BACKGROUND OF THE INVENTION
Technical Field of the Invention
[0001] The present invention relates to a cryogenic refrigerator having a cooling capacity
of cooling a cooled object up to cryogenic temperatures and a control method therefor.
Description of the Related Art
[0002] A cryogenic refrigerator (for example, a Brayton cycle refrigerator or an Ericsson
cycle refrigerator) is used to cool down high temperature superconducting (HTS) equipment
(for example, a superconducting transmission cable, a superconducting transformer,
a superconducting motor, a superconducting coil for storing superconducting power,
a large accelerator, a nuclear fusion test facility, MHD power generation, a superconducting
coil, or the like). For example, in the case of using the cryogenic refrigerator for
cooling the high temperature superconducting equipment, the lowest temperature is
65K, 40K, 30K, 20K, or the like, though it depends on the type and application of
a superconducting wire. Moreover, cooling output is 1 to 10kW or so at each temperature,
and helium (the boiling point is approx. 4K), neon (the boiling point is approx. 27K),
or a mixture gas of helium and neon is used as a refrigerant gas.
This type of cryogenic refrigerator is disclosed in, for example, Patent Documents
1 and 2 and Non-patent Document 1.
[0003] As shown in Fig. 1, the cascade-turbo helium refrigerating liquefier in Patent Document
1 includes a neon refrigeration cycle, which has a turbo type compressor 51, heat
exchangers 52a to 52e, and a turbo type expander 53, and a helium refrigeration cycle,
which has a turbo type compressor 54, heat exchangers 55a to 55c, an expansion turbine
56, and a Joule-Thomson valve 57. It is
characterized that the neon refrigeration cycle previously cools helium.
[0004] The refrigerator disclosed in Patent Document 2 is intended to prevent a cooling
medium from being solidified, to extend the maintenance period, to enable a large
output, and to eliminate vibration. As shown in Fig. 2, the refrigerator 61 includes
a centrifugal compressor 62 and a turbine 63 with a one-stage wing 64 of the compressor
62 and converts a gas 65, which is compressed by the compressor 62 and introduced
to the turbine 63, to, for example, a gas mixture of helium and argon or of helium
and nitrogen or the like.
[0005] Non-patent Document 1 discloses a cryogenic refrigerator for cooling liquid nitrogen
(the boiling point is approx. 77K) up to 65K in order to cool a high temperature superconducting
cable as shown in Fig. 3.
[0006]
[Patent Document 1]
Japanese Patent Application Laid-Open No. S59-122868
[Patent Document 2]
Japanese Patent Application Laid-Open No. H11-159898.
[0008] While the working gases (helium, neon, and the like) for use in the foregoing cryogenic
refrigerator have extremely low liquefaction temperatures and therefore are excellent
in preventing liquefaction in the inside of an expander, there is a problem that the
working gases are very expensive.
The cryogenic refrigerator using the expensive working gases is required to minimize
a gas charging weight and to stabilize the internal pressure from the start of the
refrigerator to the steady operation.
[0009] If, however, a low-pressure low-temperature portion of the running cryogenic refrigerator
is cooled from, for example, a room temperature (for example, 300 K) to a cryogenic
temperature (for example, 60 K) along with a decrease in temperature of the inside
of the refrigerator, the gas volume of the low-pressure low-temperature portion is
reduced to one fifth (1/5). Therefore, in order to maintain a predetermined pressure
(for example, one half (1/2) of the pressure on start-up), the low-pressure low-temperature
portion is required to be supplied with a working gas so that the working gas is five
halves (5/2) of the working gas on start-up.
[0010] Contrarily, the pressure rises after the stop of the operation and therefore it is
necessary to discharge the working gas to the outside or to bleed the working gas
to a pressure vessel, which is provided separately. In this case, discharging the
working gas to the outside causes a great loss of the expensive working gas, and bleeding
the working gas to the pressure vessel causes excess pressure resistance of the pressure
vessel.
[0011] Moreover, if the entire refrigerator is stopped directly without using the pressure
vessel, it is necessary to increase the pressure resistance of the entire refrigerator
in advance. In this case, there is a problem that an excess load is applied to the
compressor on start-up.
Furthermore, if the refrigerator is suddenly stopped in an emergency stop or the like,
the working gas on the high-pressure side flows backward passing through the compressor
and the compressor turns in reverse, which adversely affects a drive system or the
like in some cases.
SUMMARY OF THE INVENTION
[0012] The present invention has been devised in order to solve the above problems. Specifically,
it is an object of the present invention to provide a cryogenic refrigerator and a
control method therefor, the cryogenic refrigerator having a cooling capacity of cooling
a cooled object up to a predetermined cryogenic temperature, capable of maintaining
the pressure in a high-pressure portion at a substantially constant level from a room
temperature in a stopped state to a cryogenic temperature in an operating state without
using a pressure vessel whose pressure resistance exceeds a predetermined pressure
(for example, 1 MPa) and without discharging or supplying a working gas, and capable
of preventing a reverse rotation of a compressor even in the case of an emergency
stop.
[0013] According to the present invention, there is provided a cryogenic refrigerator which
generates a cryogenic temperature by compressing a working gas in a closed loop and
expanding the compressed working gas, the cryogenic refrigerator comprising: a bypass
line which allows a high-pressure portion and a low-pressure portion in the closed
loop to communicate with each other; a gas storage tank which is located midway in
the bypass line and has pressure regulation valves on the high-pressure side and the
low-pressure side, respectively; and a pressure control unit which controls the pressure
regulation valves, wherein the pressure control unit controls the pressure regulation
valves so that the pressure in the gas storage tank is equal to the pressure in the
closed loop at room temperature and in a stopped state and controls the pressure regulation
valves so that the pressure in the high-pressure portion is equal to a predetermined
pressure in an operating state in which the cryogenic temperature is generated.
[0014] According to a preferred embodiment of the present invention, the capacity of the
gas storage tank is set so as to enable the pressure in the gas storage tank to be
maintained at a predetermined reference pressure or lower at room temperature and
in the stopped state and so as to enable the pressure in the high-pressure portion
to be maintained at a predetermined operating pressure in the operating state in which
the cryogenic temperature is generated.
[0015] Preferably, the pressure control unit maintains the pressure regulation valves to
be fully opened in the stopped state of the cryogenic refrigerator and opens the pressure
regulation valve connected to the high-pressure side in the case where the pressure
in the high-pressure portion exceeds a predetermined maximum pressure and opens the
pressure regulation valve connected to the low-pressure side in the case where the
pressure in the high-pressure portion is equal to or lower than a predetermined minimum
pressure.
[0016] Further, according to a preferred embodiment of the present invention, the cryogenic
refrigerator further comprises: a room-temperature compressor which is installed in
a room temperature portion in the closed loop to compress the working gas from a predetermined
low pressure to a predetermined high-pressure; a first intermediate heat exchanger
which is located between a cryogenic temperature portion in the closed loop and the
room temperature portion to perform a heat exchange between the working gases; and
an expander which is installed on the cryogenic temperature portion side from the
first intermediate heat exchanger to isentropically expand the working gas.
[0017] Moreover, the room-temperature compressor includes a plurality of turbo compressors
which compress the working gas in multiple stages from the predetermined low pressure
to the high pressure; the expander includes a plurality of expansion turbines which
expand the working gas in multiple stages from the high pressure to the low pressure;
and a plurality of intermediate heat exchangers which perform a heat exchange between
working gases are disposed in the middle of the plurality of expansion turbines.
[0018] Moreover, according to the present invention, there is provided a control method
for a cryogenic refrigerator which generates a cryogenic temperature by compressing
a working gas in a closed loop and expanding the compressed working gas, the control
method comprising: providing the cryogenic refrigerator with a bypass line which allows
a high-pressure portion and a low-pressure portion in the closed loop to communicate
with each other and a gas storage tank which is located midway in the bypass line
and has pressure regulation valves on the high-pressure side and the low-pressure
side, respectively; and controlling the pressure regulation valves so that the pressure
in the gas storage tank is equal to the pressure in the closed loop at room temperature
and in a stopped state and controlling the pressure regulation valves so that the
pressure in the high-pressure portion is equal to a predetermined pressure in an operating
state in which a cryogenic temperature is generated.
[0019] Furthermore, according to a preferred embodiment of the present invention, the capacity
of the gas storage tank is set so as to enable the pressure in the gas storage tank
to be maintained at a predetermined reference pressure or lower at room temperature
in the stopped state and so as to enable the pressure in the high-pressure portion
to be maintained at a predetermined operating pressure in the operating state in which
the cryogenic temperature is generated.
[0020] According to the cryogenic refrigerator and the method of the present invention,
the cryogenic refrigerator comprises a bypass line which allows a high-pressure portion
and a low-pressure portion in the closed loop, which constitutes the cryogenic refrigerator,
to communicate with each other and a gas storage tank which is located midway in the
bypass line and has pressure regulation valves on the high-pressure side and the low-pressure
side, respectively, and therefore it is possible to set the pressure of the entire
system, which includes the closed loop, the bypass line, and the gas storage tank,
to a predetermined reference pressure or lower by controlling the pressure regulation
valves (for example, maintaining the pressure regulation valves to be fully opened
in the stopped state) so that the pressure in the gas storage tank is equal to the
pressure in the closed loop at room temperature and in a stopped state. Moreover,
this enables the pressures on the inlet side and outlet side of the compressor to
be equalized in the stopped state of the refrigerator, and therefore it is possible
to prevent a reverse rotation of the compressor caused by a pressure difference between
the inlet side and the outlet side of the compressor after the stop.
[0021] Moreover, even if the low-pressure low-temperature portion of the cryogenic refrigerator
in the operating state requires, for example, five halves of the working gas on start-up
due to a decrease in temperature and in pressure, it is possible to supply the required
working gas from the gas storage tank by controlling the pressure regulation valves
so that the pressure in the high-pressure portion is equal to a predetermined pressure
in the operating state where the cryogenic temperature is generated.
[0022] Therefore, the capacity of the gas storage tank is set so that the pressure in the
gas storage tank is able to be maintained at a predetermined reference pressure or
lower level at room temperature in the stopped state and so that the pressure in the
high-pressure portion is able to be maintained at a predetermined operating pressure
level in the operating state in which the cryogenic temperature is generated, thereby
enabling the cryogenic refrigerator to have a cooling capacity of cooling an cooled
object up to a predetermined cryogenic temperature and to maintain the pressure in
the high-pressure portion at a substantially constant level from the room temperature
in the stopped state to the cryogenic temperature in the operating state without using
a pressure vessel whose pressure resistance exceeds a predetermined pressure (for
example, 1 MPa) and without discharging or supplying the working gas.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023]
Fig. 1 is a schematic diagram of an apparatus in Patent Document 1.
Fig. 2 is a block diagram of a refrigerator in Patent Document 2.
Fig. 3 is a schematic diagram of an apparatus in Non-patent Document 1.
Fig. 4 is a diagram illustrating a first embodiment of a cryogenic refrigerator according
to the present invention.
Fig. 5 is a diagram illustrating a second embodiment of the cryogenic refrigerator
according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0024] Preferred embodiments of the present invention will be described hereinafter with
reference to the accompanying drawings. In the drawings, the same reference numerals
are used for the same parts and the overlapped description thereof is omitted.
[0025] Referring to Fig. 4, there is shown a diagram illustrating a first embodiment of
a cryogenic refrigerator according to the present invention.
The cryogenic refrigerator 10 according to the present invention is a cryogenic refrigerator
which generates a cryogenic temperature by compressing a working gas in a closed loop
11 and expanding the compressed working gas. The expansion by an expansion turbine
is preferably an isentropic expansion.
[0026] In this figure, the cryogenic refrigerator 10 according to the present invention
has the closed loop 11 in which a working gas circulates, and the closed loop 11 is
provided with a cryogenic heat exchanger 12, a room-temperature compressor 14, a first
intermediate heat exchanger 16, and an expander 18. The working gas used to circulate
in the closed loop 11 is helium (the boiling point is approx. 4K), neon (the boiling
point is approx. 27K), or a mixture gas of helium and neon.
[0027] The cryogenic heat exchanger 12 is installed in a cryogenic temperature portion of
the closed loop 11 and indirectly cools down a cooled object with the working gas.
The cooled object is high temperature superconducting (HTS) equipment (for example,
a superconducting transmission cable, a superconducting transformer, a superconducting
motor, a superconducting coil for storing superconducting power, a large accelerator,
a nuclear fusion test facility, MHD power generation, a superconducting coil, or the
like), and the outlet temperature of the cryogenic heat exchanger 12 in the cryogenic
temperature portion is, for example, 65K.
[0028] The room-temperature compressor 14 is, for example, a turbo compressor, which is
installed in a room temperature portion (for example, in a room at a temperature around
300 K) of the closed loop 11 to compress the working gas from a predetermined low
pressure to a predetermined high pressure. Preferably the predetermined low pressure
is, for example, 0.5 to 0.6 MPa, the predetermined high pressure is, for example,
1.0 to 1.2 MPa, and the compression ratio of the compressor is around 2.
A water-cooled gas cooler 15 is installed on the downstream side (high-pressure side)
of the room-temperature compressor 14 to cool the working gas, which has increased
in temperature as a result of the compression, preferably up to around 300 K by using
cooling water supplied from an external cooling water circulation unit 9.
[0029] The first intermediate heat exchanger 16 is located between the cryogenic temperature
portion and the room temperature portion to perform a heat exchange between the working
gas in the high-pressure side and the working gas in the low pressure side. The heat
exchange cools the working gas on the high-pressure side preferably up to 65 to 70
K.
The expander 18 is, for example, an expansion turbine and is installed on the cryogenic
temperature portion side from the first intermediate heat exchanger 16 to isentropically
expand the working gas, which has been cooled by the first intermediate heat exchanger
16. The expansion by the expansion turbine causes the working gas to generate a predetermined
cryogenic temperature (for example, 56 K). The expansion turbine is coaxial with the
turbo compressor, and preferably the same electric motor drives the expansion turbine
and the turbo compressor.
The working gas at the cryogenic temperature is supplied to the cryogenic heat exchanger
12 to cool the cooled object indirectly with the working gas, and cools the working
gas on the high-pressure side indirectly in the first intermediate heat exchanger
16. Subsequently, the working gas is supplied to the room-temperature compressor 14
and is compressed again.
[0030] The foregoing structure allows the cooled object up to the predetermined cryogenic
temperature by compressing the working gas in the closed loop 11 and expanding the
compressed working gas using the expander 18 to generate a cryogenic temperature.
[0031] In Fig. 4, the cryogenic refrigerator 10 according to the present invention further
includes a bypass line 22, a gas storage tank 24, and a pressure control unit 26.
[0032] The bypass line 22 allows a high-pressure portion and a low-pressure portion of the
closed loop 11 to communicate with each other directly. The above high-pressure portion
is on the downstream side from the compressor 14 in this example, and more specifically,
has the volume of the high-pressure side of the gas cooler 15 and the first intermediate
heat exchanger 16, and a connecting pipe located from the outlet of the compressor
14 to the inlet of the expander 18. The above low-pressure portion is on the upstream
side from the room-temperature compressor 14 in this example, and more specifically,
has the volume of the low-pressure side of the cryogenic heat exchanger 12 and the
first intermediate heat exchanger 16, and a connecting pipe from the outlet of the
expander 18 to the inlet of the room-temperature compressor 14.
[0033] The gas storage tank 24 is located midway in the bypass line 22, having pressure
regulation valves 23a and 23b on the high-pressure side and the low-pressure side,
respectively.
The capacity of the gas storage tank is set so that the pressure in the gas storage
tank 24 is able to be maintained at a predetermined reference pressure (for example,
1 MPa) or lower level at room temperature in a stopped state and so that the pressure
in the high-pressure portion is able to be maintained at a predetermined operating
pressure level (for example, 1.0 to 1.2 MPa) in an operating state in which a cryogenic
temperature is generated.
[0034] The capacity of the gas storage tank 24 requires a volume of the gas storage tank,
which satisfies the condition that a difference between the total mass of a gas exclusive
of the gas storage tank 24 in the closed loop 11 calculated from the temperature and
pressure in the operating state and the mass of a gas loaded at the pressure (for
example, 1 MPa) in the high-pressure portion (on the downstream side from the gas
cooler 15 in Fig. 4) in the operating state for the volume of the closed loop 11 exclusive
of the gas storage tank 24 in the stopped state and at room temperature is equal to
a difference between the mass of a gas obtained in the case where the gas storage
tank 24 is filled with the pressure in the high-pressure portion in the operating
state and the mass of a gas obtained in the case where the gas storage tank 24 is
filled with the pressure in the low-pressure portion (the upstream side from the room-temperature
compressor 14 in Fig. 4) in the operating state.
The temperature of the gas storage tank is always constant. The pressure in the gas
storage tank is maximum when it is equal to the pressure on the high-pressure side
in the operating state and is minimum when it is equal to the pressure on the low-pressure
side in the operating state. The mass of the gas which the gas storage tank is able
to absorb is obtained from the pressure difference at the constant temperature and
the volume.
Therefore, the capacity of the gas storage tank 24 is preferably set so as to be 3
or more times, preferably 4 or 5 times, the volume of the low-temperature low-pressure
portion at cryogenic temperature and low pressure.
[0035] Moreover, a pressure sensor 25 is installed in the high-pressure portion in the closed
loop 11, and detected pressure data is input to the pressure control unit 26. The
pressure control unit 26 controls the pressure
regulation valves 23a and 23b so that the pressure in the gas storage tank 24 is equal
to the pressure in the closed loop 11 at room temperature and in a stopped state on
the basis of the detected pressure data and controls the pressure regulation valves
23a and 23b so that the pressure in the gas storage tank 24 is between the pressures
in the high-pressure portion and in the low-pressure portion and close to the pressure
in the low-pressure portion (a pressure slightly higher than the pressure in the low-pressure
portion) in the operating state in which the cryogenic temperature is generated.
[0036] In the control method for the cryogenic refrigerator according to the present invention,
the pressure control unit 26 performs the following controls by using the cryogenic
refrigerator 10 having the above configuration:
(A) The pressure regulation valves 23a and 23b are maintained to be fully opened in
the stopped state of the cryogenic refrigerator 10. This operation enables the pressure
on the inlet side of the compressor 14 to be equalized with the pressure on the outlet
side of the compressor 14 in the stopped state of the refrigerator, and therefore
it is possible to prevent a reverse rotation of the compressor caused by pressure
after the stop of the refrigerator.
(B) The pressure regulation valves 23a and 23b are fully closed before the start-up
of the cryogenic refrigerator 10. This operation enables the gas storage tank 24 to
be isolated from pressure fluctuations on the high-pressure side and on the low-pressure
side caused immediately after the start-up, by which the cryogenic refrigerator 10
is able to be started only in the closed loop 11.
(C) During the start-up of the cryogenic refrigerator 10, the pressure regulation
valve 23a on the high-pressure side is opened if the pressure in the high-pressure
portion exceeds a predetermined maximum pressure (for example, 1.1 MPa). This operation
prevents the pressure in the high-pressure portion from exceeding the predetermined
maximum pressure and enables excess working gas to be collected into the gas storage
tank 24.
(D) During the start-up of the cryogenic refrigerator 10, the pressure regulation
valve 23b on the low-pressure side is opened if the pressure in the high-pressure
portion is equal to or lower than a predetermined minimum pressure (for example, 0.9
MPa). This operation enables the low-pressure portion in the closed loop 11 to be
supplied with working gas from the gas storage tank 24, thereby inhibiting the pressure
in the high-pressure portion from decreasing.
Through the operations of (B) to (D), it is possible to complete the start-up of the
cryogenic refrigerator 10 and to perform the steady operation which generates the
cryogenic temperature.
[0037] Moreover, the same controls are performed to stop the cryogenic refrigerator 10 from
the steady operation which generates the cryogenic temperature. More specifically,
the pressure on the high-pressure side rises up along with an increase in the temperature
and pressure of the low-temperature low-pressure portion at cryogenic temperature
and low pressure in the operating state, and therefore it is possible to collect excess
working gas into the gas storage tank 24 by the above operation (C).
Further, in the stopped state of the cryogenic refrigerator 10, the operation (A)
for maintaining the pressure regulation valves 23a and 23b to be fully opened enables
the pressure on the inlet side of the compressor 14 to be equalized with the pressure
on the outlet side of the compressor 14 in the stopped state of the refrigerator,
and therefore it is possible to prevent a reverse rotation of the compressor caused
by a pressure difference between the inlet side and outlet side of the compressor
14 after the stop of the refrigerator.
[0038] According to the above refrigerator and method of the present invention, the cryogenic
refrigerator 10 includes the gas storage tank 24, which is located midway in the bypass
line 22 allowing the high-pressure portion and the low-pressure portion in the closed
loop 11 to communicate with each other, and which has the pressure regulation valves
23a and 23b on the high-pressure side and the low-pressure side, respectively. Therefore,
it is possible to set the pressure in the entire system, which includes the closed
loop 11, the bypass line 22, and the gas storage tank 24, to a predetermined reference
pressure (for example, 1 MPa) or lower by controlling the pressure regulation valves
so that the pressure in the gas storage tank 24 is equal to the pressure in the closed
loop 11 at room temperature and in the stopped state (for example, by maintaining
the pressure regulation valves 23a and 23b to be fully opened in the stopped state).
Moreover, this enables the pressure on the inlet side of the compressor 14 to be equalized
with the pressure on the outlet side of the compressor 14 in the stopped state of
the refrigerator, and therefore it is possible to prevent a reverse rotation of the
compressor caused by pressure after the stop of the refrigerator.
[0039] Furthermore, the pressure regulation valves 23a and 23b are controlled so that the
pressure in the gas storage tank 24 is between the pressures in the high-pressure
portion and in the low-pressure portion and close to the pressure in the low-pressure
portion in the operating state in which the cryogenic temperature is generated, and
therefore it is possible to supply the corresponding working gas from the gas storage
tank even if the pressure of the working gas in the closed loop drops along with a
decrease in the temperature of the low-temperature portion in the refrigerator after
the start of the operation.
[0040] For example, if the capacity of the gas storage tank 24 is set so as to be 3 or more
times the volume V of the low-temperature low-pressure portion at cryogenic temperature
and low pressure in the operating state, it is necessary to supply the low-temperature
low-pressure portion with working gas so that the gas volume of the portion is five
halves (2.5) of the gas volume on start-up in order to maintain the pressure (for
example, one half of the pressure on start-up) in the low-temperature low-pressure
portion due to a decrease in temperature (for example, 300 K to 60 K) and a decrease
in pressure (for example, to one half).
Therefore, even if the working gas corresponding to the shortfall of 1.5 V is supplied
from the gas storage tank 24 to the low-temperature low-pressure portion, it is possible
to maintain the pressure in the gas storage tank 24 at one half or more of the pressure
in the stopped state.
[0041] More specifically, the capacity of the gas storage tank 24 is set so that the pressure
in the gas storage tank 24 is able to be maintained at the predetermined reference
pressure (for example, 1 MPa) or lower level at room temperature in the stopped state
and so that the pressure in the high-pressure portion is able to be maintained at
the predetermined operating pressure level in an operating state in which the cryogenic
temperature is generated, thereby enabling the cryogenic refrigerator to have a cooling
capacity of cooling the cooled object up to the predetermined cryogenic temperature
and to maintain the pressure in the high-pressure portion at a substantially constant
level from the room temperature in the stopped state to the cryogenic temperature
in the operating state without using a gas storage tank whose pressure resistance
exceeds the predetermined pressure (for example, 1 MPa) and without discharging or
supplying the working gas.
[Embodiment]
[0042] Referring to Fig. 5, there is shown a diagram illustrating a second embodiment of
the cryogenic refrigerator according to the present invention. The outlet temperature
of the cryogenic temperature portion is 65 K and the cooling capacity thereof is 3
kW in this example, where P, T and G in this figure represent the pressure (bar),
the temperature (K), and the mass flow rate (g/s), respectively.
[0043] In this example, the room-temperature compressor 14 includes a first stage compressor
14A, which compresses a working gas from a predetermined low pressure (5.57 bar) to
a first intermediate pressure (8.03 bar) between the low pressure and the high pressure,
and a second stage compressor 14B, which compresses the working gas from the first
intermediate pressure to a high pressure (11.0 bar). Water-cooled gas coolers 15 are
installed on the downstream side (the high-pressure side) of the first stage compressor
14A and the second stage compressor 14B, respectively.
Moreover, the expander 18 includes a first expander 18A, which expands the working
gas from the high pressure (11.0 bar) to a second intermediate pressure (10.29 bar)
between the low pressure and the high pressure, and a second expander 18B, which expands
the working gas from the second intermediate pressure to the low pressure (5.57 bar).
[0044] Furthermore, there is provided a second intermediate heat exchanger 17, which exchanges
heat between the low-pressure working gas and the high-pressure working gas, between
the first expander 18A and the second expander 18B. The first stage compressor 14A
and the second stage compressor 14B are turbo compressors, and the first expander
18A and the second expander 18B are expansion turbines. The first stage compressor
14A is coaxial with the second expander 18B, and the second stage compressor 14B is
coaxial with the first expander 18A. Preferably the same electric motor drives the
turbo compressors and the expansion turbines.
Other parts of the configuration are the same as in Fig. 4.
[0045] It is confirmed that this configuration enables the generation of a cryogenic temperature
of 56 K by compressing the working gas in the closed loop 11 and expanding the compressed
working gas by using the first expander 18A and the second expander 18B, thereby enabling
an absorption of 3 kW heat from the cooled object.
[0046] As described above, in the present invention, a room temperature portion is provided
with the gas storage tank 24 and is connected via a pipe (the bypass line 22) having
the pressure regulation valves 23a and 23b on the high-pressure side (the outlet side
of the compressor) and the low-pressure side (the return side) of the refrigerator,
respectively.
While both of the reference pressures in the control of the pressure regulation valves
23a and 23b are high-pressure side pressures, the pressure regulation valve 23a with
the pipe connected to the high-pressure side is "opened" when the pressure exceeds
a specified pressure and the pressure regulation valve 23b with the pipe connected
to the return side is "opened" when the high-pressure side pressure drops to a lower
value than the specified pressure to increase the pressure in the system.
Moreover, the volume of the gas storage tank 24 is set to a value as small as possible
within a scope that the pressure is maintained at a slightly higher level than the
return-side pressure in the operating state and the pressure does not exceed a design
pressure even at room temperature in the system in the stopped state.
[0047] Furthermore, the expansion turbines (the first expander 18A and the second expander
18B) are adapted to be coaxial with the turbo compressors (the first stage compressor
14A and the second stage compressor 14B) and the same electric motor drives the expansion
turbines and the turbo compressors, thereby enabling the collection of the power of
the expansion turbines so as to reduce the electric motor power and enabling the rotational
speed of the expansion turbines to be limited to that of the electric motor so as
to essentially prevent the overspeed of the expansion turbines. Therefore, there is
no need to use bypass valves for the expansion turbines or throttle valves in the
inlet and the compressors are able to operate at a rated speed from the start-up.
[0048] Moreover, both of the pressure regulation valves 23a and 23b are opened in the stopped
state of the refrigerator to equalize the pressures on the inlet side and outlet side
of the compressor, thereby preventing the reverse rotation of the compressors (the
first stage compressor 14A and the second stage compressor 14B) caused by a pressure
difference between the inlet side and the outlet side of the compressors after the
stop of the refrigerator.
[0049] According to the above configuration, the room-temperature compressor 14 increases
the pressure of the working gas, the gas cooler 15 decreases the increased temperature
of the gas up to close to a room temperature, and then the working gas passes through
the first intermediate heat exchanger 16 and the expander 18, thereby decreasing the
temperature and decreasing the pressure. A return gas, which has removed heat from
the cooled object which is a refrigeration load, increases in temperature up to close
to a room temperature while cooling the working gas on the high-pressure side in the
first intermediate heat exchanger 16 and then returns to the room-temperature compressor
14. A pressure ratio between the high-pressure side and the low-pressure side is around
2. The gas storage tank 24 is connected via the pipe (the bypass line 22) having the
pressure regulation valves 23a and 23b on the high-pressure side of the refrigerator
(the outlet side from the compressor) and the return side of the refrigerator (the
inlet side from the compressor), respectively.
[0050] While both of the reference pressures in the control of the pressure regulation
valves 23a and 23b are high-pressure side pressures, the pressure regulation valve
23a with the pipe connected to the high-pressure side is "opened" when the pressure
exceeds a specified pressure and the pressure regulation valve 23b with the pipe connected
to the return side is "opened" when the high-pressure side pressure drops to a lower
value than the specified pressure to increase the pressure in the system. Due to the
functions of the two pressure regulation valves 23a and 23b, the pressure on the high-pressure
side is maintained at a constant level in the operating state, on start-up, and in
the stopped state.
[0051] Naturally, the present invention is not limited to the embodiments described above,
but may be changed in various ways so as not to deviate from the scope of the present
invention.