A 2-watt, 4-K helium refrigerator using the Gifford-McMahon/Joule Thomson cycle is described. The unit features a removable displacer cylinder and high-efficiency, low-pressure drop heat exchangers. These improvements result in a 100 percent increase in cooling power over the existing Deep Space Network system. The effects of the heat exchanger efficiency and Gifford-McMahon expander performance on refrigerator capacity are also discussed

Britcliffe, M.

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TDA Progress Report 42-91 
1 
July-September 1987 
Two-Watt, 4-Kelvin Closed Cycle Refrigerator Performance 
M. Britcliffe 
Radio Frequency and Microwave Subsystems Section 
This article describes a 2-watt, 4-K helium refrigerator using the Gifford- 
McMahonlJoule-Thomson cycle. The unit features a removable displacer cylinder 
and high-efficiency, low-pressure-drop heat exchangers. These improvements result 
in a 100 percent increase in cooling power over the existing DSN system. The 
effects of heat exchanger efficiency and Gifford-McMahon expander performance 
on refrigerator cooling capacity are also discussed. 
1. Introduction 
The 1-watt, 4-K refrigerator currently used to cool DSN 
traveling-wave masers was developed at JPL over 20 years ago 
[ 11 . It represented an order-of-magnitude improvement in re- 
liability and performance over the commercial unit used pre- 
viously. These systems have been used with only minor modifi- 
cation since that time. 
Over the years, several problems have become apparent 
with the existing DSN design. Current DSN masers have grown 
to require more cooling power. The original 2.3-GHz maser 
required only 150 milliwatts of the 1 watt available, leaving a 
reserve of 850 milliwatts. Some current designs reduce the 
reserve to 500 milliwatts. Experience in the DSN has shown 
that systems with reduced capacities have shorter mean times 
between failures. 
Many DSN refrigerators have been in operation long enough 
to experience a reduction in cooling capacity due to wear of 
the Gifford-McMahon (GM) expansion engine displacer cylin- 
der. Replacement of the cylinder requires moving the entire 
Joule-Thomson (JT) circuit, which is a costly and time. 
consuming operation. 
The current DSN refrigerator is also difficult and expensive 
to produce. Fabrication is very labor intensive, and some 
materials are difficult to obtain. 
The cost of the refrigerator alone is a very significant part 
of a complete maser system. The new 2-watt refrigerator de- 
scribed in this article features substantially increased cooling 
capacity, simpler heat exchangers with improved efficiency, 
and a conveniently replaceable GM displacer cylinder. The unit 
will accept all current maser assemblies without modification. 
The physical size of the refrigerator and the input power for 
the system remain unchanged. 
II. Thermodynamic Considerations 
Both the existing DSN 1-watt system and the new 2-watt 
system described here use a combination of the Gifford- 
I 31 2 
https://ntrs.nasa.gov/search.jsp?R=19880003327 2020-03-20T09:15:37+00:00Z
McMahon (GM) and Joule-Thomson (JT) thermodynamic 
cycles. A gas flow schematic is shown in Fig. 1. 
A. Cycle Description 
The refrigerator uses a two-stage CTI Model 350 GM 
expansion engine that simultaneously provides 25 watts of 
cooling at 60 K and 5 watts at 15 K to precool helium gas 
flowing in the JT circuit. The JT circuit consists primarily of 
three heat exchangers and a JT expansion valve. High-pressure 
helium (300 psig) supplied by a compressor at ambient tem- 
perature (300 K) is cooled in the heat exchangers by the gas 
returning from the cold station and by the expansion engine. 
When the gas reaches the .IT expansion valve, it has been cooled 
to nearly the temperature of the 4-K stage. The JT valve is 
simply a calibrated restriction that allows the high-pressure 
gas to expand into the low-pressure JT return line. During 
this expansion, the helium cools slightly, and a fraction of 
the flow condenses into 4-K liquid. Heat from the load-in 
this case, the maser-is absorbed by vaporizing this liquid. 
6. Heat Exchanger Efficiency 
Heat exchanger efficiency is one of the most critical aspects 
in the design of a JT refrigerator. It determines the amount of 
heat that must be removed from the supply helium by the GM 
engine. It also determines the helium flow rate required in 
the JT circuit to produce a given amount of cooling at 4 K. 
Heat exchanger efficiency is defined by 
where 4max, the maximum amount of heat that can be trans- 
ferred to or from the helium, is a function of the fluid proper- 
ties of the helium for a given temperature and pressure. The 
amount of heat actually transferred, qactua,, is primarily a 
function of the design of the heat exchanger (heat transfer 
area, material thermal conductivity, flow passage size, etc.). 
The significance of heat exchanger efficiency can be seen 
by the fact that the production of 1 watt of cooling at 4K 
requires approximately 130 watts of heat to be transferred 
from the incoming gas upstream of the JT valve. Any heat not 
removed by the heat exchangers must be absorbed by the 
engine. Because the amount of heat to be removed from the 
helium is large compared to the capacity of the engine, the 
numeric efficiency must be high. Table 1 shows the estimated 
heat exchanger efficiencies for both systems. 
Pressure drop in the return path of the heat exchangers is 
also important. The operating temperature of the 4-K stage is 
determined by the helium pressure at the cold station. Any 
pressure drop in the JT return line will increase the pressure, 
and therefore the temperature, of the 4-K stage. 
C. Gifford-McMahon Expansion Engine 
Performance 
Expander performance is another crucial factor in GM/JT 
refrigerator operation. As stated earlier, the expansion engine 
absorbs any heat not removed from the supply gas stream 
before it reaches the final-stage heat exchanger. The engine 
also cools the radiation shield that intercepts thermal radia- 
tion from ambient-temperature sources. 
The expander is a reciprocating mechanical device with 
several moving parts that are subject to wear. As this wear 
occurs, the efficiency of the engine decreases and the operat- 
ing temperatures of the engine stages increase. This increase in 
engine operating temperatures has a dramatic effect on the 4-K 
cooling capacity of the refrigerator. 
Another factor that affects engine performance is external 
heat load from sources other than the helium gas in the JT 
circuit. Thermal radiation from room temperature is inter- 
cepted by radiation shields cooled by the engine first stage. 
Radiation load from large shields can equal the load from the 
helium. Heat conduction through waveguides, supports, and 
wiring also contributes to engine load. 
111. Hardware Description 
Both the DSN 1-watt refrigerator and the new 2-watt refrig- 
erator are shown in Fig. 2. They are nearly identical in physi- 
cal size and weight. The new refrigerator was designed to 
accept existing maser hardware without modification. Many of 
the components developed for the initial design are used in the 
new system. The most significant changes are the use of im- 
proved efficiency heat exchangers and the addition of a “bolt- 
in” expander cylinder. 
The new heat exchangers are based on a concept developed 
at JPL [ 11 and later refined at the National Radio Astronomy 
Observatory. They consist of a spiral coil of convoluted tubing 
wound on a Micarta mandrel and enclosed in a thin stainless 
steel tube (Fig. 3). The supply helium travels through the 
inside of the convoluted tubing, and the return gas passes 
axially over the outside. This new design contains more than 
three times the heat transfer area of the original design. 
The expander-displacer cylinder is a commercial unit sup- 
plied by CTI. The heat stations are bolted to flanges on the 
cylinder rather than being soldered. The cylinder is bolted to 
the vacuum housing end plate using an O-ring seal. The cylin- 
der is shown removed from the JT circuit in Fig. 4. 
313 
Fabrication costs and assembly time have been reduced by 
as much as 50 percent on the new refrigerator. Fabrication of 
the original displacer cylinder involved several machining, 
welding, heat treatment, and inspection processes that resulted 
in high cost and long delivery times. Interestingly, the entire 
2-watt refrigerator was built and tested while waiting for two 
of the original-design cylinder assemblies to be fabricated. 
Eight solder or weld joints have been eliminated from each 
heat exchanger. Joints of this type are often the cause of 
internal helium leaks that plague the construction of helium 
refrigerators. The materials used in the construction of the 
heat exchangers are easily obtainable. 
IV. Performance 
Performance specifications for the existing system and the 
new design are shown in Table 2. The 4-K cooling capacity of 
the new unit is more than double that of the existing design. 
The input power required remains unchanged. This results in 
a 100 percent increase in thermodynamic efficiency for the 
total closed-cycle refrigerator system. 
Figures 5 and 6 represent the effects of first- and second- 
stage engine loading on 4-K capacity. The data shows a sub- 
stantial improvement in the refrigerator’s resistance to exter- 
nal heat loads. 
V. Conclusions and Recommendations 
Cooling capacity at all three stages of refrigeration has 
been improved substantially in the new 2-watt system. This 
results in an increase in the refrigerator’s resistance to GM 
expander performance degradation and external heat loads. 
The extra cooling power should result in a marked improve- 
ment in DSN traveling-wave maser closed-cycle refrigerator- 
system mean time between failures. The reduction in fabri- 
cation expenses makes the unit a cost-effective replacement 
for aging I-watt systems. 
Future projects such as the 32-GHz maser and masers 
with cryogenically cooled feeds may require the extra cool- 
ing this refrigerator provides. HEMT amplifiers can also be 
cooled to 4 K to reduce system noise temperature. Although 
the HEMT device itself does not benefit greatly from 4-K 
operation, thermal noise contribution from microwave com- 
ponents at the device’s input (e.g., from filters and isolators) 
can be reduced by lowering their physical temperature. 
Acknowledgment 
The author would like to thank T. Hanson of Bendix Field Engineering for providing 
the superb craftsmanship that went into the fabrication of the prototype refrigerator. He 
was also responsible for the initial cryogenic testing of the unit. 
Ref e ren ce 
[l]  W. H. Higa and E. Wiebe, “One Million Hours at 4.5 Kelvins,” National Bureau of 
Standards TechnicalPublication No. 508, pp. 99-107, April 1978. 
31 4 
I 
Table 1. Estimated heat exchanger efficiency 
Heat exchanger 1-watt CCR 2-watt CCR 
First stage 
Second stage 
Third stage 
0.94 0.96 
0.93 0.96 
0.97 0.99 
Table 2. Comparison of heat exchanger specifications 
Specification 1-watt CCR 2-watt CCR 
Operating temperature 4.5 K 4.4 K 
operating temperature 
JT mass flow 1.5 SCFM 2.3 SCFM 
Cooling capacity at 0.95 w 2.2 w 
Compressor input 
power required 
8000 W 8000 W 
Net thermodynamic 8400 W/W 3600 W/W 
efficiency 
31 5 
ORIGINAC PA-GE IS 
OF POOR QUALITY 
J-T 
CI RCUlT 
COMPRESSOR 
FIRSTSTAGE 
HEAT 
EXCHANGERS 
SECOND STAGE 
J-T VALVE 
4.5-K 
STAT I ON 
GM 
EXPANSION 
ENGINE 
r OUTER SS TUBE 
FLEXIBLE METAL HOSE 
WITH HELICAL CONVOLUTIONS 
INNER MICARTA 
Fig. 3. New heat exchanger 
Fig. 1. GMlJT refrigerator gas flow schematic 
Fig. 2. Two-watt refrigerator (shown on right) and 1-watt 
refrigerator 
I 
Fig. 4. Displacer cylinder removed from refrigerator, leaving 
JT circuit intact 
1 
I 
31 6 
2.5 1 i i 
1.7 .51 
1.1 
* 1.3 
-----Po-.- 
. -  O.g'c------- 0 7 lo 15 0 5 
EXTERNAL FIRST-STAGE LOAD, watts 
Fig. 5. Four-kelvin capacity versus first-stage load 
317 


Two-watt, 4-Kelvin closed cycle refrigerator performance

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