SE531665C2 - Method and apparatus for distributing an expanding liquid - Google Patents

Description

ET! EEE 2 is entered in the channels. As a result, most of the liquid refrigerant is collected in the outermost part of the manifold channel. wherein it is fed into the channels in this farthest part.

The problem that arises as a result of this behavior is that the cooling properties will suffer, both in terms of capacity and homogeneity, as the cooling properties will vary between individual channels.

The distribution can be improved if the pressure drop across the channels is large compared with the pressure drop in the distribution channel. The higher this ratio, the smaller the pressure drop difference between the channels and the better the distribution.

Prior art solutions include distributors located near the entrance to each (or every other third) channel. The advantages usually include a fixed limitation of the cross section of the duct, which results in a pressure drop before the duct but after the mixture has been distributed along the length of the distributor duct, in contrast to the previously described situation in which the main pressure drop occurs in the valve, before mixing distributed. The pressure drop in the duct thereby increases relative to the pressure drop in the manifold channel. This type of solution solves part of the problem, but only in a static arrangement.

The flow in the conveyor channel is still a two-phase mixture, with the behavior described above.

A device for even expansion of a liquid / gas two-phase mass flow of refrigerant in a plate evaporator is described in US-A-S 806 586. The evaporator has a distribution line which on the inlet sled can be fed with a refrigerant mass flow coming from an expansion valve. The evaporator further has a plurality of exchanger sections which are substantially perpendicularly branched, spaced apart from the manifold along the latter. In order to achieve an even distribution of the mass flow to the exchanger sections, a porous body is arranged in the manifold line. With this expansion valve the distribution is improved but is still far from being ideal.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate or at least mitigate the above disadvantages and to provide an improved method and system for accurately distributing a refrigerant into each duct of a heat exchanger, especially under partial load conditions.

This object is achieved according to the invention by a method and by means of a heat exchanger system with the features stated in the following independent claims. Preferred embodiments are set forth in the appended subclaims.

By providing a number of limiters before each channel, the expansion of the liquid takes place just before each channel. Thus, the pressure drop in the manifold channel will be relatively small while the pressure drop in the channels is large, i.e. close to the differential pressure between the condensation and evaporation pressure.

According to the invention, each restrictor will be fed with a precise amount of liquid, each channel being fed with a precise amount of liquid and steam.

Optional of the above steps, or a combination of both, results in the refrigerant being prevented from boiling over as it passes the pressure changing means. This results in a single-phase failure in the manifold channel, which results in a small pressure drop and a precise distribution. Note that parameters such as condensation pressure / temperature and evaporation pressure temperature, and the like, are substance-specific and vary between different refrigerants. In one embodiment, the pressure changing means is a pump, wherein the step of changing the pressure is performed by means of said pump. A pump arranged upstream of the evaporator is an easy way to obtain an increased pressure.

The pressure increase before the evaporator makes it possible to vary the pressure upstream of the limiters within a larger range without the risk of the refrigerant boiling. Since the pressure drop across the limiters determines the fl fate through the channels, the larger interval will result in a larger flow interval and thereby a larger capacity interval. In an embodiment where the pressure generated by the pump is difficult to control, a valve is arranged downstream of the pump. The valve makes it possible to lower the pressure before the limiters and the variable pressure drop across the limiters means a changed capacity (changed flow).

According to one embodiment, the pressure generated by the pump is regulated by means of a variable speed motor. This is a straightforward way to achieve a variable pressure before the evaporator. In such an embodiment, the valve is redundant.

The step of changing the temperature is preferably performed by means of a subcooler / subcooler which is arranged upstream of the evaporator. The subcooler can lower the temperature of the coolant in such a way that the coolant remains in the liquid state, even after the pressure drop which takes place in the pressure-changing member. The pressure changing means preferably comprises a valve but could also consist of the previously mentioned pump. 10 15 20 25 30 531 E55 4 According to one embodiment, the subcooler forms a separate section of the evaporator, the refrigerant flowing from the condenser into said separate section of the evaporator, where it cools down, and then to the valve. The subcooling can also be performed with cooling medium which is diverted from the heat exchanger circuit, in a system called an "economizer" within the cooling area. In yet another embodiment, the subcooler comprises an independent cooling circuit in which a sufficiently cold cooling medium flows. An independent circuit is a flexible way to make the subcooler independent of the rest of the heat exchanger system.

A heat exchanger system according to the invention comprises means for performing the above steps.

Other objects, features, advantages, and preferred embodiments of the present invention will become more apparent from the following detailed description taken in conjunction with the drawings and appended claims.

Brief Description of the Drawings Preferred embodiments of the present invention are described below with reference to the accompanying drawings.

Fig. 1 is a schematic view of a basic cooling cycle.

Fig. 2 is a partial illustration of a plate heat exchanger according to the prior art.

Fig. 3 is an illustration of a plate heat exchanger according to Fig. 2, which is provided with a main expansion valve.

Fig. 4 is an illustration of a plate heat exchanger according to Fig. 2, which is provided with individual expansion valves for each channel.

Figs. 5-7 are illustrations of plate heat exchangers according to Fig. 2, which are provided with individual fixed limiters for each channel and a main expansion valve.

Fig. 8 is an illustration of a plate heat exchanger according to a first embodiment of the present invention.

Fig. 9 is an illustration of a plate heat exchanger according to a third embodiment of the present invention. Detailed Description of Preferred Embodiments of the Invention A basic compressor refrigeration cycle according to the prior art is shown in Fig. 1.

The actual use of the bicycle can of course be present in an air conditioning device / system, a heat pump as well as in a real cooling device / system.

In condenser 1, gaseous refrigerant condenses at high pressure. The condensed refrigerant then flows to the expansion valve 2. In the expansion valve 2, the liquid passes through a limited cross section. This causes a large pressure drop, whereby the pressure drops to a level close to the evaporation pressure. When this occurs, a portion of the liquid refrigerant evaporates and the mixture is cooled to a temperature close to the evaporation temperature. At the outlet of the valve 2, a cold two-phase mixture leaves the valve 2. The two-phase mixture is fed into the evaporator 3 in which the cold liquid coolant evaporates and cools the process fluid. There are several generally known process fluids, such as air, water, saline, a process liquid, etc. The cold gaseous refrigerant with low pressure is then fed into the compressor 4. There the refrigerant pressure is increased to a level sufficient for the intended refrigerant to condense in the cone. - densorn 1.

The regulation of the valve 2 together with the evaporator 3 is critical for a good function of the cycle. When the cooling needs change, the valve must change accordingly. If too much coolant leaves the valve, there is a risk that the liquid will not evaporate completely in the ducts, which in some cases can damage the compressor. If too little coolant passes through the valve 2, the required capacity cannot be maintained. In a heat exchanger, see Fig. 2, which consists of a number of parallel channels 6, it can be difficult to obtain a precise distribution of fluid from a distribution channel 5 to the parallel channels 6 and then into a collecting channel 7. The distribution channel 5, or the "header", is a distribution chamber from which the channels 6 are branched. Fig. 2 only shows the fate pattern, the heat exchanger is only indicated as separated surfaces 8. It can consist of any type of channels connected in parallel.

Assume that the pressure drops A-B and C-D are large compared to A-D and B-C.

Since the pressure drop must be equal from inlet to outlet, whether we follow the path A-D or A-B-C-D, it follows that the pressure drop A-D is greater than B-C. Since the pressure drop is the driving force for the flow, it follows that the coolant flow will be different for different channels. 10 15 20 25 30 35 531 E55 6 A correct distribution is even more difficult for a two-phase flow, for example for an evaporator in a cooling device / plant_ Fig. 3 shows the valve evaporator unit. Saturated or almost saturated refrigerant is fed into the valve 2 under high pressure, usually close to the condensation pressure. In the valve 2, it expands to a pressure just above the evaporation pressure, whereby part of the liquid is evaporated. The resulting two-phase fluid has a large volume, which increases the pressure drop in the manifold channel, which is the problem. If the velocity of the refrigerant is low in the manifold channel 5, the liquid part settles in the inlet part of the manifold channel 5 and is preferably fed into the first channels extending from that part of the manifold channel 5. If the refrigerant velocity is very high, the inertia will result in that the liquid refrigerant finds it difficult to change direction and is fed into the ducts. In this case, liquid coolant will collect in the farthest part of the distributor channel 5 and then be fed into the farthest channels 6. Thereby, the flow rate of the coolant in the distributor channel 5 is a parameter which adversely affects the performance of the heat exchanger.

Both of these problems could be solved if the expansion of the liquid was performed just before each channel 6. In Fig. 4 a number of variable limiters 2 'have been placed in front of each channel 6. In the distributor channel 5 there is now only liquid, and thus no problem with phase separation, and the pressure drop AB is low.

The pressure drop C-D is still relatively low while the pressure drop A-A'-D and B-B'-C is high, practically corresponding to the differential pressure between the condensation and evaporation pressure. The refrigerant flow through different channels will be precise because a precise amount of liquid refrigerant passes each variable limiter 2 ”. There is thus no distribution problem as there is only liquid coolant and no gaseous coolant in the distribution channel 5. The solution of using a large number of small, adjustable limiters is expensive and to really integrate the valves in the evaporator 3 is difficult. Furthermore, maintenance and cleaning of such valves or limiters is a difficult task.

A practical solution is to keep the main expansion valve 2 and to introduce fixed limiters 9, 10, 11 instead of the variable limiters 2 ', as shown in Figs. 5-7. The fixed limiters 9-11 can consist of a pipe with fixed limiters, the pipe being inserted into the distribution channel 5, see fi g 5. In the case of plate heat exchangers, the plates can be provided with plate-like limiters 10 at each channel inlet, see fi g 6, or discs with drilled limiters 11 inserted in the port hole of each channel, see Fig. 7. The fixed limiters 9-11, if allowed to take the entire differential pressure, will do a good job of applies to distribute the refrigerant. However, in order to function well at partial load conditions, the valve 2 must be used to achieve the pressure drop required to vary the flow through the channels, and the greater the pressure drop, the more the previously mentioned problems with two-phase flow will recur. Furthermore, the optimal size of the limiters varies with nominal capacity, pressure, type of coolant, etc., which means that each cooling system must have individually tailored limiters. Thus, the use of fixed limiters in the above context is an inflexible solution.

A first embodiment of the present invention is described below with reference to Fig. 8. The invention solves the practical problems associated with a two-phase flow. Fig. 8 shows a partial overview view of a plate heat exchanger system according to the invention in accordance with the first embodiment of the invention and shows the evaporator 3 with its distributor channel 5 and channels 6 connecting the distributor channel 5 with the collecting channel 7. Fixed limiters 12 are distributed along the distributor channel 5 length, which limiters place the inside of the distributor channel 5 in fluid communication with the channels 6. The limiters 12 may be of a type previously described with reference to Figs. 5-7. Upstream of the valve 2, a subcooler 13 is arranged for cooling the coolant.

The subcooler 13 may form part of the evaporator 3 in such a way that the coolant flows from the condenser 1 into a separate section of the evaporator 3, where it cools down, and then to the valve 2. The subcooler 13 may also comprise a separate cooling circuit or a economizer. The subcooler 13 cools the refrigerant with a cooling fluid and in general the cooling fluid can be any sufficiently cold fluid, comprising a flow from an economizer. The problem with varying the capacity of the expansion valve 2 was thus that shock boiling took place when the refrigerant expanded in the valve 2. If the liquid refrigerant boils, the problems with uneven distribution arise which are related to two-phase flow.

This first inventive embodiment solves the distribution problem in the following way: Consider a situation where shock boiling takes place at the expansion valve 2. With the system according to the invention, the coolant can be cooled down by the subcooler 13 before it is fed into the expansion valve 2. When the coolant passes the expansion valve 2 to be subjected to a pressure drop, but since the temperature is lowered, the refrigerant will not boil. Thus, there is a single-phase flow in the distributor channel 5, which eliminates said disadvantages.

The foregoing method of ensuring precise distribution utilizes the fact that when the refrigerant is cooled from the saturated condensation temperature to a temperature slightly higher than the evaporation temperature, the pressure can be reduced in the manifold channel 5 from which the restrictors 12 leads into the channels 6, without shock boiling. Note that relevant pressures and temperatures vary between different types of refrigerants and that the concept according to the invention is not limited to any particular refrigerant. Once a specific refrigerant has been selected, the temperatures and pressures follow. The variable differential pressure across the limiters 12 is the parameter that regulates the refrigerant flow and thereby the capacity.

In this first embodiment, the maximum capacity of the evaporator is present when the limiters 12 take substantially all of the differential pressure. The total differential pressure corresponds to the difference between the condensation and the evaporation pressure. The differential pressure across the limiters 12 is the driving force of the flow. To lower the capacity, the pressure drop across the valve 2 is increased, which reduces the pressure drop across the limiters 12 and thereby the flow through the channels 6.

According to a second embodiment, see Fig. 9, a pump 14 is arranged upstream of the expansion valve 2. The pump makes it possible, for example, to increase the pressure before the valve 2. In the second embodiment the temperature in the flow between the condenser 1 and the evaporator 3 is constant while the pressure is increased by means of the pump 14. The pump 14 increases the pressure (a ') after the condenser (not shown) to a higher pressure (a). The variable valve reduces this pressure (a) to a lower pressure (b). Although the pressure drops in the valve 2, the pressure of the liquid refrigerant is constantly above the saturation pressure and no shock boiling takes place before it has been fed into the restrictors 12. The regulator variable for the refrigerant flow is the variable pressure differential across the restrictors 12. In the second embodiment the restrictors 12 are designed for to provide the required minimum capacity at the entire differential pressure drop, which means that the valve 2 is completely open and the pump 14 is not used. When the pump 14 starts to increase the pressure before the limiters 12, possibly modified by the valve 2, the driving differential pressure increases and thus also the capacity.

Valve 2 is only necessary in case the pump delivers a pressure a which is constant or difficult to regulate. If the pressure provided by the pump 14 can be easily regulated, for example by a motor with variable speed (not shown), the valve 2 can be eliminated. The pump 14 may be any suitable pressure boosting device, such as a mechanical or thermal pump.

As has been pointed out above, the concept according to the invention is universal in the sense that it can be used for many types of refrigerant. The following tables, however, show a number of exemplary figures for 531 E55 9 several different cases. In the examples shown in the following tables, line pressure losses and other, irrelevant pressure losses have been excluded.

Table 1. Basic cooling system (cooling medium R507A).

Component Temperaturel ° C Printable Composition.

Condenser, in 75 18.7 Saturated steam Condensation 40 18.7 Saturated Condenser, out 37 18.7 Subcooled liquid Expansion valve, in 37 18.7 Subcooled liquid Expansion valve, out -40 1,39-t Two-phase mixture Evaporator, in -40 1.39+ Two-phase mixture Evaporation -40 1.39 Saturated Evaporator, out -35 1.39 Overheated steam Compressor, in -35 1.39 Overheated steam Compressor, out -35 1.39 Steam + means that the pressure is slightly higher before the evaporator than the evaporation pressure, which is defined at the evaporator outlet.

An example of a system according to the first embodiment is shown in Table 2, below.

Table 2. First embodiment.

Component Temperaturel ° C Pressure / bar Composition.

Condenser, in 75 18.7 Saturated steam Condensation 40 18.7 Saturated Condenser, out 37 18.7 Subcooled liquid Subcooler, in 37 18.7 Subcooled liquid Subcooler, out 7 18.7 Subcooled liquid, real Subcooler 7 7.74 Saturated condition Expansion valve, in 7 18.7 Subcooled liquid 10 10 15 20 533 E55 l O Cont. table 2 Expansion valve, out 7 7.74-17 Subcooled liquid Limiter, in 7 7.74-17 Subcooled liquid Limiter, out -40 1.39+ Two-phase mixture variable pressure drop 6.354 5.61 Evaporator, in -40 1 , 39 + Two-phase mixture Evaporation -40 1.39 Saturated Evaporator, out -35 1.39 Superheated steam Compressor, in -35 1.39 Superheated steam Compressor, out -35 1.39 Steam + means that the pressure is slightly higher before the evaporator than the evaporation pressure, which is defined at the evaporator outlet.

Note that the term “expansion valve” is no longer completely correct as no expansion takes place. Furthermore, in Table 2, the pressure drop over the expansion valve is minimal. The pressure before the fixed limits can, without shock boiling before the fixed limits, vary between 17.0 bar and 7.74 bar (saturation pressure at 7 ° C), whereby the capacity can be varied accordingly.

Note further that the subcooling can be performed with any of the devices described above. In the above example, the overheating can be used to regulate the expansion valve. In short, if the load increases, so does the overheating. This increases the pressure drop across the expansion valve and thus reduces the pressure before the fixed limiters (still within the range above), which lowers the evaporator's capacity to meet the reduced load. The use of overheating as a control parameter can also be applied to the examples below. This parameter is then used to regulate the expansion valve or, where applicable, the pump.

Table 3 exemplifies the second embodiment, in which a pump is arranged downstream of the condenser. As mentioned earlier, the expansion valve can be eliminated and the pump can provide a variable pressure. Furthermore, a subcooler can be added. 10 15 531 EEE 1 1 Table 3.

Component Temperature PC Printable Assembly.

Condenser, in 75 18.7 Saturated steam Condensation 40 18.7 Saturated Condenser, out 37 18.7 Subcooled liquid Pump, in 37 18.7 Subcooled liquid Pump. out 37 30 Subcooled liquid Expansion valve, in 37 30 Subcooled liquid Expansion valve, out 37 18.7-30 Subcooled liquid Limiter, in 37 18.7-30 Subcooled liquid Limiter, out -40 1.39 + Two-phase mixture variable pressure drop 17.31-28.61 Evaporator, in -40 1.39 + Two-phase mixture Evaporation -40 1.39 Matt Evaporator, out -35 1.39 superheated steam Compressor, in -35 1.39 Superheated steam Compressor, ut -35 1.39 Steam + means that the pressure is slightly higher before the evaporator than the evaporation pressure, which is defined at the evaporator outlet.

The use of a subcooler 13 offers certain advantages in combination with a pump. It can increase the thermodynamic efficiency of the system and furthermore it can improve the operation of the pump 14, since pumps in some cases have difficulty pumping saturated liquids. Therefore, in a third embodiment, a subcooler 13 according to the first embodiment is arranged upstream of a pump 14 according to the second embodiment. This provides the said benefits.

The limiter can be either a limiter or a number of limiters, each supplying one or more channels. In the latter configuration, the distribution will be improved, as described in the foregoing.

The invention thus relates to a method and a device for improved distribution of an expanding liquid in a heat exchanger system by changing one or more intensive variables of a cooling medium flowing in a heat exchanger system. It should be emphasized that the embodiments described above that are used as stand-alone solutions can be combined. 10 15 5534 B55 12 The invention makes it possible to regulate the coolant flow (capacity) by changing the division of pressure drop between a valve and a limiter instead of regulating the flow through a variable cross section of a valve.

Also note that the concept according to the invention is based on fl your applications where a number of parallel channels are to be fed with a two-phase flow, which applications include air coolers, etc.

Furthermore, it will be appreciated that a small amount of steam, less than that used in prior art systems, after the pressure changing member does not destroy the inventive concept and is included within the scope of the appended claims.

Claims (21)

10 15 20 25 30 35 531 555 13 PATENT REQUIREMENTS A method for distributing an expanding liquid in a heat exchanger system with a circuit comprising the components: a condenser (1), pressure changing means (2; 14), an evaporator (3) comprising a distributor channel (5) which via limiter (9-12) is in fluid contact with a plurality of parallel-connected fluid channels (6), and a compressor (4), which components each have an inlet and an outlet, the outlet of the condenser (1) being connected to the inlet. of said pressure changing means, said outlet of said pressure changing means 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), and the circuit contains refrigerant fluid and the method comprises the steps of: (a) lowering the temperature of the refrigerant downstream of the condenser (1) and upstream of said pressure changing means; and / or (b) changing the pressure of the refrigerant by means (14) capable of increasing the pressure of the refrigerant downstream of the condenser (1), thereby preventing the refrigerant from boiling until it passes the limiters (9-12), resulting in in a single-phase, liquid refrigerant flow in the manifold channel (5). A method according to claim 1, wherein the pressure changing means is a pump (14) and the step of changing the pressure is performed by means of said pump (14). A method according to claim 2, wherein the pump (14) is arranged before a further pressure changing means, and said pressure changing means is constituted by a valve (2). A method according to claim 2 or 3, wherein the pressure generated by the pump (14) is regulated by means of a variable speed motor. A method according to any preceding claim, wherein the step of changing the temperature of the coolant is performed by means of a subcooler (13). A method according to claim 5, wherein the pressure changing means comprises a valve (2). A method according to claim 5 or 6, wherein the subcooler (13) performs cooling by diverting a part of the coolant fluid from the heat exchanger circuit. A method according to claim 7, wherein the subcooler (13) is a separate section of the evaporator (3). 10 15 20 25 30 35 53? E55 14 A method according to claim 5 or 6, wherein the subcooler (13) comprises an independent circuit, separate from the heat exchanger circuit. A method according to any preceding claim, wherein said plurality of fluid channels (6) in the heat exchanger system form part of a plate heat exchanger. A heat exchanger system having a circuit comprising the components: a condenser (1), pressure changing means (2; 14), an evaporator (3) and a compressor (4), each component having an inlet and an outlet, the outlet of the condenser (1) being connected to the inlet of said pressure changing means (2; 14), the outlet of said pressure changing means being connected to the inlet of the evaporator (3), the outlet of the evaporator (3) being connected to the inlet of the compressor (4) and the outlet of the compressor (4) is connected to the inlet of the condenser (1), and the circuit contains refrigerant fluid and the system comprises: means for lowering the temperature of the refrigerant downstream of the condenser (1) and upstream of said pressure changing means (2; 14); and / or means for changing the pressure of the coolant comprising means (14) capable of increasing the pressure of the coolant downstream of the condenser (1), characterized in that the evaporator (3) comprises a distribution channel (5) which is via limiter (9-12) in fluid contact with a plurality of parallel connected fluid channels (6); and that said means for lowering the temperature of the refrigerant and / or said means for changing the pressure of the refrigerant prevent the refrigerant from boiling until it passes the limiters (9-12), resulting in a single-phase, liquid refrigerant flow in the distributor duct (5). The system of claim 11, wherein the pressure changing means is a pump (14) capable of increasing the pressure. A system according to claim 11, wherein the pump (14) is arranged before an additional pressure changing means and said pressure changing means is constituted by a valve (2) A system according to claim 12 or 13, wherein the pressure generated by the pump (14) is regulated by means of a variable speed motor. A system according to any one of claims 11-14, wherein the step of changing the temperature of the refrigerant is performed by means of a subcooler (13). A system according to claim 15, wherein the pressure changing means comprises a valve (2). A system according to claim 15 or 16, wherein the subcooler (13) performs cooling by diverting a part of the coolant iden outside the heat exchanger circuit. 10 531 E55 15 The system of claim 17, wherein the subcooler (13) is a separate section of the evaporator. A system according to claim 15 or 16, wherein the subcooler (13) comprises an independent circuit, separate from the heat exchanger circuit. A system according to any one of claims 11-19, wherein said several fluid channels (6) in the heat exchanger system constitute one of a plate heat exchanger. A system according to any one of claims 11-19, wherein said plurality of fluid channels (6) in the heat exchanger system form part of an air cooler.