GB2193799A

GB2193799A - Cryogenic cooling apparatus

Info

Publication number: GB2193799A
Application number: GB08619535A
Authority: GB
Inventors: David Neil Campbell; Michael Julian Richardson
Original assignee: Hymatic Engineering Co Ltd
Current assignee: Hymatic Engineering Co Ltd
Priority date: 1986-08-11
Filing date: 1986-08-11
Publication date: 1988-02-17
Also published as: GB8619535D0

Abstract

Cryogenic cooling apparatus includes a heat exchanger affording an inlet path defined by a finned tube 11 and communicating with the inlet of a Joule-Thomson expansion nozzle 16 and an outlet path which communicates with the outlet of the expansion nozzle and is in heat exchange relationship with the inlet path. A valve needle 18 cooperates with the orifice 17 of the nozzle 16 and together with the latter constitutes a valve which is, in use, urged towards the open position by the pressure of refrigerant gas on the inlet side of the nozzle. Biasing means which may be constituted by a coil spring 21 urges the valve towards the closed position. In use, the valve does not open until the pressure of the refrigerant gas exceeds a predetermined value. <IMAGE>

Description

SPECIFICATION Cryogenic cooling apparatus The present invention relates to cryogenic cooling apparatus and is concerned with that type of apparatus which includes a countercurrent heat exchanger having two pathways of which one communicates with a Joule Thomson expansion nozzle which in turn -communicates with the other pathway which is in heat exchange relationship with the first pathway.

Such cooling apparatus is frequently of socalled self-regulating type, e.g. as disclosed in our British Patent Specification No.2085139, in which the expansion nozzle is regulated by a valve member which is moved in response to the presence of liquefied refrigerant on a sensor bulb positioned in the space into which fluid flowing through the expansion nozzle passes. The sensor bulb is generally connected to a beliows or the like arranged to move the valve member and as an increasing proportion of the surface of the sensor bulb is contacted by liquid refrigerant the gas within it and the bellows contracts thereby progressively closing the valve and reducing the rate of production of liquid refrigerant.If such a cooler is subjected to high rates of acceleration, e.g. in airborne applications, the acceleration forces may direct the liquid refrigerant away from the load to be cooled and towards the sensor. The sensor will thus act to reduce the flow of refrigerant through the orifice despite the fact that the load may be inadequately cooled. This problem could in theory be overcome by employing a fixed orifice cooler, that is to say a cooler without a regulating valve, since such a cooler inherently produces a continuous supply of liquefied refrigerant which can be arranged to overcome the acceleration forces to which the cooler is subjected and thereby continuously extract heat from the load.However, if the orifice is a relatively large one in order to produce a satisfactorily short cool-down time in conjuntion with a bottled gas source the rate of refrigerant consumption may be unacceptably high and the temperature of the load rises somewhat above that which is expected due to the fact that the pressure downstream of the expansion nozzle rises which results in an increase in the liquefaction temperature of the refrigerant. If on the other hand the gas is provided by a compressor of limited capacity then the increased fluid density at the expansion nozzle results in the cooler operating at a lower pressure than that intended and thus rather inefficiently.If the orifice is made smaller so that the rate of production of liquid refrigerant is substantially equal to the anticipated steady state cooling load, the cooldown time may be unacceptably high and the orifice size tends to be such that it is liable to blockage by contaminant particles.

It is an object of the present invention to provide a cryogenic cooling apparatus which ensures continuous production of liquid refrigerant but which nevertheless operates at or close to its optimum condition and is not -liable to blockage by contaminant particles.

According to the present invention cryogenic cooling apparatus comprises an inler path communicating with the inlet of a Joule Thomson expansion nozzle, an outlet path communicating with the outlet of the expansion nozzle, the inlet path being arranged in heat exchange relationship with the outlet path, a valve member cooperating with the outlet of the expansion nozzle and together with the latter constituting a valve which, in use, is urged towards the open position by the pressure of refrigerant gas on the inlet side of the nozzle and biasing means urging the valve towards the closed position, whereby the valve does not open until the said pressure exceeds a predetermined value.

Thus the cooling apparatus of the present invention is not provided with a liquid refrigerant regulated valve but is instead provided with what may be considered to constitute a simple pressure-sensitive valve associated with a spring or the like which biases it into the closed position. This valve acts as a pressure maintaining valve. Thus when no refrigerant gas is flowing through it the valve is normally substantially closed and opens only when the force acting on it by virtue of the gas pressure exceeds the closing force exerted by the biasing means. This is arranged to occur at a gas pressure which is sufficient to provide a substantial Joule Thomson cooling effect, e.g.

at least 50 and more preferably 100 bar. The valve may be arranged to have a snap action whereby the valve fully opens abruptly when the gas pressure reaches a predetermined value but it is preferred that the valve opens progressively after the predetermined gas pressure has been reached as the gas pressure increases. In use, the cooling apparatus in accordance with the present invention will thus continuously produce liquefied refrigerant at a rate which is determined solely by the valve characteristics and the refrigerant mass flow rate and the biasing means can be set to ensure that the cooler operates at a viable Joule Thomson coefficient.

The nozzle may be resiliently mounted and the valve member fixed or alternatively the nozzle may be fixed and the valve member movably mounted and acted on by a spring.

Alternatively both the nozzle and the valve member may have a certain degree of movability.

As mentioned above, the gas pressure upstream of the nozzle will be determined solely by the mass flow rate and in the steady state the rate of gas flow will remain constant. It is of course essential in practice that the rate of gas flow is sufficient to maintain the load at the desired temperature and to ensure that this is the case the gas flow will in general be somewhat more than that theoretically required. The amount of refrigerant flow required to maintain the load at the desired temperature may vary with conditions, in particular ambient temperature, and thus if the gas flow is to remain constant it will sometimes have to be substantially in excess of requirements to ensure that it is never less than is required.This may be unacceptably uneconomical for certain applications and thus in a further embodiment of the invention in which the load is present and the inlet path is connected to means for supplying a refrigerant gas under pressure there are means for sensing the temperature at or adjacent the thermal load and control means responsive thereto arranged to adjust the rate of supply of refrigerant to maintain the temperature of the thermal load substantially constant.Alternatively, means may be provided for measuring or deriving a parameter indicative of the rate of thermal transfer to the load and control means responsive thereto arranged to adjust the rate of supply of refrigerant to maintain the rate of heat extraction from the load by liquefied refrigerant substantially equal to the said rate of thermal transfer to the load thereby maintaining the temperature of the load substantially constant. In the simplest form of this alternative the rate of refrigerant supply is varied in dependence on ambient temperature outside the cooler since it will be appreciated that as this rises the heat which must be extracted from the load to maintain it at a constant temperature rises also.Alternatively, the apparatus may include means for producing a temporary variation, that is to say an increase or a decrease, in the rate of supply of refrigerant and means for producing a signal indicative of the resulting state of change of temperature of the load which is then applied to the control means to vary the rate of refrigerant supply. It will be appreciated that the cooler may be connected in open circuit to either a compressor or to a pressurized gas reservoir or in closed circuit to a compressor.

In a still further embodiment in which the cooler is connected in closed circuit with a compressor, pressure sensor means are provided at the inlet side of the compressor connected to control the speed of the compressor so as to maintain the pressure at the inlet side of the compressor substantially constant.

Such an arrangement has an inherent degree of self compensation for variations in ambient temperature since maintaining the pressure constant at the inlet of the compressor means that the pressure at its outlet is a function of temperature. Thus as the temperature rises the rate of flow of refrigerant gas rises also and thus produces an increased cooling effect which counteracts the increased rate of thermal flow into the load from the exterior. if the cooler should flood with liquefied refrigerant the pressure at the outlet side of the compressor will drop thereby reducing the refrigerant flow.

Further features and details of the invention will be apparent from the following description of one specific example which is given with reference to the accompanying drawings, in which: Figure 1 is a longitudinal sectional view of a Joule Thomson cooler in accordance with the present invention; Figure 2 is a graph showing temperature against the mass flow rate of refrigerant in such a cooler; Figure 3 is a schematic representation of one method of controlling the rate of refrigerant flow through the cooler of Figure 1; and Figure 4 is a schematic representation of a second method of controlling the rate of refrigerant flow through the cooler.

For convenience of description the cooler is described herein as being in a vertical position and with a thermal load at its lower end but it will be appreciated that it will operate satisfactorily in other orientations.

Referring firstly to Figure 1, the cooler includes a heat exchanger comprising an inner tubular body 10 helically wound around which is a finned inlet tube 11 constituting the inlet path of the heat exchanger. An external coaxial tube 12 constituted by the inner wall of a Dewar vessel having an outer wall 13 extends around the inlet tube 11. The space between the inner body 11 and the external tube 12 constitutes the second or exhaust path of the heat exchanger through which exhaust gas flows to cool the incoming refrigerant gas within the tube 11.

The lower end of the tube 11 carries no fins and extends beyond the tubular body 10 and terminates with a Joule Thomson expansion nozzle 16 affording an orifice 17. A needle 18, whose tip cooperates with the orifice 17 is mounted on a sleeve 20 which is connected to a downwardly convergent frustoconical portion 9 at the end of a rod 19 within the tubular body 10. At the remote end of the rod 19 is a radially extending flange 22 which is engaged by one end of a coil spring 21 which extends around the rod 19 and whose other end engages an annular shoulder 24 connected to the interior of the tubular body 10. The spring 21 thus acts to urge the needle 18 into the orifice 17, that is to say, it urges the needle valve constituted by the nozzle 16 and needle 18 into the closed position.

When the needle valve is in the open position shown in Figure 1 the frusto-conical portion 9 engages a circular shoulder 26 and thus prevents the needle 18 from vibrating. As the valve is progressively closed the portion 9 moves progressively further away from the shoulder 26 but vibration of the needle 18 is restrained by virtue of the fact that the tip of the needle is within the orifice 17.

In use, the upper end of the helical tube 11 communicates with a coupling at the upper end of the heat exchanger to which a gaseous refrigerant gas, such as nitrogen, air or argon under pressure is supplied with a temperature below its inversion temperature. The refrigerant gas supply may constitute a pressurized gas bottle or a compressor and in the latter case the compressor may be on open circuit or more preferably in closed circuit with the heat exchanger. A thermal load 15 to be cooled, such as the active element of an infrared detector, is secured to the outer surface of the inner wall 12 of the Dewar vessel below the expansion nozzle 16.When not in operation the needle valve is closed under the action of the spring 21 and when the refrigerant gas supply is switched on the pressure within the nozzle 16 rises until the force it exerts on the needle 18 overcomes the biasing force exerted by the spring 21 whereupon the needle valve progressively opens to a position at which the forces exerted on the needle 18 by the spring 21 and the gas flowing through the orifice are equal. The gas flowing through the orifice 17 is cooled by the Joule Thomson effect and then flows out into a cooling space 14 below the nozzle 16 and then through the space defined by the tubular body 10 and the wall 12 thereby cooling the inflowing gas. The temperature of the gas flowing through the orifice 17 thus rapidly drops until a proportion of it is liquefied. The liquefied refrigerant cools the load 15 by thermal transfer through the wall 12.If the pressure of the gas supply should rise or fali the needle valve will open or close automatically to maintain the expansion of the gas at substantially the optimum condition. The production of liquefied refrigerant continues without interruption regardless of acceleration forces to which the cooler may be subjected and the load is thus reliably cooled. The diameter of the orifice 1 7 is larger than that which would be possible in the case of a fixed orifice cooler and it will be appreciated that if a contaminant particle should become lodged against the needle 18 the pressure in the nozzle 16 will rise thereby causing the valve to open and the contaminant particle to become dislodged whereafter the valve will revert to its previous position.

The opening characteristic of the valve is primarily dependent on the characteristic of the biasing spring and it will be appreciated that the valve may open abruptly when the critical pressure, e.g. between 100 and 300 bar is reached, but it is preferred that the valve opens progressively with increasing pressure in the nozzle 16.

If the refrigerant gas supply is a pressurized gas bottle or a compressor running at constant speed the rate of liquefied refrigerant production remains substantially constant and this rate should of course be least equal to that required to maintain the load 15 at the desired temperature having regard to the heat flow into it from the surroundings. In practice this generally means that the rate of liquefied refrigerant production is generally in excess of that required and this is of course wasteful of power and/or gas. If the refrigerant gas supply comprises a compressor the rate of supply can be simply controlled by varying the output of the compressor and this may be effected either in dependence on the actual temperature of or near the load or in dependence on the magnitude of the actual thermal load to which the cooler is subjected.

As illustrated in Figure 2, the graph of the temperature (Ts) of the thermal load against the mass flow rate (m) of refrigerant gas through the cooler is essentially a saturating one. As the mass flow rate increases the temperature rapidly decreases until liquefied refrigerant starts to be produced at the expansion orifice. Thereafter, the temperature rises slightly as the mass flow rate increases and this is due to the fact that an increase in the mass flow rate results in an increase in the pressure downstream of the expansion orifice which in turn results in an increase in the liquefaction temperature of the gas.Thus in order to permit a conventional control signal to be used to control the temperature by varying the mass flow rate it is necessary to operate the cooler in the substantially linear region immediately above the region A of the graph at which the characteristic relatively abruptly changes. This results in the temperature at which the cooler is operated being very slightly above the temperature which is generally attained with a self-regulating cooler which inherently operates in the region A.

In the first method of control referred to above in which the rate of refrigerant supply is varied in dependence on the temperature of or near the load, a temperature sensor is positioned adjacent the load, e.g. within the Dewar vessel but out of the line of fluid issuing through the orifice 17. The signal produced by the sensor can then simply be used to vary the rate of refrigerant supply, e.g. by varying the speed of the compressor.

In the second of the control methods referred to above, the simplest indication of the heat load to which the cooler is subjected is the ambient temperature and thus the rate of refrigerant supply may be varied in dependence on a signal produced by a temperature sensor arranged to sense the ambient temperature in the vicinity of the cooler. In a more sophisticated alternative, there is a sudden temporary change in the rate of refrigerant supply whereafter the rate in change of temperature of the load is measured and this rate of change, which is indicative of the heat load to which the cooler is subjected, is used to produce a signal which controls the speed of the compressor.

In one embodiment which is illustrated in Figure 3, the temperature of or adjacent the load 15 is monitored by a sensor, such as a diode 23. A signal indicative of this temperature, e.g. the voltage drop across the diode, is used as a feedback variable and passed to a control system 25 which calculates the rate of change of temperature of the load. The controller is connected to a solenoid valve 28 in the gas supply line, which includes a surge volume 27, to apply a refrigerant pulse to the cooler 30 whose duration is related by a control law to the rate of change of temperature of the load. The temperature of the load will thus cycle in a sawtooth manner with the amplitude of the temperature change being largely dependent on the temperature resolution which may be achieved by the diode 23 at the load 15.

In a further embodiment of the control system the control makes use of the characteristic illustrated in Figure 2. The cooler is operated at the minimum temperature condition by applying a mass flow variation or 'dither' signal to the cooler and measuring the resultant variation in temperature of the load. The variation is measured using a temperature sensor, such as a diode, as before, by a controller and by use of a control algarithm a control function is derived indicative of the slope of the characteristic. This control function is used to adjust the rate of refrigerant gas flow to maintain the cooler in the region of the characteristic immediately above the region A shown in Figure 2.

The control techniques referred to above are intended primarily for use with an open cycle system, the rate of supply of refrigerant gas being varied by adjusting the speed of the compressor.

In a still further embodiment which is illustrated in Figure 4, the cooler 30 is connected to a compressor 32 in a closed circuit. The outlet of the compressor is connected via a high pressure volume 34 to the inlet of the cooler whose outlet is connected via a low pressure volume 40 to the inlet of the compressor. The compressor is controlled by a controller 42 which is connected to a pressure transducer 36 which measures the pressure on the inlet side of the compressor, that is to say the outlet side of the cooler. The controller is arranged to maintain the pressure on the inlet side of the compressor substantially constant whereby variations in ambient temperature and in the volume of the gaseous refrigerant in the closed system are reflected by changes in pressure on the outlet side of the compressor.It will be appreciated that this means that the system is to a large extent 'selfregulating' in that an increase in the ambi- ent temperature will result in an increase in pressure on the inlet side of the cooler whereby more gas flows through the expansion nozzle and liquefied refrigerant is produced at a greater rate. Conversely, if an excessive amount of liquefied refrigerant should be produced in the cooler the pressure at the inlet to the cooler will drop because more of the total mass of gas in the system is in the low pressure side of the system thereby reducing the rate of liquefied refrigerant production.

In the embodiment described with reference to Figure 1, the expansion nozzle is fixed and the valve needle is movable against the action of a spring. It will however be appreciated that it is possible for the needle to be fixed and the nozzle to be movable against the action of biasing means. In a particularly simple construction of this latter type, the expansion nozzle is simply carried by the helical tube 11 which is sufficiently resilient that it constitutes a spring acting to urge the nozzle into contact with the valve needle.

Claims

1. Cryogenic cooling apparatus comprising an inlet path communicating with the inlet of a Joule Thomson expansion nozzle, an outlet path communicating with the outlet of the expansion nozzle, the inlet path being arranged in heat exchange relationship with the outlet path, a valve member operating with the outlet side of the expansion nozzle and together with the latter constituting a valve which, in use, is urged towards the open position by the pressure of refrigerant gas on the inlet side of the nozzle and biasing means urging the valve towards the closed position, whereby the valve does not open untikl the said pressure exceeds a predetermined value.

2. Apparatus as claimed in claim 1, wherein the nozzle is resiliently mounted and the valve member is fixed.

3. Apparatus as claimed in claim 2 wherein the inlet path comprises a metallic tube wound around a former and having a portion which extends beyond the former and carries the expansion nozzle, the said portion being resilient and constituting the biasing means.

4. Apparatus as claimed in claim 1 in which the nozzle is fixed and the valve member is movably mounted and acted on by a spring which constitutes the biasing means.

5. Apparatus as claimed in any one of the preceding claims wherein the valve member is a needle.

6. Apparatus as claimed in any one of the preceding claims, including means for supplying a refrigerant gas under pressure to the inlet path, a thermal load positioned to be cooled, in use, by the fluid flowing through the nozzle, means for sensing the temperature of the thermal load or at a position adjacent the thermal load and control means responsive thereto arranged to adjust the rate of supply of refrigerant to maintain the temperature of the thermal load substantially constant.

7. Apparatus as claimed in any one of claims 1 to 5 including means for supplying a refrigerant gas under pressure to the inlet path, a thermal load positioned to be cooled, in use, by the fluid flowing through the nozzle, means for measuring or deriving a parameter indicative of the rate of thermal transfer to the load and control means responsive thereto arranged to adjust the rate of supply of refrigerant to maintain the rate of heat extraction from the load by liquefied refrigerant substantially equal to the said rate of thermal transfer to the load.

8. Apparatus as claimed in claim 7 in which the control means is arranged to be responsive to a signal produced by a temperature sensor indicative of the ambient temperature.

9. Apparatus as claimed in claim 7 including means for producing a temporary variation in the rate of supply of refrigerant and means for producing a signal indicative of the resulting rate of change of temperature of the load and means for applying the said signal to the control means.

10. Apparatus as claimed in any one of claims 1 to 5 whose inlet and outlet paths are connected respectively to the outlet and inlet of a compressor, pressure sensor means being provided at the inlet side of the compressor connected to control the speed of the compressor so as to maintain the pressure at the inlet side of the compressor substantially constant.

11. Cryogenic cooling apparatus substantially as specifically herein described with reference to Figure 1, optionally in combination with either Figure 3 or Figure 4.