A Claude cycle refrigerator with a three stage reciprocating expansion engine is described. Instead of a cam mechanism, valves are driven directly by magnetic solenoids operated by means of a micro processor control system. A swash plate mechanism is used to convert reciprocating motion of the expander pistons to rotary motion. A refrigeration capacity of 8 watts was achieved at 4.5 K with the operating pressure of 1.1 MPa and flow rate of 2.4 g/sec.. An effect of overintake operation was studied. Experimental results show that the efficiency of the expander has a peak point in the region of overintake operation with constant cycle speed, which agrees with theoretical results. The electrically controlled valve system is useful to vary the valve timing to achieve an optimum condition of operation

Baba, Y.

Hiersaki, Y.

Kaneko, M.

Matsubara, Y.

Munekata, T.

Yasukochi, K.

English

NASA Technical Reports Server

COMPACT CLAUDE CYCLE REFRIGERATOR FOR LABORATORY USE
Y. Hiresaki, M. Kaneko, T. Munekata and Y. Baba
Cryogenic Department, Suzuki Shokan Co., Ltd.
Y. Matsubara and K. Yasukochi
Atomic Energy Research Institute, Nihon University
ABSTRACT
A Claude cycle refrigerator with a three stage reciprocating expansion
engine is described. Instead of a cam mechanism, valves are driven
directly by magnetic solenoids operated by means of a micro processor
control system. A swash plate mechanism is used to convert reciprocating
motion of the expander pistons to rotary motion. A refrigeration capacity
of 8 watts was achieved at 4.5 K with the operating pressure of I.i MPa and
flow rate of 2.4 g/sec..
An effect of overintake operation was studied. Experimental results
show that the efficiency of the expander has a peak point in the region of
overintake operation with constant cycle speed, which agrees with
theoretical results. The electrically controlled valve system is useful to
vary the valve timing to achieve an optimum condition of operation.
INTRODUCTION
Recentry the demand for a compact helium refrigerator has been
increased due to the development of applications of small scale
superconducting technology such as NMIR for medical applications,
magnetically levitated trains, and Josephson-Junction computers.
In our previous theoretical study of the multi-stage Claude cycle
helium refrigerator it was predicted that the existing optimum value of
working pressure is lower than for commercial machines (i). To operate
efficiently at low working pressure, such as below 1.0 MPa, three expanders
are needed. This is shown in another study that the efficiency of a
refrigerator increases by increasing the number of expanders while
decreasing the operating pressure (2).
Simple mechanical design is an important factor for the three-stage
Claude cycle refrigerator with three reciprocating expanders having six
valves. Electronic control techniques are a powerful method to simplify
the mechanical design. A micro processor control system provides not only
a simple mechanism but also a flexible system such as variable valve timing
169
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control system or variable cycle speed control system.
OUTLINE OF DESIGN
Figure 1 shows the main unit of the refrigerator without heat
exchangers. The magnetic solenoid assembly with actuating valve rods is
placed on the upper flange. The swash plate mounted in the crosshead
housing is used to convert reciprocating motion to rotary motion. Roary
motion of the swash plate is transmitted to a DC brake motor working as an
electric generator by the V belt. Speed of revolution of the brake motor
is maintained at a constant value by the speed control circuit. The DC
motor works in its usual motor mode at starting time. Heat exchangers are
of coiled finned tube using soldered copper finned tube. The cold box has
a diameter of 40 cm and length of 1 m.
The electrical control system includes two micro processors, the valve
driving circuit, the speed control circuit, the interface circuit to
measure temperature and pressure and another interface circuits. The system
is controlled by a host computer via a standard comunication line.
The compressor^for a commercial model has an input power of 7.5 kW and
a capacity of 52 Nm_/hour with 1.05 MPa.
EXPANDER AND SWASH PLATE
The expansion engine has three identical cylinders with a bore of 50
mm, which are arranged symmetrically above the swash plate. Reinforced
phenolic pistons are sealed by capseals at their warm end of them. A stroke
of a piston is 30 mm. Fig. 2 shows a scheme of the expansion engine,
representing the bottom part of the cylinder and piston, the swash plate
and the flywheel which also servs as a pulley. The swash plate is
lubricated by oil and connected to the rotary encoder not shown in the
figure.
VALVE MECHANISM
The valves of the expansion engine are driven directly by DC solenoid
coils which are actuated by the micro processor instead of the usual cam
mechanism. The rotary encoder connected to the axis of the swash plate is
an absolute value type encoder of which the output signal represents a
three digit number corresponding to the angle of the axis. At every one
degree of angle the encoder sends interrupt signal to the micro prosessor.
The processor has a table in the memory containing values of the angle
corresponding to the setting points of the valves. Comparing the angle
obtained from the rotary encoder with the table, the processor operates
valves through the driving circuit. This system responds with a resolution
of 1 degree for engine speeds up to 350 rpm. The absolute value type
170
rotary encoder makes the system simple in comparison with the incremental
type. Since set points of valves can be changedby changing the content of
the table, valve timing can be changedeven during operation.
Fig. 3 shows typical data of valve action recorded by an
electromagnetic oscillograph along with data of the voltage across the
solenoid coil and the signal of the angle marker. The upper level
coresponds to a closed state of the valve. The osciration which appears
during the rising and falling periods is due to a characteristic of the
transducer. Time delay during closing is i0 to 20 m sec..
ELECTRICAL CONTROL SYSTEM
Fig. 4 shows the block diagram of the electrical control system for
test operation of the prototype. The micro processor operates six valves
on three expanders. The cycle speed of the expansion engine is varied by
control of the DC brake motor. The speed control circuit compares the
reference voltage with the output voltage from the tachometer on the brake
motor, and maintains constant cycle speed by means of changing the
electrical load of the brake motor. A PDPII/V03 was used as a host
computer and also as a data acquisition system.
Fig. 5 shows the block diagram of the commercial model which includes
another micro processor CPU-I. CPU-I receives commands from the host
computer through a standard comunication line, RS-232C, or an IEEE-488 bus.
Main roles of the CPU-I are
i. setting the cycle speed on the speed control circuit
2. changing the valve timing
3. monitoring temperature and pressure
4. operating the J-T valve and other valves
5. operating the compressor unit
6. performing emergent sequence
7. sending back infomation about the status of the refrigerator to
the host computer.
The host computer contains an operator console and can be
interconnected to another computer system. In addition to using the
computer for these functions the user may also make other programs such as
a time schedule or a data logging program on the host computer. This
system is useful in both laboratory and industrial applications.
TEST RESULTS AND DISCUSSION
Fig. 6 shows typical experimental results of the test operation with
0.8 MPa pressure at the inlet of third expander. Refrigeration capacity of
8 watts at 4.5 K was obtained at 74 rpm and 2.4 g/sec, total mass flow
rate. Expander efficiencies of 60, 70 and 74 % were obtained at inlet
temperature of 129, 48, and 13 K respectively. Ideal isothermal compressor
171
work at 290 K is 3.5 KW, when the helium gas is compressed from 0.i MPa to
i.i MPa with a flow rate of 2.4 g/sec., therefore the actual input power
will be less than 7 KW with the assumption that the overall efficiency of
the compressor is more than 50 %. With this assumption the ratio of the
refrigeration capacity to the input power is l.lxl0 -_.
Fig. 7 shows the efficiency of the third expander against the filling
factor, X, with constant cycle speeds of 140 rpm and 180 rpm. Inlet
temperature of the expander is 15 K and inlet and outlet pressure are 0.8
and 0.12 MPa respectively. Each curve has a remarekable peak in the
overintake region, 0.34<X<I.0. Fig. 8 shows theoretical results with same
condition as the experiment for three different values of heat leak to the
expander. The model for the calculation (3) is illustrated in Fig. 9 on a
T-S plane. Helium gas at high pressure, point i, is expanded
isentropically to point f. Heat leak is considered only on the process
from f to ff and gas expands to pressure state Pe. The final state e is
reached by the remaining gas at point el and leaving gas at point e2. In
spite of the simplicity of this model, the theoretical results agree well
with experimental results.
Fig. i0 shows cool down curves for each expander on the normal and the
overintake mode. On the oveintake mode at 165 rpm the cool down time is
4.5 hours compared to 6 hours in the normal mode at 140 rpm. Improvement
of operation in the overintake mode is clear even though there is a
difference of speed between them.
SUMMARY
A compact three stage claude cycle helium refrigerator which produces
8 watts of refrigeration at 4.5 K has been made. The electrical control
system with micro processors provides a flexible system which permits the
operating mode to be changed, and is useful to simplify the valve
mechanism. Overintake operation increases refrigeration capacity without
decreasing the efficiency of the expanders.
REFERENCES
i. Matsubara, Y., Kaneko, M., Hiresaki, Y., and Yasukochi, K.:
"Exergetic Analysis of Multi-staged Claude Cycle Helium Refrigerator",
Cryogenic Processes and Equipment in Energy System, 131-134, ASME,
Century 2 Publication, 1980.
2. Khalil, A. and Mclntosh, G.: "Thermodynamic Optimization Study of the
Helium Multi-engine Claude Refrigeration Cycle", Adv. Cry. Eng.,
vol. 23, 431-437, 1978.
3. Kaneko, M., Hiresaki, Y., Matsubara, Y., and Yasukochi, K.: "Performance
of Reciprocating Expansion Engine with Electronic Control Valves",
Proceedings of the 9'th Int'l Cryog. Eng. Conf., 355-358, 1982.
172
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Compact Claude cycle refrigerator for laboratory use

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