US20080104975A1

US20080104975A1 - Liquid-Vapor Separator For A Minichannel Heat Exchanger

Info

Publication number: US20080104975A1
Application number: US11/794,774
Authority: US
Inventors: Mikhail B. Gorbounov; Michael K. Sahm; Parmesh Verma
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2005-02-02
Filing date: 2005-12-28
Publication date: 2008-05-08
Also published as: CN101111731A; AU2005326650B2; WO2006083445A2; MX2007009257A; JP2008528939A; CA2596331A1; EP1844271A2; AU2005326650A1; EP1844271A4; BRPI0519906A2; WO2006083445A3; KR20070091343A

Abstract

A method and apparatus for promoting uniform refrigerant flow in a minichannel heat exchanger by providing a liquid-vapor separator between an expansion device and the inlet header such that the refrigerant vapor will pass directly to the compressor and only the liquid refrigerant will pass to the inlet header. The liquid-vapor separation is accomplished by way of a float valve which prevents the flow of liquid refrigerant to the compressor and the flow of refrigerant vapor from the outlet manifold, through the valve and back to the inlet manifold. A second float valve may be provided between a downstream end of the inlet manifold and the compressor for the purpose of removing any residual vapor from the liquid.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to air conditioning and refrigeration systems and, more particularly, to parallel flow evaporators thereof.
A definition of a so-called parallel flow heat exchanger is widely used in the air conditioning and refrigeration industry now and designates a heat exchanger with a plurality of parallel passages, among which refrigerant is distributed and flown in the orientation generally substantially perpendicular to the refrigerant flow direction in the inlet and outlet manifolds. This definition is well adapted within the technical community and will be used throughout the text.
Refrigerant maldistribution in refrigerant system evaporators is a well-known phenomenon. It causes significant evaporator and overall system performance degradation over a wide range of operating conditions. Maldistribution of refrigerant may occur due to differences in flow impedances within evaporator channels, non-uniform airflow distribution over external heat transfer surfaces, improper heat exchanger orientation or poor manifold and distribution system design. Maldistribution is particularly pronounced in parallel flow evaporators due to their specific design with respect to refrigerant routing to each refrigerant circuit. Attempts to eliminate or reduce the effects of this phenomenon on the performance of parallel flow evaporators have been made with little or no success. The primary reasons for such failures have generally been related to complexity and inefficiency of the proposed technique or prohibitively high cost of the solution.
In recent years, parallel flow heat exchangers, and brazed aluminum heat exchangers in particular, have received much attention and interest, not just in the automotive field but also in the heating, ventilation, air conditioning and refrigeration (HVAC&R) industry. The primary reasons for the employment of the parallel flow technology are related to its superior performance, high degree of compactness and enhanced resistance to corrosion. Parallel flow heat exchangers are now utilized in both condenser and evaporator applications for multiple products and system designs and configurations. The evaporator applications, although promising greater benefits and rewards, are more challenging and problematic. Refrigerant maldistribution is one of the primary concerns and obstacles for the implementation of this technology in the evaporator applications.
As known, refrigerant maldistribution in parallel flow heat exchangers occurs because of unequal pressure drop inside the channels and in the inlet and outlet manifolds, as well as poor manifold and distribution system design. In the manifolds, the difference in length of refrigerant paths, phase separation and gravity are the primary factors responsible for maldistribution. Inside the heat exchanger channels, variations in the heat transfer rate, airflow distribution, manufacturing tolerances, and gravity are the dominant factors. Furthermore, the recent trend of the heat exchanger performance enhancement promoted miniaturization of its channels (so-called minichannels and microchannels), which in turn negatively impacted refrigerant distribution. Since it is extremely difficult to control all these factors, many of the previous attempts to manage refrigerant distribution, especially in parallel flow evaporators, have failed.
In the refrigerant systems utilizing parallel flow heat exchangers, the inlet and outlet manifolds or headers (these terms will be used interchangeably throughout the text) usually have a conventional cylindrical shape. When the two-phase flow enters the header, the vapor phase is usually separated from the liquid phase. Since both phases flow independently, refrigerant maldistribution tends to occur.
If the two-phase flow enters the inlet manifold at a relatively high velocity, the liquid phase (droplets of liquid) is carried by the momentum of the flow further away from the manifold entrance to the remote portion of the header. Hence, the channels closest to the manifold entrance receive predominantly the vapor phase and the channels remote from the manifold entrance receive mostly the liquid phase. If, on the other hand, the velocity of the two-phase flow entering the manifold is low, there is not enough momentum to carry the liquid phase along the header. As a result, the liquid phase enters the channels closest to the inlet and the vapor phase proceeds to the most remote ones. Also, the liquid and vapor phases in the inlet manifold can be separated by the gravity forces, causing similar maldistribution consequences. In either case, maldistribution phenomenon quickly surfaces and manifests itself in evaporator and overall system performance degradation.
In tube-and-fin type heat exchangers, it has been common practice to provide individual capillary tubes or other expansion devices leading to the respective tubes in order to get relatively uniform expansion of a refrigerant into the bank of tubes. Another approach has been to provide individual expansion devices such as so-called “dixie” cups at the entrance opening to the respective tubes, for the same purpose. Neither of these approaches are practical in minichannel or microchannel applications, wherein the channels are relatively small and closely spaced such that the individual restrictive devices could not, as a practical manner, be installed within the respective channels during the manufacturing process.
In the air conditioning and refrigeration industry, the terms “parallel flow” and “minichannel” (or “microchannel”) are often used interchangeably in reference to the abovementioned heat exchangers, and we will follow similar practice. Furthermore, minichannel and microchannel heat exchangers differ only by a channel size (or so-called hydraulic diameter) and can equally benefit from the teachings of the invention. We will refer to the entire class of these heat exchangers (minichannel and microchannel) as minichannel heat exchangers throughout the text and claims.

SUMMARY OF THE INVENTION

Briefly, in accordance with one aspect of the invention, a liquid-vapor separator is provided between the expansion device and the inlet header such that the separator causes the refrigerant vapor to pass directly to the compressor and only liquid refrigerant to pass to the inlet manifold. In this way, a more uniform distribution of liquid refrigerant to the individual parallel channels is obtained.
In accordance with another aspect of the invention, the liquid-vapor separator comprises a float valve with a float member oriented to move vertically to permit the flow of refrigerant vapor therearound but if liquid refrigerant flows into the valve, the float member will tend to seat and prevent the flow of liquid refrigerant therethrough.
By yet another aspect of the invention, a second float valve is interconnected between a downstream end of the inlet manifold and the compressor such that the residual vapor from the liquid refrigerant will pass directly to the compressor.
In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the true spirit and scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of one embodiment of the invention.

FIG. 2 is a modified version thereof.

FIG. 3 is an alternate embodiment of the present invention.

FIG. 4 is a modified version thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, the invention is shown generally at 10 as applied to a minichannel heat exchanger 11 having an inlet manifold 12, an outlet manifold 13, and a plurality of parallel microchannels 14 interconnecting the inlet manifold 12 to the outlet manifold 13.
An inlet chamber 16 is fluidly connected to the upstream end 17 of the inlet manifold 12 by way of conduit 18. At an upper portion of the inlet chamber 16 an inlet line 19 provides fluid communication from an expansion device such that a mixture of liquid and vapor refrigerant flows into the upper portion of the inlet chamber 16. The heavier liquid refrigerant tends to fall to the bottom of the inlet chamber 16 and flow through the conduit 18 to the inlet manifold 12 such that each of the parallel minichannels 14 have single phase liquid refrigerant presented at their inlet ends.
Also connected at the upper portion of the inlet chambers 16 is a bypass duct 21 for conducting the flow of refrigerant vapor to the compressor as indicated by the arrows. Disposed within the bypass duct 21 is a float valve 22 having an inlet port 23, an outlet port 24, and a float member 26.
In operation, as the two phase refrigerant enters the inlet line 19, the liquid refrigerant tends to drop into the lower portion of the inlet chamber 16 and the refrigerant vapor is drawn upwardly by the suction of the compressor. As it enters the float valve 22, it passes around the float member 26 and through the outlet port 24 to the compressor. If liquid refrigerant enters the float valve 22, it will tend to lift the float member 26 such that it will engage the outlet port 24 and seat so as to prevent the flow of liquid toward the compressor. The liquid refrigerant will then drop to the lower portion of the float valve and then flow into the inlet chamber 16.
The outlet manifold 13 is fluidly attached to the compressor by way of an outlet line 15 so that, after the liquid refrigerant is converted to vapor as it passes through the microchannels 14 and into the outlet manifold 13, the refrigerant vapor is then drawn into the compressor. It should be mentioned that, if the vapor from the outlet line 15 tends to flow into the bypass duct 21, the float member 26 will be caused to seat against the inlet port 23 and prevent vapor from entering the inlet chamber 16.
Referring now to FIG. 2, a similar arrangement is shown, but with additional features. At the downstream end 27 of the inlet manifold 12 a conduit 28 is connected to provide fluid communication to the compressor by way of a second float valve 29. This float valve separator operates in the same manner as the float valve 22 as described hereinabove to remove any residual vapor that may be in the downstream end 27 of the inlet manifold 12. That is, all liquid refrigerant in the manifold 12 should flow upwardly through the microchannels 14, and any residual vapor would pass upwardly through the conduit 28, the float valve 29 and to the compressor.
It will also be seen in the FIG. 2 embodiment, that the inlet chamber 16 of the FIG. 1 embodiment has been changed from its reservoir form to a simple piping arrangement 31. While the piping arrangement 31 will contain less liquid refrigerant, it operates in substantially the same way as the inlet chamber 16 as described hereinabove.
The FIG. 1 and FIG. 2 embodiments as described hereinabove relate to arrangements wherein the heat exchanger 11 is orientated such that the manifolds 12 and 13 are horizontal and the minichannels 14 are vertical. The FIG. 3 embodiment illustrates the invention as used in a configuration wherein the headers are orientated vertically and the minichannels are orientated horizontally.
Here the minichannel heat exchanger 32 has a manifold 33, a manifold 34 and parallel minichannels 36. The manifold 33 is divided into upper and lower sections 37 and 38, with the microchannels 36 in the lower sections 38 acting to conduct the flow of refrigerant to the manifold 34 and the minichannels in the upper section 37 acting to conduct the flow of refrigerant from the manifold 34 back to the upper section 37 of the manifold 33. Again, a float valve 39 is provided in a line 41 connecting the upper section 37 of the inlet manifold 33 to the compressor. This float valve operates in the same manner as the float valve 29 of the FIG. 2 embodiment to remove any residual vapor from the liquid.
In the FIG. 4 embodiment, there is provided a conduit 42 for fluid communication between the downstream end of manifold 34 and the compressor suction. A float valve 43 is provided and operates in the same manner as the float valve described hereinabove. Its purpose is to separate any vapor that appears after the first pass of the heat exchanger so that only liquid refrigerant is fed to the second pass of the heat exchanger.
In each of the liquid-vapor separators described hereinabove, the selection of the float member and the seats are made to be consistent with the refrigerant fluid in respect to density, buoyancy and seating force requirements, as well as for material compatibility considerations.

Claims

1. A liquid-vapor separator for a heat exchanger of the type having inlet and outlet manifolds fluidly interconnected by a plurality of parallel minichannels for conducting the flow of refrigerant therebetween, comprising:

an inlet chamber fluidly connected between said inlet manifold and an expansion device that is adapted to deliver two phase refrigerant thereto;

a bypass duct fluidly interconnected between said inlet chamber and a compressor inlet; and

a float valve disposed within said bypass duct and operable to permit the flow of refrigerant vapor to said compressor but not to permit the flow of liquid refrigerant thereto.

2. A liquid-vapor separator as set forth in claim 1 wherein said inlet chamber is connected to the expansion device by an inlet line and further wherein said inlet line is connected to said inlet chamber at an upper portion thereof.

3. A liquid-vapor separator as set forth in claim 2 wherein said inlet chamber is so constructed as to permit the flow of liquid refrigerant to said inlet manifold but the presence of liquid refrigerant in said inlet chamber prevents the flow of refrigerant vapor to said inlet manifold.

4. (canceled)

5. A liquid-vapor separator as set forth in claim 1 wherein said float valve includes an outlet port and float member moveable upwardly to seat against and seal said outlet port.

6. A liquid-vapor separator as set forth in claim 5 wherein said valve further includes an inlet port against which the float member can seat as it moves downwardly to prevent the flow of refrigerant vapor downwardly therethrough.

7. A liquid-vapor separator as set forth in claim 1 wherein said outlet manifold is fluidly connected to the compressor.

8. A liquid-vapor separator as set forth in claim 1 and including a conduit fluidly interconnecting a downstream end of said inlet manifold to said compressor and said float valve is disposed within said conduit for removal of any residual vapor from a liquid.

9. A liquid-vapor separator as set forth in claim 8 wherein said float valve and an outlet port and a float member that is upwardly moveable to seat thereagainst.

10. A liquid-vapor separator as set forth in claim 9 wherein said float valve includes an inlet port against which the float member can seat.

11. A method of promoting uniform refrigerant flow from an inlet manifold of a heat exchanger to a plurality of parallel minichannels fluidly connected thereto, comprising the steps of:

providing an inlet chamber for fluidly interconnecting the inlet manifold to an expansion device adapted to deliver two phase refrigerant thereto;

providing a bypass duct for fluidly interconnecting said inlet chamber and a compressor inlet; and

providing a float valve within said bypass duct so as to permit the flow of refrigerant vapor to said compressor but not permit the flow of liquid refrigerant thereto.

12. A method as set forth in claim 11 including the steps of connecting the expansion device to an upper portion of said inlet chamber by way of an inlet line.

13. (canceled)

14. A method as set forth in claim 11 wherein said float valve includes an outlet port and float member moveable upwardly to seat against and seal said outlet port.

15. A method as set forth in claim 14 wherein said float valve further includes an inlet port against which the float member can seat as it moves downwardly to prevent the flow of refrigerant vapor downwardly therethrough.

16. A method as set forth in claim 15 and including the further step of fluidly connecting said outlet manifold to the compressor.

17. A method as set forth in claim 11 and including the further step of fluidly interconnecting a downstream end of said inlet manifold to said compressor by way of a conduit.

18. A method as set forth in claim 17 and including the further step of providing said float valve within said conduit for removal of any residual vapor from a liquid.

19. A method as set forth in claim 18 wherein said float valve includes an outlet port and a float member that is upwardly moveable to seat thereagainst.

20. A method as set forth in claim 19 wherein said float valve includes an inlet port against which the float member can seat.