Apparatus For Liquefying A Cryogen By Isentropic Expansion

Abstract

Method and apparatus for liquefying relatively large quantities of a cryogen, helium in particular. After the high-pressure fluid has been cooled through indirect heat exchange with a cold low-pressure stream it is isentropically expanded in an expansion engine. The rate of liquefaction may be increased by as much as 30 percent by substituting isentropic expansion for the conventional isenthalpic expansion.

Parent Case Text

This application is a continuation of my application Ser. No. 81,937 filed Oct. 19, 1970 and now abandoned.

Description

This invention relates to the liquefaction of gases and more particularly to the liquefaction of cryogens such as oxygen, nitrogen, hydrogen and helium.

The liquefaction of cryogens, and particularly of helium, is now performed on a large scale to provide refrigeration for a wide range of equipment such as superconducting magnets, superconducting cavities and the like.

Although there are several known cycles for liquefying helium and a number of apparatus capable of performing these cycles, the most widely used cycle and apparatus are those embodied in the Collins helium crysotat described in U.S. Pat. No. 2,458,894. Improvements in this cryostat are disclosed in U.S. Pat. Nos. 3,250,079 and 3,438,220, the latter being directed in part to the use of a unique type of expander.

In the basic Collins liquefaction cycle a stream of high-pressure helium is precooled, by indirect heat exchange with a counterflowing stream of cold low-pressure helium, to a temperature below the inversion temperature of helium. Liquefaction is then accomplished by one or more isenthalpic expansions through one or more Joule-Thomson valves. The cold low-pressure stream is provided in part from high-pressure fluid which is withdrawn at two appropriate temperature levels from the high-pressure stream for isentropic expansion and return in the low-pressure stream.

Liquefaction of helium by the method and in the apparatus of U.S. Pat. No. 2,458,894 requires a highly efficient heat exchange system in order to precool the high-pressure helium to below its inversion temperature (30.degree.K). It is, moreover, difficult to effect complete liquefaction of the helium through isenthalpic expansion in the Joule-Thomson expansion valve. I have now discovered that by substituting a relatively slow-speed expansion engine for the Joule-Thomson expansion valve or valves in a helium liquefier of the general character described it is possible to increase the amount of helium liquefied by as much as 30 percent.

It is therefore a primary object of this invention to provide an improved apparatus for liquefying helium through the use of expansion engines. It is another object of this invention to provide apparatus of the character described which makes it possible to liquefy a greater percentage of a stream of precooled high-pressure helium.

Another primary object of this invention is to provide an improved cycle for liquefying high-pressure precooled helium by substituting isentropic expansion for isenthalpic expansion. Other objects of the invention will in part be obvious and will in part be apparent hereinafter.

The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, and the apparatus embodying features of construction, combination of elements and arrangement of parts which are adpated to effect such steps, all as exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings in which.

FIG. 1 is a diagram of one embodiment of the cycle and apparatus of this invention; and

FIGS. 2-6 are simplified diagrams of four additional embodiments of this invention.

In the following detailed presentation the cycle and apparatus of this invention will be described in terms of liquefying helium. It is, however, to be understood that the cycle and apparatus are also suitable for liquefying other cryogens, particularly hydrogen. The drawings are diagrammatic inasmuch as several different types of heat exchangers, expansions engines and work-absorbing means may be used. These will be identified in the following description.

In the apparatus of FIG. 1, high-pressure helium from compressor 10 is cooled in aftercooler 11 and then divided into two streams, a first main high-pressure stream 12 and a second branch high-pressure stream 13. The first stream 12 is directed into the high-pressure side 14a of first section 15 of the main heat exchanger 16. The main heat exchanger 16 is constructed in any suitable manner to effect highly efficient indirect heat exchange between the high-pressure fluid stream forming the high pressure side 14 and the low-pressure fluid stream forming the low-pressure side 17. One preferred type of such a heat exchanger is illustrated in U.S. Pat. No. 2,895,303. It comprises means to define an annular passage in which finned tubing is helically wound, the fins of the tubing having a diameter essentially equal to the width of the annular passage. The first or high-pressure fluid flow path 14 is preferably that passage within the finned tubing; while the second or low-pressure fluid flow path 17 is the passage defined within the annular passage around the finned tubing. The internal volume defined within the confines of such a heat exchanger is typically used to locate charcoal traps and the expansion engines.

Returning to FIG. 1, the second branch high-pressure stream 13 is directed through one side of a nitrogen precooling heat exchanger 18 adapted to effect indirect heat exchange between liquid nitrogen introduced into the other side of heat exchanger 18 through line 19 and withdrawn as gaseous nitrogen through line 20. The liquid-nitrogen cooled high-pressure helium in stream 13 is then mixed with the cooled high-pressure helium of stream 12 before forming a single precooled stream 21 which is introduced into the high-pressure side 14, which is in effect a continuation of the high-pressure side 14a, of the first section 15 of the main heat exchanger 16. At the temperature level at which the high-pressure helium is introduced into the second section 22 of the main heat exchanger 16, a portion of the precooled high-pressure helium is diverted by way of line 23, having a charcoal trap 24, into a first expansion engine 25 from where, after expansion and cooling, it is conducted by conduit 26 into the low-pressure side 17 of the main heat exchanger.

In a similar manner, a second portion of the high-pressure helium is diverted at a second, lower temperature level at the top of the third section 30 of the main heat exchanger and taken by line 31, having charcoal trap 32, into a second expansion engine 33 from where it is returned by line 34 to the low-pressure side 17 of the heat exchanger at approximately its lowest temperature level.

Below the main heat exchanger 16 is a second heat exchanger 40 designed to effect the final precooling of the high-pressure fluid just prior to liquefaction. The refrigeration for this is supplied from the helium boiled off from the liquefied helium and the unliquefied portion of the stream flowing through the engine which is returned through the low-pressure fluid flow path 17. The heat exchanger 40 is preferably constructed as coiled concentric tubing in which the high-pressure stream flows within the inner tubing and the low-pressure stream flows in the annular passage around it.

The finally precooled high-pressure helium is then passed into a surge volume 41 and then through a liquefying expansion engine 42. The liquefied helium is accumulated in a suitable liquid vessel 43 where it may be stored for cooling a load which is represented diagrammatically at 44 and which is positioned within the cryostat. Alternatively, the liquid helium may be withdrawn through drawoff line 45.

Inasmuch as the flow of the precooled high-pressure fluid through expansion engine 42 will be periodic it is preferable to incorporate the surge volume 41 in the high-pressure flow path between heat exchanger 40 and the liquefying expansion engine 42 to achieve a balanced and efficient heat exchange between the high-pressure and low-pressure streams in the heat exchangers. It is also preferable to incorporate a comparable surge volume 46 in the low-pressure side for the same reason. These surge volumes are fluid accumulators adapted to absorb the pulsations in fluid flow.

The expansion engines suitable for use in this apparatus are those which are capable of removing energy from a fluid under pressure thereby to effect isentropic expansion of the fluid and to deliver mechanical energy to some externally located work absorbing means. Expansion engines which are particularly suitable for the apparatus of this invention are those of the types described in U.S. Pat. Nos. 2,607,322 and 3,438,222. Generally, such expansion engines will be relatively slow-speed (60 to 100 strokes per minute piston engine). In FIG. 1 there are shown mechanical connections 50, 51 and 52 between expansion engines 25, 33 and 42, respectively, and a suitable work absorbing means 53 which may be a crank shaft combined with suitable driving means and valve actuating means associated with the expansion engines as illustrated and described in detail in U.S. Pat. No. 3,438,220.

It will, of course, be evident that it is necessary to provide suitable thermal insulation around the heat exchangers and expansion engines as shown by the outlet designated by the numeral 55. Such insulation will include suitably cooled radiation shielding and evacuated volumes and is known in the art.

FIG. 2-5, in which like reference numerals are used to refer to like components in FIG. 1, illustrate modifications in the manner in which the high-pressure fluid is precooled prior to its introduction into the liquefying expansion engine. In FIGS. 2-5 the auxiliary equipment such as the compressor, aftercooler, and work absorbing means, as well as the charcoal traps, insulation, etc. are omitted for the sake of simplifying the drawings. It is, of course, to be understood that these components will be integral parts of the apparatus in the same way as in FIG. 1.

In the apparatus of FIG. 2 the high-pressure helium supply line 12 is branched so that a portion of the incoming high-pressure helium is directed down through the high-pressure side or the high-pressure fluid flow path 14 while another portion of the incoming high-pressure helium is directed into a line 60 which leads into and is part of one side of the liquid nitrogen heat exchanger 18. The fluid which is cooled in the liquid nitrogen heat exchanger is then taken to an expansion engine 61 and after expansion and cooling is taken through line 62 into the low-pressure side 17 of the main heat exchanger at a point nearer the room temperature end than in FIG. 1. Thus, in effect, in the apparatus of FIG. 2 expansion engine 61 takes the place of expansion engine 25 in FIG. 1. Otherwise the apparatus of FIG. 2 is essentially equivalent to that of FIG. 1.

In the apparatus of FIG. 3 that portion of the high-pressure helium stream which is cooled in the liquid nitrogen heat exchanger 18 is mixed with the high-pressure helium in the high-pressure side of heat exchanger 16 after it has passed through the first section 15 of the main heat exchanger. Two additional sections 65 and 66 are added to the main heat exchanger 16.

The apparatus of FIG. 4 adds the expansion engine 61 to the apparatus of FIG. 3 thereby providing the apparatus of FIG. 4 with three expansion engines associated with the main heat exchanger.

Finally, the apparatus of FIG. 5 shows a modification in the use of liquid nitrogen as a precooler. In this apparatus the liquid nitrogen precooling is achieved within the main heat exchanger 16 by introducing the liquid nitrogen into suitable passages so that it may be used to augment the low-pressure helium in cooling the incoming high-pressure helium.

Returning to FIG. 1 a typical operation of the apparatus may be illustrated. The high-pressure helium is introduced into the system at a pressure of about 225 psi. The temperature of the expanded low-pressure fluid leaving expansion engine 25 will typically be about 40.degree.K. The high-pressure helium is introduced into the liquefying expansion engine 42 at about 7.degree.K and is, of course, cooled to 4.2.degree.K in expansion and liquefaction. The pressure of the helium in the low-pressure fluid flow path is typically about 3 psi. Using a cycle such as that described above it is possible to liquefy as much as 75 percent of the high-pressure helium introduced into the liquefying expansion engine 42.

In the conventional helium liquefiers, the section of the heat exchanger between the lowest point of precooling and the Joule-Thomson expansion valve is typically referred to as the Joule-Thomson heat exchanger. This would be heat exchanger 40 in FIG. 1. For any liquefaction to take place by Joule-Thomson, or isenthalpic expansion, the temperature of the gas entering the J.T. heat exchanger must be below the inversion temperature of helium, or below about 30.degree.K. In the present invention in which an expansion engine is used in place of the Joule-Thomson valve, giving isentropic rather than isenthalpic expansion, the temperature of the gas entering this heat exchanger can be well above the inversion temperature, and in fact, with an ideal heat exchanger, it can be normal room temperature. Precooling the high pressure gas stream to liquid nitrogen temperature before entering what might still be referred to as the Joule-Thomson heat exchanger permits some liquefaction of helium on isentropic expansion while using heat exchanger of practical efficiency. FIG. 6 shows such a liquid nitrogen precooled system.

It will thus be seen that the objects set forth above, among those made apparent from the preceeding description, are efficiently attained and since certain changes may be made in carrying out the above method and in the constructions set forth without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

I claim:

1. An apparatus for liquefying helium, comprising in combination

a. high-pressure fluid flow path means;

b. low-pressure fluid flow path means;

c. heat exchange means arranged to provide heat exchange between high-pressure helium flowing in said high-pressure fluid flow path means and low-pressure helium flowing in said low-pressure fluid flow path means;

d. means to divert a portion of said high-pressure helium from said high-pressure fluid flow path means, to expand the helium so diverted and to introduce the resulting cooled low-pressure helium into said low-pressure fluid flow path means at at least one temperature level within said heat exchange means;

e. liquefied fluid receptacle means;

f. a slow-speed intermittent expansion engine arranged to periodically receive high-pressure cold helium and to isentropically expand said helium with the production of mechanical energy thereby to liquefy at least one portion of said high-pressure helium and discharge the at least partially liquefied helium into said receptacle means;

g. surge chamber means in said high-pressure fluid flow path means between said heat exchange means and said expansion engine and arranged to receive and store high-pressure cold helium discharged at the cold end of said heat exchange means and to deliver said high-pressure cold helium periodically to said expansion engine; and

h. means to return nonliquefied low-pressure helium from said receptacle means through said low-pressure fluid flow path means.

2. An apparatus in accordance with claim 1 including precooling heat exchange means adapted to precool at least a portion of said high-pressure helium in said high-pressure fluid flow path means.

3. An apparatus in accordance with claim 2 including means to expand high-pressure helium precooled in said precooling heat exchange means thereby to provide further cooled low-pressure helium, and means to introduce said further cooled low-pressure helium into said low-pressure fluid flow path means.

4. An apparatus in accordance with claim 1 wherein said means to divert said portion of said high-pressure helium and to expand said helium so diverted is arranged to introduce said resulting cooled low-pressure helium into said low-pressure fluid flow path means at two different temperature levels.

5. An apparatus in accordance with claim 1 including separate, distinct surge chamber means in said low-pressure fluid flow path means between said fluid receptacle means and cold end of said heat exchange means.