WO2009062739A1

WO2009062739A1 - Suction gas heat exchanger

Info

Publication number: WO2009062739A1
Application number: PCT/EP2008/009661
Authority: WO
Inventors: Tomas Dahlberg; Svante Hoberg; Dirk Sterner; Jessica LINDSTRÖM; Fredrik STRÖMER
Original assignee: Swep International Ab
Priority date: 2007-11-14
Filing date: 2008-11-14
Publication date: 2009-05-22
Also published as: EP2223023A1

Abstract

An evaporator unit for exchanging heat between a refrigerant (C) and a low temperature heat source (B), comprises: a heat exchanger pack (610; 710) including a number of identical heat exchanger plates arranged such that spaces are formed between pairs of said plates, wherein neighboring spaces are separated such that a heat exchange can take place between the refrigerant (C) and the low temperature heat source (B) flowing in such neighboring spaces, and a suction gas heat exchanger (SGHX) exchanging heat between refrigerant (C) about to enter a throttling (700; 660, 663, 675) and refrigerant (C) exiting the heat exchanger pack (610; 710), wherein the suction gas heat exchanger (SGHX) comprises plates (620, 630) provided with ridges and grooves or nipples and dimples, wherein the heat exchanger pack (610, 710) and the suction gas heat exchanger (SGHX) are combined into a single unit.

Description

SUCTION GAS HEAT EXCHANGER

FIELD OF THE INVENTION The present invention relates to an evaporator unit for exchanging heat between a refrigerant and a low temperature heat source. It comprises a heat exchanger pack comprising a number of identical heat exchanger plates arranged such that spaces are formed between pairs of said plates, wherein neighboring spaces are separated such that a heat exchange can take place between the refrigerant and the low temperature heat source flowing in such neighboring spaces, and a suction gas heat exchanger exchanging heat between refrigerant about to enter a throttling valve and refrigerant exiting the heat exchanger pack.

PRIOR ART In the art of heat pumps and cooling equipment, the standard procedure includes compressing and expanding a refrigerant, in order to increase and decrease its boiling temperature. There are also systems working with so-called transcritical refrigerants. CO₂ used as refrigerant in heat pump applications is one example of a transcritical refrigerant, the use and function of which being well known by persons skilled in the art.

The compression is performed by a mechanical compressor, which can be powered by e.g. electricity, and the expansion is performed by an expansion valve. In automobiles, it is common to power the compressor of an air conditioning system by mechanically connecting a compressor to the engine of the automobile. Basically, a heat pump system comprises a compressor, a condenser, an expansion valve and an evaporator. The components are connected to one another in this order, and the refrigerant will flow in a closed system passing the components.

One problem with this kind of system is that the heat transfer in the evaporator must be sufficiently large to ensure that all refrigerant will leave the evaporator in gas phase. This can be achieved by using a large pressure drop over the expansion valve (a large pressure drop will decrease the boiling point of the refrigerant), but this is highly uneconomical from an energy viewpoint.

A well known solution to the problem of ensuring that all refrigerant is in gas phase after the evaporator is to arrange a heat exchange between the suction gas (i.e. the refrigerant about to be compressed in the compressor) and the refrigerant about to expand in the expansion valve. This heat exchange will increase the temperature of the refrigerant about to enter the compressor, and will hence ensure that no liquid refrigerant will enter the compressor.

In most prior art solutions, this so-called suction gas heat exchange is arranged by welding or brazing piping leading to and from the evaporator to one another. This is however not a perfect way of solving the suction gas heat exchange, since it is complicated and uneconomical to provide such piping. Also, the system will take up much space.

A typical prior art evaporator for heat exchange between air and a refrigerant can e.g. be found in EP 0 710 808.

SUMMARY OF THE INVENTION

The present invention aims to solve or alleviate this and other problems by providing an evaporator unit wherein the suction gas heat exchanger comprises plates provided with ridges and grooves or nipples and dimples, wherein the heat exchanger pack and the suction gas heat exchanger are combined into a single unit, wherein the throttling is arranged between said suction gas heat exchanger and said heat exchanger pack and wherein the plates comprised in the suction gas heat exchanger and the heat exchanger plates are brazed together to form the single unit.

In one embodiment of the invention, a throttling between the suction gas heat exchanger and the heat exchanger pack might be a fixed throttling and in another embodiment of the invention, the throttling might be a controllable throttling.

The evaporator unit is especially suited for refrigerants having a glide of more than 4K. Examples of such refrigerants are 407C and 407E.

In one embodiment of the invention, the throttling could be arranged as a distribution pipe provided with a number of openings distributing the flow of refrigerant into the spaces formed by the heat exchanger plates of the heat exchanger pack.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be explained by means of description of some embodiments, with reference to the appended drawings, wherein:

Fig. 1 is a schematic view showing a prior art system provided with a suction gas heat exchanger,

Fig. 2 is a schematic view showing a system used for utilizing a combined unit according to a first embodiment of the present invention, Fig. 3 is an exploded view of the combined unit according to the first embodiment of the present invention,

Fig. 4 is a schematic view of a system used for utilizing a combined unit according to a second embodiment of the present invention, and Fig. 5 is an exploded view of a combined unit according to the second embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

In Fig. 1, a prior art system for heating and/or cooling purposes, comprising a suction gas heat exchanger is shown. The prior art system comprises a compressor 100 having an inlet 105 and an outlet 110, a condenser 200 having an inlet 205 and an outlet 210, an expansion valve 300 having an inlet 305 and an outlet 310, an evaporator 400 having an inlet 405 and an outlet 410, and a suction gas heat exchanger SGHX. Pipes 110-205, 210-305, 310-405 and 410-105 connect the compressor 100, the condenser 200, the expansion valve 300 and the evaporator 400. In many prior art applications, the suction gas heat exchanger is only a device in which the pipes 410-105 and 310-405 are brazed or soldered to one another for enabling heat transfer between such pipes; in some applications, the pipes are simply placed close enough to one another to provide a heat exchange between such pipes. It can be noted that for systems using transcritical refrigerants, it is not entirely correct to refer to the condenser 200 as a condenser, it is rather a gas cooler.

The prior art process functions as follows (please note that the temperatures, pressures and phases given are exemplary only; they can vary within wide ranges): As well known by persons skilled in the art of heating and cooling, the previously known components and the piping connecting them are filled with a refrigerant, e.g. refrigerant R407C. In most cases, the pressure of the refrigerant in a shut-off system is about 6 bars. When the system is running, i.e. when the compressor 100 is energized, the compressor will suck in refrigerant from the pipe 410-105, compress the refrigerant to about 15 bars (which increases the temperature to about 70 degrees centigrade) and expel such refrigerant into the pipe 110-205. The refrigerant will be in gas-phase after the compressor 100. The compressed, hot gas will enter the condenser 200, in which heat will be extracted from the hot, compressed refrigerant, and be transferred to e.g. a building requiring heating or a tap water generator, or, if the system is used for cooling purposes, outdoor or ambient air. After rapid, initial cooling to about 35 degrees, the refrigerant will start to condense; when the refrigerant leaves the condenser, it should completely condensed (i.e. in liquid phase) and have a temperature of about 30 degrees.

After having condensed in the condenser 200, the refrigerant is conveyed to the expansion valve 300 via the suction gas heat exchanger SGHX. In the SGHX, the temperature of the refrigerant will drop about 5 degrees, meaning that the refrigerant will enter the expansion valve 300 at a temperature of about 25 degrees. In the expansion valve, the pressure of the refrigerant will drop to about 5 bars. The pressure reduction means that the refrigerant will start boiling at a significantly lower pressure than at high pressure; immediately after the expansion valve, the refrigerant will have a temperature of about -7 degrees centigrade; 80 percent (by weight) of the refrigerant will be in liquid phase, whereas the remainder will be in gas phase.

After the expansion valve, the refrigerant is directed to the evaporator 400 via its inlet 405. In the evaporator, the refrigerant will pick up heat from any low temperature heat source, e.g. the interior of a cooling space, from a brine solution (in heat pump applications) or outdoor air (if the outdoor air is used as heat source in a heat pump system). The refrigerant will leave the evaporator as predominantly gas, although some of the refrigerant might still be in liquid phase; refrigerant in gas phase is most common if there is a "glide" in the refrigerants boiling temperature, as is the case for e.g. R407C. The temperature of the refrigerant exiting the evaporator 400 is about -2 degrees centigrade.

Of course, it is possible to get rid of the possible liquid phase refrigerant by exaggerating the size of the evaporator, but since the temperature of the flow (brine or outdoor air) exchanging heat with the refrigerant often is very close to the refrigerant's boiling temperature, the evaporator is already rather large, and it is uneconomical to increase its size. It is also uneconomical to increase the pressure drop over the expansion valve in order to decrease the refrigerant's boiling temperature, since this means that mechanical work is wasted in the compressor.

In order to get rid of the possible remaining liquid in the refrigerant flow, it is rather common to use a suction gas heat exchanger, such as the suction gas heat exchanger SGHX, which exchanges heat between the refrigerant flow in the pipe 210- 305 and the pipe 410-105; the temperature difference between those refrigerant flows is about 30 degrees centigrade, which means that a relatively efficient heat transfer between the two refrigerant flows can be achieved even with a relatively small heat exchanger. The temperature of the refrigerant in the pipe 410-105 will increase about twice as much as the temperature in the refrigerant in the pipe 210-305 will decrease; as mentioned above, the temperature of the refrigerant in the pipe 210-305 will decrease about 5 degrees centigrade, which means that the temperature of the refrigerant in the pipe 410-105 will increase about 10 degrees centigrade, and hence reach a temperature of about 8 degrees centigrade; such a high temperature will effectively diminish all possible liquid refrigerant in the pipe 410-105; as well known by persons skilled in the art, most compressors work very poorly with a gas/liquid mixture. If the liquid content is too high, there is even a risk that the compressor might be ruined.

In fig. 2, a first embodiment of a system comprising a first embodiment of a combined unit 500 comprising a suction gas heat exchanger 510 and an evaporator 520 according to the invention is shown. Between the suction gas heat exchanger 510 and the evaporator 520, a controllable expansion valve 530 is shown.

The combined unit 500 according to the first embodiment of the invention is used in a system thermodynamically virtually identical to the prior art system; however, instead of using a separate evaporator 400, a separate expansion valve 300 and a separate suction gas heat exchanger SGHX, as in the prior art system, all these components are combined into the combined unit 500. The combined unit makes a large amount of piping unnecessary, and can hence significantly reduce manufacturing complexity.

As can be seen in fig. 2, and as mentioned above, the basic system design for the system in which the combined suction gas heat exchanger and evaporator 500 is used, is thermodynamically identical to the prior art system, except for the fact that the suction gas heat exchanger 510 of the combined unit 500 is not a counter flow heat exchanger, which is desired in most circumstances. There is, however, one benefit (except from the above-mentioned reduction of complexity) that is present if the expansion valve is placed close to the evaporator 520, namely that the two-phase flow (as mentioned above, about 20% of the refrigerant is in gas phase just after the expansion valve) will be fairly homogeneous as it enters the evaporator. In case the expansion valve is placed far from the evaporator, there is a risk that the liquid phase of the refrigerant flow will agglomerate into large droplets, which might be difficult to evaporate in the evaporator.

In fig. 3, the first embodiment of the combined unit 500 is shown in an exploded perspective view. The combined unit 500 according to the first embodiment comprises a standard heat exchanger pack 610, comprising a number of heat exchanger plates arranged in a fashion typical for heat exchanger, i.e. every other plate being turned 180 degrees with respect to its neighboring plates, a dividing plate 620, and a cover plate 630. The cover plate 630 comprises a brine inlet 635, a refrigerant inlet 640, a brine outlet 650 and a refrigerant outlet 645. These inlets and outlets are arranged such that: a. The brine inlet 635 opens in a space delimited by the heat exchanger plates of the heat exchanger pack 610, b. The refrigerant inlet 640 opens in the space delimited by the dividing plate 620 and the top plate of said heat exchanger pack, c. The refrigerant outlet 645 connects the space delimited by the dividing plate 620 and the cover plate 630 and d. The brine outlet 650 connects to the space delimited by the heat exchanger plates of the heat exchanger pack 610.

The plates used to establish these flowpaths for brine and refrigerant are of a standard type exhibiting ridges and grooves, which, when the unit is assembled, will form said flowpaths. The assembly process may e.g. be a brazing operation, wherein a layer of brazing material, e.g. a copper foil, is applied on the plates prior to a stacking of the plates on one another, after which the stack of plates is placed in a furnace and heated to a temperature exceeding the melting point of the copper foil to achieve a brazing connection between the ridges and grooves of the plates. The ridges and grooves of the heat exchanger pack and the end plate can be arranged in a herringbone pattern; the plates of the suction gas heat exchanger may also be provided in an X- pattern, such as has been described in Swedish patent application 0501908-8.

In order to establish correct pathways for the flows of refrigerant and brine in the combined unit, the dividing plate is provided with four openings 620 a-d; the pathways will be described more thoroughly below. Moreover, an expansion valve 660 is provided on the cover plate 630. The expansion valve comprises a needle 663, which extends through an opening 665 in the cover plate and the opening 620c of the dividing plate, to an opening 675 arranged in the top plate of the heat exchanger pack 610. The cooperation between the needle 663, the expansion valve 660 and the opening 675 allows a flow area through the opening 675 to be controlled responsive to an electronic control signal, wherein the control signal will control the expansion valve to retract or advance the needle 663 from or towards the opening 675. A retracted needle will provide a large opening, and hence a small pressure drop over the opening 675, whereas an advanced position of the needle 663 will give a small opening, and hence a large pressure drop over the opening 675. During operation, there is a constant flow B of brine (most commonly a mixture between an alcohol, e.g. ethanol or glycol and water, arranged to extract heat from any low-temperature heat source, such as the ground or the ground-water) flowing in through the brine inlet 635, through the opening 620b and into the heat exchanger pack 610. In the heat exchanger pack 610, the brine will flow through spaces arranged between every other plate pair, and exit the unit 600 through the opening 62Od and the brine outlet 650.

Moreover, refrigerant C will enter the refrigerant inlet 640; at the entrance, i.e. at the inlet 640, the refrigerant will be in liquid phase, have a pressure of about 15 bar, and a temperature of about 30 degrees centigrade. The liquid refrigerant will pass through an opening (not shown) arranged in the dividing plate and enter the space delimited by the cover plate and the dividing plate. It will flow in this space, having a pressure of about 15 bar, until it reaches the opening 675; depending on the temperature of the brine, the position of the needle 663 will be set such that a corresponding pressure drop of the refrigerant is achieved, i.e. a pressure drop giving a refrigerant boiling temperature allowing the refrigerant to be vaporized by the heat transferred from the brine flow B to the refrigerant C.

After passing the opening 675, the refrigerant flow into the heat exchanger package 610 at a pressure of about 5 bars; the refrigerant will flow through the heat exchanger package in the spaces delimited by neighboring heat exchanger plates, and hence be able to exchange heat with the brine flowing in the other spaces delimited by the neighboring plates. Immediately after having passed the opening 675, the temperature of the refrigerant will be about -7 degrees centigrade, and about 80% of the refrigerant will be in liquid phase. After having exchanged heat with the brine, a majority, or all, of the refrigerant should be in gas phase; and its temperature should be about -2 degrees centigrade. The refrigerant having exchanged heat with the brine flows out from the heat exchanger package and enters the space between the cover plate 630 and the dividing plate 620 through the opening 620a. The space between the cover plate and the dividing plate is in thermal contact with the space delimited by the dividing plate and the top heat exchanger plate; in this space, liquid refrigerant having a temperature of about 30 degrees is flowing, and the refrigerant having exchanged heat with the brine will exchange heat with this refrigerant, that just entered the refrigerant inlet 640. As a result of this, the temperature of the refrigerant having exchanged heat with the brine will increase. The temperature increase will secure that no liquid phase whatsoever will be present in the refrigerant exiting the space between the cover plate and the dividing plate through the refrigerant outlet 645.

In order to minimize heat transfer between the brine flowing through the space delimited by the top heat exchanger plate and its neighboring plate and the refrigerant flowing between the dividing plate and the top heat exchanger, it might be preferable to provide one or more "blind plates" in order to thermally insulate the brine from the refrigerant. The use of thermally insulating blind plate is well known by persons skilled in the art, and will hence not be more thoroughly described.

In another embodiment of the invention, shown schematically in Fig. 4, the controllable expansion valve 660 of the previous embodiment is replaced by a fixed throttle 700 giving some expansion of the flow of refrigerant. In order to control the process for operating at various temperature levels, a controllable expansion valve 710 is arranged upstream the suction gas heat exchanger of the second embodiment.

In the prior art systems, all expansion of the refrigerant takes place in the expansion valve. In the system according to the second embodiment, the expansion takes place in two different positions, namely in the fixed throttle 700 and in the controllable expansion valve 710.

Hereinafter, the system in which a combined unit 800 according to the second embodiment can be used will be thoroughly described. A system in which a combined unit 800 according to the second embodiment can be used comprises a compressor 720, a condenser 730, the controllable expansion valve 710, the suction gas heat exchanger SGHX, the fixed throttle 700, and a heat exchanger 740. The components are connected by piping resembling what has been describe above in connection with the description of the prior art system and the system of the first embodiment of the invention.

In the system of the second embodiment, the compressor 720 will compress the refrigerant to about 15 bars, which means the temperature will reach about 70 degrees centigrade (just as in the earlier described systems). Just like in the earlier described systems, the compressed refrigerant will enter the condenser 730, in which the refrigerant will condense at a temperature of about 30 degrees (still having a pressure of about 15 bar). Thereafter, the refrigerant will enter the controllable expansion valve, which will decrease the pressure of the refrigerant to about 8 bars. At this pressure, the temperature of the refrigerant will be around 13 degrees centigrade, and about 15% of the refrigerant will be in gas phase. The refrigerant having a pressure of about 8 bars will enter the suction gas heat exchanger and continue to the fixed throttle 700, over which its pressure will decrease to about 5 bars. The resulting gas temperature after the expansion to 5 bars will be about -7 degrees, which means that the state of the refrigerant will be about the same as in the previously described systems. Just like in the previously described system, the refrigerant will vaporize in the evaporator and exit the evaporator at a temperature of about -2 degrees. After the evaporator, the refrigerant exiting the evaporator will exchange heat with the refrigerant having entered the suction gas heat exchanger from the controllable expansion valve 710.

In Fig. 5, the combined unit 800 according to the second embodiment is shown in an exploded view. In the second embodiment, the openings 620 and 665 and the expansion valve 660 of the first embodiment is omitted; hence, the refrigerant outlet 645 in the second embodiment can be moved to the position of the expansion valve in the first embodiment, which increases the heat exchange of the suction gas heat exchanger.

The seat 675 of the first embodiment is replaced by the fixed throttling 700 in the second embodiment. In one embodiment, this throttling is an opening in form of a hole having a diameter giving the desired pressure reduction of e.g. 2 bars at a certain refrigerant flow.

In another preferred embodiment, the fixed throttling might be a distribution pipe 780 extending from the position of the fixed throttling 700. Such a distribution pipe is provided with a number of small holes 790, wherein each hole is located and directed such that the refrigerant exiting such hole is directed towards a specific space between two plates in the heat exchanger package. One benefit of using a distribution pipe is that the distribution of refrigerant between different spaces can be controlled. Usually, it is desired that the distribution is as even as possible such that the flow of refrigerant is equal over all spaces defined by the heat exchanger plates. Please note that it is possible to use the distribution pipe 780 both as a single unit performing both the expansion, in combination with the fixed throttling 700 and in combination with the controllable expansion valve 660.

Above, some preferred embodiments of the present invention have been presented. It is however possible to vary these embodiments without falling outside the scope of the invention such as it is defined by the appended claims.

Claims

1. Evaporator unit for exchanging heat between a refrigerant (C) and a low temperature heat source (B), comprising: a heat exchanger pack (610; 710) including a number of identical heat exchanger plates arranged such that spaces are formed between pairs of said plates, wherein neighboring spaces are separated such that a heat exchange can take place between the refrigerant (C) and the low temperature heat source (B) flowing in such neighboring spaces, and a suction gas heat exchanger (SGHX) exchanging heat between refrigerant (C) about to enter a throttling (700; 660, 663, 675) and refrigerant (C) exiting the heat exchanger pack (610; 710), characterized in that the suction gas heat exchanger (SGHX) comprises plates (620, 630) provided with ridges and grooves or nipples and dimples, in that the heat exchanger pack (610, 710) and the suction gas heat exchanger (SGHX) are combined into a single unit, wherein the throttling (700; 660, 663, 675) is arranged between said suction gas heat exchanger (SGHX) and said heat exchanger pack (610; 710) and in that the plates (620, 630) comprised in the suction gas heat exchanger (SGHX) and the heat exchanger plates are brazed together to form the single unit.

2. The evaporator unit (600; 800) according to claim 1, wherein the throttling (700; 660, 663, 675) is a fixed throttling (700).

3. The evaporator unit (600, 800) according to claim 1, wherein the throttling (700; 660, 663, 675) is a controllable throttling (660, 663, 675).

4. The evaporator unit (600; 800) according to any of the preceding claims, wherein the refrigerant (C) has a glide of more than 4K.

5. The evaporator unit (600; 800) according to claim 4, wherein the refrigerant

(C) is 407C or 407E.

6. The evaporator unit (600; 800) according to claim 2, wherein the throttling (700; 660, 663, 675) is arranged as a distribution pipe (780) provided with a number of openings (790) distributing the flow of refrigerant into the spaces formed by the heat exchanger plates of the heat exchanger pack.

7. The evaporator unit (600, 800) according to any of the preceding claims, wherein