GB2250085A - Joule-Thornson cooling apparatus - Google Patents

Abstract

Joule-Thomson cooling apparatus operates by expansion of gaseous refrigerant through an orifice (4) which is fed via a counterflow heat exchanger (2). The orifice is obstructed by a needle (6) which is biased against it by means of a spring (8). The effective area of the orifice is thus made dependent on the pressure of the refrigerant. In a closed cycle cooling apparatus high pressure refrigerant gas is supplied by a two-stage compressor. A temperature-sensing diode adjacent the infra-red detector (16) provides information for controlling the speed of the compressor. If the temperature is too high, the compressor speed is increased. <IMAGE>

Description

COOLING APPARATUS This invention relates to cooling apparatus comprising a counterflow heat exchanger having an input path for conducting refrigerant gas to an expansion orifice to produce cooling in a chamber by means of the Joule-Thomson effect and an ouput path for refrigerant which has expanded through said orifice, said output path being in thermally conductive relationship to said input path.

Cooling apparatuses capable of producing very low temperatures find applications in many different fields, such as in electronics. Electronics devices and their associated cooling apparatuses must often operate under remote or adverse environmental conditions, such as aboard spacecraft, aircraft or other vehicles wherein space and weight are at a premium and where high reliability and long operating life are required in order to satisfy these requirements, open cycle cooling apparatuses commonly have been employed.

One such open cycle cooling apparatus stores refrigerant gas under high pressure in a pressure vessel. Upon initiation the gas flows through a heat exchanger to a Joule Thomson expansion orifice where expansion causes liquification of the refrigerant. The liquid refrigerant is puddled to cool the device. Cold refrigerant vapour is then routed back through the heat exchanger to cool the incoming gas and is then vented out of the apparatus. The pressure vessel used for storing the refrigerant must be removed, refilled, and returned to the apparatus quite frequently. The apparatus has a rather low efficiency, requires high maintenance, and there is a possibility of contamination entering the system, which can cause it to malfunction. Moreover, such frequent maintenance may be highly impractical depending upon the environment in which the device is located.

Closed cycle cooling apparatuses, which recompress and reuse refrigerant gas rather than venting it, are capable of higher efficiency than open cycle apparatuses and do not require the same type of maintenance.

In such apparatuses the temperature reduction of the gaseous refrigerant under pressure is achieved before ideally isenthalpic expansion of the gas through a small orifice causes the gas to cool and in particular conditions partially liquify.

In most applications of such apparatuses it is desirable to use the minimum mass of refrigerant gas whilst achieving the required cooling effect. In pursuance of this aim the cooling apparatus is generally installed in a low heat load encapsulation and the optimum Joule Thomson orifice is selected involving a trade-off between cool down time, and steady state gas consumption. If the cool down time is not critical the orifice can be made very small reducing the consumption of refrigerant gas under steady state conditions. In practice the limit in this direction is normally set by susceptibility to plugging by condensible and/or particulate contaminants.

An extension of this technology is to replace the pressure vessel with a compressor, the output of which is controlled by a pressure transducer. The temperature control remains at the orifice of the Joule Thomson cooler. This apparatus then has two control systems, one to ensure an adequate supply of refrigerant the other, a complex mechanism, to control the temperature at the detector..

In addition a fixed throttling orifice does not give the required performance. Therefore controlling the flow of the refrigerant gas by incorporating a temperature sensitive mechanism at the orifice is now established practice.

There are two established methods of temperature sensitive control which are well known.

In the first of these, the effective orifice size of the Joule Thomson cooler is controlled by bellows, the pressure upon which is determined by a sensor comprising a vapour bulb with an extended tail, extending down to the pool of liquified refrigerant. The sensor temperature depends partly upon the level of the liquid refrigerant and partly upon the temperature of the vapour escaping from the nozzle. When the desired temperature is reached a needle attached to the bellows moves into the orifice restricting the flow. British Patent Application 1230079 and French Patent Application Number 2176544 describe such self regulation devices.

Precise temperature control is not achieved with this type of design due to the on/off nature of the control system and reliable operation is dependent upon containment of the vapour within the bellows and sensor. In addition, in high acceleration environments failure often occurs due to the difficulty of producing a rugged mechanism in a small volume.

Hence such designs are generally complex, expensive and their temperature stability and reliability are not adequate for many applications.

The second of the methods of temperature sensitive regulation of the flow is achieved as a result of differential expansion controlling the effective orifice size.

There have been several configurations of this technique but they all involve the use of at least two materials of widely differing expansion coefficients which are configured to reduce the effective area of the orifice as the temperature falls.

French Patent Application Numbers 2136023 and 2322336 describe such configurations.

Although these designs are less expensive and more rugged than those mentioned previously, they are slower to respond to temperature changes.

According to the present invention there is provided cooling apparatus comprising a counterflow heat exchanger having an input path for conducting refrigerant gas to an expansion orifice to produce cooling in a chamber by means of the Joule Thomson effect and an output path for refrigerant which has expanded through said orifice, said output path being in thermally conductive relationship to said input path, the apparatus further comprising pressure-sensitive means for adjusting the effective area of said orifice in dependence upon the excess of pressure of refrigerant on the input path side of said orifice over the pressure of refrigerant on the output path side of said orifice in such manner that said effective area will be reduced when said excess has a first comparatively low value as compared with when said excess has a second comparatively high value.

Said pressure-sensitive means may comprise an obstruction, for example taking the form of a needle or a sphere, which is urged against the output path side of said orifice by means of a spring.

The apparatus may include a temperature sensor in thermally conductive relationship with said chamber, and pressure control means for controlling said excess of pressure in dependence upon temperature sensed by said sensor in such manner that said excess will be reduced when said temperature has a first comparatively low value as compared with when said temperature has a second comparatively high value.

Said pressure control means may comprise a compressor for refrigerant gas on the input path side of said orifice, and means for controlling the speed of said compressor in dependence upon temperature sensed by said sensor.

In order that the present invention may be more clearly understood, a specific embodiment will now be described, by way of example only, with reference to the accompanying diagrammatic drawings of which: Figure 1 shows a detail of the embodiment, and Figure 2 is a diagram of the complete embodiment.

In Figure 1 an assembly 1, shown partly cut away, comprises a counterflow heat exchanger 2, having an input path 3 for conducting refrigerant gas to an expansion orifice 4 to produce cooling in a chamber 5 by means of the Joule Thomson effect.

The exchanger 2 has an output path 9 for refrigerant which has expanded through the orifice 4, the path 9 being in thermally conductive relationship to the path 3. The expansion orifice 4 is obstructed by an obstruction, in this example a needle valve 6 which is held in position urged against the output path side of the orifice 4, by the action of a biasing means in the form of a spring 8 which bears against an open support 11. The effective area of the orifice 4 and the force provided by the spring 8 is determined to ensure that the orifice 4 remains completely obstructed until the excess of pressure of the gaseous refrigerant is such that an efficient cooling effect can be obtained from the isenthalpic expansion of the gas. At this point the orifice 4 begins to open allowing the gas to flow.

The pressure excess may continue to rise increasing the effective orifice area and thus the mass flow of refrigerant gas. When the desired temperature is achieved the pressure excess can be reduced resulting in a reduction in the effective orifice area. The assembly may then operate at a pressure excess just above that required to open the orifice 4, reducing the mass flow of refrigerant. In the present example the assembly 1 is used to cool an infra-red detector 16 mounted on the outside wall of the chamber 5 so as to be in thermally conductive relationship therewith.

The part of the assembly so far described is encapsulated by a vacuum encapsulation 7. If condensible impurities solidify at the orifice 4, the flow is restricted. A small increase in pressure will cause the needle 6 to be further displaced to increase the effective area and compensating for the partial obstruction.

In the complete cooling apparatus shown in Figure 2, the high pressure refrigerant gas is supplied by a compressor 10.

This compressor 10 is formed of a first stage 12 into which low pressure gas is returned and a second stage 14 out of which high pressure gas is supplied. Such a compressor 10 is capable of controlling both the flow rate and the resultant temperature by variation of the gas pressure. Prior art self regulation systems have the temperature control at the orifice and a secondary, normally independent control of the compressor output.

In the closed cycle cooling apparatus shown in Figure 2, the cooling effect is controlled by variation of the compressor output. In order to effect such control the infra red detector 16 has a temperature-sensing diode (not shown) intimately associated with it. This diode provides information 18 for a speed controller 20 for the compressor 10 and thereby ensures an adequate compressor output, via a motor 22, under all conditions while maintaining high efficiency. If the detector temperature is too high the compressor speed is increased, and conversely.

In the particular embodiment described an input power of 12 watts provided 260mW of cooling power for the detector 16 and 0 encapsulation at about 85 K.

The apparatus may incorporate a filter 24 for impurities to substantially prevent them from entering the assembly 1.

In any particular application the orifice size and the spring force should be chosen so that the opening pressure matches the optimum operating pressure of the gaseous refrigerant. To this end, it may be desirable for the shape of the orifice and needle to be varied to optimise the rate at which the effective orifice area changes with respect to gas pressure.

Depending upon application, the apparatus may work more efficiently if effectively the needle valve 6 can take up only two positions. In such a case it may be arranged that, when the apparatus is activated, the high pressure holds the needle open during cool-down and as the pressure drops the needle 6 moves to a second position restricting the orifice 4 to reduce the flow of refrigerant.

It will be appreciated that the invention is not limited to the above identified embodiment; other embodiments will be apparent to those skilled in the art. For example, the apparatus may be of the open cycle type. Alternatively or in addition, the obstruction may take the form of a sphere.

Claims (7)

1. Cooling apparatus comprising a counterflow heat exchanger having an input path for conducting refrigerant gas to an expansion orifice to produce cooling in a chamber by means of the Joule-Thomson effect and an output path for refrigerant which has expanded through said orifice, said output path being in thermally conductive relationship to said input path, the apparatus further comprising pressure-sensitive means for adjusting the effective area of said orifice in dependence upon the excess of pressure of refrigerant on the input path side of said orifice over the pressure of refrigerant on the output path side of said orifice in such manner that said effective area will be reduced when said excess has a first comparatively low value as compared with when said excess has a second comparatively high value.

2. Apparatus as claimed in Claim 1, wherein said pressure-sensitive means comprises an obstruction which is urged against the output path side of said orifice by means of a spring.

3. Apparatus as claimed in Claim 2, wherein said obstruction takes the form of a needle.

4. Apparatus as claimed in Claim 2, wherein said obstruction takes the form of a sphere.

5. Apparatus as claimed in any preceding claim, including a temperature sensor in thermally conductive relationship with said chamber, and pressure control means for controlling said excess of pressure in dependence upon temperature sensed by said sensor in such manner that said excess will be reduced when said temperature has a first comparatively low value as compared with when said temperature has a second comparatively high value.

6. Apparatus as claimed in Claim 5, wherein said pressure control means comprises a compressor for refrigerant gas on the input path side of said orifice, and means for controlling the speed of said compressor in dependence upon temperature sensed by said sensor.

7. Cooling apparatus substantially as described herein with reference to Fig. 1 or Fig 2 of the drawing.