US 3310955 A
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United States Patent 3,310,955 LIQUID HYDROGEN REFRIGERATION George K. Sneden, Jr., Spotswood, Irwin Rowe, North Plainfield, and Jorgen Helm, Tenafly, Ni, assignors to Air Reduction Company, Incorporated, New York,
N.Y., a corporation of New York Filed Apr. 12, 1965, Ser. No. 447,143 3 Claims. (Cl. 62-45) This invention relates to refrigeration apparatus and methods and more particularly to the use of liquid hydrogen as a primary refrigerant, together with helium gas as a secondary refrigerant for maintaining temperatures in the range substantially between the boiling point of nitrogen and the freezing point of hydrogen; that is, from about 77 K. down to about 14 K.
An object of the invention is to reduce or avoid danger in the handling and use of liquid hydrogen for refrigeration when cooling below about 77 K. is desired.
Another object is to physically separate the main voltune or all of the hydrogen from the neighborhood of the operating personnel and from the apparatus to be refrigerated, with interposition of a secondary refrigerant in the form of helium gas.
Another object is to supply refrigeration capable of cryopumping and/or thermal space simulation in a safe manner and more economically than heretofore.
A further object is to utilize liquid hydrogen to provide versatile thermal systems that are safe and practical for use at temperatures below about 77 K.
A feature of the invention is that the liquid hydrogen can be stored and used in an open area which is remote from personnel and equipment.
Another feature is that the liquid hydrogen can be stored in a standard container or tank and used in heat exchangers located at the site of the hydrogen storage, rather than at the location of the device to be refrigerated.
A further feature is the elimination of expensive and complex system components heretofore required in low temperature refrigeration; comprising compressors, numerous complicated heat exchangers, and expanders.
It has been possible when maintaining temperatures below about 77 K. to employ hydrogen directly as a refrigerant but such use is dangerous due to the highly explosive nature of hydrogen. Accordingly, elaborate precautions have been required and the use of hydrogen as a refrigerant has generally been restricted to cases where it is practically and commercially feasible to adequately protect operating personnel and valuable equipment. The danger has militated against the use of hydrogen refrigeration, particularly in testing equipment in laboratories, including such uses as in test chambers for simulating conditions of temperature and vacuum in outer space. Since it has been feasible to use hydrogen refrigeration for refrigeration, closed cycle gaseous refrigerators containing expensive compressors, special heat exchangers, and expanders have been used.
In accordance with the invention, liquid hydrogen is stored in a tank or Dewar of conventional design and is conveyed as needed to the cold end of a multiple tube complex heat exchanger where the hydrogen evaporates to provide refrigeration. The heat exchanger is comprised preferably of this evaporating circuit, gas to gas regenerating circuits, and circuits to utilize the refrigeration effect of the cold vent helium gas. The hydrogen gas is pumped continuously to a sufficiently low pressure to lower the temperature of the boiling hydrogen to the desired level. A vacuum pump may be located outside the cold box. The hydrogen storage tank, the cold box and the vacuum pump are, for safety considerations, preferably located in an open area remote from personnel and M: ice
valuable equipment. Additionally, an evaporating liquid nitrogen circuit may be utilized for initial cooling and to provide additional refrigeration that will reduce the overall size of the heat exchange assembly. A closed circuit is provided for helium gas, extending between the cold box and the point of utilization of the refrigeration. The helium gas is precooled by gaseous hydrogen, return stream helium and, if utilized, liquid nitrogen. The helium gas is then further cooled 'by hydrogen gas, return stream gaseous helium and finally liquid hydrogen. Optionally, the helium gas may be cooled by the liquid hydrogen by passing the helium gas through a boiler coil immersed in the liquid hydrogen. The liquid hydrogen w-ill evaporate as a result of such heat exchange contact. The cooled helium gas goes thence to the utilization device, for example, a test chamber for simulating conditions of temperature and vacuum in outer space. The helium gas leaving the utilization device is preheated to a temperature suitable for pumping, by heat exchange with helium gas on passing through the heat exchanger as above noted, and is pumped back to the heat exchanger for precooling against the hydrogen vapor :as above noted.
The heat exchanger providing for hydrogen evaporation (hydrogen evaporator) is preferably pumped out to such a low pressure as to reduce the boiling point of the liquid hydrogen to about 14 K., whereby the temperature in the utilization device may be readily maintained in a range between 15 K. to 20 K.
As will be discussed more completely hereinafter, the invention provides adequate means for effecting cryogenic pumping down of the utilization device to a pressure of about 10* mm. of mercury, which is generally a sufficiently low pressure for space vacuum simulation in the testing of equipment which is to be used in space exploration and the like. Furthermore, the system disclosed is much less expensive both in capital and operating costs than systems which employ liquid helium, or other expensive refrigerating techniques.
Other features, objects and advantages will appear from the following more detailed description of an illustrative embodiment of the invention, which will now be given in conjunction with the accompanying drawing, the single figure of which is a schematic diagram or flow sheet of an embodiment of the invention.
Referring to the drawing, there is shown a chamber 20 containing cryopanels 54 to be refrigerated, which chamber is shown by way of illustration as a cryogenically pumped chamber. Primary sources of refrigeration are provided in the form of a supply of liquid hydrogen contained in a storage tank or Dewar 22 and, preferably a supply of liquid nitrogen contained in a storage vessel or Dewar 17. A secondary source of refrigeration is provided in the form of a helium ga-s circuit 24 in heat exchange relationship with hydrogen in a cold box 26.
Liquid hydrogen is supplied through a valve 28 to a hydrogen evaporator 30. Gaseous hydrogen is pumped from the evaporator 30 into the cold end of a countercurrent heat exchanger 35 and thence to the cold end of a countercurrent heat exchanger 32 by means of a vacuum pump 34 into the top of the tank 22 by means of a vacuum pump 34 into the top of the tank 22 through a conduit 36 and/ or exhausted to the atmosphere by way of a vent 38.
The helium gas circuit 24 contains pressurized helium gas in a closed circuit, movable in the direction of the arrow 40 by means of a gas pump 42, which latter imparts an operating pressure increase to the gas. The resulting heat of compression is at least partially removed in a water cooled after-coo1er 44 from which the helium gas is introduced into the warm end of the heat exchanger 32 in countercurrent relationship to the hydrogen gas therein. From the cold end of the exchanger 32, the helium gas is introduced into the warm end of a regenerative heat exchanger 46 in countercurrent relationship to another portion of the helium gas stream which is returning from the cryopanels 50. From the cold end of the heat exchanger 46, the helium gas stream goes to a liquid nitrogen evaporator 37 supplied from the liquid nitrogen storage tank 17. From the liquid nitrogen evaporator 37, the gas enters the warm end of heat exchanger 35, countercurrent to the hydrogen gas stream, thence to another regenerative exchanger 39 where the supply helium gas passes through the exchanger, in a countercurrent relationship with helium gas returning from the cryopanels 50, and finally into the liquid hydrogen evaporator 30, which is supplied from the liquid hydrogen storage tank 22.
In the drawing, the heat exchangers 30, 32, 35, 37, 39 and 46 have the warm and cold ends labeled W and C, respectively, and the direction of gas flow in the respective passages is indicated by vertical arrows. The direction of heat flow in each exchanger is indicated by a horizontal arrow in the direction from one passage to the other.
The hydrogen and nitrogen evaporators 30 and 37 and the heat exchangers 32, 35, 39, and 46 are installed in the cold box 26 whereby they are well insulated against heat leak from their surroundings.
From the cold end of the evaporator 30, the helium V stream is fed to cryopanels 50 to provide refrigeration for cryopumping. The refrigerated cryopanels 50 condense residual molecules impinging thereon in fiight within the highly evacuated interior of the chamber 29. Means for evacuating the chamber 20 to a pressure sufliciently low to make cryopumping effective is shown as a vacuum pump 52, which may comprise several stages or a combination of pumps of different types such as mechanical, mercury ejector, or dilfusion, in known manner. The vacuum pump 52 serves to continuously remove such molecules that are not condensable on the refrigerated cry-opanels 50 due to their pressure temperature characteristics.
The warmed helium gas stream, warmed by heat of condensation, direct and reflected radiant heat emitted by the test article and the test chamber 20 walls, and radiant heat emitted by the cryopanel 50 thermal shields, passes from the cryopanels 50 into the cold end of the heat exchanger 39 thence to the cold end of heat exchanger 46 from which it is drawn into the inlet of the gas pump 42 to be recirculated.
While only two cryopanels are shown in the schematic representation made in the drawing, it will be understood that many such panels will ordinarily be used, arranged so as to present a relatively large surface for intercepting gas molecules moving within the chamber 20.
In the operation of the specific refrigerating system illustrated in the drawing in the evaporator 30, the liquid hydrogen which is at a temperature near 14 K. is evaporated under reduced pressure maintained by the vacuum pump 34, providing hydrogen gas entering the heat exchanger at a temperature of about 15 K., and thence entering the heat exchanger 32 at a significantly higher temperature. Heat of exaporation in the eavporator 30 is provided by the helium gas. In the evaporator 30, the helium gas is reduced to a temperature of about 15 K. and is pumped thence to the cryopanels 50. In the chamber 20, the cryopanels provide cryogenic pumping which we have found to be effective down to a pressure of about 10' mm. of mercury.
The helium gas leaving the cryopanels, at a temperature of about 20 K. is preheated in the heat exchangers 39 and 46 to a suitable temperature for introduction into the gas pump 42. After-cooled helium gas (in after-cooler 44) from the gas pump 42 is cooled additionally, first in the hydrogen-helium heat exchanger 32 and then in the helium-helium heat exchanger 46. The gas is further cooled in the liquid nitrogen evaporator 37, in the hydrogen-helium heat exchanger 35, and then in the heliumhelium heat exchanger 39. The helium gas is then given the main cooling in the liquid hydrogen evaporator 30, from which the gas is recirculated.
The system may be operated either continuously or intermittently under the control of the valve 28 and preferably valve 51. The size of the plant may be designed to meet the needs of use. For example, it may be desired to use the plant in eight hour stretches, or for some other definite period.
While illustrative forms of apparatus and methods in accordance with the invention have been described and shown herein, and specific details disclosed relating to the illustrated embodiment shown in the drawing, it will be understood that numerous changes may be made without departing from the scope of the invention, as defined by the following claims.
1. The method of supplying refrigeration in the temperature range below 77 K. which comprises the steps of evaporating liquid hydrogen in a heat exchanger assembly remote from a location of a utilization device in which the said refrigeration is to be utilized, circulating gaseous helium in a closed circuit extending between the location of said assembly and the location of said utilization device by impressing thereon an operating pressure, said operating pressure remaining substantially constant throughout the circuit, transferring refrigeration from said evaporating hydrogen to said helium gas by means of said heat exchanger assembly, said closed circuit thereby transporting refrigerated helium gas to the location of the utilization device and returning warmed helium gas to the location of the heat exchanger assembly.
2. The method of supplying refrigeration in the temperature range below 77 K. which comprises the steps of evaporating liquid hydrogen in a heat exchanger assembly remote from a location of a utilization device in which the said refrigeration is to be utilized, circulating gaseous helium in .a closed circuit extending between the location of said assembly and the location of said utilization device, cooling said helium in stages, first by heat exchange with hydrogen vapor efiluent from said evaporating vessel, then by heat exchange with a portion of said helium returning from the utilization device, further by heat exchange with evaporating liquid nitrogen, again by heat exchange with hydrogen vapor efiiuent, thence by heat exchange with another portion of the return helium gas, and finally by subjecting it to the cooling effect of the evaporating liquid hydrogen, and delivering the t-hus cooled helium to the utilization device, whereby risk to personnel and equipment at the utilization location due to hazards inherent in the use of liquid hydrogen is avoided.
3. In apparatus for supplying refrigeration to a utilization device in a temperature range between the boiling point of liquid nitrogen and the freezing point of liquid hydrogen, in combination, a hydrogen circuit including a storage tank for liquid hydrogen, an evaporator for evaporating said hydrogen, .a first and second countercurrent indirect heat exchanger, and a vacuum pump connected serially in the order named; a nitrogen circuit including a storage tank for liquid nitrogen and evaporator for said liquid nitrogen, a closed circuit for helium gas including a gas pump, an after-cooler, said first countercurrent heat exchanger, a first regenerative heat exchanger, said liquid nitrogen evaporator, said second countercurrent heat exchanger, a second regenerative heat exchanger, said liquid hydrogen evaporator, and a utilization device for said refrigeration, connected serially in the order named; said first countercurrent indirect exchanger being arra-nged to transfer heat from said helium gas to hydrogen vapor in said hydrogen circuit, said first regenerative heat exchanger being arranged to transfer heat from said helium gas to helium gas returning from said second regenerative heat exchanger, said liquid nitrogen exchanger being arranged to transfer heat from said helium gas to evaporating liquid nitrogen in said liquid nitrogen circuit, said second countercurrent indirect heat exchanger being arranged to transfer heat from said helium gas to hydrogen vapor in said hydrogen circuit, said second regenerative heat exchanger being arranged to transfer heat from said helium gas to said helium gas returning from said utilization device, said hydrogen evaporator being arranged to transfer heat from said helium gas to said vaporizing liquid hydrogen, and said gas pump serving to circulate said helium gas under pressure through said helium circuit; and a cold box for insulating said evaporators and said first and second countercurrent indirect heat exchangers and said first and second regenerative heat exchangers against heat leak from their surroundings, said cold box, said vacuum pump, and said hydrogen storage tank being located at a substantial distance from said utilization device.
References (Iited by the Examiner UNITED STATES PATENTS O THER REFERENCES Advances in Cryogenic Engineering, volume 5, Design, Construction and Performance of a Laboratory- Size Helium Liquefier, chapter 6-5, 7 pages.
LLOYD L. KING, Primary Examiner.