Storable cryogenic heat pipe

Abstract

A cryogenic heat pipe that may be conveniently stored at normal room temperatures and pressures is disclosed. The cryogenic working fluid is caused to expand into a storage reservoir under these room conditions of temperature and pressure since all cryogenic fluids exceed critical pressure and temperature conditions at room temperatures. To activate the heat pipe from storage conditions, the condenser region must be cooled down to cryogenic temperatures. The cryogenic working fluid vapor in the system forms a condensate in the wick and only superheated gas remains in the reservoir. In one embodiment an adsorption reservoir is employed to reduce the external volume of the unit from that which would be obtained using a purely volumetric reservoir. The adiabatic region of the heat pipe is covered with a multi-foil-vacuum, super-insulation section to minimize ambient temperature effects when the device is activated and operating.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to cryogenic heat transfer devices, and more particularly to cryogenic heat pipes capable of safe, convenient storage at room temperatures.

2. Description of the Prior Art

A heat pipe is fundamentally an evacuated chamber employing a volatile working fluid that occupies the interstices of a capillary wick. Heat is transferred from a heated evaporator section at one end of the heat pipe to a condenser section at the other end by a process called vapor heat transfer. When vaporization of the working fluid in a heat pipe occurs, two important phenomena come into play. First, upon evaporating, the working fluid absorbs from the heated evaporator surface a quantity of heat equal to its latent heat of vaporization. Second, as the working fluid changes its state from liquid to vapor, the vapor pressure increases in the evaporator section of the heat pipe causing the working fluid vapor to move toward the condenser section. Since the temperature in the condenser section is lower than that in the evaporator section, condensation of working fluid vapor occurs on the capillary wick in the condenser section. Upon condensation, the working fluid vapor gives up the thermal energy stored as its latent heat of vaporization. In addition, as the working fluid vapor changes state from vapor to liquid the vapor pressure decreases in the condenser section, thereby maintaining a pressure differential between the evaporator section and the condenser section so as to continue repetition of the vapor heat transfer cycle. It is important to note that the working fluid stores heat energy at the temperature at which it evaporates, retaining that heat energy at that temperature until the working fluid condenses.

Vaporization of the liquid working fluid at the evaporator end of the heat pipe causes depletion of the liquid in that area. Movement of the condensed working fluid from the condenser section back to the evaporator section is accomplished by means of capillary action within the wick. The capillarity of the wick, therefore, causes the liquid to pump itself back to the evaporator section so as to repeat the heat transfer cycle. There are, however, factors which limit the operation of heat pipes. An insufficient volume of working fluid within the system inhibits efficient heat transfer within the heat pipe. If the wick does not permit sufficiently rapid movement of the condensed working fluid back to the evaporator section, all of the incident heat energy cannot be absorbed. Either circumstance results in a large increase in the temperature of the evaporator section. Increasing the temperature of the heat pipe as a whole results in a rise in the vapor pressure of the working fluid. Temperatures in excess of a maximum allowable operating temperature may result in a sufficiently high vapor pressure to rupture the walls of the heat pipe.

One obvious solution to the foregoing problem is to make the walls of the heat pipe thick enough and the structural bonds strong enough to withstand such high vapor pressure. However, thick walls increase the temperature gradients through the walls of the evaporator and condenser regions, thus lowering the overall efficiency of heat transfer.

SUMMARY OF THE INVENTION

A cryogenic device according to the invention is an hermetically sealed unit comprising a cryogenic heat pipe, enough cryogenic working fluid to completely saturate its wick when the working fluid is in its liquid state and an expansion chamber. Examples of particular cryogenic working fluids which may be employed and their boiling points are: Oxygen at -183.degree.C, liquid nitrogen at -196.degree.C and freon at -81.degree.C. Insufficient fluid will limit performance of the heat pipe, on the other hand, if the heat pipe is over-filled and allowed to warm up, all liquid will become gas at the critical pressure and this buildup of pressure can cause an explosion. It has already been pointed out that increasing the structural strength of the heat pipe is an unsatisfactory technique. The present invention provides an expansion chamber to furnish a reservoir for the safe storage of the vaporized working fluid at room temperatures. To activate the device for use, it is only necessary to cool the condenser region down to cryogenic temperatures thus causing the gas contained both in the heat pipe region and the reservoir region to condense out in the wick.

A chamber formed about the longitudinal dimension of the heat pipe, wrapped with foil layers and then evacuated forms the super-insulation section of the device. This superinsulation section helps to provide efficient heat transfer from the evaporator section to the condenser section. During operation of the device, there will be negligible heat transfer to the environment through that region of the heat pipe protected by the super-insulation section.

OBJECTS

It is, accordingly, an object of this invention to increase the efficiency of operation of a cryogenic heat pipe device.

It is a further object of this invention to provide an optimum amount of working fluid that can be employed with a cryogenic heat pipe.

It is still another object of this invention to provide a heat pipe for use at cryogenic temperatures capable of safe storage at room temperatures.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cut away side elevational view of a device according to one embodiment of the invention;

FIG. 2 is a simplified partially cut away side elevational view of a device according to another embodiment of the invention;

FIG. 3 is a graph illustrating the functional relationships between temperature, pressure and the arrival rate of the adsorbate at the surface of the adsorbent in an adsorptive reservoir;

FIG. 4 is a partial cut-away side elevational view of an adsorption reservoir showing the adsorbent as a metallic powder.

DETAILED DESCRIPTION

Referring more specifically to the drawings, FIG. 1 is a simplified pictorial view of a storable cryogenic heat pipe. In FIG. 1 an elongated cylindrical chamber is provided having an evaporator section 11, a vapor transfer section 17 and a condenser section 12. A capillary wick 30 is shown disposed along the inner surface of the elongated cylindrical chamber. A multifoil-vacuum insulation section 13 surrounds vapor transfer section 17. Expansion chamber 16 is connected to the elongated cylindrical chamber through tube 15.

In FIG. 2 a different form of the device is illustrated in that expansion chamber 16' is shown folded back upon and surrounding the body of the device instead of being an appendage thereto. Such a construction affords increased structural integrity over the construction illustrated in FIG. 1 in addition to the greater efficiency of insulation provided for vapor transfer region 17.

Assuming quiescent conditions of storage at room temperature and pressure, it is necessary to activate the device before operating conditions may be attained. Under room conditions of temperature and pressure the working fluid is in the vapor state both in the heat pipe chamber and in the reservoir 16. It is necessary to cool the condenser section 12 down to cryogenic temperatures, thus working fluid vapor will condense on the wick 30 of the heat pipe in the vicinity of condenser section 12 and migrate under capillary action to evaporator section 11. Because of the high temperature gradient now existing between the reservoir 16 and the condenser section 12 of the heat pipe, all cryogenic vapors condense in the condenser section 12 and only superheated gas remains in the reservoir 16. Exposure of evaporator section 11 to a source of heat will initiate the vapor heat transfer cycle by causing the liquid working fluid in that section of the heat pipe to vaporize. Vaporization of working fluid causes an increase in vapor pressure in the vicinity of evaporator section 11 and a subsequent migration of the working fluid vapor toward condenser section 12, a region of comparatively low vapor pressure. The liquid working fluid, upon vaporizing, retains its latent heat of vaporization as taken in from the evaporator surface 11. Upon the subsequent condensation of the working fluid vapor within condenser region 12 this latent heat of vaporization is released. The condensate thus formed in region 12 collects on the wick 30 where it is transported through capillary action to region 11 where the supply of liquid working fluid has been depleted through evaporation. The vapor heat transfer cycle thus described will continue so as to maintain evaporator section 11 at approximately the temperature at which the liquid working fluid evaporates. Multifoil-vacuum insulation section 13 prevents heat transfer through the walls of the heat pipe in vapor transfer section 17, thus heat is transferred from evaporator section 11 to condenser section 12 without attendant losses through the walls of region 17. Expansion chamber 16 is connected to condenser section 12 through low thermal conductivity tubing 15.

In one embodiment, the expansion reservoir utilized with the invention is an adsorption chamber. The phenomenon of adsorption is shown functionally by the curves of FIG. 3. Some solids can take up several times their own volume of gases by a process known as sorption, a generic term referring to and including both absorption and adsorption. The solid which takes up the gas is known as the sorbent; the gas or vapor removed from the gas phase is called the sorbate; and the process of removing gas from a sorbent is usually referred to as desorption. The gas or vapor may, however, interact only with the surface of the solid in which case the process involved is known as adsorption which may be defined as the deposition of a layer having a thickness of one or more molecules on the surface of a solid. The solid taking up the gas or vapor is referred to as the adsorbent; the gas or vapor removed from the gas phase is called the adsorbate; and the process of removing gas from the adsorbent is referred to, as before, as desorption. An adsorbent is generally a material with large surface-to-mass ratio. The quantity of gas or vapor contained by adsorption is a function of the temperature and pressure of the environment surrounding the adsorbent as well as the gas or vapor to which it is exposed. In FIG. 3 the arrival rate of the adsorbate at the surface of the adsorbent is plotted as the ordinate in atoms/cm.sup.2 -sec, while the absolute temperature of the system is plotted as the abscissa. A coverage coefficient 0 provides a measure of the coverage of the adsorbent by the absorbate. A value of .theta. equal to unity corresponds to one complete monolayer coverage. The term "monolayer" has reference to a layer of the adsorbate that is one molecule thick. Thus, a reservoir can be designed to contain large numbers of vapor molecules which can, when needed, be transferred to a heat pipe section by creating a pressure gradient between the heat pipe and the reservoir. Such pressure gradient may be created by cooling the condenser section of the heat pipe to cryogenic temperatures.

FIG. 4 shows a cut-away view of an adsorption type reservoir wherein 15' is low conductivity tubing connecting the reservoir to the heat pipe proper, 21 is the envelope of the reservoir and 22 represents the adsorbent in the form of a metallic power. The required storage area for a reservoir of this type is given by the relation: ##EQU1## where V= volume d= particle diameter

We may estimate the storage capabilities of an adsorption type reservoir in comparison to a volume type reservoir from a consideration of the general gas equation:

Pv=n RT

where P=pressure

V=volume

T=absolute temperature

n=the number of molecules, and

R=the gas constant

According to this relation, the number of molecules, n, will be given as: ##EQU2## and the resultant volume required to contain these molecules as: ##EQU3## in the volume type of reservoir. In the adsorption reservoir, storage capability is increased by the added term: ##EQU4## so that ##EQU5## gives the number of molecules capable of storage in the adsorption type reservoir. The resultant volume required to contain these molecules is given by ##EQU6## which may be rewritten in the form ##EQU7## thereby showing that the required volume for an adsorption reservoir is less by the second term than that for a volume reservoir.

It should be understood, of course, that the foregoing disclosure relates to only preferred embodiments of the invention and that it is intended to cover all changes and modifications of the examples of the invention herein chosen for the purposes of the disclosure, which do not constitute departures from the spirit and scope of the invention.

Claims

What is claimed is:

1. A cryogenic heat transfer device comprising:

an hermetically sealed, evacuated elongated housing;

a capillary wick longitudinally disposed within said housing;

an expansion reservoir in fluid communication with said elongated housing;

an insulation shell, longitudinally disposed about a substantial portion of the outer lateral surface of said elongated housing; said reservoir is a shell-like structure enclosing said insulation shell; and

a quantity of cryogenic working fluid disposed within said capillary wick and said expansion reservoir.

2. A cryogenic heat transfer device in accordance with claim 1 wherein said insulation comprises an hermetically sealed evacuated chamber containing foil wrappings.

3. A cryogenic heat transfer device comprising:

an hermetically sealed, evacuated elongated housing;

a capillary wick longitudinally disposed within said housing;

an adsorption reservoir in fluid communication with said elongated housing;

an insulation shell, longitudinally disposed about a substantial portion of the outer lateral surface of said elongated housing; and

a quantity of cryogenic working fluid disposed within said capillary wick and said expansion reservoir.

4. A cryogenic heat transfer device in accordance with claim 2 wherein said adsorption reservoir is a shell-like structure enclosing said insulation shell.