US20090229282A1

US20090229282A1 - Parallel-flow evaporators with liquid trap for providing better flow distribution

Info

Publication number: US20090229282A1
Application number: US11/909,086
Authority: US
Inventors: Michael F. Taras; Alexander Lifson
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2005-05-24
Filing date: 2005-05-24
Publication date: 2009-09-17
Also published as: MX2007012510A; HK1120600A1; EP1883771A4; WO2006127001A3; AU2005332040A1; CA2604466A1; EP1883771A2; CN100554833C; JP2008542677A; AU2005332040B2; WO2006127001A2; CN101180506A; BRPI0520260A2

Abstract

A parallel-flow evaporator has a liquid trap for regulating velocity of refrigerant delivered to an evaporator from an expansion device. In its simplest configuration, the liquid trap is a u-shaped pipe positioned vertically and connected to an inlet manifold of the evaporator. By providing a liquid trap, a small amount of liquid refrigerant separates from the vapor phase at certain conditions. This separated liquid will tend to collect in the trap, and reduce a flow cross-sectional area of the line leading to the inlet manifold of the evaporator. As this cross-sectional area decreases, the velocity of the refrigerant passing through the line will increase. In this sense, as a small amount of liquid phase separates out, it will ensure that the velocity of the remaining refrigerant will increase such that further separation will be significantly reduced or entirely avoided. As a result, homogeneous refrigerant flow is provided to the evaporator, resulting in its performance enhancement and system reliability improvement.

Description

BACKGROUND OF THE INVENTION

This invention relates to a parallel-flow evaporator wherein a liquid trap is positioned upstream of an inlet manifold to provide better flow distribution among parallel channels, improved heat transfer and enhanced system reliability.
Refrigerant systems are utilized to control the temperature and humidity of air in various indoor environments to be conditioned. In a typical refrigerant system operating in the cooling mode, a refrigerant is compressed in a compressor and delivered to a condenser (or an outdoor heat exchanger in this case). In the condenser, heat is exchanged between outside ambient air and the refrigerant. From the condenser, the refrigerant passes to an expansion device, at which the refrigerant is expanded to a lower pressure and temperature, and then to an evaporator (or an indoor heat exchanger if the system operates in the cooling mode). In the evaporator, heat is exchanged between the refrigerant and the indoor air, to condition the indoor air. When the refrigerant system is operating in the cooling mode, the evaporator cools and typically dehumidifies the air that is being supplied to the indoor environment.
One type of evaporator that could be utilized in refrigerant systems is a parallel-flow evaporator. Such evaporators have several parallel channels for communicating refrigerant between an inlet manifold and an outlet manifold. Each channel typically has numerous parallel internal paths of various cross-sectional shape separated by internal walls. Corrugated fins are disposed in between the channels for heat transfer enhancement and structural rigidity. Usually, the channels, manifolds and fins are constructed from similar materials such as aluminum and are attached to each other by furnace brazing. Recently, parallel-flow evaporators have attracted a lot of attention and interest in the air-conditioning field due to their superior performance, compactness, rigid construction, and enhanced resistance to corrosion. However, one concern with parallel-flow evaporators is maldistribution of the refrigerant among their channels. The maldistribution problem in the parallel-flow evaporators is typically caused by the liquid phase separating from the vapor in the inlet manifold due to gravity combined with insufficient refrigerant velocity, and thus manifests itself in unequal amounts of vapor and liquid refrigerant passing through the evaporator channels. Additional phenomena effecting maldistribution can be attributed to different distances the refrigerant must flow to reach various channels and to exit them, unequal pressure impedances and variations in the heat transfer rates between the channels, etc.
Known parallel-flow evaporators typically have inlet and outlet manifolds that are cylindrical in shape. The channels are typically made of identical aluminum extrusions that form flat tubes. As the two-phase refrigerant enters the inlet manifold, the vapor phase is often separated from the liquid phase. Since the two phases will move independently from each other after separation, the problem of refrigerant maldistribution often arises.
When such maldistribution occurs, the heat exchanger performance drops significantly, frequently resulting in liquid refrigerant leaving the outlet manifold. This liquid refrigerant can cause serious reliability problems and permanent compressor damage. Obviously, this is undesirable.

SUMMARY OF THE INVENTION

In a disclosed embodiment of this invention, a parallel-flow evaporator is provided with a liquid trap upstream of its inlet manifold. In this manner, should the refrigerant be moving at a speed such that the liquid phase will not separate from the vapor phase, it can flow through the trap, into the manifold, and into the evaporator channels in a generally equal distribution. However, should the refrigerant be moving at reduced speed, such that separation of liquid is likely to occur, then the liquid will tend to separate and accumulate in the liquid trap. As the liquid accumulates in the liquid trap, the flow cross-sectional area for the remainder of the refrigerant will become smaller. Since the flow cross-sectional area becomes smaller, then the refrigerant velocity will increase, creating a jetting effect that will carry droplets of liquid into the inlet manifold and will limit further phase separation. This phenomenon will be self-regulating, to ensure that an adequate refrigerant velocity will be maintained such that the refrigerant liquid will tend not to separate from the vapor.
In one embodiment, rather than having a single u-shaped trap, a serpentine path provides by a number of such u-shaped structures is utilized.
In another disclosed embodiment, the refrigerant system is provided with an economizer circuit, and the liquid trap is utilized on a line directing the tapped two-phase refrigerant mixture into the economizer heat exchanger. This embodiment will provide the benefit and function as with regard to the first disclosed embodiment.
These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an evaporator incorporating the present invention.

FIG. 2 shows the FIG. 1 evaporator in a different flow condition.

FIG. 3 shows another embodiment.

FIG. 4 shows yet another embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A refrigerant system 20 is illustrated in FIG. 1 having a parallel-flow evaporator 22. As is known, refrigerant moves from the evaporator 22 downstream to a compressor 24, a condenser 26, through an expansion device 28, and back to the evaporator 22. The refrigerant leaving the expansion device 28 is in a mixed vapor and liquid state. The evaporator 22 has a plurality of parallel channels 32 spaced along an inlet manifold 34. The channels 32 and inlet manifold 34 are in fluid communication with each other. Further, the channels 32 are similarly positioned and communicated with an outlet manifold 35. Fins 30 are disposed between the channels 32. The channels 32, fins 30, inlet manifold 34, and outlet manifold 35 are typically attached to each other by furnace brazing. As is known, air is passed over the fins 30 and channels 32 to be conditioned. Due to heat transfer interaction with air supplied to a conditioned space, refrigerant evaporates inside the channels 32.
As mentioned above, should the velocity of the refrigerant approaching the inlet manifold 34 be insufficiently low, it may cause liquid refrigerant to separate from the vapor. This can result in a poor distribution of the two refrigerant phases among the channels 32. As shown in FIG. 1, the refrigerant is moving at an adequate velocity, and little or no separation of refrigerant phases occurs.
A tube 36 leading into the inlet manifold 34 is positioned downstream of a liquid trap 38. As illustrated, the liquid trap 38 generally extends vertically in a u-shape. Thus, any liquid that tends to separate will collect in the liquid trap 38.
As shown in FIG. 2, the refrigerant velocity is insufficiently low in comparison to the FIG. 1 condition to prevent phase separation, and a certain quantity of liquid refrigerant 40 has collected in the trap 38. As a result, the cross-sectional area 42 remaining for the flow of refrigerant decreases significantly. This in turn increases the velocity of the refrigerant passing to the inlet manifold 34. As the velocity of the refrigerant flow increases, the vapor refrigerant will tend to carry its liquid phase to the channels 32 in a homogeneous manner to ensure generally equal distribution. In effect, a jetting zone is created to increase velocity and limit additional phase separation. Thus, by including the liquid trap 38 upstream of the header 34, the present invention self-regulates the velocity of the refrigerant and ensures that other than the initial separation of a small quantity of liquid refrigerant 40, the remaining liquid refrigerant will tend not to separate form the vapor phase resulting in homogeneous flow conditions in the inlet manifold 34. Of course, the inlet manifold 34 should be of an appropriate cross-sectional area and length to sustain this flow homogeneity. Also, the liquid trap 38 should be positioned in close proximity to the inlet manifold 34. Preferably, the liquid trap 38 should be located within 5 inches from the entrance to the inlet manifold 34 and extend vertically beneath it. Consequently, the evaporator performance is improved. This will also result in no liquid refrigerant in the evaporator outlet manifold 35 and system reliability enhancement.
While this invention is disclosed in a conventional evaporator, other heat exchangers, for instance economizer heat exchangers (or so-called brazed plate heat exchangers) also performing an evaporator function, may equally benefit from this invention.
Further, although the liquid trap 38 is shown in its simplest configuration, other arrangements (such as multiple u-shape segments connected together, local flow impedances, etc.) are also feasible.
Another embodiment 100 shown in FIG. 3 has a plurality of serial u-shaped traps 102 upstream of the portion 104 leading into the inlet manifold 34. Each liquid trap 102 can collect small amount of liquid refrigerant, increasing velocity of the vapor phase and promoting homogeneous conditions at the entrance of the inlet manifold 34.
Another refrigerant system embodiment 110 is illustrated in FIG. 4. In this embodiment, a compressor 112 delivers a compressed refrigerant to a condenser 114. A line 116 is tapped off of a main refrigerant flow line 126, and passed through an economizer expansion device 118. A liquid trap 120 regulates the refrigerant passing through an inlet 122, to an economizer heat exchanger 124. The liquid trap 120 will provide the function and will operate as described with regard to the FIG. 1 and FIG. 2 embodiments. It should be understood that the economizer heat exchanger 124 is structured to have adjacent channels such that heat is exchanged between the refrigerant in the tap line 116 and the refrigerant in the main flow line 126. The main flow line 126 delivers refrigerant to an outlet 128 and passes it through a main expansion device 130 to an evaporator 132. The present invention can utilize the liquid trap with both the economizer heat exchanger 124, and the evaporator 132. The refrigerant returns from the evaporator 132 back to the compressor 112. A line 134 downstream of the economizer heat exchanger 124 returns the tapped refrigerant back to an intermediate compression point in the compressor 112.
It has to be pointed out that although all inlet manifolds are shown in a horizontal configuration, the maldistribution phenomenon is more pronounced in a vertical orientation. In such circumstances, the benefits of the present invention become even more pronounced.
Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.

Claims

1. A refrigerant system comprising:

a compressor delivering a compressed refrigerant to a condenser, refrigerant passing from said condenser to an expansion device, and from said expansion device to an evaporator, said evaporator comprising an inlet manifold, outlet manifold, a plurality of channels receiving refrigerant from said inlet manifold and delivering it to said outlet manifold, and fins disposed between said channels; and

a line connecting said expansion device and said evaporator, said line being provided with a liquid trap to collect liquid separated out of a vapor refrigerant passing from said expansion device to said evaporator.

2. The refrigerant system as set forth in claim 1, wherein said liquid trap extends vertically beneath said inlet manifold.

3. The refrigerant system as set forth in claim 1, wherein said liquid trap is generally provided by a u-shape downwardly extending portion of said line.

4. The refrigerant system as set forth in claim 1, wherein said liquid trap is positioned within 5 inches from said inlet manifold.

5. The refrigerant system as set forth in claim 1, wherein said refrigerant system is also provided with an economizer circuit, said economizer circuit having an economizer heat exchanger, and said economizer heat exchanger being provided with a tap line connecting a main flow line through an economizer expansion device, and then into said economizer heat exchanger, said tap line being returned to an intermediate compression point in said compressor downstream of said economizer heat exchanger, and a liquid trap to collect liquid separated out of a vapor refrigerant passing from said economizer expansion device to said economizer heat exchanger.

6. The refrigerant system as set forth in claim 1, wherein said liquid trap includes a plurality of serially spaced u-shaped liquid trap portions.

7. A method of operating a refrigerant system comprising the steps of:

providing an evaporator having a plurality of tubes receiving refrigerant from an inlet manifold, and delivering said refrigerant to an outlet manifold, and from said outlet manifold to compressor, said compressor delivering the refrigerant to a condenser, and said refrigerant passing from said condenser to an expansion device, and then back to said evaporator, and providing a fluid line connecting said expansion device to said evaporator, said fluid line being provided with a liquid trap to capture liquid that has separated from a vapor refrigerant; and

passing refrigerant through said refrigerant system and such that liquid trap self-regulates a velocity of refrigerant as the liquid separates from the vapor refrigerant to deliver refrigerant into said inlet manifold in a predominantly homogeneous state.

8. The method as set forth in claim 7, wherein the refrigerant system further being provided with an economizer circuit, said economizer circuit including an economizer heat exchanger, and tapping refrigerant and passing the tapped refrigerant through an economizer expansion device into said economizer heat exchanger, and a liquid trap provided to capture liquid that has separated from a vapor passing from said economizer expansion device into said economizer heat exchanger, and further including the steps of passing refrigerant through said economizer expansion device, and to said economizer heat exchanger, such that said liquid trap self-regulates a velocity of refrigerant as the liquid separates from the vapor refrigerant to deliver refrigerant into said economizer heat exchanger in a predominantly homogeneously state.

9. A heat exchanger and fluid line system comprising:

a fluid line leading into an inlet manifold;

a liquid trap on said fluid line; and

a heat exchanger having a plurality of channels receiving a fluid from said inlet manifold.

10. The heat exchanger and fluid line system as set forth in claim 9, wherein said heat exchanger is a refrigerant system evaporator.

11. The heat exchanger and fluid line system as set forth in claim 9, wherein said heat exchanger is a refrigerant system economizer heat exchanger.

12. The heat exchanger and fluid line system as set forth in claim 9, wherein said liquid trap extends vertically beneath said inlet manifold.

13. The heat exchanger and fluid line system as set forth in claim 9, wherein said liquid trap is generally provided by a u-shape downwardly extending portion of said line.

14. The heat exchanger and fluid line system as set forth in claim 9, wherein said liquid trap is positioned within 5 inches from said inlet manifold.

15. The heat exchanger and fluid line system as set forth in claim 9, wherein said liquid trap is provided by a plurality of serially spaced u-shaped structures.