US2996893A - Low temperature liquid transfer apparatus - Google Patents

Description

Aug. 22, 1961 J. G. GOODENOUGH ETAL 2,996,893

LOW TEMPERATURE LIQUID TRANSFER APPARATUS Filed July 3, 1958 PRESSURE Power p y AIR James G. Goodenough lrvmg H. Swift.

INVENTORS.

A T TOR/V5 Y.

United States Patent '0 2,996,893 LOW TEMPERATURE LIQUID TRANSFER APPARATUS James G. Goodenough and Irvin H. Swift, Santa Barbara,

Calif., assignors to Santa Barbara Research Center,

Goleta, Califl, a corporation of California Filed July 3, 1958, Ser. No. 747,555 4 Claims. (Cl. 62-52) ,This invention relates to apparatus and methods for handling and transferring low temperature liquefied gases from one point to another. More particularly, the invention relates to apparatus and techniques for transferring low temperature liquefied gases from a thermally-insulated supply through a relatively higher temperature me dium to a point of utilization.

It is known, for example, that infrared radiation detection devices are preferably maintained at an extremely low temperature in order to increase the sensitivity of the devices to the infrared radiation desired to be detected. The desired low temperatures are usually provided by cooling the infrared sensitive elements with liquefied gases such as nitrogen or helium. The liquefied gas is generally held in a specially-constructed container in order to reduce the absorption of heat by the gas from the surrounding apparatus and atmosphere. When the infrared sensitive cell is supplied with liquefied gas by piping it thereto from a supply container, the transfer almost invariably requires a transit of the liquefied gas through an intervening atmosphere and/ or adjacent apparatus the ambient temperatures of which may be and generally are much higher than the temperature of the liquefied gas. Hence, in order to reduce the absorption of heat during such transfer it has been thought necessary to utilize extensively insulated tubing which in turn results in an undesirably bulky and heavy apparatus. In some applications of infrared detecting apparatus (i.e., an infrared tracking system in a guided missile), the weight and bulkiness of such heavily-insulated transfer tubing is a considerably unfavorable factor. As a matter of fact, the problems and disadvantages associated with directly transferring liquid nitrogen through bulky insulated tubing have been so great as to discourage any attempts to employ this method of cooling infrared sensitive cells in many applications. Rather, the approach now employed in most instances is to pipe warm, high-pressured gas to the point of utilization and then liquefy the gas at this point by a Joule- Thompson Cryostat. This technique requires a heavy vessel to store compressed gas at about 2,000 to 3,000 p.s.i. in order to supply from 3 to 5 liters per minute of nitrogen at atmospheric pressure. A typical installation according to this technique employs a supply tank weighing from to pounds for perhaps three hours of operation. In addition to the weight and bulkiness involved, the requisite high pressure tank also constitutes a safety hazard. 1

Another scheme that has been suggested but never put into practice is to provide a vacuum-jacketed transfer tube. Such a transfer line would not only be large and rigid but would also probably be made of glass and therefore would constitute a rather fragile installation.

. It is therefore an object of the instant invention to provide improved methods and means for transferring low temperature liquefied gases through a higher temperature medium without requiring thermally insulated transfer apparatus or tubing.

f This and other objects and advantages of the invention are achieved by providing droplets of the liquefied gas from a thermally-insulated supply thereof with a thermally-insulating envelope during passage of the droplets through an uninsulated tube to a point of utilization whereat the thermally-insulating envelope around each droplet ice is removed and the droplets collected to form a pool of liquefied gas. The thermally-insulating envelope around each droplet is formed by permitting external portions of each liquefied gas droplet to vaporize and the flow of such thermally-insulated droplets from one point to another is achieved by maintaining a pressure dilferential between the points.

The invention will be described in greater detail by reference to the drawings in which:

FIG. 1 is an elevational view in section of one embodiment of the invention; and

FIG. 2 is an elevational view partly in section and partly schematic, of another embodiment of the invention.

Referring now to FIG. I, the apparatus to which the invention pertains comprises a thermally-insulated container 2 in which is mounted an infrared detector cell 4. The container 2 may be in the form of a Dewar flask having an outer Wall 6 and a re-entrant portion 8, with the chamber '10 formed therebetween evacuated to provide thermal insulation around the re-entrant portion 8. The infrared detector cell 4 is preferably mounted in the vacuum chamber 10 on the outside of the end wall of the re-entrant portion 8. The outer Wall 6 of the flask opposite the detector cell .4 may include an infrared transparent window (not shown) of silicon or the like, for example. In order to maintain the detector cell 4 at a very low temperature during operation thereof, the reentrant portion -8 is filled with a liquefied gas 12 such as liquid nitrogen or helium. The mouth of the re-entrant portion 8 is generally closed off with a thermally-insulating stopper 14 of rubber, for example, through which is inserted a tube 16 for delivering the liquefied gas 12 from a supply thereof maintained in a second thermallyinsulated container 18.

The supply flask 18 in general corresponds to the detector cell flask 2, just described, having an outer wall 20, a re-entrant portion 22, an evacuated chamber 24 therebetween, a supply 26 of liquefied gas in the re-entrant portion '22, and a thermally-insulating stopper 28 closing ofi the mouth of the re-entrant portion 22. The transfer tube 16 is likewise inserted through the stopper 28 and extends down near the bottom of the r e-entrant portion 22. A portion of the liquefied gas is made to move through the tube 16 from the supply flask 18 to the detector'flask 2 by establishing a pressure differential between the flasks. This may be accomplished by providing a higher gas pressure over the supply 26 of liquefied gas in the supply flask than the gas pressure in the detector flask 2. The detector flask 2 is supplied with a spring-loaded pressure relief valve 30 which is connected by a tube 32 to the interior of the re-entrant portion 8 in order to permit the maintenance of a predetermined gas pressure therein.

Such apparatus is extremely useful, and indeed required, in infrared detection apparatus and infrared tracking systems employed in aircraft and missiles. It will be appreciated that the weight and size of this apparatus in such application is of prime importance. H-eretofore the necessity of providing the transfer tube 16 with extensive thermal insulation has resulted in a significant proportion of the total weight of the apparatus being devoted to this purpose alone. Furthermore, in many applicaions, it is essential to provide the detector cell and hence its flask at a location remote from the supply flask. For example, in certain infrared tracking missiles, the detector cell is mounted in the nose of the missile. In such missiles, not only is the Weight of the insulation for the tube 16 an unfavorable factor but also the bulk of the insulation poses severe problems in design and installation due to the restrictions on space within the missile.

According to the invention, liquefied gas may be transferred from the supply container 18 to the detector flask 2 without such insulation of the transfer tube 16, thus resulting in a substantial reduction in the total weight of the apparatus as well as simplifying the installation thereof, since a tube as small as 0.050" in diameter may be used. This is accomplished by causing a stream of gas to flow from the supply container 18 through the tube 16 to the detector flask 2. Droplets of liquefied gas in the supply flask 18 are also caused to be formed and each droplet is provided with a thermally-insulating envelope. The insulated droplets are then carried by the gas stream to the detector flask and collected therein to form a pool of liquefied gas as shown in FIG. 1. In order to accomplish these results it is required that a pressure differential be maintained between the two flasks and that the temperature of the transfer tube 16 be maintained at or above theboiling point of the liquefied gas.

The pressure differential is achieved by increasing the gas pressure in the supply flask somewhat above atmospheric pressure, for example, while maintaining the gas pressure in the detector flask at atmospheric pressure, for example. The gas pressure in the supply flask may be maintained at about 1-2 pounds over atmospheric pressure, for example, while the spring-loaded pressure relief valve may be adjusted to maintain /2-1 pound over atmospheric pressure in the detector flask, for example.

The pressure differential thus established causes some of the liquefied gas 26 in the supply flask to rise in the transfer tube 16 which is at a temperature at or above the boiling point of the liquefied gas. The temperature of the tube 16 is achieved by disposing the tube in a medium (such as the atmosphere) with as little thermal insulation for the tube as possible. To this end the transfer tube is made of a good thermal conductive material such as aluminum, for example. The rising liquefied gas, upon encountering the relatively warm tube, boils and forms a gas which, again due to the pressure differential established in the system, flows through the tube 16 to the detector flask 2. At the same time, the boiling of the liquefied gas causes the formation of unnumerable droplets of the liquid each of which are completely surrounded by an envelope of thermally-insulating gas. These droplets are then carried through the tubing, their liquefied state being maintained by the thermally-insulating effect of their gaseous envelopes. Upon entering the thermally-insulated detector flask, the droplets are collected and form a liquid pool. Thus, in this manner it is possible to transfer the liquefied gas through a tube to a point of utilization without the necessity of any heavy or bulky thermal insulation for the transfer tube.

It will be appreciated that the efficiency of this method of transferring a liquefied gas depends primarily upon the length and diameter of the transfer tube and the velocity 'of the gas stream flowing therethrough. Thus a relatively large diameter tube with a relatively low velocity gas stream will not deliver, over the same distance, as large a proportion of liquefied gas as a smaller diameter tube carrying a gas stream moving at the same velocity. In general, the efficiency of transfer is an increasing function of the velocity of the gas stream and a decreasing function of the length and diameter of the transfer tube. In one demonstration of the invention wherein an aluminum transfer tube having an internal diameter of 0.075" was employed, with a 1.5 pound pressure differential maintained in the system, a liquid displacement rate of 70 cc. per hour was established, and the approximate percentage of delivered liquefied gas for various lengths of tubing was as follows:

Delivery distance Percent of liquefied Delivery distance Percent of liquefied (tube length, ft.) Na delivered It is generally true that the transport efliciency decreases With an increase in the diameter of the transfer tubing for diameters above a critical value. The critical diameter depends primarily upon the droplet diameter which in turn depends upon the surface tension of the liquid. In the case of nitrogen, for example, if the tubing diameter is smaller than 0.075" it begins to approach the droplet diameter and the droplets begin to touch the Walls of the tube and thus lose their insulating blankets. In the case of nitrogen it was found experimentally that increases or decreases in the diameter of the transfer tubing up to 30% of the critical diameter (0.075") could be tolerated without drastically affecting the efiiciency of the system.

Another embodiment of the invention is shown in'FIG. 2 wherein substantially the same apparatus is utilized except that the means for establishing and controlling the pressure differential in the system is different. In this embodiment, a small electrical resistance heating filament 36 of nichrome, for example, is mounted within the reentrant portion 22 of the supply flask 18 so as to be immersed in the liquefied gas pool therein. Alternatively, the heater may be situated above the surface of the liquefied gas pool. Upon energization of the resistance heater 36, some of the liquid will boil within the re-entrant portion 22 and establish a gas pressure therein over the surface of the liquefied gas pool 26. This will cause the liquid to rise in the transfer tube 16 and boil as described previously. There will thus be a flow of gas from the supply flask 18 to the detector flask 2 through the transfer tube 16. Likewise the formation of liquid droplets with their thermally-insulating gaseous envelopes will occur and the thus-insulated droplets will be carried in and by the gas stream to the detector flask 2.

In order to maintain a relatively constant pressure differential in the system, a gas-pressure-actuated bellows 38 is provided and communicatm with the re-entrant portion 22 of the supply flask by means of the tube 34. As the gas pressure in the re-entrant portion 22 over the liquefied gas pool 26 increases, the bellows expands, eventually opening the energization circuit for the resistance heater 36 by means of the switch 40. This results in no further heating of the liquefied gas pool 26 and hence no further increase in the gas pressure within the re-entrant portion 22 of the supply flask. This condition obtains until the gas pressure in the re-entrant portion 22 drops, permitting the bellows to contract and close the heater energization circuit. In this manner, the pressure differential within the system may be maintained substantially constant and at any arbitrarily fixed value by adjustment of the pressure-actuated bellows-switch system described. It will also be appreciated that some means for preventing a pressure build-up in the detector flask 2 must be provided. This may be established by simply venting the re-entrant portion 8 to the atmosphere, as shown in FIG. 2, by employing an adjustable pressurerelief valve 30 as shown in FIG. 1 and described in connection therewith, or by substituting an orifice of fixed size in the vent to atmosphere. As the pressure in supply vessel 22 rises under the influence of the heating element 36, the pressure in the vessel 8 will also rise, and the flow rate will increase until the pressure drop across the orifice equals the pressure at which the bellows 38 has been set to open the switch 40. The heat input from power supply 42 to heater 36 is thus a function of the pressure drop across the orifice in tube 32, and hence likewise of the flow rate.

By this means, and by properly adjusting the size of orifice, and the pressures at which the switch is opened and re-closed, the flow rate can be adjusted to the cooling requirements of any particular system and will be automatically maintained substantially at this value. With the apparatus of this embodiment, substantially the same transfer efiiciencies are obtained as described in connection with the embodiment shown in FIG. 1.

There thus has been described novel and useful apparatus and methods for transferring a liquefied gas from one point to another without the use of extensive thermal insulation of the transfer means.

What is claimed is:

1. In apparatus having means for utilizing a liquefied gas in the liquid state, a thermally conductive member maintained at a temperature above the boiling point of said liquefied gas connected between said utilizing means and a source of said liquefied gas for transferring a portion of said liquefied gas in the liquid state from said source to said utilizing means and extending below the surface of said liquefied gas in the liquid state in said source, electrical heating means for heating said source of liquefied gas whereby to cause the vaporization of a portion thereof to establish a gas pressure at the input portion of said thermally conductive member relatively higher than the gas pressure at said utilizing means to form liquid droplets of liquefied gas conveyed and thermally insulated by said vaporized portion of liquefied gas, and a pressure-actuated switch responsive to the gas pressure at said input portion of said thermally conductive member to de-energize said electrical means when said gas pressure exceeds a predetermined value.

2. In apparatus having means for utilizing a liquefied gas in the liquid state, a thermally conductive member maintained at a temperature above the boiling temperature of said liquefied gas connected between said utilizing 6 means and a source of said liquefied gas for transferring a portion of said liquefied gas in the liquid state from said source to said utilizing means and extending below the surface of said liquefied gas in the liquid state in said source, means for heating said source of liquefied gas whereby to cause the vaporization of a portion thereof to establish a gas pressure at the input portion of said thermally conductive member relatively higher than the gas pressure at said utilizing means to form liquid droplets of liquefied gas conveyed and thermally insulated by said vaporized portion of liquefied gas, and means responsive to the gas pressure at said input portion of said thermally conductive member to control said heating means.

3. In apparatus having means for utilizing a liquefied gas in the liquid state, a thermally conductive conduit having an inside diameter in the range from about 0.05 to about 0.1 in. maintained at a temperature above the boiling temperature of said liquefied gas connected between said utilizing means and a source of said liquefied gas for transferring a portion of said liquefied gas in the liquid state from said source to said utilizing means and extending below the surface of said liquefied gas in the liquid state in said source, means for heating said source of liquefied gas whereby to cause the vaporization of a portion thereof to establish a gas pressure at the input portion of said thermally conductive member relatively higher than the gas pressure at said utilizing means to form liquid droplets of liquefied gas conveyed and thermally insulated by said vaporized portion of liquefied gas, and means responsive to the gas pressure at said input portion of said thermally conductive conduit to control said heating means.

4. Apparatus according to claim 3, wherein said thermally conductive conduit is a bare, uninsulated aluminum transfer tube having an internal diameter of about 0.075 in., and said apparatus is adapted for utilizing liquid nitrogen.

References (Zited in the file of this patent UNITED STATES PATENTS 1,895,442 Henderson May 24, 1932 2,368,680 Riise Feb. 6, 1945 2,618,935 Malir Nov. 25, 1952 2,671,154 Burstein Mar. 2, 1954 FOREIGN PATENTS 736,486 Germany June 18, 1943

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