EP2171372A1

EP2171372A1 - Device for distribution of an expanding liquid

Info

Publication number: EP2171372A1
Application number: EP08779378A
Authority: EP
Inventors: Claes Stenhede; Klas Bertilsson
Original assignee: Alfa Laval Corporate AB
Current assignee: Alfa Laval Corporate AB
Priority date: 2007-06-25
Filing date: 2008-06-11
Publication date: 2010-04-07
Also published as: CN101802514B; EP2171372A4; CN101802514A; SE0701526L; SE531780C2; WO2009002256A1

Abstract

A pressure reducing valve for expansion of a refrigerant fluid comprises: an inlet (104) for reception of said fluid, and an outlet (106) for discharging of a resulting expanded liquid/gas mixture, and a channel (108) fluidly connecting the inlet (104) and the outlet (106), The valve is characterized in that the length of said channel (108) is variable in order to obtain a specific degree of expansion.

Description

DEVICE FOR DISTRIBUTION OF AN EXPANDING LIQUID

Field of the Invention

The present invention relates to a pressure reducing valve for expansion of a refrigerant fluid and to a plate heat exchanger comprising said valve.

Background

In a plate heat exchanger adapted for direct expansion, a mixture of liquid and gas, i.e. a two-phase flow, enters a port into a header and is subsequently distributed to channels, and as the mixture passes the channels of the plate heat exchanger, heat from a medium surrounding the channels is absorbed by the mixture through evaporation of the mixture. In the case of a two-phase flow the liquid refrigerant enters an expansion valve upstream the port at a high pressure, normally a pressure close to the condensing pressure. In the valve the liquid expands or flashes to just above the evaporation pressure and a part of the liquid vaporizes. In order to achieve a precise distribution of capacity in the plate heat exchanger each channel should ideally be charged with a precise amount of liquid and gaseous components.

It is important to know that the expansion valve does not control the evaporation pressure, and the pres- sure drop over the valve is constant over short periods and are given by the refrigeration duty and the ambient conditions but not by the valve. The valve controls the flow (capacity) by changing the cross section, that is, the internal resistance. A precise distribution can either be an equal distribution between the various channels, or any other well defined distribution corresponding to a certain case, e.g., when the evaporator has two sections, which are used to cool two different fluids, having different prop- erties. In this case the channels in the two sections shall have different, yet precise, flow rates.

A precise distribution is difficult to obtain since, at low velocities, the two-phase flow tends to separate, resulting in that the liquid portion settles at the entrance of the header while the gaseous portion is distributed in the remainder of the space. As a consequence the liquid preferentially enters into the first channels. On the other hand, at high velocities, the inertia makes it difficult for the liquid to change direction and enter the channels. As a consequence thereof, most of the liquid refrigerant is collected in the furthermost part of the header, and thus enters the channels in said furthermost part. The problem resulting from this behaviour is that the cooling properties will suffer, both in terms of capacity and homogeneity, since the cooling properties will vary between individual channels.

The distribution can be improved if the pressure drops over the channels are high compared to the pressure drop in the header. The higher this ratio is, the less is the pressure drop difference between the channels and the better the distribution will be.

Prior art solutions include distributors that are arranged close to the entrance of each (or every second/third) channel. The distributors generally comprise a fixed restriction of the channel cross section, which results in a pressure drop prior to the channel but after the mixture has been distributed along the length of the header, as opposed to the previously described situation in which the main pressure drop occurs in the valve, before the mixture being distributed. The channel pressure drop is now increased as compared to the header pressure drop. This type of solution solves part of the problem, yet only in a static arrangement. The flow in the header is still a two-phase mixture, with the behaviour described above. A device for uniform expansion of a liquid/gas two- phase refrigerant mass flow in a plate evaporator is described in US-A-5 806 586. The evaporator has a distribution duct, which is capable of being loaded on the inlet side with the refrigerant mass flow coming from an expansion valve. The evaporator further has a plurality of exchanger sections branched off essentially perpendicularly from the distribution duct along the latter at a distance from one another. In order to achieve a uniform distribu- tion of the mass flow to the exchanger sections a porous body is arranged in the distribution duct. By this expansion valve the distribution is improved but is still far from ideal .

Summary of the Invention

The object of the present invention is to eliminate or at least alleviate the above referenced drawbacks by the provision of a novel pressure reducing valve in accordance with claim 1. The inventive valve enables variations in expansion by variation of the distance over which a refrigerant travels, as opposed to regular valves where the expansion (or pressure drop) is effected by variation of a cross section. The inventive solution makes it possible to con- struct economical valves that eliminates or at least alleviate the drawbacks of prior art. With the inventive valve it is possible to maintain a one-phase, liquid, flow until any relevant spatial distribution of the flow is effected. In one or more embodiments the pressure reducing valve comprises inlet which is arranged in an inner cylinder, said inner cylinder being rotatably mounted in a cavity having an inner wall comprising the outlet, wherein the channel is formed by a clearance between said peripheral wall and said inner wall and wherein rotation of said inner cylinder varies the length of the channel, that is the distance between the inlet and the outlet. The use of an inner cylinder and a rotational movement makes it possible to vary the shape of the channel, as well as the behaviour of the valve in an expedient manner. The inner cylinder is easily machined using standard equipment and arranged in the cavity using standard measures .

In one or more embodiments the inner cylinder is hollow and orifices are arranged along the longitudinal axis of the cylinder fluidly connect the interior of the cylinder with the channel. This arrangement facilitates the use of the valve in that the inlet of the valve, and thereby connection tubing, can be arranged concentrically without elaborate constructions. Also, the distribution of the refrigerant can be performed before said refriger- ant is discharged through the orifices.

In one or more embodiments the inner cylinder has an axially extending groove on its outside into which groove the orifices debouch. The groove will constitute a second distribution channel, such that pressure differences can be equalized and the flow even better distributed. The arrangement of a groove is also a straightforward action in order to obtain uniform conditions for the various channels .

In one or more embodiments the channel has a cross section which varies as the length of the channel varies. This construction will result in an expansion which is not directly proportional to the length of the channel, i.e. to a rotation angle, and it enables the creation of a expansion valve with exponential behaviour, or any other suitable behaviour.

In one or more further embodiments the orifices are arranged essentially in pairs, diametrically arranged on opposite sides of the rotational axis of the inner cylinder. This makes it possible to equalize a bending force acting on the inner cylinder due to a difference in pressure on opposing sides of said inner cylinder. In certain designs this bending force could deform the cross section of the channel and thereby inflict an unwanted variation in expansion and/or flow rate of the refrigerant.

In one or more embodiments the inner wall has a projection extending inwardly into the channel, wherein ro- tation of the inner cylinder varies a distance between a tip of said projection and the inner cylinder and thus varies a cross section of a free passage in the channel, wherein a pressure drop is varied. In this embodiment the variation in expansion is effected by the variation of the length of the channel. However, in some operating conditions the pressure drop occurring in the free passage practically represents the entire pressure drop of the valve.

In one or more embodiments the valve comprises an assembly of an inner cylinder and an outer cylinder, said assembly being insertable in a cavity in a heat exchanger system. This construction is particularly suitable for use in heat exchanger systems since all operational parts can be preassembled and subsequently arranged in the heat exchanger system, without any precision work being performed on the heat exchanger system as such. Inlets, channels and outlets, defining the characteristics of the valve are all preassembled.

In one or more embodiments the valve is an assembly of a stationary part and a part that is rotatable around a rotational axis, respectively, said valve having a longitudinal dimension, L, along the direction of the rotational axis, wherein at least one of said parts is constructed from discrete, compatible, elements of a length 1, KL. The use of construction elements of a certain length increases the standardization of the system in that identical construction elements could be used for different applications, e.g. the total length of the valve can easily increased by adding a few elements. When constructing the valve from smaller elements other types of machining processes could be used. In one or more embodiments the valve is arranged in a heat exchanger system comprising a circuit including a condenser, an evaporator containing a distribution header fluidly connected to several fluid channels coupled in parallel and a compressor, each having an inlet and an outlet, wherein the outlet of the condenser is connected to the inlet of the valve, the outlet of the valve is connected to the inlet of the evaporator, the outlet of the evapo- rator is connected to the inlet of the compressor, and the outlet of the compressor is connected to the inlet of the condenser, such that the valve is arranged in or constitutes the distribution header.

One inventive concept relates to a plate heat ex- changer comprising a valve according to the invention. The plate heat exchanger comprises at least one base plate and a pressure plate, wherein the pressure plate has an interior indentation for positioning of one end of a valve according to the invention. The indentation makes it possible to position the inner end of the valve securely, without addition of further components. The indentation may have an essentially circular base section and is dimensioned to receive a portion of the end of said valve. As an additional benefit the indentation also serves as a constraint in the axial direction so that, e.g., in cases where the valve is an assembly of several elements the indentation acts as an abutment surface, which aids in holding the assembly together. The manufacture of the indentation is cost efficient and can readily be incorporated in a production process. In an alternative embodiment the indentation is provided in a separate plate which may be arranged either on the inside or the outside of an end or pressure plate. Brief Description of the Drawings

Preferred embodiments of the invention will now be described in more detail below, reference being made to the accompanying drawings, in which Fig. 1 is a schematic of a basic refrigeration cycle;

Fig. 2 is a partial schematic of a plate heat exchanger according to prior art;

Fig. 3 is a schematic of a plate heat exchanger in accordance with Fig. 2, provided with a main expansion valve;

Fig. 4 is a schematic of a plate heat exchanger in accordance with Fig. 2, provided with individual expansion valves for each channel; Figs. 5-7 are schematics of a plate heat exchanger in accordance with Fig. 2, provided with individual fixed restrictors for each channel and a main expansion valve;

Fig. 8 is a cross section in a radial direction of a valve according to a first embodiment of the invention. Fig. 9 illustrates various examples of cross sections, in an axial direction, that are possible for the different embodiments of the invention.

Fig. 10 is a cross section similar to Fig. 8 of a valve according to a second embodiment of the invention. Fig. 11 is a cross section similar to Fig. 8 of a valve according to a third embodiment of the invention.

Fig. 12 is a cross section similar to Fig. 8 of a valve according to a fourth embodiment of the invention.

Fig. 13 is a schematic exploded perspective view of an inventive valve according to a fifth embodiment of the invention .

Fig. 14 illustrates how the valve of one or more embodiments can be assembled from elements.

Fig. 15 is an exploded view of a valve according to the inventive concept arranged in a plate-heat exchanger.

Fig. 16 is a cross section of a plate heat exchanger in accordance with one inventive concept. Detailed Description of Embodiments of the Invention

The basic compressor refrigeration cycle according to prior art is shown in Fig. 1. The actual use of the cycle can obviously be in an air conditioning apparatus/plant, a heat pump as well as in a proper refrigeration apparatus/plant or any other process where a refrigerant is vaporized, such as a power cycle.

In the condenser 1, high pressure gaseous refriger- ant condenses. The condensed refrigerant then flows to the expansion valve 2. In the expansion valve 2 the liquid passes a restricted cross section. This imparts a high pressure drop, so that the pressure falls to close to the evaporation pressure. In the process, part of the liquid refrigerant evaporates and the mixture is cooled down to close to the evaporation temperature. At the exit of the valve 2, a cold two-phase mixture leaves the valve 2. The two-phase mixture enters the evaporator 3 in which the cold liquid refrigerant evaporates and cools down the process fluid. There are several commonly used process fluids, such as air, water, brine, a process liquid, etc. The cold low pressure gaseous refrigerant then enters the compressor 4. Here the refrigerant pressure is increased to a pressure level, which is sufficiently high for the intended refrigerant medium to be able to condense in the condenser 1.

The control of the valve 2 in conjunction with the evaporator 3 is crucial for a good functioning of the cycle. When the cooling requirements change, the valve has to be adjusted accordingly. If too much refrigerant leaves the valve, liquid might not evaporate completely in the channels. This results in liquid leaving the evaporator, which in certain cases can damage the compressor. If too little refrigerant passes through the valve 2, the required capacity cannot be kept.

In a heat exchanger, see fig 2, composed of a number of parallel channels 6, it can be difficult to obtain a precise distribution of a fluid from a distribution header 5 to the parallel channels 6 and then into a collection header 7. The distribution header 5, or "header", is a distribution manifold from which the channels 6 are branched off. In Fig. 2 only the flow pattern is drawn, the heat exchanger is just indicated as separated surfaces 8. It can be composed of any type of parallel connected channels.

Assume that the pressure drops A - B and C - D are large compared to A - D and B - C . As the pressure drop must be equal from inlet to exit regardless of whether we follow the path A - D or A - B - C - D, it follows that the pressure drop A - D is higher than B - C. As the pressure drop is the driving force for the flow, it fol- lows that the flow of refrigerant will be different for different channels.

A correct distribution is still more difficult for a two-phase flow, e.g. for an evaporator in a refrigeration apparatus/plant. Fig. 3 shows the valve/evaporator assem- bly. Saturated or almost saturated liquid refrigerant enters the valve 2 at a high pressure, usually close to the condensing pressure. In the valve 2 it expands to just above the evaporation pressure, whereby a part of the liquid vaporizes. The resulting two-phase fluid has a large volume, which increases the pressure drop in the header, which compounds the problem. If the refrigerant velocity is low in the distribution header 5, the liquid part settles at the entrance part of the distribution header 5 and enters preferentially in the first channels, extending from that part of the distribution header 5. If the refrigerant velocity is very high, inertia will result in that the liquid refrigerant will have difficulty to change direction and enter the channels. In this case, liquid refrigerant will build up in the furthermost part of the distribution header 5 and subsequently enter the furthermost channels 6. Consequently, the refrigerant flow velocity in the header 5 is a parameter that affects the performance of the heat exchanger in an unwanted fashion .

Both problems could be solved if the expansion of the liquid is made just before each channel 6. In Fig. 4 a number of variable restrictors 2' are placed before each channel 6. In the distribution header 5 there is now only liquid, thus no problem with phase separation, and the pressure drop A - B will be low. The pressure drop C- D is still comparatively low, while the pressure drop A- A' -D and B-B' -C is high, practically corresponding to the differential pressure between the condensing and evaporation pressures. The flow of refrigerant through different channels will be precise, since there is a precise amount of liquid refrigerant passing each variable restrictor 2' . There is consequently no distribution problem as there is only liquid refrigerant and no gaseous refrigerant in the distribution header 5. The solution of using a large number of small, adjustable restrictors of known type is costly, and to actually integrate the individual valves into the evaporator 3 is cumbersome. Further the maintenance and cleaning of such valves or restrictors is a difficult task.

A practical solution is then to keep the main expansion valve 2 and to introduce fixed restrictors 9, 10, 11 instead of the variable restrictors 2', as illustrated in Figs. 5-7. The fixed restrictors 9-11 can be in the form of a pipe with fixed restrictors 9 in its peripheral wall, said pipe being inserted in the distribution header 5, see Fig. 5. In the case of a plate heat exchanger the plates can be formed to a plate-like restrictor 10 at each channel inlet, see Fig. 6, or disks with drilled restrictors 11, inserted in a port hole of each channel, see Fig. 7. The fixed restrictors 9-11, if allowed to take the full differential pressure, will do a good job distributing the refrigerant. However, in order to operate at part load the valve 2 has to be used to realise the necessary pressure drop in order to vary the flow through the channels, and the larger the pressure drop, the more of the previously mentioned problems with the two-phase flow will recur. Further, the optimal size of the restrictors 9-11 varies with nominal capacity, pres- sure, type of refrigerant, etc, i.e. each refrigeration system needs individually tailored restrictors. Thus, the use of fixed restrictors in the above context will be an inflexible solution.

The invention alleviates the above mentioned prob- lems and a first embodiment of the present invention will now be described referring to Fig. 8 and 9. This embodiment solves the practical problems related to the occurrence of a two-phase flow. The valve 100 is composed of two concentric tubes 101, 102. The tubes are preferably made of metal, such as a brass alloy, bronze alloy or stainless steel but may of course be made of other materials such as PTFE, etc. The inner tube 101 has an external thread profile and the outer tube 102 has a mating internal thread profile, which ensures a secure and re- peatable fit between the tubes 101 and 102. The ridges of these threads have a frustoconical shape so as to form defined channels between the inner and the outer tube. The circumferential grooves 108, which start at the groove/header 110, continue along the periphery but stop just before they reach the groove/header 110 again. The grooves 108 can have a rectangular, V-shaped, semi spherical or any suitable cross section, as exemplified in Fig. 9. The grooves can be made individually in the form of circumferential ridges, or as threads on a screw. The outer tube 102 can be equipped with internal grooves 111 as described above but is preferably smooth, for cost reasons. If equipped with internal grooves 111, the grooves on the two tubes 101 and 102 should be made as threads and the inner cylinder 101 screwed into the outer 102. The outer circumference of the inner tube could also be smooth, so that the channel 108 essentially consists of a clearance between the inner and outer tube. The inner tube 101 further has an inlet, not shown, leading to its hollow interior 112. Refrigerant entering through the inlet will be distributed along the axial direction of the hollow interior 112, the latter opera- tively working as a distribution header. The refrigerant will then follow the path indicated by the arrow 114 in Fig. 8, radially out through orifices 104 in the inner cylinder 101, circumferentially along the clearance/channels 108 ("channels" in the following) between the inner cylinder 101 and the outer cylinder 102 and will subsequently be discharged from the valve 100 through the outlets 108 in the outer cylinder 102. As the refrigerant flows from the orifices 104 in the inner tube 101, through the channels 108 and out through the open- ings 106 in the outer tube 102, it experiences a pressure drop. The dimensions of the channels 108 is determined so that the pressure drop from the condensing pressure to the evaporation pressure takes place along said channels 108. As pointed out earlier this pressure drop varies with the type of refrigerant, which is one of the reasons for the valve assembly to be adjustable. When the pressure decreases because of the pressure drop, the liquid starts to flash and a two-phase mixture of gaseous and liquid refrigerant emerges from the openings 106 of the outer tube 102.

The length of the flow path from the orifices 105 in the inner tube 101, through the channels 108 and out through the openings 106 of the outer tube 102 is varied by turning the inner tube 101, so that the peripheral distance between the orifices 104 and the openings 106 is varies. As the length of the flow path is varied so is the pressure drop and consequently the flow through the valve 100. A physical block 107 prevents the refrigerant from following an alternative flow path along the periph- ery of the channels 108. In Fig. 10 the block 107 is integrated with the inner tube 101, the block 107 corresponding to a section of the inner tube 101 having an outer diameter corresponding to the inner diameter of the outer tube 102. Consequently, said physical block 107 only permits the refrigerant to flow along one flow path/direction through the channels 108. The refrigerant flowing through the channels 108 will have a dedicated flow direction. This can introduce a relative torque between the two tubes 101 and 102. Also, the refrigerant will enter the channels 108 at relatively high pressure and exit the channels 108 at a relatively lower pressure. This can result in a bias and thus asymmetric control characteristics. If such asymmetries are considered to be a problem, they can be dealt with by more elaborate constructions, as will be described in the following. In the design shown in Fig. 8 the biasing sources do not cancel out, and to enhance manoeuvrability of the valve 100 ball bearings (not shown) can be arranged between the inner 101 and the outer 102 tube. The ball bearings are arranged in a groove formed close to each end of at least one of the tubes. Note that the number of orifices 104, channels 108 and outlets 106 does not have to be one and the same. Further, when the valve 100 is used in an evaporator of a heat exchanger system, the number of outlets 106 does not have to equal the number of evaporator channels in order for a good distribution to be effected. One outlet 106 per two or three evaporator channels will generally suffice. The orifices 104 and outlets 106 are generally circular, to simplify manufacture, but they could equally well be realised by slots, oblong openings etc. Fig. 10a illustrates a valve 100 according to a second embodiment. In this embodiment the orifices 104 leading from the interior 112 of the inner cylinder are diametrically arranged on opposing sides of a rotational axis R. In this way the inner cylinder 101 is balanced and there are no bending force acting on its longitudinal axis. Further, as shown in Fig. 10a the channels 108 do not need to be of the same height along the channel length. By letting the channel height increase when its length is decreased by the rotation of the inner cylinder 101, the change in flow can be reasonably proportional to the total flow for a give change in the channel length. This means that a valve 100 with a logarithmic characteristic has been obtained. There are obviously numerous possible variations of this second embodiment. Common to these are that the control range, the possibility to alter the pressure drop, is larger than if a constant chan- nel height, or channel cross section, is used.

A variation of the second embodiment is shown in Fig. 10b. Here the outer circumference of the inner cylinder has an oval cross section, e.g. defined by two interconnected half circles, thus forming a channel of varying height along the channel length. The inner cylinder bear against the outer cylinder in two diametrically opposed positions on the largest radii of the inner cylinder. The orifices 104 are in this embodiment arranged in diametrically opposed positions of the smallest radii of the inner cylinder. This provides for a balanced valve assembly.

A third embodiment of the invention is illustrated in Fig. 11. As for the embodiment of Fig. 10 the valve 100 of Fig. 11 is balanced regarding bending forces act- ing on the inner tube 101 due to pressure differences. Further, in use the flow will follow the path indicated by the lines and arrows indicated by 114, making the flow perfectly symmetrical, as evident form Fig. 11. This also balances the possible torque induced by the refrigerant flow. Note at the outer tube has diametrically opposed outlets. Since there are two sets of outlets a third tube (not shown) can be arranged as a casing outside the outer tube. This casing only comprises one set of outlets, said outlets being positioned at a suitable location in terms of the intended use of the valve 100. The use of an outer casing is also applicable to the second embodiment, as well as the first, if so desired. A fourth embodiment is illustrated in Fig. 12. This embodiment is similar to the second embodiment, though the major part of each channel 108 has a large cross section, such that it does not induce any significant pres- sure drop. The outer tube 102, however, has a projection 116 extending inwardly into each channel 108 (as before there might be only one channel) thus creating a restriction. Further, the inner tube 101 has a varying diameter, such that rotation of the inner tube 101 will not only change the length of the channel 108, as described before, but will also cause the cross section of the restriction to vary. The major part of the expansion will then occur over the restriction, being a part of the channel . A fifth embodiment is shown in Figs. 13a and b. in this embodiment channel 108 between the inner tube 101 and the outer tube 102 has a varying height, or cross section, similar to the second embodiment. In this fifth embodiment, however, the inner tube 101 is arranged to "roll" along the inner periphery of the outer tube 102 as the inner tube 101 rotates. This could be a way to decrease abrasion and thus make the device more durable. Fig. 13a shows a closed position and Fig. 13b shows a partly open position. Variations of a sixth embodiment are illustrated in Fig. 14. This embodiment embraces all the other embodiments in that it illustrates an alternative construction that can be applied for all embodiments. In this fifth embodiment the structure of the inner tube is similar to the inner tubes described in the other embodiments, preferably a cylinder with a smooth outer circumference. The outer tube 102, however, differs in that it is assembled from discrete elements, rings 102' . The use of discrete rings 102' enables the use of alternative materials and methods of manufacture. In this embodiment the inner tube 101 is metal as before while the rings 102' are made of extruded polytetrafluoroethylene (PTFE) . PTFE also has the advantage of being a lubricant so the rings also act as bearings, so that no external bearings are necessary. Fig. 14 illustrates three different embodiments of said rings of which only the rightmost has been provided with reference numbers. Fig. 14 comprises, from top to bottom, an axial section view of the three embodiments, a radial section view, and a perspective view, respectively, the various embodiments being illustrated from left to right in each view. It can be noted that the outer casing 118 is shown in Fig. 14. For clarity it should be noted that the portion marked with 108 in the lowermost part of Fig. 14 corresponds to a recess in the inner portion of the outer tube element 102' . Once the valve is assembled this recess will form the channel 108, hence the notation. When the valve assembly 100 is mounted into the distribution header 5 or port hole of a plate heat exchanger, the outer tube, or, when applicable, the outer casing 18, is mounted stationary with respect to the port hole/plates while the inner tube 101 is rotatable. The valve 100 can also be combined with a heat- exchanger system, comprising a circuit comprising a condenser, pressure altering means, an evaporator containing a distribution header in which the valve is arranged in fluid connection with several fluid channels coupled in parallel, and a compressor, each having an inlet and an outlet. The outlet of the condenser is connected to an inlet of the valve and the outlet of valve is connected to the inlet of the evaporator, the outlet of the evaporator is connected to the inlet of the compressor, and the outlet of the compressor is connected to the inlet of the condenser. Said circuit comprises a fluid refrigerant and the refrigerant is kept from flashing until it has passed the valve by any of the measures of: lowering the temperature of the refrigerant down- stream the condenser and upstream the valve, and changing the pressure of the refrigerant with means capable of increasing the pressure of the refrigerant downstream the condenser.

The temperature is preferably lowered by using a precooler, and the pressure is preferably altered with a pump. The valve according to the invention could also be preceded by a traditional expansion valve, arranged in the circuit, between the valve and the condenser. Though the valve according to the invention can be used as a standalone unit, the described setup makes it possible to tune the inventive valve to a suitable setting during the start-up of a heat exchanger apparatus/plant, thus adjusting it to a certain capacity range, refrigerant medium, surrounding temperature and so forth. The day-to- day tuning of the capacity is thereafter realised by other means, such as variation in precooling, pressure and further expansion prior to the inventive valve. By using the valve in this context it is possible to increase the usability of existing heat-exchanger systems. Fig. 15 shows an exploded view of the general assembly of a valve according any of the embodiments, when used in a plate heat exchanger 128. A housing 120 has an inlet opening 130 through which refrigerant flows (indicated by the arrow) . The housing further comprises drive and transmission means, exemplified by a step motor 132. The outer cylinder 102 is fitted with bearings 134, such as ball bearings, roll bearings etc. receiving the inner cylinder 101 in rotatably. The housing 120 is sealingly mounted to a sleeve 136 of the heat exchanger 128, by welding, threading etc. A drive shaft 136 of the motor

120 is coupled to a shaft 138 suspended in the inner cylinder 101, so as to enable rotation of the inner cylinder 101 by means of the motor 120. The arrows 140 indicate the refrigerant flow entering the housing 120 and leaving the outer cylinder 102. Obviously the bearing surface between the inner and the outer cylinder needs to be sealed from the incoming flow, in order to prevent leakage to or from the outermost channel. Note that this seal, however, does not have to be perfect, as long as the preferred path for the flow is into the inlet, through the channels and out through the outlets. Fig. 16 is a cross section of a plate heat exchanger 118 adapted to receive a valve according to the inventive concept. The heat exchanger 118 comprises a base plate 142 and a pressure plate 144 and a sleeve 136 into which the valve is inserted and over which the valve assembly is mounted. The pressure plate 144 comprises an indentation 146 adapted to receive and position one end of the valve. The use of an indentation is a reliable and cost efficient way to position the valve in the heat exchanger. It is envisaged that this use of an indentation is useful for positioning of other types of valves. In an alternative embodiment the indentation may be provided in separate plate which can be arranged either on the inside or on the outside of the pressure plate.

A person skilled in the art realizes that there are several materials possible for the production of the inventive device or components of the same, such as machined brass, extruded aluminium, PTFE and so forth.

Claims

1. A pressure reducing valve for expansion of a refrigerant fluid, said valve comprising an inlet (104) for reception of said fluid, and an outlet (106) for discharging of a resulting expanded liquid/gas mixture, and a channel (108) fluidly connecting the inlet and the outlet, characterized in that the length of said channel (108) is variable in order to obtain a specific degree of expansion and wherein the inlet (104) is arranged in an inner cylinder (101), said inner cylinder (101) being rotatably mounted in a cavity having an inner wall comprising the outlet (106), wherein the channel (108) is formed by a clearance between a peripheral wall of the inner cylinder (101) and said inner wall and wherein rotation of said inner cylinder (101) varies the length of the channel (108), that is the distance between the inlet (104) and the outlet (106) .

2. The valve of claim 1, wherein the inner cylinder (101) is hollow and wherein orifices (104) arranged along the longitudinal axis of the cylinder (101) fluidly connect the interior of the cylinder (101) with the channel (108) .

3. The valve of claim 2, wherein the inner cylinder (101) has an axially extending groove (108) on its outside into which groove the orifices (104) debouch.

4. The valve of any preceding claim, wherein the channel (108) has a cross section which varies as the length of the channel (108) varies.

5. The valve of claim 4, wherein the channel (108) cross section is tapered towards the outlet.

6. The valve according to any one of claims 2-5, wherein the orifices (104) are arranged essentially in pairs, diametrically arranged on opposite sides of the rotational axis of the inner cylinder (101) .

7. The valve according to any one of claims 2-7, wherein said inner wall has a projection (116) extending inwardly into the channel (108), wherein rotation of the inner cylinder (101) varies a distance between a tip of said projection (116) and the inner cylinder (101) and thus varies a cross section of a free passage in the channel (108), wherein a pressure drop is varied.

8. The valve of any preceding claim, wherein the valve comprises an assembly of an inner cylinder (101) and an outer cylinder (102), said assembly being insert- able in a cavity in a heat exchanger system (128) .

9. The valve of any preceding claim, wherein the valve is an assembly of a stationary part and a part that is rotatable around a rotational axis, respectively, said valve having a longitudinal dimension, L, along the direction of the rotational axis, wherein at least one of said parts is constructed from discrete, compatible, elements (118) of a length KL.

10. The valve of any preceding claim arranged in a heat exchanger system comprising a circuit including a condenser, an evaporator containing a distribution header fluidly connected to several fluid channels coupled in parallel and a compressor (4), each having an inlet and an outlet, wherein the outlet of the condenser (1) is connected to the inlet of the valve, the outlet of the valve is connected to the inlet of the evaporator (3) , the outlet of the evaporator (3) is connected to the inlet of the compressor (4), and the outlet of the compressor (4) is connected to the inlet of the condenser (1), such that the valve is arranged in or constitutes the distribution header .

11. A plate heat exchanger, comprising a valve according to any one of the preceding claims.

12. A plate heat exchanger, comprising a plate (144) having an interior indentation (146) for positioning of one end of a valve according to any one of the preceding claims .

13. The plate heat exchanger of claim 12, wherein the plate (144) is a pressure plate.

14. The plate heat exchanger of claim 12 or 13, wherein the indentation has an essentially circular base section and is dimensioned to receive a portion of the end of said valve.