Refrigeration – Gas compression – heat regeneration and expansion – e.g.,...
Reexamination Certificate
2000-07-05
2001-12-18
Capossela, Ronald (Department: 3744)
Refrigeration
Gas compression, heat regeneration and expansion, e.g.,...
C060S520000
Reexamination Certificate
active
06330800
ABSTRACT:
This invention relates to a cryocooler and, more particularly, to a two-stage cryocooler whose heat loading varies during operation and which is to be thermally stabilized.
BACKGROUND OF THE INVENTION
Some sensors and other components of spacecraft and aircraft must be cooled to cryogenic temperatures of about 77° K or less to function properly. A number of approaches are available, including thermal contact to liquefied gases and cryogenic refrigerators, usually termed cryocoolers. The use of a liquefied gas is ordinarily limited to short-term missions. Cryocoolers typically function by the expansion of a gas, which absorbs heat from the surroundings. Intermediate temperatures in the cooled component may be reached using a single-stage expansion. To reach colder temperatures required for the operation of some sensors, such as about 40° K or less, a multiple-stage expansion cooler may be used. The present inventors are concerned with applications requiring continuous cooling to such very low temperatures over extended periods of time.
One of the problems encountered in some applications is that the total heat load which must be removed by the cryocooler, from the object being cooled and due to heat leakage, may vary over wide ranges during normal and abnormal operating conditions. The heat loading is normally at a steady-state level, but it occasionally peaks to higher levels before falling back to the steady-state level. The cryocooler must be capable of maintaining the component being cooled at its required operating temperature, regardless of this variation in heat loading and the temporary high levels. While it handles this variation in heat loading, the cryocooler desirably would draw a roughly constant power level, so that there are not wide swings in the power requirements that would necessitate designing the power source to accommodate the variation.
One possible solution to the problem is to size the cryocooler to handle the maximum possible heat loading. This solution has the drawback that the cryocooler is built larger than necessary for steady-state conditions, adding unnecessarily to the size and weight of the vehicle. Such an oversize cryocooler also would require a power level that varies widely responsive to the variations in heat loading.
There is a need for an improved approach to the cooling of sensors and other components to very low temperatures. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides a cryocooler which cools a component to a low temperature while accommodating wide variations in the heat loading. The cryocooler is sized to the steady-state heat loading requirement, not the maximum heat loading requirement. It continuously draws power at about the level required to maintain the component at the required temperature with the steady-state heat loading, although some variation is permitted, while it accommodates the variations in heat loading.
In accordance with the invention, a hybrid two-stage cryocooler comprises a first-stage Stirling expander having a first-stage interface and a Stirling expander outlet, a second-stage pulse tube expander having a pulse tube inlet, a gas flow path extending between the Stirling expander outlet and the pulse tube inlet, and a heat exchanger in thermal contact with the gas flow path. A thermal-energy storage device is in thermal communication with the first-stage interface. The thermal-energy storage device may be of any operable type, and preferably is a triple-point cooler. The triple-point cooler may utilize any operable working fluid, such as nitrogen, argon, methane, or neon.
The first-stage Stirling expander preferably has an expansion volume having an expander inlet, a first-stage regenerator, and the Stirling expander outlet, a displacer which forces a working gas through the expander inlet, into the expansion volume, and thence into the gas flow path, and a motor that drives the displacer. There is a motor controller for the motor, and the motor controller is operable to alter at least one of the stroke and the phase angle of the displacer (where the displacer phase is measured against pressure).
The pulse tube expander preferably comprises a pulse tube inlet, and a pulse tube gas volume in gaseous communication with the pulse tube inlet. The pulse tube gas volume includes a second-stage regenerator, a pulse tube gas column, a flow restriction, and a surge tank. A second-stage heat exchanger is in thermal communication with the second-stage regenerator and the pulse tube gas column.
Thus, most preferably, a hybrid two-stage cryocooler comprises a first-stage Stirling expander comprising an expansion volume having an expander inlet, a first-stage regenerator, and an outlet, and a displacer which forces a working gas through the expander inlet and into the expansion volume. There is a thermal-energy storage device in thermal communication with the expansion volume of the first-stage Stirling expander. A second-stage pulse tube expander comprises a pulse tube inlet, a pulse tube gas volume in gaseous communication with the pulse tube inlet, the gas volume including a second-stage regenerator, a pulse tube gas column, a flow restriction, and a surge tank, and a second-stage heat exchanger in thermal communication with the second-stage regenerator and the pulse tube gas column. A gas flow path establishes gaseous communication between the outlet of the expansion volume of the Stirling expander and the pulse tube inlet, and a flow-through heat exchanger is disposed along the gas flow path between the output of the expansion volume of the Stirling expander and the pulse tube inlet.
This multistage cryocooler has the ability to allocate cooling power between the first-stage Stirling expander and the second-stage pulse tube expander by the manner of operation of the motor that drives the displacer of the first-stage Stirling expander. If an increased heat loading is sensed, the motor allocates increased cooling power to the second-stage pulse tube expander so that the component being cooled is retained within its temperature requirements. The cooling power to the first-stage Stirling expander is reduced, but the thermal-energy storage device temporarily absorbs that portion of the heat at the hot end of the second-stage pulse tube expander which cannot be removed by the first-stage Stirling expander operating with reduced cooling power. When the heat loading on the second-stage pulse tube expander returns back to more nearly steady-state levels, the cooling power is reallocated from the second-stage pulse tube expander to the first-stage Stirling expander, which removes the temporarily stored heat from the thermal-energy storage device to restore and prepare it for subsequent thermal loading peaks. Throughout these cycles, the power level consumed by the cryocooler remains approximately constant, although the cooling power is reallocated as necessary.
The present invention thus provides an advance over conventional cryocoolers. The cryocooler of the invention is sized to a steady-state heat loading requirement, and the thermal-energy storage device acts as a buffer. Significantly, the thermal-energy storage device stabilizes the cryocooler at the first-stage Stirling expander, while maintaining the temperature within operating limits at the heat load of the second-stage pulse tube expander. The thermal-energy storage device thus functions at a substantially higher temperature than the cooled component, but allows the temperature of the cooled component to remain approximately constant.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.
REFERENCES:
patent: 4711650 (1987-12-01), Faria et al.
patent: 5519999 (1996-05-01), Harpole
Kirkconnell Carl S.
Price Kenneth D.
Alkov Leonard A.
Capossela Ronald
Lenzen, Jr. Glenn H.
Raufer Colin M.
Raytheon Company
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