listed below. 1. Working volume of chamber: 1.5 m diameter × 2 m long 2. Chamber diameter: 2 m. 3. Chamber operating pressure--with shroud hot: 1 x 10-s torr; with shroud cold: 1 x 10-btorr 4. Shroud temperature--mode 1 : below 100°K; mode 2: 150-300°K; mode 3: 300-400°K 5. Temperature distribution over shroud: + 2.5°C max. from mean.
Vacuum chambers. Each of the identical twin chambers is constructed with the longitudinal axis in the vertical plane and comprises four main sections. The major body section has external brackets to accommodate support legs and is surmounted by a narrow ring section with feed-through ports and provision internally for mounting the satellite or component to be tested. To complete the chamber two covers are provided for the top and bottom of the vessel. Each section is connected with flange joints utilizing double O-ring seals. The vessels are made from austenitic grade stainless steel, type EN58J, and designed in accordance with "/4
BS1500 class I pressure vessel code with all welds fully radiographed. To minimize heat transfer between the chamber and shroud the inner surface of the chamber wall is mechanically polished to give a near mirror finish (about 10 micro-inch surface finish), and emissivity not greater than 0.1. On completion of the fabrication the leak rate was below 1 x 10-4torr litres/s measured with a helium mass spectrometer. Provision of the narrow ring section offers an easy means of satellite loading; the ring section is placed on a separate supporting frame with unrestricted access for mounting the satellite and wiring it up. During this period the top cover is replaced on the main body section so that the internal cleanliness of the chamber is preserved.
Vacuum system. Two identical pumping units are attached to each chamber and these consist of 610 mm oil vapour diffusion pumps, high vacuum baffle valves, refrigerated chevron baffles, single stage, air ballasted, rotary mechanical roughing pump, and single stage, air ballasted, rotary mechanical backing pumps. The overall pumping speed is greater than 4 000 litres/s at 1 × 10-4 torr and the ultimate pressure when pumping a clean uncooled chamber is less than 5 x 10-6 torr. With the shroud cold, a chamber pressure below 1 x 10-7 torr can be achieved. A simplified flow diagram of the vacuum complex is shown in Figure 12. The sequence of events from atmospheric pressure to the high vacuum state is followed entirely automatically once initiation has been made. A pirani gauge monitors the roughing and backing pressure and controls the opening of the high vacuum valve at 0.1 torr. The high vacuum is measured with a Penning gauge over the range 1 x 10-3 torr to less than 1 x 10-6 torr while a record is made using an ionization gauge over the range 5 x 10-3 torr and 5 x 10-9 torr. Cryogenic or heating and cooling system Because of the wide range of temperatures required, a combined variable temperature gas cycle and single temperature liquid cycle is used. The primary refrigerant for all low temperature conditions is liquid nitrogen and
Pirani gauge Penning gauge
Ionization gauge I / J _ 6 ~ " ~ I I
Diffusion pump
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Penning
gauge7
High vacuum/ valve /
Pirani J_~ gauge ~1 Holding I pump j Roughing pump"
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/
Figure 12. Schematic flow diagram of vacuum system CRYOGENICS • APRIL 1968
Liquid
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Nitrogen gas is circulated by the booster 6 through the exchanger 5 and part through and part around t.he evaporator 4 in order to cool to the appropriate temperature. It then traverses the shroud, where heat is absorbed and returns by way of the exchanger 5 to the booster 6 for recycling. Mode 3 (300-400°K). During this mode of operation heat is not absorbed by the system but is rejected. Consequently refrigeration is not used. Heat is provided electrically by the heater 7. The gas is circulated by the booster 6 from which it passes through the exchanger 5, heater 7, shroud 3, and exchanger 5 during each cycle. In this case the shroud cools down the gas.
(5 ® 3ct,3b,3 ¢: s h r o u d
Figure 13, S c h e m a t i c f l o w d i a g r a m of c r y o g e n i c s y s t e m
the secondary refrigerant is nitrogen for all duties; liquid nitrogen is used in mode 1 while gaseous nitrogen is used in modes 2 and 3. The major difficulty confronting the designer is in ensuring that the temperature difference between A and B (Figure 13) is never greater than 5°C and that the required mean temperature is midway between A and B. The former is a function of accurate determination of the heat absorbed by the system while the latter is governed by the control engineering and instrumentation accuracy. These two forces are of equal importance as the purpose of the cryogenic system is to reach, and maintain accurately, selected environmental conditions. Figure 13 shows the cryogenic system adopted which operates in the following manner. Mode 1 (below 100°K). Liquid nitrogen passes from a storage installation into the separator vessel 1 where any nitrogen evaporated in transit is removed. The liquid then fills the system completely, i.e. pump 2, shroud 3, and evaporator 4, and is pressurized to 4 atm. With the nitrogen feed isolated, the pump circulates the liquid through the cycle where it is sub-cooled in the evaporator 4 and heated in the shroud 3. In this mode of operation it is of no consequence to maintain accurate temperature distribution but rather to ensure that the maximum temperature nowhere exceeds 100°K. Mode 2 (150-300°K). The liquid nitrogen pump is isolated as gaseous nitrogen is used for the secondary refrigerant. Since a small temperature rise is allowed in the shroud it is necessary to circulate a large mass flow, and since the specific heat of nitrogen gas is low, it is necessary to circulate the refrigerant at a generally elevated press u r e - i n this particular case 7 atm. In theory it is quite feasible to circulate at low pressure, but this would be unsatisfactory because very large flow passages would be required. This would lead to increased capital charges because the shroud would be larger and consequently the chamber would be larger; moreoever, the cryogenic cycle would obviously be much more bulky and require increased building space. CRYOGENICS
• APRIL
1968
Control
The entire control system is very complex and includes electronic, electro-pneumatic, and pneumatic instruments of the indicating, recording, and control types with mimic diagrams showing the state of the facility at all times. Extensive interlocks and fail safe mechanisms prevent maloperation, hazards, or damage to the expensive equipment under test. Conventional control techniques are used in the liquid cycle for the mode 1 requirement whereas less conventional instrumentation practice is utilized for the thermal stability required during modes 2 and 3. During operation over the temperature range 150 to 400°K a number of interesting conditions can arise. If the temperature is raised, the specific volume of the circulating gas will increase, and consequently the pressure in the system will also increase. Conversely, when the temperature is lowered the pressure will fall. Furthermore, at low temperatures the amount of gas passing through the refrigerator (nitrogen evaporator) may be small and there will be tendency for it to liquefy; this can occur because the nitrogen boils at about 79°K and the condensing temperature of the circulating stream is 100°K. Consequently, there will be a rapid reduction in the system pressure causing instability. Moreover, the frictional resistance in the cryogenic system is directly related to the temperature and for a constant circulating pressure there is a larger differential pressure across the booster at higher temperatures. Thus, to ensure a constant booster pressure, it is necessary to vary the flow conditions, otherwise over-pressurization and overheating of the booster would take place. The control system provided ensures that stability is automatically preserved at any selected temperature in the operating range. A split range pressure control loop maintains the pressure constant. This is a pneumatic control device which operates with a conventional instrument air supply over the range 3 to 15 lb/in 2. The charging and venting valves have opposite actions and one is open at 3 lb/in z and closed at 8 lb/in 2, whereas the other is closed at 10 lb/in 2 and open at 15 lb/in 2. Thus, automatic charging and venting is achieved and, because of the 8-10 lb/in 2 inactive band, hunting of the valves is prevented. The booster delivery pressure is maintained constant by a differential control unit fitted across the compressor; this actuates a control valve fitted in the booster bypass. During low temperature operation the temperature of 76
Cooling
FsWOter &
Local start
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the gas entering the shroud is fixed by the quantities of gas passing through the evaporator and its bypass respectively. The control valves in the evaporator feed and bypass line have opposite actions and are positioned so as to ensure a mixed gas temperature at the particular value selected. To prevent condensation of the circulating nitrogen, a temperature controller in cascade with the level control of the evaporating liquid resets the level of nitrogen on the boiling side of the heat exchanger. The temperature setting on the cascade unit is about 5°C above the condensing temperature of the high pressure
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Figure 14. Circuit diagram of booster motor starter ---;Command flashing suppl Fault flashing supply
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Figure 15. ' G o - n o go' circuit diagram for booster
Distribution of gas through each section of the shroud is fixed by temperature control loops situated in the lines leaving the shroud; these are set 5°C above the temperature of the feed gas for the low temperature duties. For high temperature duties heat is introduced automatically by energizing and de-energizing the electrical heater with a mercury switch system built into the standard electro-pneumatic temperature controller. In this instance the temperature of the gas leaving the shroud is set 5°C below the inlet. The entire plant can be operated from a remote control centre or from local control stations. Interlocks prevent maloperation: the gas booster cannot function unless the liquid nitrogen pump is isolated and vice-versa; refrigerant cannot enter the shroud unless the pressure within the chamber is at or below 1 x 10-4 torr; the pressure inside the chamber cannot be raised unless the shroud is at or above ambient temperature; refrigeration and heat cannot be introduced concurrently. A foolproof
Figure 16. Machinery floor showing switchgear and cold box 76
CRYOGENICS
• A P R I L 1968
Figure 17. T e s t floor and gallery showing twin thermal vacuum chambers and part of control panel
"go-no go' system incorporates these functions together with many other features including the state of the system at all times. All important functions and parameters are displayed on the mimic diagram with amber and green signal lights which either flash or are steady. The sequence is as follows: Amber steady - - initiation has not been made Amber flashing - - a fault has occurred Green flashing - - initiation has taken place and the system is moving towards the stationary state Green s t e a d y - the system is operating satisfactorily in the stationary state. The electronic system for controlling and monitoring these functions is very complex but follows the principles described below for an individual module. The module described concerns one of the gas boosters used for circulating nitrogen during modes 2 and 3. As can be seen from Figures 14 and 15 it is not possible to energize the booster motor unless the following conditions are met: 1. 2. 3. 4.
The correct mode has been selected Cooling water is circulating through the booster The liquid nitrogen pump is inoperative The motorized valves isolating the liquid and gas cycles are in the correct position 5. The pressure in the chamber is below 1 x 10-4 torr 6. The overload contact is closed 7. Power to the 'go-no go' system is on.
CRYOGENICS
• A P R I L 1968
After all these conditions have been satisfied the 'go-no go' system will indicate amber steady; if the conditions are not met an amber flashing signal will be indicated. When initiation has been given, the amber signal will be replaced by a green steady signal indicating that the booster is operating correctly. If a fault develops after start-up the green light will be extinguished and the amber light will flash; at the same time the booster will shut down automatically and cannot be restarted until the fault has been corrected. The command stage utilizing the green flashing signal is not built into the booster circuit since the time to run up to speed is very short. On completion of all command signals when the stationary condition has been achieved, the lights on the mimic diagram will show steady green with the exception of those showing steady amber by intent. For example, when the booster is in operation, the liquid nitrogen pump will be inactive and indicated as such by a steady amber signal.
Operation The plant operation is simple and requires very little attention from the operator. In principle it is necessary only to select the required mode by means of a three position switch on the panel, open or close a few hand valves as appropriate, set the required temperature on the control instruments, and start-up the plant. Apart from adjusting the manual valves (which is necessary only when changing from one mode to another) all the operations can be carried out at the remote control centre. 7"/
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Figure 19. Typical heating and cooling curves for thermal vacuum chamber without satellite
Naturally, the vacuum system must be operational before the cryogenic system can function and while the vacuum is being achieved, the operator is free to carry out the setting up procedure for the cryogenic system. The plant is installed in a building with three levels: a basement for machinery (see Figure 16), a test floor through which the upper section of each chamber protrudes, and a gallery containing the remote control units. This form of plant layout facilitates a clear and unhindered test floor allowing complete freedom when
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1
mounting and wiring up a satellite inside the chambers (see Figure 17). Figure 18 shows a typical pump-down curve for the chambers and shows the effect on the vacuum of the shroud temperature. A typical heating and cooling curve for the shroud is shown in Figure 19 and this covers the cycle of events from ambient temperature to the extremities of each operational mode. The thermal distribution over the shroud during the cool-down to 150°K is shown in Figure 20, while Figure 21 shows the temperature difference over the shroud through the range 150 to 400°K. Since the two chambers were handed over in March 1967, tests have been conducted continuously and these have included thermal cycling tests for the ESRO 1, ESRO II, and HEOS satellites.
CRYOGENICS
• APRIL
1968