US3407615A - Low temperature heat exchanger - Google Patents

Description

Oct. 29, 1968 G. KLIPPING LOW TEMPERATURE HEAT EXCHANGER Filed sept. 14,v 196e INV ENIOR au u Gustav Klpping ATTORNEYS United States Patent O v 3,407,615 LOW TEMPERATURE HEAT EXCHANGER Gustav Klipping, Berlin-Zehlendorf, Germany, assigner to Max-Planck-Gesellscllaft zur Forderung der Wissenschaften eN., Gottingen, Germany Filed Sept. 14, 1966, Ser. No. 579,244 Claims priority, application Germany, Sept. 14, 1965, M 66,623 4 Claims. (Cl. 62-45) ABSTRACT F THE DISCLOSURE A heat exchanger for use in low temperature heat exchange devices using low boiling fluids and, particularly, helium, hydrogen and nitrogen. The heat exchanger has a housing whose walls are shrink-fitted onto a sintered metal insert, and is therefore characterized by an extremely high heat conductivity.

The present invention relates generally to the cryogenic field, and, more particularly, to a heat exchanger which uses the low temperature of low boiling refrigerants, and particularly helium, hydrogen, and nitrogen, which are fed in the liquid phase to the interior of the heat exchanger in which a sintered metal insert or charge is disposed for improving heat transfer.

In known continuous flow cryostats for producing low temperature, the low boiling refrigerant is fed by a vacuum pump into the vaporizing chamber of the cooling head Where it evaporates and brings about the cooling of the cooling head which is provided, for example, with a probe holder, and thus cools the probe which is supported thereon. For many applications such devices are required to have a high heat load at low temperatures of down to 2.5" K. As a prerequisite for a high heat load, however, the heat exchange between the refrigerant and the probe holder must be as complete as possible. Previously known heat exchangers were provided as hollow chambers, and in order to improve the heat transfer battles or fillers, such as sintered metal bodies, were introduced therein. Gther heat exchangers were provided as a spiral pipe or as a solid body having flow chanels and it was sought to provide as ygood a heat transfer as possible by branching off and providing countercurrently directed pipes and channels.

With this in mind, it is a main object of the present invention to provide a heat exchanger for low boiling refrigerants having considerably greater heat load at low temperature than previous devices.

Another object of the present invention is to provide a heat exchanger including a sintered metal which has an extremely good heat conducting connection between sintered metal and the body which supports it.

A further object of the present invention is to provide a heat exchanger in which a housing is shrunk onto a sintered metal insert to provide good heat exchange contact between the insert and the housing.

These objects and others ancillary thereto are accomplished in accordance with preferred embodiments of the invention wherein the heat exchanger is shrunk with its wall portions onto a sintered metal insert or charge within the area of its circumference. In accordance with a further feature of the invention chambers are provided both in front of and to the rear of the sintered metal insert or charge and these chambers have cross-sectional areas which correspond approximately to the free surface of the sintered metal insert or charge.

It may be particularly advantageous in this arrangement to provide the housing of the heat exchanger to have a pot-like construction. The free opening receives the rce cylindrical sintered metal insert or charge and the supply line for the refrigerant terminates at a closing element of the heat exchanger which closes off the free opening of the pot-like member.

Additional objects and advantages of the present invention will become apparent upon consideration of the following description when taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a schematic cross-sectional view through a sintered metal heat exchanger for cooling the probe.

FIGURE 2 is a schematic view of a sintered metal heat exchanger in a continuous ow cryostat disposed on the refrigerant storage tank.

As can be seen in FIGURE l a pot-shaped housing 2 which may be of copper -is shrunk onto a preformed sintered metal insert or charge which may be of copper too to get an extremely good heat conductive connection with the housing 2 over the entire periphery or circumference.

A sintered metal, eg., copper, brass, silver or the like, is a porous material which is made by the sintering of a powder of spherical or spattered particles of the same size. The porosity can be Widely varied by variations of sintering temperature and sintering time. A sintered metal is machinable as a compact metal and can be soldered or welded like a compact material.

Shrinking the housing 2 onto the sintered metal insert 1 lrneans that the housing with a bore of given diameter is warmed up to about C. whereby it is expanded. At the same time the sintered metal insert which has a diameter slightly bigger than the diameter of the bore inside the housing is cooled to about -200 C. (liquid nitrogen) whereby it is shrunk to a diameter which is smaller than the diameter of the enlarged bore inside the warm housing. Both parts are then tted into each other. When the combined parts have reached room tem perature, the molecular forces operating between the housing and the sintered metal insert will provide an optimum connection which is similar to an intermetallic compound or a solid solution, and which results in optimum heat conduction between both parts shrunk together. The housing will thus be in a. shrinlotted relationship to the insert. The difference between the diameters of both parts in the initial separated state at room temperature, which was described above as having the sintered metal insert slightly bigger depends on the materials of both parts and their expansion coeicients, on the diameter, and on the temperature difference that can be achieved before the parts are fitted into each other. Preferably the insert and the housing have approximately the same coeicient of thermal expansion.

As FIGURE l shows the pot-shaped housing 2 is closed off at its free opening thereof by a closure member or cover 3 and a refrigerant supply line 4 is disposed in cover 3. An exhaust gas line 5 is connected to the upper end of the housing 2. The housing 2 is provided as a probe holder in which the probe 6 is held in place by means of a dip soldered connection 7. A temperature sensor 8 is disposed in the housing 2 in proximity to the probe 6. In the ernbodiment illustrated the temperature sensor 8 is connected as part of a vapor pressure thermometer, but it is equally possible to use instead electrical sensors or both types of sensors.

During operation the refrigerant enters from the supply line 4 into the distributing chamber 9 for the refrigerant and fiows from there to the interior of 'the sintered metal insert or charge 1. Here, heat exchange takes place While the refrigerant evaporates, and possibly the cold gas too absorbs some heat. The resultant cold gas flows into the collector chamber 10 and from there into the exhaust gas line 5. The heat which is to be conducted away from the probe 6 is conducted through the housing 2, which must be suitably designed, and is conducted through the sintered metal insert or charge 1 to the refrigerant. A suitably designed housing means that the wall thickness must be sufficient enough so that the maximum heat load can flow through this wall to the sintered metal, i.e., the necessary wall thickness depends on the material of the housing and its coeicients of heat conduction at low temperatures and on the maximum heat load. For better understanding a numerical example will be given: for a housing made of copper (coeicient of heat conduction at 42 K. is \=4W/cm.2 K.), a height of the housing of 4 cm., a temperature difference between inside and outside surface of 1 and a heat load of 10W, the wall thickness should be about 1 cm.

FIGURE 2 shows a sintered metal heat exchanger 11 disposed in the cooling head 12 of a continuous flow cryostat for probe cooling. In a known manner the cooling head is disposed in a vacuum housing 13 and cornmunicates with a refrigerant storage tank 15 by means of riser 14. It also communicates via exhaust gas line 16 which is arranged in the form of a radiation shield 16a by being coiled around the cooling head. It is provided with a regulating valve 18 which is thermostatically controlled in dependence upon the evaporator temperature and this is accomplished by means of a temperature sensor 17. It is also connected with a feed pump 19. This could be used with the apparatus disclosed in U.S. Patent No. 3,166,915.

The heat transfer which may be accomplished in a heat exchanger depends among other things upon the size of the exchange surface. In order to obtain a high heat load of the heat exchanger the exchange surface Should be as large as possible. Sintered metals have a large specific surface due to their porosity and, since they are made from the metals copper, silver, brass and the like, they also have relatively good heat conduction. Also, even with the smallest pore width or diameter they have sufcient permeability for the low boiling refrigerants due to the low viscosity of the latter. Moreover, particularly good heat transfer can take place between low boiling uids and a metal within a sintered metal body. On the other hand the quality of the heat exchange between the refrigerant and the probe which is supported at the surface of the heat exchanger depends to a large extent upon the heat transfer between the sintered metal insert or charge and the housing of the heat exchanger which encloses it. The sintered metal body which itself has relatively good heat conduction must therefore be brought into a good conducting connection with the cooling head which is the probe holder. It is particularly advantageous to form the sintered metal body, which may be machined as a solid material, true to size or measure on a part of the surface thereof and to shrink the cooling head onto it, because there results a connection with optimum heat conduction characteristics. Advantageously according to this method the pores at the free surface of the sintered metal body remain intact, i.e., the free surface remains fully permeable for the refrigerant. Thus, the heat exchanger provided, as proposed by the present invention has a considerably greater heat load at low temperatures than do the known devices.

It will be understood that the above description of the present invention is susceptible to various modifications, changes, and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

What is claimed is:

1. A heat exchanger for use in low temperature heat exchange devices using low boiling uids and, particularly, helium, hydrogen and nitrogen, comprising, in combination, a heat exchanger housing having walls; and a preformed sintered metal insert disposed within said housing and forming therewith an intermetallic compound or a solid solution; said housing being in shrink-fitted relationship to said insert, thereby providing an element having an extremely high heat conductivity.

2. A heat exchanger as dened in claim 1 wherein in the direction of the iiow of refrigerant a distributing chamber is disposed before the sintered metal insert in the housing and a collection chamber is disposed after the sintered metal insert in the housing, the cross-sectional surfaces of said chambers corresponding approximately to the free surface of the sintered metal insert.

3. A heat exchanger as dened in claim 2 wherein the housing is pot-shaped and in its free opening receives the sintered metal insert which is cylindrical, a cover closing olf the free opening of said housing, and a refrigerant supply line being connected to said cover so as to communicate with the distribution chamber.

4. A continuous flow cryostat with a cooling head having a heat exchanger as defined in claim 1.

References Cited UNITED STATES PATENTS 2,267,339 12/1941 Paulsen 29-447 2,448,315 s/1948 Kunzog -154 x 2,663,626 12/1953 Spangler 62-48 X 3,302,415 2/1967 Royer 62-48 x ROBERT A. OLEARY, Primary Examiner.

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