Patent Publication Number: US-9851126-B2

Title: Cooling apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application is a U.S. National Stage Application filed under 35 U.S.C. §371 of International Application PCT/AT2011/000298, filed Jul. 12, 2011, designating the United States, which claims priority from Austrian Patent Application A 1177/2010, filed Jul. 12, 2010, the complete disclosures of which are hereby incorporated herein by reference in their entirety for all purposes. 
     The invention relates to a cooling apparatus with a closed cooling circuit for cooling objects to semi-cryogenic or cryogenic temperatures of 230 K to 80 K, comprising a compressor for compressing a coolant, to which the coolant is supplied in a gaseous state, and from which the coolant exits in a compressed gaseous state, an after-cooler connected downstream from the compressor, a counterflow heat exchanger comprising a feed line and return line, which are arranged in such a way that the compressed coolant can be liquefied in the feed line as the relieved coolant flowing through the return line is being heated, and a cooling head that is connected with the feed line and return line and has coolant flowing through it, in which the coolant evaporates. 
     For example, such a cooling apparatus may be gleaned from Document EP 650574 A1. 
     In commercial cooling systems, the counterflow heat exchanger and cooling head are combined into a structural unit, and arranged in a vacuum chamber, in which the object to be cooled is also placed. The counterflow heat exchanger situated in the vacuum chamber is here integrated into the cooling circuit via flexible gas feed lines. The fact that the coolant in the flexible gas feed lines is at room temperature, and is only brought to semi-cryogenic or cryogenic temperatures in the vacuum chamber, eliminates the requirement that these lines be insulated. This avoids problems associated with transporting coolant cooled to semi-cryogenic or cryogenic temperatures via flexible lines, e.g., icing, formation of condensate, as well as heat losses. However, the disadvantage is that the devices require a large vacuum chamber, which is undesired or even unusable for many processes. This is because the heat exchangers used in such devices are generally several meters long and wound into a spiral to achieve a certain level of compactness for the cooling head with the heat exchanger. Nevertheless, the heat exchangers are already relatively large (approx. 200 mm high with a diameter of 80 mm) at low outputs (e.g., −130° C. at 30 W). In addition, such a cooling system requires a good vacuum system, since the volume of the vacuum chamber must be correspondingly large. In existing devices (e.g., Polycold-Cryotigers) that use the mixed gas Joule Thomson process (see EP 650574 A1) and have the counterflow heat exchanger with cooling head in the vacuum chamber, an increased cooling capacity is accompanied by a distinctly larger heat exchanger. In particular at higher cooling capacities (approx. &gt;50 W), such devices are no longer economically viable or attractive, in particular for scientific applications (e.g., heavy duty laser amplifiers, e.g., 150 W at 130 K, laser crystals measuring 3 mm×6 mm, for example), in which small vacuum chambers are desired. 
     Therefore, the object of the invention is to improve a cooling apparatus in such a way that an object can be efficiently cooled to the lowest possible semi-cryogenic or cryogenic temperatures, wherein the vacuum chamber is simultaneously to exhibit as small and convenient as possible, while at the same time significantly raising the capacity (e.g., 200 W/140 K instead of 30 W/140 K). In addition, losses incurred while transporting the coolant are to be minimized. 
     This object is achieved by further developing the cooling apparatus of the kind mentioned at the outset according to the invention essentially in such a way as to arrange the cooling head in a vacuum chamber that can be connected with a low-pressure source, and is joined by flexible connecting lines with the feed line and return line of the counterflow heat exchanger, so that the counterflow heat exchanger is situated outside the vacuum chamber. Therefore, the invention is based on the idea of using the conventional mixed JT method, and executing the process of liquefying the coolant with the counterflow heat exchanger separately from the vacuum chamber. As a consequence, the coolant is liquefied outside of the vacuum chamber, wherein the liquefied coolant is supplied to the vacuum chamber by way of flexible connecting lines. One must here only make sure that the heat exchanger and connecting lines have been thermally insulated in a suitable manner. For example, the heat exchanger can be insulated with the help of a separate vacuum chamber with vacuum pump or more simply with insulating materials made out of expanded polystyrene (EPS), insulating materials made out of extruded polystyrene (XPS), insulating materials made out of polyurethane (PUR), or by means of a vacuum insulating plate (VIP). Since the configuration according to the invention makes it possible to multiply the cooling capacities, any losses owing to at best a more poorly insulated heat exchanger only play a secondary role. For example, capacities were realized on a cooling head with (cylinder: H 35 mm×D 35 mm) 300 W/140 K. This corresponds to a tenfold increase in cooling capacity given an approx. thirtyfold volumetric reduction. In order to improve the thermal insulation, in particular at lower temperatures, a thermal radiation reflecting layer made out of aluminium foil or equivalent material can be introduced into the insulation of the heat exchanger. For example, this yields a structural design for the layer following the pattern EPS, aluminium film, EPS, aluminium film, . . . . 
     The configuration according to the invention makes it possible to design the cooling aggregate and cooling head as functionally separate units, so that the cooling head itself need not incorporate any large components of the cooling aggregate, for example a counterflow heat exchanger or the like. This further makes it possible to provide a cooling aggregate with the respectively required cooling capacity that reflects the respective requirements, without the cooling head having to be adjusted in any way whatsoever, and without impairing the operability of the cooling head in any way whatsoever. 
     In a preferred further development, an especially simple operability is ensured by arranging the compressor, after-cooler and counterflow heat exchanger together in a floor-mounted device, the housing of which exhibits a lead-through for the connecting lines that join the counterflow heat exchanger with the vacuum chamber. 
     In the mixed gas Joule Thomson cooling aggregate, it is crucial that the cooler be connected to the feed line of the counterflow heat exchanger with a throttle interspersed, so that the necessary pressure reduction of the coolant takes place, and the liquefied coolant can evaporate in the cooling head. In this conjunction, an especially advantageous structural design provides that the connecting line joining the feed line of the counterflow heat exchanger with the cooling head forms the throttle. 
     In order to minimize the thermal losses while transporting the coolant through the connecting lines, a preferred further development of the invention provides that the connecting lines exhibit vacuum insulation. It can preferably be provided here that the vacuum chamber and vacuum insulation of the connecting lines are joined directly together, and can be joined with a shared low-pressure source. The advantage to this is that the vacuum system of the vacuum chamber is used for suitably insulating the coolant during its transport between the vacuum chamber or cooling head and the cooling reservoir, and set up a vacuum lead-through. The vacuum chamber itself incorporates the cooling head (e.g., made out of copper), which is secured to the connecting lines and carries the coolant (e.g., liquid nitrogen) coming from the connecting lines. The existing vacuum chamber is here expanded by the relative slight volume of the vacuum insulation for the connecting lines, and a vacuum connection is simultaneously established between the vacuum insulation for the connecting lines and the vacuum chamber. This resolves the problem associated with the vacuum lead-through and insulation for the connecting lines at a low outlay and cost. It is here preferably provided that the vacuum chamber exhibits a lead-through for the connecting lines, which is configured in such a way that the hollow space of the vacuum insulation for the connecting lines is joined with the interior space of the vacuum chamber. 
     The coolant-carrying line is guided out of the vacuum insulation at the other end of the connecting lines, i.e., on the side of the cooling reservoir, as also suitable in vacuum-insulated tubes known from prior art. Attention must here be paid in particular to the thermal conductivity of the cladding tube of the vacuum insulation and the thermal transfer surface. Focus must likewise be placed on a good vacuum welding at the transition. This transition should trigger slight heat transfer losses, and can be additionally protected against condensate or ice by means of conventional insulation. 
     Based on the embodiment described, it now becomes possible to introduce any cooling capacities desired in a vacuum chamber in an extremely efficient, space-saving (volume-saving) manner, depending on the dimensioning of the respective cooling aggregate. 
     In a preferred further development of the invention, an especially simple structural design for the vacuum insulation for the connecting lines is achieved if the vacuum insulation comprises a cladding tube that envelops the connecting lines, with the formation of a preferably essentially annular hollow space. The cladding tube can be just as flexible as the connecting lines. To prevent the connecting lines from contacting the cladding tube, which would lead to an undesirable heat transfer, the embodiment is preferably configured in such a way as to arrange at least one spacer in the hollow space between the connecting lines and the cladding tube. Having the spacer exhibit a preferably provided corrugated outer and inner contour ensures that only point-shaped or linear contacts arise between the spacer on the one hand and the cladding tube and connecting lines on the other, wherein such Hertzian contacts make it possible to further diminish the introduction of heat from outside. 
     A particularly simple structural design is achieved according to a preferred further development by joining the shared low-pressure source to the vacuum chamber. 
     It is further preferably provided that the vacuum chamber incorporate in particular a tubular spacer, which envelops the lead-through, and defines the distance between the cooling head and the inner wall of the vacuum chamber, wherein the spacer exhibits radial through holes, so that the interior space of the vacuum chamber is joined with the hollow space of the vacuum insulation for the connecting lines. 
     The coolant preferably comprises butane and/or isobutane and/or propane and/or propene and/or ethyne and/or ethane and/or ethene and/or methane and/or argon and/or nitrogen. 
    
    
     
       The invention will now be explained in greater detail based on exemplary embodiments schematically depicted on the drawing. In the latter, 
         FIG. 1  presents a closed coolant circuit with a cooling aggregate and a cooling head, 
         FIG. 2  a sectional view of the cooling head with the connecting lines, and 
         FIG. 3  a section according to line III-III on  FIG. 2 . 
     
    
    
     The coolant circuit shown on  FIG. 1  is most often referred to as a mixed gas Joule Thomson cooling process, and is described, for example, in Document EP 650574 A1. The coolant circuit comprises a compressor  1  for compressing the coolant supplied in gaseous form at  2 . For example, the coolant can be a gas mixture consisting of propane, ethane, methane and nitrogen. The compressed coolant is fed via a line  3  to an oil separator  4 , with which the oil potentially being mixed with the coolant in the compressor  1  is separated out. The coolant cleansed of oil is subsequently fed to an after-cooler  5 , in which the heat supplied to the compressor  1  is removed from the coolant. The cooled, compressed, but still most often gaseous coolant is then fed via a line  6  to a counterflow heat exchanger  7 , in which the coolant flowing through the coolant feed line  8  is cooled and liquefied by the coolant flowing in the coolant return line  9 . In practice, the coolant feed line  8  and coolant return line  9  can be several meters long, and are often helically or spirally wound, so as to achieve a certain compactness of the thermal flow heat exchanger. The liquefied coolant is relieved by way of a throttle  10 , so that the coolant can evaporate in the cooling head  11 , allowing it to remove heat of evaporation from the environment. Since coolant passes through the cooling head  11 , the latter is designed as a hollow cylinder, for example. As a consequence, the coolant flowing back from the cooling head  11  is heated to room temperature in the counterflow heat exchanger  7 , wherein the returning coolant cools the inflowing coolant. In order to cool an object schematically marked  12 , the latter is brought into contact with the cooling head  11 . Therefore, the cooling head  11  consists of a thermally conductive material, such as copper. 
     According to the invention, the cooling head  11  is joined with the counterflow heat exchanger  7  by connecting lines  13  and  14 , so that the cooling aggregate  15  and the cooling head  11  arranged in a vacuum chamber  16  can be realized as separate structural units. The configuration according to the invention requires that the coolant cooled and liquefied in the heat exchanger  7  be transported over a more or less long distance by way of the connecting lines  13  and  14 , so that a sufficient insulation must be ensured for the connecting lines. 
       FIG. 2  provides a more detailed view of the cooling head with vacuum chamber, along with the connecting lines. As evident, the connecting lines  13  and  14  exhibit a vacuum insulation  17 , the evacuated interior space of which is connected with the interior space of the vacuum chamber  16 . The connecting lines  13  and  14  can here be designed as flexible tubes, so as to improve operability. The vacuum insulation  17  of the connecting line exhibits a flexible cladding tube  18 , for example which can be designed as a stainless steel corrugated tube that preferably exhibits a steel jacket. The connecting lines  13  and  14  can also exhibit a corrugated outer contour, and have arranged between them spacers  19  that can also be flexible in design. The spacers  19  preferably exhibit a corrugated outer contour, so that the resultantly achieved linear contacts with the cladding tube  18  or connecting lines  13  and  14  minimize the heat transfer. As a consequence, the spacer  19  is used to mechanically, and hence thermally, decouple the connecting lines  13  and  14  to the cladding tube  18 . It should be sufficiently flexible, temperature stabile, ageing resistant and outgassing free (e.g., Teflon, plastic, stainless steel). The connecting lines  13  and  14  are routed out of the vacuum insulation  17  at point  20 . An eye must be kept out for slight thermal losses at the transition point  20 . This can be achieved using materials with a slight thermal conductivity and a slight transition cross section (e.g., stainless steel). In addition, the transition point  20  can also be protected by conventional materials for heat insulation (e.g., foamed polystyrene, Armaflex). 
     The connecting lines  13  and  14  can be thermally coupled. Alternatively, the connecting lines  13  and  14  can also be telescoped into each other. Depending on the cross section and length of the connecting line  13 , the coolant can undergo a pressure reduction along the feed line, so that the coolant evaporates in the cooling head, just as in compression refrigeration machines, and heat is dissipated. In this case, the feed line simultaneously acts as the throttle. 
     The vacuum insulation  17  is joined with a vacuum flange  21 , through which the connecting lines  13  and  14  are passed, and routed to the cooling head  11 . In order to improve the mechanical stability of the cooling head  11 , the cooling head  11  and vacuum flange  21  have arranged between them a spacer  22 , for example which can consist of Teflon, ceramic or stainless steel, and should be outgassing-resistant, low temperature-suitable, embrittlement-resistant and ageing-resistant. Care must here be taken to ensure that the spacer  22  is thermally decoupled from the vacuum flange  21  to a sufficient extent, and that there is a good atmospheric permeability relative to the cladding tube  18 . As evident from the cross sectional view on  FIG. 3 , the spacer  22  exhibits several radial through holes  24 , so that the evacuated interior space of the vacuum insulation for the connecting lines is conductively connected with the evacuated interior space of the vacuum chamber  16 . A flange or port for connection to a vacuum pump is marked  23 . 
     The cooling head becomes extremely low-vibration if coolant flows uniformly through the cooling head and the cooling head is mechanically stabilized by means of spacers. 
     Typical areas of application for the invention include the cooling of heavy duty laser amplifiers as well as various cooling operations in analytical chemistry, in the field of superconductivity, astronomy, and also generally in research and development, along with medical diagnostics.