GB2127137A - Cryogenic cooling apparatus - Google Patents

Abstract

Cryogenic cooling apparatus includes a heat exchanger comprising a finned tube 11 around a tubular body 10 and within a tubular body 12 and affording first and second paths through the first of which refrigerant gas flows from a supply under pressure to a Joule Thomson expansion nozzle 29 whence the low pressure gas returns through the second path. The expansion nozzle is accommodated in a space defined by the side wall and an end wall of the container 12, which space also accommodates a porous body 42 in contact with the side wall and the said end wall of the container but exposing the central portion of the said end wall. <IMAGE>

Description

SPECIFICATION Cryogenic cooling apparatus The present invention relates to cryogenic cooling apparatus and is concerned with that type of such apparatus including a heat exchanger accommodated within a substantially tubular container and affording first and second paths through the first of which, in use, refrigerant glass flows from a supply under pressure to a Joule Thompson expansion nozzle whence the low pressure gas returns through the second path, the expansion nozzle being accommodated in a space defined by the side wall and an end wall of the container.

Coolers of this general type are disclosed in British Patent Specifications 1164276 and 1230079 and are used to produce liquefied gas which is then used to cool a load, such as an infra-red detector or a laser, which is usually placed in contact with the exterior of the container, down to a cryogenic temperature.

Such coolers frequently include a valve operated by a temperature sensitive control mechanism. In a typical construction this comprises a vapour bulb communicating with a larger volume of gas on the interior or exterior of a bellows. The sensor is generally provided on the side of the expansion nozzle remote from the heat exchanger and is intended to respond to the depth and thus quantity of liquid refrigerant in the container. Thus the volume of sensing gas has heat flowing into it from the warm end of the heat exchanger, i.e. the end remote from the expansion nozzle, and heat is extracted from it through the vapour bulb. Since the heat transfer coefficient of liquid refrigerant is far greater than that of gaseous refrigerant the rate of heat extraction through the vapour bulb is substantially proportional to the proportion of its surface in contact with liquid refrigerant.Thus as the level of liquid refrigerant rises in the container the rate of heat extraction through the vapor bulb from the volume of sensing gas increases which leads to a contraction and perhaps partial condensation of this gas and thus a movement of the bellows, which is coupled to the valve or its seating defining the expansion nozzle, to reduce the area of the nozzle and thus the rate of production of liquid refrigerant.

It is preferred that such cooling apparatus is used in its vertical position with the cold (nozzle) end at the bottom such that a pool of liquid forms in the container and the sensor responds to the depth of liquid in the container. However, in some applications it is necessary to use the apparatus in an orientation which is nearer to horizontal and at times even inverted. In these attitudes the gas and liquid droplets emerging from the nozzle will mainly be directed towards the load, but there may be little or no accumulated liquid in the container ajacent to the load and the sensor may respond to liquid droplets impinging on it, thus reducing the flow of refrigerant even though the load may be considerably above liquid temperature.

Cryogenic cooling apparatus of the type referred to above is also known without a sensor bulb, e.g.

from U.S. Patents Nos. 3269140 and 3320755. In these constructions control of the valve is effected by a temperature sensitive element, such as a bellows or bimetallic strip, which is situated "above" the nozzle and which is splashed or flooded by liquid refrigerant.

It has been proposed, for instance in British Patent Specifications Nos. 1330837 and 1311003 of the present applicants, that the problem of the valve being controlled to restrict the flow of refrigerant gas even though the load is in fact considerably above the desired operating temperature may be overcome by positioning a body of absorbent material against that part of the container, which, in use, is contacted by the load thereby preferentially holding the liquid refrigerant in intimate thermal contact with the load.

However, such constructions suffer from the disadvantage that the absorbent material is situated between the load and the liquid refrigerant issuing from the nozzle and thus decreases the rate at which the load cools down to its operating temperature.

For many applications it is important that the load be cooled down as rapidly as possible and it is thus an object of the invention to provide a cryogenic cooling apparatus which preferentially retains liquid refrigerant in thermal contact with the load whilst not slowing down the rate at which a load is cooled down to the working temperature.

According to the present invention cryogenic cooling apparatus of the type referred to above includes a porous body within the said space contacting the side wall and the said end wall of the container but exposing the central portion of the said end wall. The porous body is preferably of anular shape and it is preferred that its thermal conductivity and thermal mass are low. It may be of various constructions but in the preferred embodiment it is fibrous since this minimises thermal conductivity and mass. Alternatively the porous body may be of sintered metallic gauze.

Thus in the apparatus in accordance with the invention the central area is exposed to the fluid jet discharged from the expansion nozzle and can thus cool down rapidly. In the initial cool-down phase of the apparatus, that is to say when the fluid jet comprises only gas and liquid refrigerant has not yet started to form, the porous body will insulate that part of the container which it contacts which will therefore not cool down fully during this phase. The rate of heat extraction from the apparatus is thus reduced during the cool down phase which therefore increases the rate at which the load is cooled down to the desired working temperature. Once liquid refrigerant starts to be formed, it will wet the end wall of the container and be absorbed in the porous body.In steady-state operation the majority of the thermal load on the apparatus occurs by conduction through the container towards the load and the bulk of this is intercepted by liquid refrigerant held in the porous member which assists in the reduction of temperature gradients in the load once steady state operation has been reached. This is of particular importance. when the load comprises two or more elements, such as a multi-element infra-red detector.

In the preferred embodiment the container, which is preferably made of glass or metal, constitutes the innerwall of a Dewarvessel. In such a construction the load is connected to the exterior of the container and is thus situated between the inner and outer walls of the Dewarvessel.

In the most preferred construction the said space within the container also contains a valve cooperating with the nozzle and a sensor responsive to the amount of liquid refrigerant contacting it and arranged automatically to operate the valve to vary the effective area of the nozzle. Thus in this construction the rate of production of liquid refrigerant is varied in dependence on the amount of liquid refrigerant in the container and thus as this amount reaches a predetermined level the expansion orifice is progressively restricted thereby avoiding forming an amount of liquid refrigerant in excess of requirements and avoiding excess gas consumption.

Preferably the sensor constitutes a vapour bulb, the interior of which communicates with a vapour space, variations in the pressure of which act on a bellows automatically to vary the effective area of the expansion nozzle.

Preferably the major portion of the sensor is embedded in the porous body. This shields the sensor from direct impingement by droplets of liquid refrigerant issuing from the expansion nozzle and thus ensures that the nozzle is more truly responsive to the quantity of liquid refrigerant in the container rather than merely to the presence of liquid refrigerant droplets in the fluid stream issuing from the expansion nozzle.

Further features and details of the present invention will be apparent from the following description of one specific embodiment of a cryogenic cooling apparatus in accordance with the present invention which is given by way of example only with reference to the single accompanying drawing which is a diagrammatic side sectional elevation of the apparatus.

The cooling apparatus is of elongate tubular form and will be described for convenience in one particular orientation, that is to say with its axis vertical and its cold end, i.e. the end at which the expansion nozzle is situated, at the bottom. It will however be appreciated that the apparatus may be used in orientations other than that illustrated and indeed the advantages of the present invention are most necessary when the cold end of the apparatus is level with or higher than the warm end.

The apparatus includes a tubular heat exchanger comprising an inner tubular body 10 around which is helically wound a finned inlet bue 11 forming the inlet path for the working fluid of the heat exchanger.

An external coaxial tube 12, which in this embodiment is the inner wall of a Dewar flask 13, is located around the finned coil 11 and the space between the inner body 10 and the external tube 12 provides the second or exhaust path of the heat exchanger for exhaust gas flowing past the fins to cool the incoming high pressure refrigerant within the helical cooled tube 11 forming the inlet path. The lower end of the external tube 12 is closed by a horizontal wall 40 thus forming a container for the heat exchanger, the lower end of which constitutes a reservoir in which liquid refrigerant can accumulate. The upper end of the helical finned tube 11 communicates with a central bore (not shown) in the upper end of the apparatus to which, in use, working fluid under pressure is supplied at a temperature below its inversion temperature.

At its lower end the inner tubular body 10 has welded to zit a reinforcing ring 16 having a sensor 17 in the form of a vapour tube or bulb projecting substantially parallei to the axis of the heat exchanger, and having a threaded stud 20 mounting a seating member 24.

The seating member 24 comprises a disc 25, one face of which has projecting eccentrically from it a part-circular boss 26 from which in turn a smaller circular boss 27 projects even further. The disc has in it a hole which receives the threaded stud 20 and is held in place by a nut 21. The small circular boss 27 projects coaxially up into the cold end of the heat exchanger, whilst the part-circular boss 26 is also coaxial with the heat exchanger and fits snugly into the reinforcing ring 16. The small circular boss 27 of the seating member 24 has a coaxial bore 28 extending through it from its upper end to a valve orifice 29 opening through its lower end, and a transverse bore 30 which opens into the axial bore 28 and contains a filter 31, and of which the outer end is closed by a screw plug (not shown).A further transverse bore (not shown) opens into this transverse bore 30 and the lower end of the helical heat exchanger tube 11 is sealed into this last transverse bore. The upper end of the axial bore 28 is closed.

The effective area of the expansion nozzle 29 is arranged to be controlled by means of a needle valve 34 which is itself controlled by a bellows 35.

The bellows 35 has its lower open end secured to the reinforcing ring 16 whilst its movable closed upper end is secured to the upper end of a depending actuating tube 36. This extends down beyond the seating member 24 and half the circumference of its lower portion is cutaway whilst the remaining half receives and is secured to and reinforced by a tubular valve carrier 37. The valve carrier is also cut away for half its circumference except at its outer end portion. Thus the seating member 24 projects into the open half of the actuating tube 36 and the valve carrier 37 from the side, and the small cylindrical boss 27 of the seating member 24 projects up into and fits in the upper end of the valve carrier to guide it. The lower end of the valve carrier carries the needle valve 34 which has a lower cylindrical portion and an upper tapered portion projecting into the expansion orifice 29 of the seating member 24.

As referred to above, the reinforcing ring 16 carries a sensor 17 and this is in the form of a metal tube 18 sealed in a hole extending through the reinforcing ring 16 parallel to the axis of the heat exchanger and having its lower end portion squashed flat to form an extended heat conducting tail 19. The sensor tube 17 and the space outside the bellows 35 inside the tubular body 10 are filled with a gas which is condensible at the operating temperature under pressure. The gas may be e.g. air or nitrogen and in the controlled phase of operation liquid is in equilibrium with vapour.

Situated within the lower end of the container 12 is a porous annular body 42 made of fibrous material, e.g. a dense felt. This body contacts the inner surface of the tubular wall 12 up to a point slightly below the expansion nozzle 29 and also contacts the outer peripheral edge of the lower container wall 40. The central portion of the wall 40 over the area to which the load 14 is connected is exposed and can thus be contacted directly by droplets of liquid refrigerant issuing from the expansion nozzle. The sensor tube 17 is accommodated in an axial hole in the porous body 42 and is thus protected from direct impingement by liquid refrigerant issuing from the expansion nozzle.

In use, a gas such as nitrogen or air below its inversion temperature is supplied to the heat exchangerfrom a pressurised supply (not shown) and flows through the first heat exchanger path, is expanded through the nozzle 29 and then is ex hausted through the second heat exchanger path cooling the incoming gas as it does so. As operation proceeds, the gas issuing through the nozzle 29 progressively approaches its liquefaction temperature and impinges directly on the bottom wall 40 of the container 12 thereby cooling it rapidly. The side wall of the container 12 is insulated by the porous body 42 and this reduces the thermal drain from the apparatus and thus increases the rate at which the load is cooled down.As the temperature of the gas issuing through the expansion nozzle reaches its liquefaction temperature droplets of liquid refrigerant are formed and these impinge directly on the bottom wall 40 of the container thereby cooling the load very rapidly indeed. A quantity of liquid refrigerant gradually forms in the container and this is preferentially held in contact with the side wall 12 by virtue of the capillary effect of the porous body 42.

As the liquid refigerant wets an increasing proportion of the volume of the porous body, an increasing proportion of the sensor tube 17 is wetted by liquid refrigerant thereby increasing the rate of heat extraction from the sensor gas volume surrounding the bellow 35. This leads to a pressure reduction and perhaps partial liquefaction of the sensor gas which results in the bellows moving the needle valve upwards thereby progressively throttling the expansion nozzle and thus reducing the rate of production of liquid refrigerant. Heat conducted down the container 12 is intercepted by the liquid refrigerant retained in contact with its inner surface by the porous body 42 thus reducing temperature differentials across the load 14.

CLAIMS (Filed on 15Sept.83) 1. Cryogenic cooling apparatus including a heat exchanger accommodated within a substantially tubular container and affording first and second paths through the first of which, in use, a refrigerant gas flows from a supply under pressure to a Joule Thomson expansion nozzle whence the low pressure gas returns through the second path, the expansion nozzle being accommodated in a space defined by the side wall and an end wall of the container, the said space also accommodating a porous body in contact with the side wall and the said end wall of the container but exposing the central portion of the said end wall.

2. Apparatus as claimed in Claim 1 in which the porous body is of annular shape.

3. Apparatus as claimed in Claim 1 or Claim 2 in which a thermal load is connected to the external surface of the said end wall of the container.

4. Apparatus as claimed in any one of the preceding claims in which the container constitutes the innerwall of a Dewarvessel.

5. Apparatus as claimed in any one of the preceding claims, including means for varying the effective area of the nozzle when a predetermined amount of liquid is present in the said space.

6. Apparatus as claimed in Claim 5 in which the said space contains a valve co-operating with the expansion nozzle and a sensor responsive to the amount of liquid refrigerant contacting it and arranged automatically to operate the valve to vary the effective area of the nozzle.

7. Apparatus as claimed in Claim 6 in which the sensor comprises a vapour bulb, the interior of which communicates with a sensing vapour space, variations in the volume of which acts on a bellows automatically to vary the effective area of the expansion nozzle.

8. Apparatus as claimed in Claim 6 or Claim 7 in which the major portion of the sensor is embedded in the porous body.

9. Cryogenic cooling apparatus substantially as specifically herein described with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. Situated within the lower end of the container 12 is a porous annular body 42 made of fibrous material, e.g. a dense felt. This body contacts the inner surface of the tubular wall 12 up to a point slightly below the expansion nozzle 29 and also contacts the outer peripheral edge of the lower container wall 40. The central portion of the wall 40 over the area to which the load 14 is connected is exposed and can thus be contacted directly by droplets of liquid refrigerant issuing from the expansion nozzle. The sensor tube 17 is accommodated in an axial hole in the porous body 42 and is thus protected from direct impingement by liquid refrigerant issuing from the expansion nozzle. In use, a gas such as nitrogen or air below its inversion temperature is supplied to the heat exchangerfrom a pressurised supply (not shown) and flows through the first heat exchanger path, is expanded through the nozzle 29 and then is ex hausted through the second heat exchanger path cooling the incoming gas as it does so. As operation proceeds, the gas issuing through the nozzle 29 progressively approaches its liquefaction temperature and impinges directly on the bottom wall 40 of the container 12 thereby cooling it rapidly. The side wall of the container 12 is insulated by the porous body 42 and this reduces the thermal drain from the apparatus and thus increases the rate at which the load is cooled down.As the temperature of the gas issuing through the expansion nozzle reaches its liquefaction temperature droplets of liquid refrigerant are formed and these impinge directly on the bottom wall 40 of the container thereby cooling the load very rapidly indeed. A quantity of liquid refrigerant gradually forms in the container and this is preferentially held in contact with the side wall 12 by virtue of the capillary effect of the porous body 42. As the liquid refigerant wets an increasing proportion of the volume of the porous body, an increasing proportion of the sensor tube 17 is wetted by liquid refrigerant thereby increasing the rate of heat extraction from the sensor gas volume surrounding the bellow 35. This leads to a pressure reduction and perhaps partial liquefaction of the sensor gas which results in the bellows moving the needle valve upwards thereby progressively throttling the expansion nozzle and thus reducing the rate of production of liquid refrigerant. Heat conducted down the container 12 is intercepted by the liquid refrigerant retained in contact with its inner surface by the porous body 42 thus reducing temperature differentials across the load 14. CLAIMS (Filed on 15Sept.83)

1. Cryogenic cooling apparatus including a heat exchanger accommodated within a substantially tubular container and affording first and second paths through the first of which, in use, a refrigerant gas flows from a supply under pressure to a Joule Thomson expansion nozzle whence the low pressure gas returns through the second path, the expansion nozzle being accommodated in a space defined by the side wall and an end wall of the container, the said space also accommodating a porous body in contact with the side wall and the said end wall of the container but exposing the central portion of the said end wall.

2. Apparatus as claimed in Claim 1 in which the porous body is of annular shape.

3. Apparatus as claimed in Claim 1 or Claim 2 in which a thermal load is connected to the external surface of the said end wall of the container.

4. Apparatus as claimed in any one of the preceding claims in which the container constitutes the innerwall of a Dewarvessel.

5. Apparatus as claimed in any one of the preceding claims, including means for varying the effective area of the nozzle when a predetermined amount of liquid is present in the said space.

6. Apparatus as claimed in Claim 5 in which the said space contains a valve co-operating with the expansion nozzle and a sensor responsive to the amount of liquid refrigerant contacting it and arranged automatically to operate the valve to vary the effective area of the nozzle.

7. Apparatus as claimed in Claim 6 in which the sensor comprises a vapour bulb, the interior of which communicates with a sensing vapour space, variations in the volume of which acts on a bellows automatically to vary the effective area of the expansion nozzle.

8. Apparatus as claimed in Claim 6 or Claim 7 in which the major portion of the sensor is embedded in the porous body.

9. Cryogenic cooling apparatus substantially as specifically herein described with reference to the accompanying drawing.