Patent Publication Number: US-2010108294-A1

Title: Heat transfer unit for heating systems and surefaces and railway  point heater

Description:
The invention relates to a heat exchanger unit for heating systems and surfaces, according to the preamble of claim  1 , and to a track switch heater having a heat exchanger unit, according to the preamble of claim  21 . 
     Document EP 1 529 880 A1 and WO 2005/045134 A1 relate to a thermal ground probe which serves heat directly to traffic facilities, the heat flow from which thermal ground probe is conducted via at least one heat pipe from the heat source via a transport zone and, in order to provide a supply to a plurality of heat sinks, is split up already in the transport zone, long before reaching the heat sinks, in such a way that a split takes place in each case from one to two heat flows in order to ensure a uniform distribution or a distribution of the heat flow according to the respective power demands of the connected heat sinks. The splitting-up of the heat flows from one pipe to two is restricted in that the sum of the cross sections of the two pipes after the distribution must be equal to the cross section before the distribution, that is to say the cross section of the pipe which is closer to the probe is approximately equal in size to the sum of the cross sections of the two distribution pipes. Here, the cross sections of the distribution pipes are proportional to the ratio of the power demands of the heat sinks connected downstream. 
     As a heat exchanger, provision is made in each case of merely a plate under which the up to three heat pipes are fastened in a heat-conducting fashion longitudinally with respect to the rail body, but are not integrated. Said pipes are therefore not an independent heat exchanger, but rather are described as being fastened to heat distribution plates in a heat-conducting fashion. 
     The targeted splitting-up of a gas flow in the transport zone requires that the calculated geometrical dimensions of the distribution pipes be adhered to very precisely, and it is not possible for said geometrical dimensions to be realized in assembly under the conditions prevailing at the track. No weld or solder seams should project into the interior space of the pipe, nor should any burrs protrude into said interior space, since such seams or burrs hinder the gas flow and the condensate flow. The splitting-up of the large heat flow from the probe into a plurality of relatively small flows by means of a further interposed heat exchanger, as proposed in EP 1 529 880 A1, is associated with temperature losses, which would not necessarily be advantageous in the case of the small temperature difference which is present. The reference to capillary pipes in document EP 1 529 880 A1, which utilize a capillary effect in some arbitrary way, cannot be regarded as being equivalent to pipes of small diameter at least in connection with the use of CO 2 , since at such small diameters, the pressure drop in the pipe with the length of up to 5 m specified in EP 1 529 880 A1 is so high that transportation of gas no longer takes place. It is in fact prior art for pipes with a capillary internal structure and diameters of 10 mm to be used to generate a backward flow of condensate in the horizontal, or even counter to a slight gradient up to a length of 5 m. With a smooth internal structure and a diameter which does not generate a pressure drop, a backward flow of a condensate is not physically possible. 
     The heating of traffic signs using geothermal heat, as is known from DE 40 36 729 A1, requires only very small heat quantities and can be realized in a relatively simple manner. A significant difference with respect to the invention proposed here is the power ranges, which are higher here by factors of 20 to several hundred. An increase in power of the described method into the range involved here is not possible. Of equal significance for the lack of comparability is the fact that said document involves the prevention of an obstruction to visibility caused by frost. A further fundamental difference also lies in the fact that, in a traffic sign, only vertical distances must be overcome in the distribution of heat, and only vertical surfaces need be thawed. Here, gravity performs the significant task of causing the precipitation, snow, sleet or frost only in the thawed state to slide down. The heated traffic sign is duly a heat pipe application, but the realization of a multi-duct heat exchanger is designed explicitly for water/glycol mixtures. The proposed design having a so-called heat pipe has only one pipe with a plate, which is fastened thereto and which conducts the heat, as a heat sink. 
     Already widespread, and known for example from DE 43 25 002 A1, are devices for heating track switch parts by means of electric heating elements which are arranged locally and which are intended to ensure that the track switch can be operated even at low temperatures below the freezing point. 
     The object on which the invention is based is that of developing a heat exchanger unit which is adapted to the specific demands of an application and, here, constitutes an efficient and economical solution. 
     The invention is realized with regard to a heat exchanger unit by means of the features of claim  1 , and with regard to a track switch heater by means of the features of claim  21 . The further dependent claims relate to advantageous embodiments and refinements of the invention. 
     The invention encompasses the technical teaching with regard to a heat exchanger unit for heating systems and surfaces, which heat exchanger unit can be connected at least to a thermal ground probe, which is operated with a multi-phase working medium, as a heat source and to a component to be heated, and which heat exchanger unit is composed of at least one heat exchanger and transport lines for the gaseous or liquid working medium. According to the invention, for the transfer of the latent heat originating from the heat source from the heated gaseous component of the working medium to the component to be heated, at least one gas-tight heat exchanger is formed, in a modular fashion, with a multiplicity of directly adjacent, depending on the power requirement, mini-ducts in a laminar arrangement, which mini-ducts are connected to at least one supply duct, and the mini-ducts have a diameter of 0.3 to 6 mm. 
     That diameter of the mini-ducts which is provided as a lower limit is distinct from the dimensions of micro-ducts, for which the literature specifies a diameter of &lt;0.3 mm. In the literature, a dimension of a mini-duct is often specified as being in the range between 0.3 mm and 3 mm diameter. In the case of relatively large surfaces to be heated, the diameters of the mini-ducts in this context may even be up to 6 mm. Even at such diameters, the ducts are integrated into the component to be heated; the component to be heated therefore is itself the heat exchanger. 
     Two variants of heat exchangers are possible, the parallel flow principle and the counterflow principle. 
     For both principles, at a temperature of the working medium of 7° C. in the heat exchanger, the desired specific power range of the heat exchanger unit lies between 0.4 and 4 kW/m 2 . 
     To increase the size of the duct surface and therefore of the heat transfer area between the gas and the heat exchanger, it is possible for a plurality of layers of ducts to be arranged one above the other while maintaining the minimum spacing of the ducts with respect to one another which must be adhered to for mechanical reasons (strength). With said measure, it is also possible for an even faster heating response to be obtained. 
     For the counterflow principle: the ratio of the cross sections of the feeding distributor pipes to the overall cross section of the respectively connected ducts should be selected to be between 0.1 and 0.25 depending on the power requirement of the surface to be heated and the selected duct cross sections. The sum of the cross sections of the mini-ducts is accordingly significantly greater than that of an associated distributor pipe. The mini-ducts conventionally have a smooth inner wall in order to assist the gas and liquid flow as effectively as possible. 
     It is possible to specify a minimum diameter on the capillary side of 0.3 mm, and a maximum diameter in the case of aluminum heat exchangers, for reasons of strength and economy (excessively high wall thicknesses), of 5 mm. The minimum diameter therefore also should not be undershot, such that the return flow of the liquid working medium is not hindered by the inflowing gas. Intermittent behavior of the medium may occur even with a diameter of 1 mm and at high power. 
     On the distributor side, the maximum diameter should not exceed 20 mm. The Product Safety Act sets limits on production monitoring, and demands frequent safety checks, for significantly greater diameters. The Act seizes the exceedance of limit values from the product of pressure and volume of a reservoir. Furthermore, the wall thicknesses then become uneconomically thick. 
     When using CO 2 , the dimensions of the distributor pipes and the mini-ducts at power levels required for such applications (&lt;2 kW per m 2  and distributor pipe) are substantially non-critical. The high enthalpy content of CO 2  entails a low flow speed. A metallic heat exchanger, on account of its good thermal conductivity, is effective at dissipating the latent heat of the gas, as a result of which a suction effect, so to speak, is generated which even permits unequal capillary tube cross sections without this leading to critical power losses. 
     If the power requirements are increased disproportionately in relation to the cross section of the ducts, a build-up of condensate can occur, that is to say the outflowing condensate hinders the inflow of gaseous medium, such that power-reducing, intermittent behavior of the heat transfer is generated. 
     For the parallel flow principle, the cross sections of the distributor pipes and ducts can be selected to be smaller. 
     Modules of 25 W to 200 W have been proposed for use in track switch heaters. The dimensions are coordinated with the type of track switch and the required snow-melting power, and may encompass heating of the slide chairs. 
     Here, the invention is based on the notion of specifying a heat exchanger unit for utilizing, collecting and distributing low-temperature heat which preferably originates from a thermal ground probe, which heat exchanger unit is operated with a liquid-gaseous working medium. As a suitable heat source, it is preferable to use the latent heat of a working medium even in the case of an only slight temperature gradient between the phase change point of the working medium and the freezing point of water, with it being possible to maximize the power which can be transmitted to the heat sink, and to significantly improve efficiency. 
     The heat exchanger is designed such that preferably geothermal heat at a low temperature level is used for heating and temperature control in such a way that, even with the small temperature differences between the working medium and the heat sink, such as are present if the temperature level of the working medium is not raised by any interposed heat pump, it is possible for a large quantity of heat to be transferred over a limited area, that is to say a high energy density is obtained. 
     It can be expected that up to 95% of the heat extracted from the probe is not utilized for melting snow or ice, but rather that considerable heat quantities are dissipated by convection at temperatures below 10° C. and by radiation into the open air. 
     By means of the invention, therefore, for the control of the extraction of heat from the heat source, a minimization of the uncontrolled extraction of heat is passively obtained by means of the configuration of the surface of the heat exchanger. This is provided by means of the configuration of the surface and/or the material selection of the heat exchanger. 
     It is additionally possible for the flow of the working medium to be reduced or even interrupted by means of temperature-dependent regulation. This is preferably achieved in that, at a position which is protected from wind and radiation, a temperature-controlled actuator raises the heat exchanger at one side, or raises the feed pipe by a small amount, in such a way that the return flow of the medium is interrupted. 
     One significant innovation of the invention over known solutions relates to improvements in the efficiency of the core components, particularly with regard to the distribution of heat, which improvements may also be used in other applications and even permit effective low-temperature utilization in said applications for the first time. With the invention, the efficiency of previous known solutions is surpassed with regard to power, material use and economy. Furthermore, additional system components are proposed, such as for example the integration of heat accumulators and additional heat sources, or an additional integration of heat accumulators into the heat exchanger itself. 
     Microstructure heat exchangers differ from other heat exchangers by their high power density. It is preferable, depending on the required power and the resulting overall duct cross section which is defined by the overall duct length and the duct cross section, for 2 to approximately 100 ducts to be arranged per 10 centimeters width of the exchanger. The heat exchanger operates with only a small temperature difference and around the condensation point of the working medium, with the operating points of the two fluid phases lying close to one of their phase change points. In particular, the combination of a microstructure heat exchanger according to the invention in connection with the utilization of geothermal heat offers particular advantages, viewed as an overall system, with regard to system integration, operational reliability and freedom from maintenance. 
     The heat exchanger unit serves in particular for heating traffic surfaces, traffic infrastructure and traffic facilities. In this connection, the heat exchanger unit is particularly suitable for railroad equipment, in particular for track switches and also for railroad crossing segments or platform segments designed as heat exchangers and similar units designed as heat exchangers, since the solution according to the invention can be operated without additional pumps driven by external energy and without moving parts. The heat exchangers can be connected, preferably in a releasable and non-destructive fashion, to in each case one or more supply lines. 
     For heating traffic facilities, such as railroad track switches, and for the clearance of snow and ice and therefore applications for increasing safety in high-risk regions such as platform edges, railroad crossings or grade crossings, it is therefore possible to dispense with the use of for example electric heating, or the use of gritting means or means for reducing the melting point of the snow. 
     Furthermore, it is possible to minimize the flow losses and pressure losses in the heat exchanger resulting from the mutual hindrance of the gas phase and the condensate returning to the heat source. 
     The particular advantage is that the heat exchanger unit according to the invention has a level of efficiency, with regard to heat utilization, material use and economy, which surpasses that of the present state of the art. The efficiency with regard to the utilization of heat results from the optimized dissipation of the heat flowing out across a temperature gradient. 
     In a preferred embodiment of the invention, the mini-ducts may be arranged, in the case of a particularly high heat requirement, in a plurality of planes. To be able, with the small temperature difference, to transfer a heat quantity which is required for the intended purpose, the heat transfer area on the side of the working medium must be maximized. Said heat exchanger is formed in one plane, or in a plurality of planes depending on the energy requirement, of a plurality of parallel-running ducts, but preferably 2 to 100 ducts per 10 centimeters of width of the heat exchanger and per plane. The duct diameter preferably lies between 0.5 mm and 5 mm. In this way, the transfer area from the working medium to the heat exchanger, and therefore the possible energy density, is maximized. At the same time, however, the surface area with respect to the environment must be kept as small as possible in order to minimize the radiation losses to the environment. In selecting the material of the heat exchanger, it is sought to obtain good conductivity and a high level of corrosion resistance. 
     In this way, the heat exchanger may take the form of a microstructure so-called plate-type heat exchanger which, using the micro-channel principle, is optimized according to the working medium which is used, preferably CO 2 , and the application. 
     When using CO 2  as a working medium, a configuration of the inflow conditions, as is known in heat exchangers, is only necessary in the case of extreme power demands, since with simple branching of the ducts from the distributor pipe, the temperature difference in the heat exchanger with respect to optimum inflow conditions is only a few tenths of a degree. The design from  FIG. 1  or  FIG. 3  is adequate. Flaring of the capillary pipes is not necessary with CO 2  even in counterflow heat exchangers if said minimum diameters of the ducts are adhered to and the power does not exceed an output power of 5 kW/m 2 . 
     It is advantageously possible, in order to improve the return flow of the condensate in counterflow heat exchangers, for the closed “ends” of the ducts to be curved upward. In this way, it is achieved that the condensate from said region is provided with an impetus and improves the overall return flow. This is advantageous in particular with relatively long ducts, such as may be provided in platform edge heaters or railroad crossing heaters. 
     It is advantageously possible for the upper apex line of a mini-duct to run in both a rising and falling manner in sections proceeding from the inlet point of the gaseous component of the working medium. The transport mechanism of the gaseous medium is based on pressure gradients which are generated on account of the reduction in the volume of the gaseous working medium as a result of condensation. The guidance of the gaseous medium is basically independent of the spatial guidance of the ducts. 
     In particular, however, it is advantageously possible for the upper apex line of a mini-duct to rise proceeding from the inlet point of the gaseous component of the working medium. In this way, the gaseous component of the working medium in the mini-duct is guided more effectively overall. 
     Furthermore, the lower apex line of a mini-duct or of a duct section may advantageously be inclined in the direction of the outlet point of the liquid component of the working medium. This ensures an improved outflow of condensate, since the return flow of the liquid working medium is effected primarily by gravity. To reliably discharge the condensate, the ducts should generally have an inclination in the flow direction which is sufficient but nevertheless as slight as possible, and a cross section which is adapted to the fluid being used. As a result of its very low viscosity, it is possible for very small inclinations and cross sections to be selected for the fluid CO 2  which is preferably provided. 
     In a preferred refinement of the invention, a mini-duct may widen along the longitudinal axis. This may also take place conically. If a heat exchanger to which the supply of heat takes place via a gaseous working medium is installed horizontally or approximately horizontally and use is made predominantly of the stored latent heat, the problem may occur that, as a result of a small gradient, the return flow of the condensate is only assisted to a small degree and further disruptive influences must be eliminated. As one measure for improving the return flow, it is proposed that the heat exchanger be designed such that at least the inflow of the gas into the heat exchanger does not prevent or only minimally prevents the return flow of the condensate. 
     In one preferred embodiment, the feed duct may taper along the longitudinal axis in the flow direction of the gaseous fluid. This may take place conically. Such a cross-sectional narrowing makes allowance for the fact that, as a result of the distribution between the individual mini-ducts, gaseous working medium is constantly extracted from a supply duct in the flow direction. 
     The discharge duct may advantageously widen along the longitudinal axis L in the flow direction of the fluid. Such a cross-sectional widening makes allowance for the fact that liquid working medium is constantly passing from the individual mini-ducts into the discharge ducts in the flow direction. 
     It is advantageously possible for the feed duct and discharge duct to be formed on one side of the mini-ducts as a supply duct, or for the feed duct and discharge duct to be arranged spaced apart from one another. It is possible in this way for the length and efficiency of the feed line or discharge line paths to be optimized according to the structural requirements. 
     In the case of the design with the ports at different sides, there is practically no mutual hindrance, since the gas flow runs above the outflowing condensate in the mini-duct, and not oppositely but rather in the same direction. 
     It is also possible for the condensate return flow to be arranged in the profile of a duct. In this case, the cross-section increases up to the outlet. The gradient angle of the upper envelope line with respect to the horizontal is positive over the entire length, that is to say is aligned upward in the flow direction. The angle of the lower envelope line is negative up to the outlet, that is to say directed downward, following gravity, in the flow direction. In said design, the gradient angle of the section from the outlet to the closed end may be formed with a different, preferably greater gradient. This has the effect that the returning condensate is accelerated and, with the kinetic energy which is introduced in this way, more easily overcomes any horizontal sections in the profile. 
     As a result of the arrangement of the outlet and inlet—the outlet is situated in any case lower than the inlet—gas is prevented from escaping without having dissipated the latent heat contained therein. At the same time, the outflowing condensate blocks any possible undesired inflow of gas at the outlet side. Furthermore, it is obtained by means of said arrangement that the return may extend back in parallel in a separate pipe, separated from the gas flow, into the probe pipe expediently no further than up to the boundary between the transport and heat absorption zones. 
     If the entire heat exchanger is installed with a slight inclination, then it should be ensured that the installation inclination and the inclination of the ducts at least do not fully compensate one another, but rather supplement one another to the greatest possible extent. 
     This is provided in that the individual ducts of the heat exchanger are of slightly conical design, with the design having both ports, the gas inlet and condensate outlet on the same side and having the ports, gas inlet and condensate outlet on different sides, preferably at opposite sides, being different. If both ports are on one side, then the widened end is situated at the port side, with the gradient angle of the longitudinal axis of the pipe, measured from the port side, with respect to the horizontal being greater than that of the upper pipe envelope line but smaller than that of the lower pipe envelope line, which should be less than 0°. If the ports are situated at different sides, then the widened side of the pipe is situated on the side of the condensate outlet, wherein the gradient angle of the upper pipe envelope line with respect to the horizontal, measured from the gas inlet side, should be at least greater than 0°. The gradient of the longitudinal axis with respect to the horizontal should be negative. The gradient angle need not remain constant over the length of the ducts, that is to say said gradient angle may differ within the specified boundary conditions and may thereby be adapted, for example, to the installation conditions. 
     In both embodiments, the port for the gas supply is situated higher than that of the condensate outlet. In the embodiment with the ports on one side, the hindrance of the flows is minimized, since the gas flow runs above the condensate return flow. 
     In one preferred embodiment of the invention, a layer which inhibits the transfer of heat may be at least partially arranged between the working medium and the outer surface of the heat exchanger. An aluminum heat exchanger may possibly have too high a degree of thermal conductivity, such that under some conditions, the thermal probe dissipates too much heat. With a thin insulating layer, the thermal conductivity on the path from the fluid to the heat exchanger surface may be reduced by virtually any desired order of magnitude. The heat exchanger and also further parts of the heat exchanger, such as for example transport lines, may be provided with a layer which inhibits the transfer of heat, such that the region in particular below the heat exchanger as viewed in the flow direction is not heated, and the probe is not subjected to unnecessary loading. 
     Furthermore, it may be advantageous for the mini-ducts which are traversed by the working medium to be formed into a housing, wherein the heat conductance values of the materials of the two assemblies—duct system and housing—may differ. Here, the production of the distribution and of the ducts composed of plastic is correspondingly cost-effective. 
     In one preferred embodiment of the invention, a heat accumulator may be integrated into at least one transport line for the return flow of the liquid working medium to the heat source and/or in at least one heat exchanger. In this way, ambient heat or heat from solar radiation is stored and introduced into the circuit of the working medium. 
     In one preferred refinement of the invention, a return transport line may be arranged from the heat accumulator to the heat exchanger for the gaseous working medium formed as a result of the absorption of heat in the heat accumulator. 
     Here, one advantage is the reduction in the extraction of heat from the heat source, or the regeneration of said heat. A further reduction in the extraction of heat from the thermal ground probe and the improvement of the regeneration of said heat may be obtained in this way. It is thereby provided that, by utilizing ambient heat, and in particular heat from solar radiation, the temperature of the working medium at the heat sink is temporarily increased to such an extent that the circulation of the working medium and the extraction of heat from the ground are interrupted for said period of time. As a result of said reduction in the operating duration of a thermal ground probe as a primary heat source, it is provided that said thermal ground probe may be designed more cost-effectively. If sufficient installation space is available, the accumulator may also be formed with silica gel or zeolite as accumulator material. 
     In one preferred embodiment, an auxiliary heater may be integrated as a further heat source. In this way, the heat exchanger unit is designed for an extreme extraction of heat. 
     The upper side of a heat exchanger may advantageously be installed so as to be at least slightly inclined with respect to the horizontal. If the heat exchanger is installed in a horizontal or virtually horizontal position and for the purpose of thawing frozen precipitate, the surface should be designed so as to ensure reliable drainage of the thawed precipitate. This measure also serves to reduce the undesired dissipation of heat to the environment. 
     Standing water on the heat exchanger would, on account of its high heat storage capacity, absorb considerable amounts of power, rendering said power useless. In contrast, if water, melted snow or melted ice is prevented from remaining on the surface in any more than a thin film of moisture, then thermal power is no longer extracted from the heat source for the further melting or heating of the melt water, but rather only a significantly lower thermal power is extracted as a result of an air flow or radiation. The surface which is therefore proposed is accordingly designed such that the snow or sleet which falls thereon is immediately melted as completely as possible, at least partially melted, and the melt water flows off from the surface as a result of an at least slight gravitational component, preferably from a slightly oblique plane which runs parallel to the pipes which conduct the working medium. For the use of the heat exchanger in track switch heaters, a material is selected which has good thermal conductivity, preferably a metal with a smooth surface, for example aluminum, which at the same time has very small heat dissipation values. By means of said measures, it is provided that more heat than is necessary for clearing snow and ice is extracted from the heat exchanger only by convection, such as a cold air flow, and as a result of the pure radiation losses, for example to the cold environment. The radiation and convection losses are however kept to the lowest possible level as a result of the minimization of the heat exchanger surface and the material selection. 
     In one preferred embodiment of the invention, a thermal ground probe which is fixedly installed in the ground can be separated in terms of vibration by means of a permanently elastic connecting element. If a heat exchanger which is used is exposed to vibration and shock loading, for example in transportation equipment, and in particular if the working medium is gaseous and a high working pressure prevails (approximately 40 bar if CO 2  is used), said heat exchanger must be separated in terms of vibration from a fixedly installed thermal ground probe, and here, must withstand said high pressure over a long period of time. Here, the working pressure may be monitored by means of a pressure or temperature display. 
     Since it must be possible to carry out maintenance work on the traffic facilities and traffic surfaces, possibly also in order to replace damaged parts, it must preferably be provided that the elastic element can be dismounted in a non-destructive fashion, such that, for example, the heat exchanger can be separated from the thermal ground probe and replaced in the event of damage or during maintenance work in the surroundings of the heat exchanger. Since the heat exchanger is not necessarily rigidly connected to the surface or equipment to be heated, it is also possible for the elastic fastening of said heat exchanger to form a decoupling in terms of vibration. 
     If the heat exchanger is not the surface or system to be heated but rather must be fixedly connected thereto, it is proposed, in order to improve the transfer of heat by means of the use of thermally conductive pastes, that the contact pressure of the heat exchanger against the heat sink be increased by means of a screw connection or that a metallic connection be provided, for example by soldering. 
     It is advantageously possible for the component to be heated to itself be designed as a heat exchanger. In a further advantageous refinement, the mini-ducts may be formed at least as co-supporting elements of the component to be heated. In the event that the heat exchanger constitutes the component to be heated, the surface of which cannot be formed from metal, and the radiation losses are therefore greater than in the case of a metallic surface, the higher thermal energy requirement must be taken into consideration in the design of the probe. Here, for this purpose, the metallic pipes are designed at least as co-supporting elements of the heat exchanger. 
     It is advantageously possible for the heat exchanger with its mini-ducts to be designed such that it can be tilted with respect to the horizontal by means of a control unit. The power losses as a result of radiation and convection can be counteracted by means of a temperature-controlled circuit. This takes place in that the inclination in the condensate return flow, preferably in the counterflow embodiment, can be varied such that the condensate can no longer flow back. For this purpose, a control element which expands or contracts in the event of a temperature change may be arranged on the heat exchanger. In this way, the extraction of energy from the heat source can be controlled in a targeted fashion as a function of the ambient temperature. This counteracts an unnecessary extraction of heat from the heat source. 
     It is advantageously possible for at least one return transport line to be raised by means of a control unit, thereby preventing the return flow of condensate. Said control unit may in particular be arranged under the condensate return line, with the aim of raising the return transport line for example only locally in order to prevent the return flow of the condensate and to interrupt the gas flow. Here, the heat exchanger need not necessarily be designed to be tiltable. 
     In a preferred embodiment of the invention as a track switch heater, at least one heat exchanger can be integrated at least in a slide chair, in the locking crib, in the region of the track switch tongue on the rail web and/or on the rail base of the track switch. 
     The heat exchangers are installed such that, in the simplest case, they longitudinally cover a so-called tie crib in a width of approximately the adjustment travel of the track switch tongue, and so as to lie approximately over half of the width of the inner rail base. If the heat exchanger is additionally used for heating the adjacent slide chairs, then a sufficient transfer of heat of the required heat quantity per unit time is ensured by means of a sufficient contact pressure against the slide chairs. 
     The ties of a track change their position over time after being laid as a result of the occurring load collective. Here, the movements in the longitudinal direction are of particular significance for the use of heat exchangers, because as a result of said movements, the installation space of the heat exchanger changes. 
     It is advantageously possible for at least one heat exchanger to be integrated in a slide chair of the track switch. The solution according to the invention makes it possible for a slide chair of a track switch to be heated and also for the rail head to be heated. If it is necessary for two or more heat exchangers which are supplied with working medium from one heat source to be connected in series, the connection must comprise an elastic element in order that opposing movements are compensated and do not lead to the destruction of the heat exchanger. 
     A preload is often generated by means of a resiliently mounted connection. The preload is selected such that a sufficient contact pressure against the respectively delimiting slide chairs is generated in all positions between the minimum and maximum change in position of the ties. 
     For heating a slide chair in track switches, it is important for said slide chair to be connected in a heat-conducting fashion to a heat exchanger. Alternatively, in particular where the demands on heating power are relatively high, slide chairs are proposed into which a heat exchanger is integrated. Said heat exchanger may be designed as a removable plate, or the mini-ducts are guided in grooves, which are for example milled or forged, under or on the slide chair. It is also possible for the slide chair and heat exchanger to be formed as one component. 
     It is advantageously possible for at least one heat exchanger to be arranged in the region of the track switch tongue on the rail web and/or on the rail foot. 
     To prevent the track switch tongue from possibly becoming fixedly frozen to the stock rail, it is proposed that heating be provided by means of the corresponding heat exchanger with guidance on the rail web and ducts which run in the transverse direction with respect to the rail. The fastening of the heat exchanger and the increase in the contact pressure by means of screws, is preferably done with a bore through the neutral axis of the rail web. In the case of increased heat requirements, an additional heat exchanger may be arranged on the outer channel of the rail profile below the rail head. 
     With powers of approximately 120 W per side and crib interval, adequate melting power is provided even for snowfall of more than 10 cm/h. It is to be assumed that up to 95% of the power output does not take the form of melting power but rather results from radiation and convection. A minimization of the power which is not utilized for melting is therefore expedient and economical. 
     Likewise, to prevent radiation and convection there, the web and the base of the stock rail may be thermally insulated to the outside, and the web may also be thermally insulated to the inside if the structure of the track switch permits this. The insulation may be fastened to the rail base and, on the outer side, surrounds the additional heat exchanger which may be provided in the outer channel of the rail profile. 
     Heat exchangers which are not intended to heat the slide chair or the rail head are fastened with clamps to the rail base and lie against the ties. Said heat exchangers may also additionally be fastened to a tie. Said heat exchangers are designed in terms of their dimensions such that, in the one extreme case, an increase in the tie interval, the heat exchangers do not fall between the ties, and in the other extreme case, a reduction in the tie interval, the heat exchangers are not damaged by the slide chairs. It is advantageously possible for at least one heat exchanger to also be integrated in the locking crib of the track switch and/or designed as a cover of the locking crib. 
     In a further preferred embodiment of the invention as a track switch heater, the heat exchanger unit may be mounted on the support points on the track switch by means of deformable elements. If the heat exchangers are used in railroad engineering for heating track switches in order to clear the latter of snow and ice, then the proposed technology must be adapted to the more difficult requirements. The heat exchangers must thus be designed such that a change in length of the installation space does not have an influence on function and operational reliability, and does not lead to the destruction of the heat exchanger. 
     If the heat exchanger is designed such that it is a component which is mounted in each case on a tie and covers and heats approximately half of a tie crib at both sides of said tie, the expenditure for vibration decoupling and for the compensation of length variations of the installation space to the next heat exchanger can be minimized. 
     The device according to the invention for the improved utilization of geothermal heat for heating systems at low temperature comprises at least one of the following improvements, such that a heat exchanger is specified which can be used for temperature control or for heating purposes, preferably for heating transportation equipment. In relation to the prior art, said device constitutes a more efficient and more economical method. 
     In contrast to the known heat exchangers which utilize geothermal heat and which are used in combination with heat pumps and in which the heat is transferred in a controlled fashion from a positively-driven medium to a second positively-driven medium, it is preferably proposed here that the latent heat contained in a working medium be dissipated directly to the environment. 
     If the heat transfer area on the side of the working medium is maximized, then it is possible for the required heat quantity to be transferred even at the low temperature difference from thermal ground probes. 
     In this way, the transfer surface area from the working medium to the heat exchanger, and therefore the possible energy density, is maximized. At the same time, however, the surface with respect to the environment is kept as small as possible as a result of a compact design, so as to minimize the radiation losses to the environment. 
     The reduction in the extraction of heat from a thermal ground probe, as has hitherto been realized by means of regulating or actuating elements such as valves, takes place in a passive fashion in the solution according to the invention, specifically in that a minimization of the heat extraction to that which is required is obtained by means of the material selection and the configuration of the surface of the heat exchanger. 
    
    
     
       Exemplary embodiments of the invention will be explained in more detail on the basis of schematic drawings, in which, in each case schematically: 
         FIG. 1  shows a plan view of a heat exchanger having duct structures and the flow characteristic of the working medium, 
         FIG. 2  shows a cross section through a heat exchanger according to  FIG. 1  along a mini-duct, 
         FIG. 3  shows a cross section through a further embodiment of a heat exchanger according to  FIG. 1  along a mini-duct, 
         FIG. 4  shows a plan view of a further embodiment of a heat exchanger having duct structures and the flow characteristic of the working medium, 
         FIG. 5  shows a cross section through the heat exchanger according to  FIG. 4  along a mini-duct, 
         FIG. 6  shows a plan view of a further embodiment of a heat exchanger having duct structures and the flow characteristic of the working medium, 
         FIG. 7  shows a cross section through the heat exchanger according to  FIG. 6  along a mini-duct, 
         FIG. 8  shows a detailed view of the constituent parts of a heat exchanger for forming mini-ducts, 
         FIG. 9  shows a cross section, running perpendicular to the mini-ducts, of the heat exchanger formed from the constituent parts in  FIG. 8 , 
         FIG. 10  shows a partial view of a tie interval heater having transport lines leading from a thermal ground probe, 
         FIG. 11  shows a partial view of a tie interval heater having an elastic connecting element for pressure monitoring, and branching transport lines, 
         FIG. 12  shows a partial view of the tie interval heater having an elastic connecting element for pressure monitoring, and branching transport lines, as per  FIG. 11  at a different pressure, 
         FIG. 13  shows a partial view of a further embodiment of a slide chair heater having duct structures, 
         FIG. 14  shows a partial view of a slide chair heater having a heat exchanger with duct structures additionally integrated in the slide chair, 
         FIG. 15  shows a cross section through the slide chair heater according to  FIG. 14 , perpendicular to the mini-ducts, 
         FIG. 16  shows a partial view of a slide chair heater having two heat exchangers which are supplied from one heat source and which have duct structures and branching transport lines and deformable elements, 
         FIG. 17  shows a partial view of a slide chair heater which is of complex design with a plurality of heat exchangers, 
         FIG. 18  shows a plan view of the duct guidance in a heat accumulator, 
         FIG. 19  shows an arrangement of the individual system elements with a heat exchanger and heat accumulator of a heat exchanger unit, 
         FIG. 20  shows an arrangement of the individual system elements with a heat exchanger and heat accumulator of a heat exchanger unit in connection with a thermal ground probe, 
         FIG. 21  shows a partial view of a locking crib heater having duct structures, 
         FIG. 22  shows a partial view of a further embodiment of a locking crib heater having duct structures, 
         FIG. 23  shows a cross section through a heat exchanger along a mini-duct which operates on the counterflow principle, 
         FIG. 24  shows a cross section through a heat exchanger along a mini-duct which operates on the counterflow principle, in the working position, 
         FIG. 25  shows a cross section through a heat exchanger along a mini-duct which operates on the counterflow principle, in the standby position. 
     
    
    
     Corresponding parts are denoted by the same reference symbols in all the figures. 
       FIG. 1  shows a plan view of a heat exchanger  11  having duct structures  111 ,  114 ,  115  and the flow characteristic FG, FF of the working medium. 
     The heat exchanger  11  has, in the transverse direction, a multiplicity of mini-ducts  111  which run parallel to one another. The gas inflow and the condensate return flow of the working medium take place in directly adjacent feed ducts  114  and discharge ducts  115  which are situated on one side of the mini-ducts  111 . 
     Arrows denoted by FG indicate the flow direction of the gaseous medium. Arrows denoted by FF indicate the flow direction of the liquid medium after having dissipated heat in the heat exchanger  11 . 
     In the heat exchanger  11 , the parallel mini-ducts  111  may of course also run obliquely or may for example be curved in an S-shape or run in a spiral fashion. 
       FIG. 2  shows a cross section through a heat exchanger  11  according to  FIG. 1  along a mini-duct  111 . The gas inflow and the condensate return flow of the working medium take place in directly adjacent feed ducts  114  and discharge ducts  115  which are situated on one side of the mini-ducts  111 . In this case, the mini-ducts are formed with a constant cross section along the longitudinal axis L. Such mini-duct arrangements are preferably installed with a longitudinal axis L which is inclined slightly with respect to the horizontal, so as primarily to ensure the condensate return flow and thereby improve the power. 
       FIG. 3  shows a cross section through a further embodiment of a heat exchanger  11  according to  FIG. 1  along a mini-duct  111 . In this case, the upper apex line OS of the mini-duct  111  rises slightly proceeding from the feed duct  114  for conducting the gaseous component of the working medium. The lower apex line US is inclined in the direction of the discharge duct  115  for discharging the liquid component of the working medium. In this case, the normally preferred inclination of the longitudinal axis L of the mini-ducts  111  is already integrated in the heat exchanger  11  itself. 
       FIG. 4  shows a plan view of a further embodiment of a heat exchanger  11  having duct structures  111 ,  114 ,  115  and the flow characteristic FG, FF of the working medium. 
     The associated cross section through the heat exchanger  11  according to  FIG. 4  along a mini-duct  111  is shown in  FIG. 5 . In said embodiment, the feed duct  114  and the discharge duct  115  are situated in each case on opposite sides of the mini-ducts  111 . The profile of a mini-duct  111  again has an upper apex line OS which rises continuously from the inlet point  112  of the gaseous component of the working medium. The lower apex line US, in contrast, is inclined in the direction of the outlet point  113  of the liquid component of the working medium. 
       FIG. 6  shows a plan view of a further embodiment of a heat exchanger  11  having duct structures  111 ,  114 ,  115  and the flow characteristic FG, FF of the working medium. In said embodiment, the discharge duct  115  divides the mini-ducts  111 , which run parallel to one another, into two halves. 
       FIG. 7  shows a cross section through the heat exchanger  11  according to  FIG. 6  along a mini-duct  111 . Here, the upper apex line OS of the mini-duct  111  rises continuously from the feed duct  114  for conducting the gaseous component of the working medium. The lower apex line US is inclined, in its partial sections, in each case in the direction of the discharge duct  115  for discharging the liquid component of the working medium. 
       FIG. 8  shows a detailed view of the constituent parts of a heat exchanger  11  for forming mini-ducts  111 . The heat exchanger  11  is composed of an upper shell  116  and a lower shell  117 , between which a punched corrugated plate  118  is arranged to form the mini-ducts  111 . The heat exchanger, formed from an upper shell  116 , lower shell  117  and corrugated plate  118 , may also be produced in one piece from an extruded profile. 
     The described multi-part configuration of the heat exchanger  11  may also be utilized in the application such that, for example, the upper shell  116  or lower shell  117  individually may be integrally connected to the component to be heated. In this case, the final assembly of the heat exchanger  11  is then carried out on site. The described single-piece configuration serves to ensure a minimum heat transfer resistance from the heat exchanger to the component to be heated. 
       FIG. 9  shows a cross section, running perpendicular to the mini-ducts  111 , of the heat exchanger  111  formed from the constituent parts in  FIG. 8 . Here, the upper shell  116  and the lower shell  117  are joined together with the corrugated plate  118 . The corrugated plate  118  is connected to the upper shell  116  and lower shell  117  in a pressure-tight manner and is in good thermally conductive contact with the housing formed from the lower shell  117  and upper shell  116 . 
       FIG. 10  shows a partial view of a tie crib heater having a thermal ground probe  21  and transport lines  12 . In  FIG. 10A , the transport lines  12  are guided along the side surfaces of the tie  37  to the rail  38 . In  FIG. 10B , the transport lines  12  are arranged along one side surface of the tie  37 . 
       FIGS. 11  A and B show a partial view of a tie crib heater having an elastic connecting element  14  for pressure monitoring, and branching transport lines  12 . The transport lines  12  are guided partially over the outer region of the tie  37  and, in the further profile, toward the side surface and under the rail  38 . The connecting element  14  is designed as an integrated pressure display which signals a possible pressure drop in the system, which manifests itself in a change in length of the elastic connecting element  14 .  FIGS. 12  A and B show a partial view of a further embodiment of the tie crib heater having an elastic connecting element  14  for pressure monitoring, and branching transport lines  12  which are arranged on one side surface of the tie  37 . 
     The functional reliability of the heat exchanger, which is dependent on the operating pressure being adhered to, is monitored by means of a pressure display which is integrated into the elastic element of the vibration isolation arrangement. Said elastic element is a corrugated pipe which is arranged in the transport zone so as to be visible adjacent to the end of the tie. If the operating pressure in the pipe is adequate, the corrugated pipe is extended; if the operating pressure falls as a result of damage, then the corrugated pipe is shortened. The damage is therefore evident from viewing the corrugated pipe, or the damage is signaled by means of an electrical contact which is triggered when the corrugated pipe is shortened. A comparison of  FIGS. 11  A and B and of  FIGS. 12  A and B shows the effect of the pressure-dependent change in length, which is indicated by the elastic connecting element  14 . The pressure display may also be provided by virtue, for example, of a pressurized supply pipe being composed of elastic material, which sags in the event of a significant pressure drop. 
       FIG. 13  shows a partial view of a further embodiment of a track switch and slide chair heater having duct structures  111 ,  114 ,  115 . The heating takes place by means of outer ducts  111  which are guided around the slide chair  31  and which are arranged on the tie  37 . The individual ducts  111  may also have different cross sections in order to provide a uniform distribution of the gaseous working medium. The heat exchanger  11  runs parallel to the rail  38  so as to also heat the latter in partial sections. 
       FIG. 14  shows a partial view of a track switch and slide chair heater additionally having a heat exchanger  11  with duct structures  111  integrated in the slide chair. The heat exchanger  11  again runs parallel to the rail  38  so as to also heat the latter in sections. The slide chair  31  has, at the top in the center on the tie  37 , a sufficient cutout into which are formed additional grooves for holding mini-ducts  111 . 
     As can be seen from  FIG. 15  in a cross section through the slide chair heater according to  FIG. 14  perpendicular to the mini-ducts, an additional plate is placed as an upper shell  116  into the cutout, which upper shell  116  absorbs the pressure forces originating from the track switch tongue. The slide chair  31  is itself designed, in effect, as a heat exchanger  11 . 
       FIG. 16  shows a partial view of a track switch and slide chair heater having two heat exchangers  11 , which are supplied from one heat source, with duct structures  111 ,  114 ,  115  and branching transport lines  12 . The arrangement of two heat exchangers, which are fed from one probe pipe, with longitudinally running ducts also has a deformable element  36  on the slide chair  31  for length compensation of the installation space in the tie interval. The transport line  12  has a slightly bulged shape at the base side for the improved separation of the gas phase and of the condensate by the lower condensate duct. 
       FIG. 17  shows a partial view of a track switch and slide chair heater which is of complex design and has a plurality of heat exchangers  11  fastened to the tie or to the slide chair. The figure illustrates a juxtaposed arrangement of heat exchangers  11  with mini-ducts running longitudinally with respect to the rail  38 . Said mini-ducts may also be arranged transversely with respect to the rail. The ties of a rail and the slide chairs change their positions over time after having been laid on account of the occurring load collective. Therefore, in each case two heat exchangers  11  are arranged in pairs on one slide chair  31 , with a certain gap remaining between the two heat exchangers  11  for length expansion. The working medium is supplied from a probe pipe (not illustrated in the figure) to the heat exchangers  11  which are arranged in series, with the heat exchangers  11  being supplied from branching transport lines  12 . In this case, the feed and return transport lines are realized by one pipe. 
       FIG. 18  shows a plan view of the duct guidance in a heat accumulator  13 . Here, said heat accumulator  13  is for example a latent heat accumulator from which the condensate flowing back from the heat sink absorbs the heat contained in the heat accumulator, evaporates and flows to the heat sink—the heat exchanger  11 —again without heat having to be extracted from the thermal ground probe as a source. In the lower-lying section plane A, one possible arrangement of zones of accumulator material which run parallel to one another is illustrated. 
     In order that the return-flowing gas and the condensate running in the opposite direction influence one another only to a small extent, the ducts are widened in the direction of the gas flow, preferably horizontally. The ducts preferably have, corresponding to the heat which can be absorbed in the heat source, a plurality of outlets situated in the profile of the condensate flow, through which outlets the gas escapes from the duct and flows back to the heat exchanger. The heat accumulator  13  may be split into a plurality of regions. Therefore, the heat in the respective region is extracted parallel thereto. If the heat source is cooled in the first section, then the condensate here is no longer evaporated, but rather passes into the second region, absorbs the heat from said second region, evaporates and flows back through the next outlet to the heat sink. The processes are similar in the further regions. The separation into a plurality of regions has the advantage that the gas does not flow in the opposite direction to the condensate over the entire pipe length in the heat source, and the two phases thus hinder one another only to a small extent. 
       FIG. 19  shows an arrangement of the individual system elements with a heat exchanger  11  and heat accumulator  13  of a heat exchanger unit. The arrangement of the system elements has the connection of the heat exchanger  11  and of the heat accumulator  13  via the transport lines  14  with elastic connecting elements  14 . The flow directions of the working medium through the system are also shown. 
       FIG. 20  shows the arrangement of the individual system elements with a heat exchanger  11  and heat accumulator  13  of a heat exchanger unit  1  in connection with a thermal ground probe  21  as a heat source  2 . For illustration, the flow directions of the working medium through the system are again shown. 
       FIGS. 21 and 22  show an arrangement of the heat exchanger  11  for a locking crib heater of a track switch  3 . The locking crib  35  is covered by a heat exchanger  11  so as to minimize the infiltration of snow or ice. A further heat exchanger which prevents freezing of the melt water may be arranged on the base of the locking crib, and the water may be discharged in this way. 
       FIG. 23  shows a cross section through a heat exchanger  11  along a mini-duct  111  which operates on the counterflow principle. To improve the return flow of the condensate in a virtually horizontally arranged heat exchanger, the closed end is turned upward in the counterflow embodiment. The feed  114  and discharge  115  ducts are formed in one as a unitary supply duct. 
       FIGS. 24 and 25  show a cross section through a heat exchanger  11 , having a tilting mechanism  4 , along a mini-duct  111  which operates on the counterflow principle. In  FIG. 24 , the duct is tilted in the working position. On account of the inclination in said position, the liquid working medium can flow out to the heat source unhindered. In contrast, in  FIG. 25 , the heat exchanger  11  is in a standby position. The mini-ducts  111  of the heat exchanger  11  are inclined by the tilting mechanism in such a way that the condensed working medium can no longer flow back to the heat source. The extraction of energy from the heat source can be controlled in a targeted fashion in this way. 
     LIST OF REFERENCE SYMBOLS 
     
         
           1  Heat exchanger unit 
           11  Heat exchanger 
           111  Mini-ducts 
           112  Inlet point 
           113  Outlet point 
           114  Feed duct, supply duct 
           115  Discharge duct, supply duct 
           116  Upper shell 
           117  Lower shell 
           118  Corrugated plate 
           12  Transport lines 
           121  Feed transport line 
           122  Return transport line 
           13  Heat accumulator 
           14  Connecting element 
           2  Heat source 
           21  Thermal ground probe 
           22  Auxiliary heater 
           3  Component to be heated; track switch 
           31  Slide chair 
           32  Track switch tongue 
           33  Rail web 
           34  Rail base 
           35  locking crib 
           36  Deformable elements 
           37  Tie 
           38  Rail 
           4  Tilting mechanism, control unit 
         OS Upper apex line of a mini-duct 
         US Lower apex line of a mini-duct 
         L Longitudinal axis of a mini-duct 
         FG Flow direction of gaseous medium 
         FF Flow direction of liquid medium 
         A Lower section plane