Cooling Apparatus Employing The Joule-thomson Effect

Abstract

The second stage in a miniature cryogenic two-stage cooler, working on the Joule-Thomson principle, is supplied with gaseous refrigerant which passes from a supply under pressure through one path of a tubular heat exchanger, to an expansion nozzle, after which the expanded gas returns through the other path of the heat exchanger to cool the incoming second stage gas. In the first stage, a precooling refrigerant in liquid form is supplied to a metering nozzle which, having no heat exchanger, is accommodated within the heat exchanger of the second stage, and this refrigerant evaporates to precool the second stage refrigerant. The nozzle of each stage is automatically controlled to vary the flow of refrigerant in accordance with the demand for cooling.

Description

This invention relates to cooling apparatus of the type employing the Joule-Thomson effect, that is to say in which cooling is produced by expansion, through a nozzle, of a refrigerant in gaseous form under pressure, which before expansion at the nozzle is at a temperature below its inversion temperature.

The invention is particularly, though not exclusively, concerned with such coolers which are of miniature size, for example for cooling infrared detectors to the temperature of liquid nitrogen. In a typical construction the complete cooler is largely situated within a cylindrical heat exchanger having a length of between 2 and 3 inches and a diameter of about a third of an inch and wound with a helical coil of finned tubing of which the diameter over the fins is less than one-twentieth of an inch. In general it is a requirement not only that the cooler itself should be of extremely small size, but in addition that the supply of compressed refrigerant required should also be as small and light as possible.

It is known that each refrigerant has a temperature, known as its inversion temperature, at which no cooling is produced by the Joule-Thomson effect, that is to say by expanding the refrigerant gas through an expansion nozzle. Above the inversion temperature such expansion actually produces a rise of temperature, and it is only below it that it produces a cooling effect. Moreover at temperatures only slightly below the inversion temperature the cooling effect is correspondingly slight and if a substantial cooling effect is required the expansion must be carried out from a temperature substantially below the inversion temperature.

Thus in the case of Joule-Thomson coolers employing refrigerants with an inversion temperature below atmospheric temperature it is essential to employ more than one stage, so as to precool the refrigerant before expansion, but even in the case of refrigerants, such as air or nitrogen, having an inversion temperature substantially above atmospheric temperatures, the efficiency can be considerably improved by employing a precooling stage. This, however, has generally not been done in coolers of the type with which the invention is concerned since the space available has not been sufficient to accommodate a precooling stage.

According to the present invention cooling apparatus includes two stages in the second of which a second stage refrigerant from a supply in gaseous form is passed through one path of a heat exchanger, expanded through an expansion nozzle and passed back through the other path of the heat exchanger to cool the incoming second stage refrigerant, and a precooling first stage in which a precooling refrigerant is supplied in liquid form under pressure through a metering nozzle to a space at reduced pressure where it evaporates to cool the second stage refrigerant before the latter passes through the heat exchanger, the effective area of the nozzle of each stage being automatically controlled in accordance with a parameter depending upon the relationship between cooling supplied and cooling demanded at that stage.

In one form of the invention the apparatus is generally of tubular form, the heat exchanger comprising a finned tube wound to helical form with the second stage refrigerant flowing through it in one direction and past its outside in the opposite direction, the second stage expansion nozzle being at the cold end of the heat exchanger while the first stage metering nozzle is enclosed within its warm end.

In the above-noted cooling apparatus designed, for example, for cooling infrared detectors to the temperature of liquid nitrogen, the effective area of the nozzle of a Joule-Thomson cooler is automatically controlled, as by means of a needle valve, in accordance with a parameter depending on the relationship between cooling supplied and cooling demanded. This may be in accordance with a temperature at a point in the cold end of the cooling apparatus. In general, however, the apparatus will serve to produce a pool of liquid refrigerant, generally at atmospheric pressure, and in this case there cannot be any variation of temperature so long as the liquid is in equilibrium with its vapor. Thus in the preferred arrangements described, the control is not directly in accordance with the temperature of the refrigerant but in accordance with the level of the liquid refrigerant which comes into contact with a sensor as the level rises, and controls the temperature of the sensor. Other arrangements may be employed in which the control is in accordance with some other or intermediate function depending upon the supply and demand for cooling, for example the sensor may have an extended tail affording a path for the flow of heat to the liquid refrigerant, the length of which path varies with the level of refrigerant. Again the sensor may be in heat exchange relationship with parts of the apparatus above the pool of liquid refrigerant so that its temperature will depend partly upon the level of liquid refrigerant and partly on the temperature of the vapor escaping from the nozzle.

In one arrangement in accordance with the present invention the second stage expansion nozzle is arranged and controlled as described above and includes a needle valve controlled by a bellows the pressure upon which is determined by a sensor comprising a vapor bulb with an extended tail extending down to the pool of liquefied refrigerant. The first stage is generally similar but in this case the sensor bulb is secured to a curved plate fitting within the body of the heat exchanger at about the level of the metering nozzle so as to respond primarily to the temperature of the heat exchanger at that part of the apparatus. Just below the metering nozzle a cup is secured to the inside of the body of the heat exchanger to collect liquid refrigerant formed by the first stage refrigerant, and at the corresponding part of the length of the heat exchanger the helical finned tube is not finned but is soldered to the outer surface of the heat exchanger tube so as to transmit heat from the second stage refrigerant within it to the liquified first stage refrigerant in the cup within the body.

The invention may be put into practice in various ways but one specific embodiment will be described by way of example with reference to the accompanying drawing which shows a two-stage cooling device.

In this embodiment the invention is applied to a two-stage cooler of miniature size, perhaps 3 inches in length and one-third inch in diameter comprising two stages. The first, precooling, stage is supplied with Freon which is stored in liquid form, under pressure but at atmospheric temperature, and is admitted past a metering orifice to a space where it can expand to atmospheric pressure, thereby effecting precooling. It operates with nitrogen in gaseous form under pressure which is fed through the inner path of a helical tube heat exchanger to an expansion nozzle where it expands and cools due to the Joule-Thomson effect, producing a pool of liquid nitrogen, while the remaining gas escapes by the outer path of the heat exchanger, i.e. past the outside of the helical finned tube, cooling the incoming nitrogen.

The cooling apparatus is of elongated form, and for purposes of description it will be assumed that it is placed vertically with its cold end at the bottom. The apparatus includes an annular heat exchanger 10 comprising a tubular body 11 around which is helically wound a finned coiled tube 12. An external coaxial tube 13, which may be the inner wall of a Dewar flask, is located around the finned coil and the space 14 between the body and the external tube provides a path for exhaust gas flowing past the fins to cool the incoming high-pressure working fluid within the helical tube. The lower end of the external tube is closed to provide a reservoir (not shown) in which the liquid working fluid can accumulate. This lower end may be combined with the device to be cooled so that the liquid nitrogen is in fact in contact with that device. The upper end of the helical finned tube 12 communicates with a lateral coupling 18 for connection to the supply of nitrogen under pressure. Its lower end communicates with an expansion nozzle 22 which is arranged to be controlled by means of a needle valve 23 which is itself controlled by a bellows 24.

Thus at its lower end the tubular body of the heat exchanger has secured in it a stout metal ring 28. Secured to a point under this ring, and thus offset from the axis of the heat exchanger, is the expansion nozzle 22, while a sensor bulb 30 depends from a diametrically opposite point. To support the expansion nozzle 22 a threaded socket 32 is secured to the ring and receives the upper end of a screwed sleeve 33 on the lower end of which is threaded a screwed plug 34 in which the expansion nozzle 22 is formed. The screwed sleeve also serves to retain a porous plug 35 or filter through which the working fluid is compelled to pass on its way to the expansion nozzle 22 so as to condense and filter out any impurities which might otherwise block the expansion nozzle.

The bellows 24, situated coaxially within the lower part of the tubular body 11 of the heat exchanger, has its lower end secured to the ring 28 while its upper end is closed by and secured to a tubular operating member 25 which extends down through it and through the ring and at its lower end carries the moving part of a valve incorporating the floating needle valve 23. The upper end of the needle valve projects into the expansion orifice while its lower end is of conical form engaging a seating formed in a screwed plug 26 threaded into a thimble 27 which is rigidly secured as by welding to the lower end of the tubular operating rod 25. The thimble has secured to it a shroud 39 in the form of a partial cage formed of wire gauge which allows free escape of exhaust vapor while intercepting any liquid droplets to prevent them from impinging on the sensor. The tubular operating rod 25 is provided with an S-shaped guide blade 29 serving to keep it centered and prevent it from being tilted by the offset pressure on the needle valve.

The sensor bulb 30 communicates through a hole in the ring 28 with the interior of the tubular body of the heat exchanger surrounding the bellows, so that the pressure in the vapor bulb of the sensor is communicated to the outside of the bellows, and as the pressure rises it tends to push the bellows down, withdrawing the needle valve 23 from the expansion orifice 22 and increasing the flow of refrigerant to increase the amount of cooling. The sensor bulb terminates a little below the level of the orifice and is provided with an extended tail 31 of metal projecting down some distance further. Thus as the level of liquid refrigerant rises it first comes into contact with the end of the extended tail so that heat flowing from the sensor bulb has to flow through its whole length. As the level of liquid refrigerant rises the extent of cooling of the sensor bulb increases progressively, giving a smooth control of the needle valve, and preventing hunting.

In accordance with the present invention the upper part of the heat exchanger contains a first stage precooler supplied with Freon kept liquid under pressure. The first stage is provided with a control valve 53 which constructionally is very similar to that of the second stage, although it should be noted that in this case it is merely a metering valve for liquid, and not an expansion valve employing the Joule-Thomson effect as in the second stage. The first stage valve is situated coaxially with the heat exchanger and cooperates with a seating 54 carried by the lower end of an admission tube 55 the upper end of which is secured in a head 56 at the top of the exchanger and communicates with a lateral coupling 57 through which liquid Freon is supplied to it. As in the second stage a thimble 61 seating the lower end of the needle valve 53, is carried at the lower end of a tubular operating member 62 the upper end of which is secured to the upper end of a bellows 63. In this case the bellows is offset from the axis of the heat exchanger and is accommodated in a chamber surmounting the head 56 at the top of the heat exchanger. Thus the operating member extends up on one side of the tubular inlet pipe 55 carrying the seating 54 at its lower end, while at a diametrically opposite position a tube 67 forming a sensor bulb 69 extends down from an opening 70 in the head and has its lower end flattened and secured to a curved shoe 72 engaging the inner face of the tubular body of the heat exchanger. The sensor bulb 69 contains a volatile liquid, for example Freon, so that the outside of the bellows 63 is subjected to its vapor pressure which varies in accordance with the temperature of the heat exchanger in the region of the first-stage orifice formed by the seating 54.

Within the tubular body of the heat exchanger, just below the first stage orifice, there is mounted a cup 75 the cylindrical wall of which is in heat exchange relationship with the body of the heat exchanger and which accommodates any Freon which may be liquefied by the first stage. The cup contains a perforated heat collector 76 of tubular form so as to assist in transmitting heat from the body of the heat exchanger to any liquid Freon in the cup.

The part of the helical tube 12 surrounding the cup is deprived of its fins and wound in a close-pitched helix 77 round the body of the heat exchanger to which it is soldered so as to provide effective heat exchange between it and the cup.

It is believed that the operation of the apparatus will be clear from the description given above. In an apparatus having no automatic control it is of course necessary to provide a flow of refrigerant sufficient to provide for the least advantageous conditions, that is to say when the maximum cooling is required by the load and the ambient temperature is at a maximum. Under other conditions a much smaller flow of refrigerant would suffice for requirements, and accordingly the amount of refrigerant that must be provided in the supply is in fact far greater than is necessary. Thus considerable economy is possible by providing automatic control even of a single stage Joule-Thomson cooler. As indicated above, however, the efficiency of a Joule-Thomson cooler is rather low if the refrigerant starts at a temperature only a little below the inversion temperature, and hence in these conditions waste of refrigerant can still occur even though there is automatic control. The present arrangement, by providing precooling, increases the efficiency of the Joule-Thomson second stage and thereby economizes in the supply of compressed nitrogen, while adding little or nothing to the bulk or weight of the cooler. In this case the automatic control is at least equally important in economizing the supply of liquid Freon. This is particularly the case since in normal ambient temperatures the precooling may not be required, since the exhaust nitrogen from the second stage may sufficiently precool the incoming nitrogen for that stage to give efficient operation of the Joule-Thomson expansion cooling. Thus the precooling by liquid Freon is primarily required when the apparatus is used at ambient temperatures where the efficiency of Joule-Thomson cooling would otherwise diminish in a single stage cooler.

Claims

What we claim as our invention and desire to secure by Letters Patent is:

1. Cooling apparatus of the type in which cooling is produced by expansion, through a nozzle, of a refrigerant in gaseous form under pressure, which before expansion at the nozzle is at a temperature below its inversion temperature, including two stages in the second of which a second stage refrigerant from a supply in gaseous form is passed through one path of a heat exchanger, expanded through an expansion nozzle and passed back through the other path of the heat exchanger to cool the incoming second stage refrigerant, a precooling first stage in which a precooling refrigerant is supplied in liquid form under pressure through a metering nozzle to a space at reduced pressure where it evaporates to cool the second stage refrigerant before the latter passes through the heat exchanger, and means for automatically controlling the effective area of the nozzle of each stage so that the amount of refrigerant flowing through each nozzle may be governed by the requirement of cooling at that stage.

2. Cooling apparatus as claimed in claim 1 in which the apparatus is generally of tubular form, the heat exchanger comprising a finned tube wound to helical form with the second stage refrigerant flowing through it in one direction and past its outside in the opposite direction, the second stage expansion nozzle being at the cold end of the heat exchanger while the first stage metering nozzle is enclosed within its warm end.

3. Cooling apparatus as claimed in claim 2 in which the second stage expansion nozzle includes a needle valve controlled by a bellows the pressure upon which is determined by said automatic control means which includes a sensor comprising a vapor bulb with an extended tail extending down to a pool of liquefied refrigerant.

4. Cooling apparatus as claimed in claim 3 in which the first stage metering nozzle includes a needle valve controlled by a bellows the pressure upon which is determined by said automatic control means which also includes a sensor bulb secured to a curved plate fitting within the body of the heat exchanger at about the level of the metering nozzle so as to respond primarily to the temperature of the heat exchanger at that part of the apparatus.

5. Cooling apparatus as claimed in claim 4 in which a cup is secured to the inside of the body of the heat exchanger, just below the metering nozzle, to collect liquid refrigerant formed by the first stage refrigerant, and at the corresponding part of the length of the heat exchanger the helical finned tube is not finned and is soldered to the outer surface of the heat exchanger tube so as to transmit heat from the second stage refrigerant in the cup within the body.

6. Cooling apparatus as claimed in claim 5 wherein the refrigerant is Freon in liquid form supplied under pressure and at atmospheric temperature.