Title:
Variable pneumatic volume for cryogenic coolers
United States Patent 3906739
Abstract:
Free displacer cryogenic cooler having a free displacer regenerator posited in a housing having a cooling end. The displacer having an opposite end in communication with a pneumatic volume; the pneumatic volume being adjustable to optimize this volume and achieve maximum cooling capability.
US Patent References:
Pulse tube method of refrigeration and apparatus therefor
Gifford - March 1966 - 3237421
HOT-GAS RECIPROCATING ENGINE
Kohler - October 1969 - 3472037
DEVICE FOR CONVERTING MECHANICAL ENERGY INTO HEAT ENERGY OR CONVERSELY
Prast - January 1970 - 3487635
STIRLING CYCLE-TYPE THERMAL DEVICE SERVO PUMP
Beale - February 1972 - 3645649
PULSE TUBE REFRIGERATOR
Daniels - June 1974 - 3817044
Pulse tube method of refrigeration and apparatus therefor
Gifford - March 1966 - 3237421
HOT-GAS RECIPROCATING ENGINE
Kohler - October 1969 - 3472037
DEVICE FOR CONVERTING MECHANICAL ENERGY INTO HEAT ENERGY OR CONVERSELY
Prast - January 1970 - 3487635
STIRLING CYCLE-TYPE THERMAL DEVICE SERVO PUMP
Beale - February 1972 - 3645649
PULSE TUBE REFRIGERATOR
Daniels - June 1974 - 3817044
Application Number:
05/500837
Publication Date:
09/23/1975
Filing Date:
08/26/1974
View Patent Images:
Export Citation:
Assignee:
The United States of America as represented by the Secretary of the Army (Washington, DC)
Primary Class:
Other Classes:
62/86
International Classes:
F25B9/14; F25B9/00
Field of Search:
62/6,86
Primary Examiner:
Wye, William J.
Attorney, Agent or Firm:
Lee, Milton Edelberg Nathan Gibson Robert W. P.
Claims:
What is claimed is
1. A free displacer cryogenic cooler including a displacer housing having a cold head end for contacting an object to be cooled and in which a free displacer is positioned to reciprocate in response to pressure pulses provided by a remote compressor connected by conduit means to a region of said displacer housing sealed from said cold head end by sealing means and from the end of said displacer opposite the end proximate said cold head end by further sealing means, and further housing means joined in a sealed manner to said displacer housing forming a pneumatic volume proximate the opposite end of said displacer, wherein the improvement comprises: means associated with said pneumatic volume for variably adjusting and maintaining the size of said pneumatic volume so that as the free displacer is reciprocated by said pressure pulses, the cooling at the cold head end of the cooler can be maximized by simultaneously adjusting the pneumatic volume.
2. The device according to claim 1 wherein the means for variably adjusting the pneumatic volume comprise:
3. The device according to claim 2 wherein said drive means is a micrometer shaft and wherein calibration on said shaft provide a measure of the variation of the pneumatic volume as the micrometer shaft is turned to adjust the position of said piston means.
4. The device according to claim 1 wherein the means for variably adjusting the size of the pneumatic volume comprises:
5. The device according to claim 1 wherein the pneumatic volume comprises a deformable structure and wherein the means for adjusting the size of said pneumatic volume comprise vise means clamping said structure, said vise means having jaws and means for varying the spacing therebetween, whereby the volume of said pneumatic volume is varied by adjusting said jaws.
1. A free displacer cryogenic cooler including a displacer housing having a cold head end for contacting an object to be cooled and in which a free displacer is positioned to reciprocate in response to pressure pulses provided by a remote compressor connected by conduit means to a region of said displacer housing sealed from said cold head end by sealing means and from the end of said displacer opposite the end proximate said cold head end by further sealing means, and further housing means joined in a sealed manner to said displacer housing forming a pneumatic volume proximate the opposite end of said displacer, wherein the improvement comprises: means associated with said pneumatic volume for variably adjusting and maintaining the size of said pneumatic volume so that as the free displacer is reciprocated by said pressure pulses, the cooling at the cold head end of the cooler can be maximized by simultaneously adjusting the pneumatic volume.
2. The device according to claim 1 wherein the means for variably adjusting the pneumatic volume comprise:
3. The device according to claim 2 wherein said drive means is a micrometer shaft and wherein calibration on said shaft provide a measure of the variation of the pneumatic volume as the micrometer shaft is turned to adjust the position of said piston means.
4. The device according to claim 1 wherein the means for variably adjusting the size of the pneumatic volume comprises:
5. The device according to claim 1 wherein the pneumatic volume comprises a deformable structure and wherein the means for adjusting the size of said pneumatic volume comprise vise means clamping said structure, said vise means having jaws and means for varying the spacing therebetween, whereby the volume of said pneumatic volume is varied by adjusting said jaws.
Description:
BACKGROUND
Presently, infrared detectors operate in three major temperature ranges; 195°K, 170°K to 145°K and 77°K. The detectors in the 195°K and 170°K to 145°K ranges are cooled by thermoelectric coolers while those at 77°K must be cooled by liquid nitrogen. Nitrogen under high pressure through a Joule-Thompson device can be used to cool these detectors, but this involves the ever-present danger of high pressure bottles. Vulleumier coolers or Split-Sterling devices have so far, proved infeasible due to high input power requirements and physical size.
The use of a free displacer cooler has proven effective. In such a free displacer cooler, the displacer is the entire moving assembly while the regenerator is a heat exchanger located within the body of the displacer. The cooler functions in the following manner. As a pressure wave (generated by a compressor and applied to the cooler) reaches a peak the displacer moves upward aided by the pressure in the pneumatic volume. Conversely, as the pressure wave reaches a minimum the displacer returns downward but is hindered by the pressure in the pneumatic space. Therefore this pneumatic volume acts as a controlling force influencing the motion of the displacer.
It has been found though, that the size of the pneumatic volume strongly influences the temperature that can be achieved with such a cooler.
Present practice is to have this pneumatic volume machined separately and to bolt it to the cooler. Any changes made to the volume must be accomplished by making a new part. Another approach is to make a series of small pneumatic tanks and interchange these by trial and error to achieve the optimum volume needed. Attempts to determine the exact pneumatic volume theoretically, as by computer analysis, have not been satisfactory and other solutions to the problem have been sought.
SUMMARY
The solution to the above stated problem, which is the basis of the disclosed invention, involves making the pneumatic volume adjustable. By varying the volume, the precise value that yields the lowest temperature at the cold head of the cooler can be determined.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a shows the invention employing a micrometer measuring device for accurately varying and measuring the pneumatic volume;
FIG. 1b shows the cooler of FIG. 1a with a bolt-on end cap in place of the micrometer device;
FIG. 2 discloses a pneumatic volume control employing a screw-like device for varying the pneumatic volume;
FIG. 3 presents the cooler employing a deformable penumatic volume for volume control.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
By referring to FIG. 1a, the technique for implementing the disclosed invention can be readily understood. The cooler 10 consists of a displacer housing 11 in which the displacer 12 is positioned for reciprocating motion. Seals 13 and 14 isolate the void volume 15 from the cold end volume 16 having a cold head 17 and from the pneumatic volume 18. A further housing 19 forming the variable volume of the pneumatic volume 18 is joined in a gas-tight relation to the displacer housing 11. A piston-like member 20 is located therein and affixed to a shaft 21. This shaft is the arm of a micrometer drive 22. Sealing rings 14 and 23 seal the pneumatic volume. A line 24 carries the driving pressure pulses from a compressor (not shown) to the cooler 10.
In operation, the compressor is actuated thereby sending pressure pulses to the cooler and causing the displacer 12 to reciprocate. The position of the piston-like member 20 is then varied by means of the micrometer drive 22 while the temperature changes resulting from the changes in the pneumatic volume 18 are observed. The micrometer reading at the lowest temperature achieved is then used to calculate the optimum pneumatic volume (the cylinder area of the variable volume being a known quantity). Of course, the micrometer drive itself could be calibrated in terms of a volume scale. Having determined the optimum volume, it only remains to provide an end member cap 25, as shown in FIG. 1b, in place of the housing 19; where the end member is of the desired volume.
In FIG. 2 the cooler device 10 is constructed with a pneumatic volume 26 already a part of the housing. A screw-like element 27 is provided which projects into the volume 26. The volume 26 is purposely made slightly larger than needed. The screw element 27, when adjusted inwardly, yields the optimum pneumatic volume and is then fixed in that position as by a sealing material.
In FIG. 3, a pneumatic volume tank 30 is positioned remotely from the displacer housing 11 and is made deformable. The tank volume is varied by applying a force to deform the tank. A precision vice 31, having soft jaw plates 32 and 33, is shown by way of example to indicate one mode of accomplishing the type of control of the pneumatic volume. By adjusting the threaded member 34, the volume is varied.
While several embodiments of the contemplated invention have been described it is to be understood that other variations, substitutions and alterations may be made while remaining within the spirit and scope of the invention which is limited only by the following claims.
Presently, infrared detectors operate in three major temperature ranges; 195°K, 170°K to 145°K and 77°K. The detectors in the 195°K and 170°K to 145°K ranges are cooled by thermoelectric coolers while those at 77°K must be cooled by liquid nitrogen. Nitrogen under high pressure through a Joule-Thompson device can be used to cool these detectors, but this involves the ever-present danger of high pressure bottles. Vulleumier coolers or Split-Sterling devices have so far, proved infeasible due to high input power requirements and physical size.
The use of a free displacer cooler has proven effective. In such a free displacer cooler, the displacer is the entire moving assembly while the regenerator is a heat exchanger located within the body of the displacer. The cooler functions in the following manner. As a pressure wave (generated by a compressor and applied to the cooler) reaches a peak the displacer moves upward aided by the pressure in the pneumatic volume. Conversely, as the pressure wave reaches a minimum the displacer returns downward but is hindered by the pressure in the pneumatic space. Therefore this pneumatic volume acts as a controlling force influencing the motion of the displacer.
It has been found though, that the size of the pneumatic volume strongly influences the temperature that can be achieved with such a cooler.
Present practice is to have this pneumatic volume machined separately and to bolt it to the cooler. Any changes made to the volume must be accomplished by making a new part. Another approach is to make a series of small pneumatic tanks and interchange these by trial and error to achieve the optimum volume needed. Attempts to determine the exact pneumatic volume theoretically, as by computer analysis, have not been satisfactory and other solutions to the problem have been sought.
SUMMARY
The solution to the above stated problem, which is the basis of the disclosed invention, involves making the pneumatic volume adjustable. By varying the volume, the precise value that yields the lowest temperature at the cold head of the cooler can be determined.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a shows the invention employing a micrometer measuring device for accurately varying and measuring the pneumatic volume;
FIG. 1b shows the cooler of FIG. 1a with a bolt-on end cap in place of the micrometer device;
FIG. 2 discloses a pneumatic volume control employing a screw-like device for varying the pneumatic volume;
FIG. 3 presents the cooler employing a deformable penumatic volume for volume control.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
By referring to FIG. 1a, the technique for implementing the disclosed invention can be readily understood. The cooler 10 consists of a displacer housing 11 in which the displacer 12 is positioned for reciprocating motion. Seals 13 and 14 isolate the void volume 15 from the cold end volume 16 having a cold head 17 and from the pneumatic volume 18. A further housing 19 forming the variable volume of the pneumatic volume 18 is joined in a gas-tight relation to the displacer housing 11. A piston-like member 20 is located therein and affixed to a shaft 21. This shaft is the arm of a micrometer drive 22. Sealing rings 14 and 23 seal the pneumatic volume. A line 24 carries the driving pressure pulses from a compressor (not shown) to the cooler 10.
In operation, the compressor is actuated thereby sending pressure pulses to the cooler and causing the displacer 12 to reciprocate. The position of the piston-like member 20 is then varied by means of the micrometer drive 22 while the temperature changes resulting from the changes in the pneumatic volume 18 are observed. The micrometer reading at the lowest temperature achieved is then used to calculate the optimum pneumatic volume (the cylinder area of the variable volume being a known quantity). Of course, the micrometer drive itself could be calibrated in terms of a volume scale. Having determined the optimum volume, it only remains to provide an end member cap 25, as shown in FIG. 1b, in place of the housing 19; where the end member is of the desired volume.
In FIG. 2 the cooler device 10 is constructed with a pneumatic volume 26 already a part of the housing. A screw-like element 27 is provided which projects into the volume 26. The volume 26 is purposely made slightly larger than needed. The screw element 27, when adjusted inwardly, yields the optimum pneumatic volume and is then fixed in that position as by a sealing material.
In FIG. 3, a pneumatic volume tank 30 is positioned remotely from the displacer housing 11 and is made deformable. The tank volume is varied by applying a force to deform the tank. A precision vice 31, having soft jaw plates 32 and 33, is shown by way of example to indicate one mode of accomplishing the type of control of the pneumatic volume. By adjusting the threaded member 34, the volume is varied.
While several embodiments of the contemplated invention have been described it is to be understood that other variations, substitutions and alterations may be made while remaining within the spirit and scope of the invention which is limited only by the following claims.