State of the art Thermosiphon circuits can be found as temperature control systems for instance for circuit board assemblies in outdoor cupboards. Fig 1 shows the basic principle of such a thermosiphon circuit. In a hermetical circuit 101 there is an evaporator 103 and a condenser 105 connecte by a rise pipe 107 and a fall pipe 109. The circuit contains some suitable refrigerant medium. The medium boils in the evaporator whereby it takes up heat 111. The formed gas rises to the condenser where it con- denses and emits heat 113. The fluid formed in the condenser then flows back to the evaporator. Accordingly, in order to make a thermosiphon work the condenser 105 must be placed higher than the evaporator 103.

A great avantage of the thermosiphon is that it can transport large quantities of heat over long distances by means of a small temperature difference. A further avantage is that the evaporator can be loaded with high surface effects.

In outdoor environments it is required that circuit board assemblies in a system are protected from direct contact with the ambient air. Fig. 2 shows an example of how this can be arrange by means of thermosiphon refrigeration and internal fan circu- lation. An outdoor cupboard 201 is refrigerated by a thermosiphon 203 whose evaporator 205 is arrange in the vicinity of circuit board assembly magazines 207

localise in the outdoor cupboard and whose condenser 209 is arrange elsewhere.

At each one of the magazines 207 a fan 211 is placed. The fans effect an air circula- tion 213 whereby hot air from the circuit board assembly magazines passes the evaporator 205 of the thermosiphon. The evaporator takes up heat from the air when the refrigerant medium is evaporated. Cold air is brought back to the magazines and warmed up. The heat being supplie to the evaporator is emitted at the condenser 209 of the thermosiphon and is taken up by for example the ambient air. Note that the evaporator and the condenser of the thermosiphon shall be placed apart from each other.

An alternative to this type consists of outdoor cupboards with inherent convection whereby the evaporator is placed in direct connection to the circuit board assemblies (not shown in the figures). The condenser is placed higher in a self-ventilating space, i. e. in direct contact with the ambient air.

A further problem with outdoor applications is that the cupboards sometimes have to be refrigerated and sometimes warmed up depending on the temperature of the surrounding air. The solution used up to now is to dimension the refrigeration for the highest intended outdoor temperature, but at the same time provide the cup- boards with electrical heating elements, so that the lowest intended outdoor tem- perature does not cause break downs. To use outdoor cupboards, which are at the same time both warmed up and refrigerated, is naturally an expensive as well as a high power consuming solution.

Yet another problem with a thermosiphon circuit is that the refrigerant medium may leak out. This can however be compensated for to some extent by somewhat over- dimensioning the amount of the refrigerant medium.

Description of the invention An object of the present invention is to obtain a simple and reliable temperature control system, especially a thermosiphon system whose refrigeration capacity can be regulated.

Another object of the invention is to obtain a power saving temperature control system.

A further object is to obtain a temperature control system with a built-in refrigerant medium buffer.

These objects and others are attained according to one side of the invention with a temperature control system, especially a thermosiphon system with controllable re- frigeration capacity, including an evaporator, a condenser, a rise pipe, a fall pipe and a refrigerant medium, where the refrigerant medium is arrange to evaporate and take up heat in the evaporator, rise in the rise pipe, condense and emit heat in the condenser and fall back to the evaporator in the fall pipe. The invention comprises a container, a branch pipe, a heater, a contact body and a cooling medium. The branch pipe is connecte to the fall pipe and to the container. The heater is thermally con- nected to the container and the contact body is thermally connecte to both the con- tainer and the cooling medium. The temperature control system is arrange to dy- namically adapt its refrigeration capacity to the current refrigeration need by regu- lation of the power of the heater.

The cooling medium may be condense refrigerant medium in the fall pipe or an external cooling medium, especially surrounding air.

The refrigeration capacity depends on the amount of refrigerant medium present in the system evaporator-condenser-rise pipe-fall pipe, which is controlled by the power of the heater: the higher power the more refrigerant medium is boiled-off and the more fluid is forced out to the system evaporator-condenser-rise pipe, fall pipe and the lower refrigeration effect is obtained.

Thus the heater is preferably arrange to heat the container with an increasing power in case of a decreasing refrigeration need and with a decreasing power in case of an increasing refrigeration need.

The container, the branch pipe, the heater and the contact body are preferably ther- mally insulated.

The heater may be controlled by a thermostat, which senses some temperature in the system. Alternatively the heater may be an in an electrical circuit connecte tem- perature depending resistor, especially a PTC-resistor.

According to another side of the invention the mentioned temperature control sys- tem can be used as refrigerant medium buffer in case of a leak. In this respect the cooling medium must be condense refrigerant medium in the fall pipe. The heater is arrange to always heat the container with at least a small threshold power, which power is adjusted such that the temperature in the container is under the boiling point when operating at high temperature. When the amount of fluid is getting smaller in the system evaporator-condenser-rise pipe-fall pipe the undercooling in the fall pipe decreases. The result is that the temperature increases in the container and then fluid is boiled-off whereby, due to the increased pressure, fluid is forced out to the system evaporator-condenser-rise pipe-fall pipe. With appropriately cho- sen parameters concerning threshold power, refrigerant medium, contact body and container the forced out fluid compensates the leakage.

One avantage of the present invention is that it is simple, inexpensive and reliable as it lacks movable mechanical parts.

A further avantage of the invention is that a combine temperature regulation and refrigerant medium buffer can be obtained.

Further avantages of the invention will appear in the following description.

Description of the figures The invention is further described below with reference to fig. 3-8, which are shown only to illustrate the invention and shall therefore not in any way restrict it.

Fig. 1 shows a thermosiphon according to prior art.

Fig. 2 shows an outdoor cupboard refrigerated by a thermosiphon and with internal fan circulation according to prior art.

Fig. 3 shows a thermosiphon with refrigeration capacity regulation according to one embodiment of the present invention.

Fig 4 shows a thermosiphon with refrigeration capacity regulation according to an alternative embodiment of the present invention.

Fig. 5 shows details of a thermosiphon with refrigeration capacity regulation and also examples of temperature conditions and levels of fluid during regulation ac- cording to the invention.

Figs. 6a-c show different examples of how a container of a thermosiphon can be re- frigerated according to the invention.

Fig. 7 shows an example of how circuit board assemblies mounted in magazines can be refrigerated by a thermosiphon according to the invention.

Fig. 8 shows an example of how circuit board assemblies in a radio base station can be refrigerated by a thermosiphon according to the invention.

Preferred embodiments In the following description, in the purpose to explain and not to restrict the inven- tion, specific details are stated such as specific applications, techniques etc. to give a clear understanding of the invention. However, it will be apparent for those skilled in the art that the invention can be performed in other embodiments than the one being found in the description.

In fig. 3 there is shown a thermosiphon system 301 with temperature control ac- cording to a preferred embodiment of the present invention filled with a two-phase refrigerant medium 321. The system comprises an evaporator 303, a condenser 305, a rise pipe 307 for evaporated fluid, a fall pipe 309 for condense vapour and also a refrigerant medium container 311.

The container is connecte to the fall pipe 309 via a branch pipe 313. In contact with the container 311 there is an electrical heating element 315 and a thermally conducting contact body 317, which also bears against the fall pipe. Further, to pre- vent exchange of heat with the surroundings the container is insulated 319.

The branch pipe 313 may alternatively be connecte to the container 311 and the fall pipe 309 such as shown in fig. 4.

The function of the thermosiphon according to the invention is described below in connection with fig. 5, which shows the fall pipe 309, the contact body 317, the re- frigerant medium container 311 and the heater 315 of the thermosiphon. Further- more, the condenser 305 of the thermosiphon is shown separated from the other components. A temperature diagram shows the temperature in the fall pipe, in the contact body, and in the refrigerant medium container for two different power levels (D and (Z of the heater. Corresponding levels of fluid in the refrigerant medium container and the condenser are also indicated.

The fluid flowing through the fall pipe 309 has always a temperature some degrees below the boiling point of the fluid. If only a small threshold power is emitted from the electrical heater (i. e. at power level 0)) all gas in the container will condense, i. e. the container will be completely filled with fluid (level (D). The quantity of fluid in the system evaporator-condenser-rise pipe-fall pipe is then adapted to a minimal quantity of fluid in the condenser (level (D). In this state the thermosiphon has its maximum refrigeration function.

When the electrical heater is activated and if the emitted power is higher than the power lost to the surroundings (i. e. power level (Z) heat will be led to the fall pipe via the contact body. This flow of heat creates a temperature difference in the con- tact body which means that the temperature of the fall pipe always is lower than the temperature of the refrigerant medium container, see the temperature diagram in fig.

5.

When the power of the heater increases the temperature in the refrigerant medium container rises. Some of the refrigerant medium then boils-off, which results in that a pillow of gas will be formed at the top of the container. The fluid displaced by the gas pillow is hereby forced out to the system evaporator-condenser-rise pipe-fall

pipe whereby the level of fluid sinks (to level (Z). The level of fluid in the con- denser of the thermosiphon will then rise (to level (Z), which results in a greater un- dercooling of the fluid. When the undercooling becomes just as large as the tem- perature fall in the contact body the boil-off in the container stops.

Since the surface of the condenser in contact with the gas decreases will the tem- perature difference relative to the surroundings increase which causes the system to stabilise around a higher boiling point, see the temperature diagram in fig. 5.

The power regulation can in principle be driven so far that the whole condenser be- comes filled with fluid, whereby the function of the thermosiphon ceases.

This simple and uncomplicated regulation principle causes the downregulation of the refrigeration effect to be approximately proportional to the power supplie to the container, which gives a stable regulation.

It shall be noted that the temperature difference that arises between the refrigerant medium in the container and the undercooled fluid in the fall pipe is given by the heat resistance of the contact body in series with the heat resistance for boiling in the container and the heat resistance for convection in the fall pipe. The two last mentioned heat resistances are however small in relation to the heat resistance in the contact body. The regulation principle according to the invention would however work even if that was not the case. The condition is that the total heat resistance, when it is flowed through by a power that is less than the undercooling power, cre- ates a temperature difference equal to the undercooling of the fluid.

Also observe that the refrigerant medium container ought to be insulated against the outside surroundings. Otherwise there is a risk that the container will not be com- pletely filled with fluid when the heater is turned off and then the regulation will not work as intended. This risk occurs when the surroundings have a high temperature

and there is an inflow of heat to the container. On the other hand, if the refrigerant medium in the container has a higher temperature than the temperature of the sur- roundings there will be an outflow of heat from the container to the surroundings.

This can however be compensated for by a higher power on the heater.

The power needed to force the fluid out from the container depends on how rapidly the device shall regulate. For an outdoor cupboard a thermal time constant of about ten minutes can be a reasonable value. In the case of correct dimensioning of the thermal resistance in the contact body and the insulation, the power need of the heater will then be very small in relation to the effect generated in the cupboard. For instance a heat power of 10 W should be more than enough for a total refrigeration effect of 500 W.

An obvious way of controlling the heater 315 of the container is to use a thermostat that senses some temperature in the system. Another very simple temperature regu- lation is obtained by letting the heater consist of a PTC resistor connecte to an electrical circuit. This type of resistor is depending on the temperature and has in principal a switch temperature over which the resistance increases considerably with the temperature. The regulation device will then be very simple.

The dimensioning of the thermosiphon system according to the invention, i. e. of the under cooling, the contact body, the insulation of the container, the time constant of the regulation and the heat supplie to the container have to be adapted for an opti- mum function.

Fig. 6a-6c show different forms of cooling medium for cooling of the container 311.

Fig. 6a shows the already described alternative with cooling by undercooled fluid in the fall pipe 309, which is in thermal contact with the insulated container via the contact body 317. This alternative works well if the temperature of the container is

higher than the temperature of the surroundings. If this is not the case problems with external heating may occur, especially if small containers are used.

Fig. 6b shows an alternative with an external cooling medium 601 which for in- stance can be formed of inherent convection of the surrounding air especially if the thermosiphon is placed outdoors. This cooling medium is in thermal contact with the container 311 via a contact body 603. The disadvantage of this type of cooling is that it becomes very irregular since the external cooling medium varies considerably in temperature. It must therefore be possible to regulate the power of the heater within a large interval. Simple solutions as for instance heating with a PCT-resistor can then be hard to make working.

Fig. 6c shows an alternative with cooling by undercooled fluid partly surrounding the container 311. The fall pipe 611 with undercooled fluid of the thermosiphon has a spiral part wherein the container is placed. The container is surrounded by a metal casing 613, which in turn is surrounded by a filling 615 working as a contact body and against which the fall pipe is in contact. Solutions like this with undercooled fluid, partly or totally surrounding the container, work very well and safely in all surroundings.

A further alternative (not shown in the figures) is that the whole assembly fall pipe- container is integrated in one single so-called roll-bond plate. Such an alternative is especially suitable for containers having small volume.

Another problem with a thermosiphon according to the prior art is that the refriger- ant medium for some reason may leak out. Large leaks are easily discovered in the production. Extremely small leaks whose influence is observe only after several years are on the other hand considerably more difficult to discover. In addition, damages may occur after the installation. In a normal household refrigerator the amount of refrigerant medium is dimensioned such that a reasonable length of life is

managed with a leakage of a few grams of refrigerant medium per year. Some type of refrigerant medium buffer is always needed in a thermosiphon system.

It would be an avantage if the container in the thermosiphon system according to the invention also can be used as refrigerant medium buffer. This is provided if the heater always emits a certain small threshold power. Normally this power is so small that the temperature in the container is under the boiling point. However, if the amount of fluid in the system evaporator-condenser-rise pipe-fall pipe becomes very small the undercooling in the fall pipe decreases. Then the temperature in the container rises and at a given level of fluid the boiling point is reached whereby fluid is boiled-off. Some of the fluid in the container will then be pressed out to the system evaporator-condenser-rise pipe-fall pipe.

In a thermosiphon system without a fluid buffer the amount of fluid can be checked by measuring the undercooling of the fluid coming from the condenser. The under- cooling decreases if the amount of fluid in the thermosiphon system and so the con- denser decreases. The connection is however influence to a certain extent by the total heat load in the system. At a high heat load the share of fluid in the condenser will be a bit higher than at low heat load. Therefore comparative measurings should be done at equal heat load.

If the thermosiphon system according to the invention is used this type of checking can be done if the heater is completely turned off. In order to discover leaks in a working system it should be enough to accomplis such a check measurement once to twice a year, and that should be easily practicable. The checking can also be per- formed with a certain small threshold power on the heater. The regulation itself should however be turned off which means that the measurement preferably is per- formed when the external surrounding air temperature is high.

Fig. 7 shows an application for the thermosiphon with refrigerant medium container according to the invention. Circuit board assemblies 701 mounted in magazines 703 are refrigerated by the evaporators 705 of the thermosiphon in that these are in con- tact with side members 707 in which the magazines 703 are mounted.

The losses in a pile 709 of magazines can be rather high. The condenser 711 of the thermosiphon should then be refrigerated by a fan 713. The evaporators 705 are preferably made of roll-bond plates.

The refrigerant medium container 715 of the thermosiphon may very well be placed together with the evaporators and the pile of magazines in a space with insulation 717 in order not to obtain too large disturbances from a varying outdoor tempera- ture.

At the top of fig. 8 another application is shown for the thermosiphon with refriger- ant medium container according to the invention. Here the thermosiphon is used in a smaller radio base station 800 comprising only one circuit board assembly. The condenser 801 of the thermosiphon is refrigerated by inherent convection (with an airflow 802 from below) and integrated in the upper half of a cooling flange element 803. The evaporator 805 is preferably compose of a roll-bond plate contacting cir- cuit board assemblies 806 localise in the base station or individual components 806a. The refrigerant medium container 807 is shown greatly magnifie in fig. 8.

The heat emitted from the lower part of the flange element may be led through the bottom plate of the flange element (not shown in fig. 8). Despite the level for the outlet of the condenser being below the highest point of the evaporator a satisfying circulation of refrigerant medium is obtained. This has been verified experimentally for a thermosiphon. Notice however that the thermosiphon shall contain a very small amount of refrigerant medium.

The evaporator can for instance be embodied according to the alternatives shown in the detail views in fig. 8, down to the left 805'and down to the right 805"respec- tively. According to the first alternative a roll-bond plate 805'is folded around the circuit board assembly 806 with components 806a. If certain components require extra refrigeration a contact body 809 can be placed between these components and the evaporator. In this way high point effects can be led away.

If the point effects are extremely high can, according to the latter alternative, a roll- bond plate 805"a on the back of the circuit board assembly and specially formed evaporators 805"b in contact with the components 806a on the front of the circuit board assembly be used.

The present invention as herein described solves the problems associated with prior art.

The temperature control system according to the invention is simple, reliable and power saving. Furthermore it comprises a built-in refrigerant medium buffer.

The invention is obviously not restricted to the embodiments described above and showed in the figures but can be modifie within the limits of the appende claims.