Cryogenic Refrigeration System With Dual Circuit Heat Exchanger

Abstract

Disclosed is a miniature cryogenic refrigeration system employing a dual circuit heat exchanger to effect rapid cooldown of the system. The heat exchanger is characterized in that the cooldown circuit is disposed within the run or steady state circuit with both circuits employing the same high pressure gas as the refrigerant.

Description

BACKGROUND OF THE INVENTION

This invention pertains to cryogenic refrigeration systems, most commonly referred to as cryostats, used in cryo-electronic systems such as cooling infra-red detectors and the like. These systems are useful in both fixed ground operations and in airborne detection systems. Such systems produce refrigeration by expansion of a gas through an orifice which is the well-known Joule-Thompson effect or cooling cycle.

A system of the type herein disclosed is shown in the U.S. Pat. No. 3,320,755 owned by the assignee of the present invention. The patentee's disclosed a method of overcoming a major problem with prior art cryostats, namely, achieving initial rapid cool-down of the system. This is accomplished by employing a valve so that initially the refrigerant flows rapidly through the orifice to provide a rapid cooldown of the system and then the flow is valved down to the lesser rate to maintain the required level of refrigeration. Although the patentees valving system achieves the required rapid cooldown it requires the cryostat to have a large number of movable parts which in turn makes assembly of such devices difficult.

SUMMARY OF THE INVENTION

In order to simplify assembly of cryostats and avoid the problems of slow initial cooldown of the system it has been discovered that when a dual circuit heat exchanger in the form of a tube within a tube is used in place of the single heat exchanger heretofore used it is possible to achieve initial rapid cool-down of the system followed by reduced flow of refrigerant to maintain the level of refrigeration without internal valving of the cryostat. It has been further discovered that extension of the cool-down circuit tube beyond where it is intimately associated with the run or steady-state circuit has the same effect as including a Joule-Thompson orifice in the end of the tube.

Therefore, it is the primary object of this invention to provide an improved cryogenic refrigeration system.

It is another object of this invention to provide an improved cryogenic refrigeration system capable of rapid initial cool-down.

It is still another object of this invention to provide an improved refrigeration system employing a dual circuit heat exchanger wherein each circuit of the heat exchanger uses the same refrigerant.

It is yet another object of this invention to provide a cryostat capable of initial rapid cool-down without the need for internal valving of the refrigerant.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a greatly enlarged cross-sectional view, taken along line 1--1 of FIG. 2, of the heat exchanger according to the present invention.

FIG. 2 is a view taken along line 2--2 of FIG. 1.

FIG. 3 is a cross sectional view of a cryostat according to the invention inserted in a vacuum test dewar.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing there is shown in FIG. 1 a cryostat heat exchanger shown generally as 10. The heat exchanger 10 consists of an outer tube 12 preferably of stainless steel. Disposed perpendicular to the axis of tube 12 are a plurality of thermal conduction devices 14 commonly referred to as fins. At the discharge end 16 of tube 12 there is a plug 18 wherein is disposed a nozzle 20. The nozzle 20 acts as a Joule-Thompson orifice so that the high pressure gas exiting from tube 12 produces refrigeration. At the entry end of tube 12 is a fitting 22 containing an inlet conduit 24 for tube 12 and an opening 13 for tube 26. Tube 26 is placed inside of tube 12 and projects at both ends through cap 18 and fitting 22 respectively. The end 28 projecting through fitting 22 is connected to a source of high pressure gas (not shown). The discharge end 30 projects beyond tube 12 sufficiently so that there is a pressure drop of the high pressure fluid in this portion of tube 26 and in effect the projection acts as a Joule-Thompson orifice. This projection of tube 26 obviates the necessity for a nozzle as is necessary for tube 12.

With the device of FIG. 1 and 2 refrigeration is achieved by connecting the entry end of conduit 24 and conduit 26 to a source of high pressure gas. Both conduits can be connected to the same source but must have individual flow control valves. Initially the gas is allowed to flow through both conduits but at a much greater rate through conduit 26. The greater flow rate of gas through conduit 26 causes a rapid cool-down of the system. Once the operating or steady-state temperature is reached the flow of gas through conduit 26 can be reduced significantly and in most cases stopped entirely. The flow of gas through conduit 12 then maintains the level of refrigeration in the system. Normally the refrigerant exiting from the end 30 of conduit 26 and nozzle 20 is caused to flow toward the entry end 28 of conduit 12 where it is collected and recycled. In this manner the fins 14 are cooled by the recirculating refrigerant.

In FIG. 3 there is shown a cryostat 10' according to the invention disposed in a glass test dewar 40. The cryostat 10' is disposed around a hollow mandrel 42. The mandrel 42 is sealed at the end 44 inserted in the dewar 40 and has disposed therein a thermocouple 46 of the copper-constantan type.

The cryostat 10' contains an inner tube 26' connected through a porous filter 48 in a one branch of tee fitting 50 to a source of high pressure gas (not shown) such as Nitrogen. The tube is fitted in the Tee 50 with a gas tight bushing 51 to prevent gas leakage. The outer tube 12' of cryostat 10' is connected through a tube connector 52 and bushing 53 to a second branch of Tee fitting 50 which communicates with a second supply path of high pressure gas. Disposed between the source of gas (not shown) and the entry end 28' of tube 12' is a porous filter 54. The Tee fitting 50 now has two available branches 56 and 58 for connection to a source of gas (not shown). The gas source can be the same for both connections, however, there must be suitable flow control means, e.g., a valve and flow meter, between each connection 56, 58 and the source of high pressure gas.

Enclosing the open end of dewar 40 is a cap 60 wherein is disposed a suitable sealing member 62 such as an O ring to assure a gas tight fit. The cap 60 holds the tube fitting 52, mandrel 42 and a return gas conduit 64 in fixed position and in a fluid tight arrangement with the dewar 40.

The fins 14' of cryostat 10' terminate about two-thirds of the length of the mandrel 42 projecting into the dewar 40. The nozzle 20' of tube 12' and the projecting end of tube 26' continue beyond the finned portion of the cryostat to the end 44 of mandrel 42 with tube 26' being disposed adjacent the mandrel 42 and nozzle 20' surrounding the projecting length of tube 26'. The discharge ends 30' and 64 of tube 26' and nozzle 20' respectively project into the space 66 defined by the bottom of the inner flask 68 of dewar 40 and the end of mandrel 42.

The voids 70,72 created by the wrapping of the tubes and finned tube against the walls of flask 68 can be filled with fiberglass string in order to force the returning refrigerant along the mandrel and into cap 60 and out through conduit 64 where it can be recycled.

In operation the device of FIG. 3 is connected to the source of high pressure gas (not shown) using branches 56 and 58 of Tee 50. Return conduit 64 can be put in the gas circuit or not as desired. The gas is adjusted so that a high flow rate is present in tube 26' and the normal steady state of refrigeration rate is flowing in conduit 12'. When the thermocouple 46 indicates operating temperature has been achieved the flow in conduit 26' can be reduced or stopped entirely. An inventory of liquefied gas will be formed and maintained in the space 66 defined by the bottom of flask 68 in dewar 40. As long as the flow of gas is maintained in tube 12' the level of refrigeration the system should be constant.

It is also possible to use argon, air, oxygen, or halogenated hydrocarbons containing one or more fluorine atoms in place of nitrogen as the refrigerant depending upon the level of temperature desired.

Claims

While I have described my invention in the foregoing specification it is defined in the following claims:

1. A cryostat refrigeration system comprising:

a mandrel with temperature sensing means disposed therein, said temperature sensing means having electrical conduit means extending outwardly of said mandrel for connection to a temperature indicating means;

a dual circuit heat exchanger disposed around said mandrel;

said heat exchanger comprising a first tube with a first end to be connected to a source of high pressure gas and a second end containing a Joule-Thompson orifice having a plurality of fins disposed thereon generally perpendicular to the axis of the first tube, a second tube smaller in diameter than said first tube and disposed within said first tube, said second tube having an inlet end connected to a source of high pressure gas and an exit end projecting beyond the end of said first tube containing the expansion orifice said projection being of a length to create a pressure drop of the high pressure gas in the projecting end of said second tube thereby achieving refrigeration;

means for introducing high pressure gas into each of said tubes;

whereby when operating said second tube promotes rapid cooldown of the system under an initial high rate of gas flow and after low temperature is indicated by the thermocouple the level of refrigeration can be maintained by throttling back the flow in the second tube substantially while maintaining a lesser flow in the first tube.

2. A refrigeration system according to claim 1 wherein the gas supplied to both tubes is the same and is selected from the group consisting of nitrogen, argon, air, oxygen, and halogenated hydrocarbons containing at least one fluorine atom.

3. A cryogenic refrigeration system comprising:

a coolant chamber having therein a reservoir for receiving a liquefied gaseous refrigerant;

disposed within said chamber a cryostat for supplying said refrigerant to said reservoir in said chamber;

said cryostat comprising a mandrel, disposed around said mandrel a first elongated finned tube with a first end connected to a source of high pressure gas and a second end containing a Joule-Thompson expansion orifice, a second elongated tube smaller in diameter than said first tube and disposed within said first tube, said second tube having an inlet end connected to a source of high pressure gas and an exit end projecting beyond the end of said first tube containing the expansion orifice said projection being of a length to create a pressure drop of the high pressure gas in the projecting end of said second tube to achieve refrigeration by expansion;

means for introducing high pressure gas into each of said tubes thus providing an initial flow of high pressure gas in said second conduit promotes rapid cooldown of the chamber and refrigeration thereof is maintained by a lesser flow of high pressure gas only in the first tube.