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Application of the thermosiphon for precooling apparatus
APPLICATION OF THE THERMOSIPHON PRECOOLING APPARATUS L.. BEWiLOGUA, Lehrstuhl,f‘iir
Plysik
tit-fer
R. KNONER,
Tetnperaturetl Received
cooling of large pieces of apparatus from room temperature down to 20 or 4” K by means of liquid hydrogen or liquid helium would require very large quantrties of these liquid refrigerants. It is therefore usual to precool such apparatus with liquid nitrogen or with nitrogen and hydrogen, so that the quantities of hydrogen or helium necessary become essentially reduced. Precooling can be carried out in different ways. e.g. by an exchange gas, by windings passed by precooled gas, or by filling the measuring vessel with the precooling agent and evaporating the residual liquid before the final refrigerant is put in. Expedient and sure precooling is also obtainable by applying the thermosiphon principle.‘-3 In Figure I the method is shown schematically. Vessels for liquid nitrogen. for liquid hydrogen or neon, and for liquid helium are arranged in vacuum. The measuring device is attached to the helium vessel. The whole system is cooled with minimum expense. First all three vessels are cooled to liquid nitrogen temperature; then the hydrogen (or neon) vessel and the helium vessel with the measuring device are cooled to liquid hydrogen (or neon) temperature; finally the helium vessel with the measuring device is cooled to helium temperature. Cooling in this sequence can be performed almost automatically when heat exchangers working according to the thermosiphon principle are inserted between the vessels. When the nitrogen vessel is filled with liquid nitrogen. the other vessels will also be cooled down to the temperature of liquid nitrogen via the heat exchangers I and 2 which are filled with gaseous nitrogen under slight excess pressure. After the temperature in the system has reached equilibrium. the next vessel is filled with liquid hydrogen or liquid neon. At these temperatures. nitrogen condenses in heat exchangers 1 and 2 so that only a negligible heat transfer via the tubes will remain. Now, the heat exchanger 3 filled with hydrogen or neon operates to cool the helium vessel. As the specific heat is generally already highly reduced, only a small amount of helium will be required to cool the helium vessel. Figure 2 is a schematic diagram of the method of operation of a heat exchanger according to the THE
34
and G. KAPPLER
der Technischet~ 15 October
FOR
Unirlersittit
Dresden,
Germatl,v
1965
thermosiphon principle. Condenser 1 and evaporator 2 are made of a high heat conducting material. Both are connected by a thin-walled tube 3 made of a material of low heat conductivity (stainless steel or german silver). The system can be filled with gas through line 4.
N2
I Figure
1. Precooling
method
By suitable blect‘ ,n of filling pressure (manometer 5). and possibly ‘7. provision of an additional volume 6. sufficient liquid is supplied to the heat exchanger during operation. When the condenser is cooled sufficiently. the gas condenses and flows into the evaporator and the heat exchanger operates. As soon as temperature equilibrium is obtained between condenser and evaporator and the connected part of the apparatus, heat contact must be broken before further cooling takes place. This is automatically effected by the thermosiphon. A test model is arranged according to Figure 2. It operates at nitrogen temperatures. Condenser and evaporator have a surface of 28 cm2 each. They are connected by a stainless steel tube having a length of CRYOGENICS.
F E I3 R U A R 1’
I ‘I b 6
10 cm, a diameter of 6 mm, and a wall thickness of O-2 mm. The thermosiphon could be operated with or without the additional volume 6. Because of this fact and because of variation of the filling pressure, it was possible to vary the quantity of liquid in the system. A copper disc of 400 g was attached to the evaporator. With the aid of.this apparatus the cooling rate of the evaporator as a .function of the quantity
increasing the volume of\ liquid. The amount of refrigerant required to cool the liquid in the thermosiphon can be pract@lly neglected, and heat input through the wall of the tube has no influence. In Dresden, the thermosiphon hq been used for preco’oling two pieces of apparatus. The liquid container of a cjopump (1200 g copper) had to be precooled from room temperature to 80’ K; it is designed for using liquid neon or liquid hydrogen. The surfaces for heat exchange had areas of 20 cm2 each. In about 45 ruin, a temperature .of 85” IS was obtained. In an apparatus,aesigned for measurements on ferroelkctrics in the helium range, 700 g copper was cooled from 80 to 20” K in 90 min with the thermosiphon; the area for heat elichange was only 5 cm*. The suggested method is distinguished by the fact that the transfer system is a cli%ed circuit which after having been filled with pure gas operates under adequate pressure without any external control and without any moving parts, so troubles encountered with other precooling methods are avoided. Cooling duration and consumption of refrigerant are nearly equal in the thermosiphon and winding methods. P
Figure
/
0 Figure
Quantity of 3. Heat transfer
2 liqui:
in the CRYOGE.NICS
. FEBRUARY
-
as a function thermosiphon 1966
./
I
2. Thermosiphon
of liquid contained in the system was measured. At equilibrium the temperature difference between condenser and evaporator could be measured as a function of heat supplied to the evaporator. The transferred thermal efficiency can be computed from the cooling rate; it is shown in Figure 3 as a function of the quantity of liquid contained in the system for the range of interest here. At first efficiency increases rapidly with the quantity of liquid. At a liquid quantity of O-8 cm3 a constant heat transfer is already obtained, determined by the heat transfer in the dondenser and evaporator. Cooling can therefore be accelerated only by enlarging the areas in condenser and evaporator and not by IO
L
of liquid
b-ri3~
quantity
I IO
0 Figure
4. Rise
I 30
20 Heat input
of temperature
-
as function
(W)
of heat
input
After the final temperature has been reached with such an equipment, a considerable quantity of heat, e.g. for measuring, can be transferred, and with only a small temperature difference. The dependence on heat supply of a system having about the dimensions specified above is shown in Figure 4 for various liquid quantities in the thermosiphon. As soon as sufficient quantity of liquid is available in tEe thermosiphon, high quantities of heat can he transferred. When the quantity of liquid is only 0.2 cm3, the evaporator warms up at a heat supply of some few watts. When the quantity of liquid in the tbermosiphon exceeds O-6 cmJ, more than 30 W can be transferred. This is equal to a formal heat conductivity of the thermosiphon of about 70 Wcm-1 deg-1. REFERENCES 1. BEWILOGUA L., and KN~NNER, R. Cryogenics 2,. 46 (1961) 2. GIFFORD, W. E. Ado. cryog. Engng 7, 551 (t962) 3. KN~NER R. Proc. 3rd Regional Conference, Prctpus, 1963,
p.
199
35
Plysik
tit-fer
R. KNONER,
Tetnperaturetl Received
cooling of large pieces of apparatus from room temperature down to 20 or 4” K by means of liquid hydrogen or liquid helium would require very large quantrties of these liquid refrigerants. It is therefore usual to precool such apparatus with liquid nitrogen or with nitrogen and hydrogen, so that the quantities of hydrogen or helium necessary become essentially reduced. Precooling can be carried out in different ways. e.g. by an exchange gas, by windings passed by precooled gas, or by filling the measuring vessel with the precooling agent and evaporating the residual liquid before the final refrigerant is put in. Expedient and sure precooling is also obtainable by applying the thermosiphon principle.‘-3 In Figure I the method is shown schematically. Vessels for liquid nitrogen. for liquid hydrogen or neon, and for liquid helium are arranged in vacuum. The measuring device is attached to the helium vessel. The whole system is cooled with minimum expense. First all three vessels are cooled to liquid nitrogen temperature; then the hydrogen (or neon) vessel and the helium vessel with the measuring device are cooled to liquid hydrogen (or neon) temperature; finally the helium vessel with the measuring device is cooled to helium temperature. Cooling in this sequence can be performed almost automatically when heat exchangers working according to the thermosiphon principle are inserted between the vessels. When the nitrogen vessel is filled with liquid nitrogen. the other vessels will also be cooled down to the temperature of liquid nitrogen via the heat exchangers I and 2 which are filled with gaseous nitrogen under slight excess pressure. After the temperature in the system has reached equilibrium. the next vessel is filled with liquid hydrogen or liquid neon. At these temperatures. nitrogen condenses in heat exchangers 1 and 2 so that only a negligible heat transfer via the tubes will remain. Now, the heat exchanger 3 filled with hydrogen or neon operates to cool the helium vessel. As the specific heat is generally already highly reduced, only a small amount of helium will be required to cool the helium vessel. Figure 2 is a schematic diagram of the method of operation of a heat exchanger according to the THE
34
and G. KAPPLER
der Technischet~ 15 October
FOR
Unirlersittit
Dresden,
Germatl,v
1965
thermosiphon principle. Condenser 1 and evaporator 2 are made of a high heat conducting material. Both are connected by a thin-walled tube 3 made of a material of low heat conductivity (stainless steel or german silver). The system can be filled with gas through line 4.
N2
I Figure
1. Precooling
method
By suitable blect‘ ,n of filling pressure (manometer 5). and possibly ‘7. provision of an additional volume 6. sufficient liquid is supplied to the heat exchanger during operation. When the condenser is cooled sufficiently. the gas condenses and flows into the evaporator and the heat exchanger operates. As soon as temperature equilibrium is obtained between condenser and evaporator and the connected part of the apparatus, heat contact must be broken before further cooling takes place. This is automatically effected by the thermosiphon. A test model is arranged according to Figure 2. It operates at nitrogen temperatures. Condenser and evaporator have a surface of 28 cm2 each. They are connected by a stainless steel tube having a length of CRYOGENICS.
F E I3 R U A R 1’
I ‘I b 6
10 cm, a diameter of 6 mm, and a wall thickness of O-2 mm. The thermosiphon could be operated with or without the additional volume 6. Because of this fact and because of variation of the filling pressure, it was possible to vary the quantity of liquid in the system. A copper disc of 400 g was attached to the evaporator. With the aid of.this apparatus the cooling rate of the evaporator as a .function of the quantity
increasing the volume of\ liquid. The amount of refrigerant required to cool the liquid in the thermosiphon can be pract@lly neglected, and heat input through the wall of the tube has no influence. In Dresden, the thermosiphon hq been used for preco’oling two pieces of apparatus. The liquid container of a cjopump (1200 g copper) had to be precooled from room temperature to 80’ K; it is designed for using liquid neon or liquid hydrogen. The surfaces for heat exchange had areas of 20 cm2 each. In about 45 ruin, a temperature .of 85” IS was obtained. In an apparatus,aesigned for measurements on ferroelkctrics in the helium range, 700 g copper was cooled from 80 to 20” K in 90 min with the thermosiphon; the area for heat elichange was only 5 cm*. The suggested method is distinguished by the fact that the transfer system is a cli%ed circuit which after having been filled with pure gas operates under adequate pressure without any external control and without any moving parts, so troubles encountered with other precooling methods are avoided. Cooling duration and consumption of refrigerant are nearly equal in the thermosiphon and winding methods. P
Figure
/
0 Figure
Quantity of 3. Heat transfer
2 liqui:
in the CRYOGE.NICS
. FEBRUARY
-
as a function thermosiphon 1966
./
I
2. Thermosiphon
of liquid contained in the system was measured. At equilibrium the temperature difference between condenser and evaporator could be measured as a function of heat supplied to the evaporator. The transferred thermal efficiency can be computed from the cooling rate; it is shown in Figure 3 as a function of the quantity of liquid contained in the system for the range of interest here. At first efficiency increases rapidly with the quantity of liquid. At a liquid quantity of O-8 cm3 a constant heat transfer is already obtained, determined by the heat transfer in the dondenser and evaporator. Cooling can therefore be accelerated only by enlarging the areas in condenser and evaporator and not by IO
L
of liquid
b-ri3~
quantity
I IO
0 Figure
4. Rise
I 30
20 Heat input
of temperature
-
as function
(W)
of heat
input
After the final temperature has been reached with such an equipment, a considerable quantity of heat, e.g. for measuring, can be transferred, and with only a small temperature difference. The dependence on heat supply of a system having about the dimensions specified above is shown in Figure 4 for various liquid quantities in the thermosiphon. As soon as sufficient quantity of liquid is available in tEe thermosiphon, high quantities of heat can he transferred. When the quantity of liquid is only 0.2 cm3, the evaporator warms up at a heat supply of some few watts. When the quantity of liquid in the tbermosiphon exceeds O-6 cmJ, more than 30 W can be transferred. This is equal to a formal heat conductivity of the thermosiphon of about 70 Wcm-1 deg-1. REFERENCES 1. BEWILOGUA L., and KN~NNER, R. Cryogenics 2,. 46 (1961) 2. GIFFORD, W. E. Ado. cryog. Engng 7, 551 (t962) 3. KN~NER R. Proc. 3rd Regional Conference, Prctpus, 1963,
p.
199
35