US20050262872A1

US20050262872A1 - Two-phase refrigerant distribution system for parallel tube evaporator coils

Info

Publication number: US20050262872A1
Application number: US10/854,603
Authority: US
Inventors: Paul Sacks; Steven Spencer; Neelkanth Gupte
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2004-05-26
Filing date: 2004-05-26
Publication date: 2005-12-01
Also published as: WO2005119157A3; WO2005119157A2

Abstract

A parallel tube heat exchanger receiving two-phase refrigerant flow from a common manifold is provided with an eductor nozzle in one end of the manifold so as to cause the flow of refrigerant to pass along one leg of the manifold and down another leg thereof so as to reenter the upstream end of the first leg by way of a crossover tube. In this way, the two-phase refrigerant flow to the parallel tubes is more uniformly distributed. In one embodiment of the invention, the manifold is a unitary tube with a centrally disposed, longitudinally extending partition to define two longitudinally extending chambers that fluidly communicate by gaps at the ends of the central partition.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to heat exchangers and particularly to the distribution of two-phase, gas-liquid flows among the several parallel circuits of multitube heat exchangers used as refrigerant evaporators. More particularly, the invention relates to this type of multitube evaporator wherein the distribution of gas and liquid flows are made to the multiple parallel circuits from a common manifold or “header”.
It is desirable that this distribution of the two-phase refrigerant flow among the circuits be uniform with regards to both the total mass flows and the relative mass flows of gas and liquid. Unequal total flows and especially unequal flows of liquid among the parallel circuits can result in incomplete evaporation in some circuits and inefficient use of the surface in others.
The difficulty associated with the achieving the desired refrigerant distribution lies with the nature of two-phase gas-liquid flow wherein the two phases tend to separate spatially and chronologically on a macro scale. Good distribution is especially difficult from low flow or stagnating regions of a manifold that are remote from the refrigerant entry. The tendency for flow separation is exacerbated if the distributing manifold has a vertical component to its orientation wherein gravitational forces contribute further to flow separation.
Evaporators with very few parallel circuits can be fed successfully by methods that avoid the use of the manifold header. A common established approach uses a high velocity jet impingement on a target area that causes a local homogeneous flow region from which individual feeder tubes lead to the first tube of each circuit. This approach is expensive from both the material content and the complexity of assembly, especially as the number of circuits increases. Another common approach discards the conventional expansion device and substitutes individual capillary tubes or orifices for each circuit. This latter approach is similarly expensive and becomes inapplicable to evaporators with very large numbers of circuits.
The usual approach to achieving the desired distribution from a manifold header is to cause the refrigerant in the distributor to flow in a homogenous micro scale mode, that is, one having very short variations in space and time. This type of two-phase flow is associated with high velocities. The invention described herein uses this approach. It is especially appropriate for the flat tube multiport construction pioneered by the automotive industry for condensing duty, wherein when applied as a refrigerant evaporator there are an extraordinary number of parallel circuits to be fed.

SUMMARY OF THE INVENTION

The unique characteristic of the distribution system that constitutes the invention is a manifold/header that facilitates a continuously circulating two phase flow having a sufficiently homogenous character to feed the parallel circuits with the desired uniformity of mass flows and relative mass flows of gas and liquid. Motive force for the circulating two-phase flow is provided by a high velocity jet flow of refrigerant through a nozzle from the high pressure side of the refrigerant system. Low fluid static pressure in the region of the high velocity jet induces the recirculation of refrigerant from what would otherwise be the stagnating downstream end of the manifold through a return passage.
A generic embodiment of the invention is a linear manifold/header system comprised of two separate conduits arranged parallel to one another and connected fluidly at both ends. The two-phase refrigerant is fed at very high velocity through a nozzle into one of the conduits and is directed down the length thereof, passing the entrances of the multiple parallel circuits and feeding them in the desired uniform fashion. Return passage of the remaining flow to the nozzle region is facilitated by the second conduit and the fluidly connected ends. In later descriptions, the linear two conduit manifold/header system can be referred to as being a two pass device. That is, there are two distinct conduits connected fluidly at their ends, the first carrying the flow in one direction and the other returning the flow to complete the circulating loop. Variations in the locations of the nozzle and circuit entrances around the circulating loop are possible.
Actual embodiments of the invention will fall into two main categories. In the first category, each pass of the two-pass system is comprised of a separate conduit. In the second category, the two passes are a single construction.
Briefly, in accordance with one aspect of the invention, a two-pass manifold is provided for conducting the flow of two-phase refrigerant first along a first pass and then back along a second pass, with the end of the second pass fluidly communicating with the entrance to the first pass. The parallel tubes are fluidly interconnected to either of the first or second passes and are generally orthogonally orientated relative thereto. A draft tube eductor is installed in the first pass and is provided a source of two-phase refrigerant flow such that a flow of refrigerant from the eductor not only pumps refrigerant along the first pass but also draws the flow of refrigerant flow from the end of the second pass to the beginning of the first pass.
By another aspect of the invention, the two-pass manifold comprises a single conduit with an internal, longitudinally extending partition, with a partition terminating short of each end of the manifold. The refrigerant flow is then caused to flow into the manifold, on one side of the partition, pass between the end of the partition and the end of the manifold into the other side of the partition, and pass back along the other side of the manifold to reenter the first side at the other end of the partition.
In the drawings as hereinafter described, there are two embodiments 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 present invention.
FIG. 2 is a schematic illustration of an alternate embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1 there is shown a heat exchanger with a vertically orientated manifold 11 that comprises a first pass tube 12 and a second pass tube 13 interconnected at their ends by a return bend 14. A plurality of parallel tubes 16 are fluidly connected to and extend orthogonally from the second pass tube 13 as shown or, alternatively, they could extend orthogonally from the first pass tube 12. The end of the second pass 13 is fluidly interconnected to an entrance to the first pass 12 by way of a connector pipe 17.
If the flow of refrigerant were introduced into the first pass 12 without further modification of the design so as to flow around the return bend 14 and down the second pass tube 13, there would likely be a mal-distribution of two-phase refrigerant to various parallel tubes 16. That is, those near the top of the second pass tube can be starved for refrigerant flow, whereas those closer to the bottom of the second pass tube 13 are more likely to be flooded with the liquid refrigerant.
To reduce the likelihood of this occurrence, a draft tube eductor 18, having an inlet end 19 and a discharge end 21 is provided at the inlet end of the first pass tube 12. Operation of the heat exchanger within the circuit operates as follows. The refrigerant passes from a compressor 22 to a condenser 23 and an expansion device 24 before passing into the inlet 19 of the draft tube eductor. The motive fluid is then caused to flow into the first pass tube 12 as shown by the arrows, around the return bend 14, down the second pass tube 13 and across the connector pipe 17 to reenter the first pass tube 12. In this way, the flow of two-phase refrigerant to the parallel tubes 16 is more uniformly distributed because of the improved circulation caused by the draft tube eductor 18.
Referring now to FIG. 2, there is shown a plurality of parallel flat tubes 26 orientated vertically and fluidly interconnected to a manifold 27 orientated horizontally. For simplicity, the tubes are shown as a small number of tubes, but in actuality, each of these tubes has multiple ports such that the refrigerant flow passes to a relatively large number of ports from the manifold 27. The manifold 27 is a unitary structure having end walls 28 and 29 with upper longitudinal wall 31 and lower longitudinal wall 32. A longitudinally extending partition 33 is disposed within the manifold 27, with the partition 33 having ends 34 and 36 which do not extend all the way to the end walls 28 and 29 but which, with those end walls, define respective end gaps 37 and 38. The partition 33 separates a lower chamber 41 from an upper chamber 42. A draft tube eductor 39 extends into the lower chamber 41 defined by the partition 33 and the lower longitudinal wall 32, and is supplied with the source of two-phase refrigerant as described hereinabove. Operation of the heat exchanger then occurs as follows.
Two-phase refrigerant from the expansion valve is fed through an inlet of the draft tube eductor 39 to the lower chamber 41 of the manifold 27, thereby inducing additional flow of two-phase refrigerant from the upper chamber 42 and through the end gap 37. The disturbed flow region associated with the draft tube eductor 39 is contained below the partition 33 and does not unduly affect the flow distribution among the flat tube inlets that are connected to the upper chamber 42. As the refrigerant flow proceeds along the lower chamber 41, the two-phase flow stabilizes by the time it reaches the end gap 38 passing into the upper chamber 42. Thus, as the refrigerant flow passes into the upper chamber 42 it is stabilized when it feeds the individual flat tubes 26 as it travels in a counterclockwise direction. The stabilized two-phase refrigerant flow continues at a diminishing rate as it flows to the left end of the upper chamber 42. The end gap 37 allows the diminished two-phase flow of refrigerant to be drawn downwardly into the lower chamber 41 where it is induced into the high velocity flow coming out of the inlet of the draft tube eductor 39 to be recirculated.
It should be mentioned that in order for the draft tube eductor 39 to operate in such a manner as to draw the flow downwardly from the upper chamber 42, its discharge stream, and preferably the tube itself, must project to a point downstream of the end gap 37. Further, it should be recognized that the stabilization of the refrigerant flow does not require the full length of the manifold 27. Accordingly, it is possible to have the tube eductor 39 extend substantially along the length of the lower chamber 41. Alternatively, rather then extending into one end thereof as shown, the draft tube eductor 39 could be inserted into the lower longitudinal wall 32 and so orientated that its discharge flow would be directed so as to cause counterclockwise flow within the manifold.
While the present invention has been particularly shown and described with reference to preferred and alternate embodiments as illustrated in the drawings, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the true spirit and scope of the invention as defined in the claims.

Claims

1. A heat exchanger having a plurality of parallel tubes for receiving two-phase refrigerant flow from a common manifold, comprising:

a plurality of tubes aligned in substantially parallel relationship for simultaneously conducting the flow of two-phase refrigerant downstream from inlet ends thereof;

a manifold having first and second passes for serially conducting the flow of two-phase refrigerant along the length of said first pass and then along the length of said second pass to an entrance of said first pass;

said plurality of tubes being fluidly interconnected at their entrance ends to said second pass for receiving two-phase refrigerant flow therefrom; and

an inducer nozzle fluidly connected with said first pass, said nozzle having inlet and discharge ends and being connected by a source of two-phase refrigerant at its inlet end for providing motive flow of two-phase refrigerant into said first pass.

2. A heat exchanger as set forth in claim 1 wherein said first and second passes are substantially parallel to each other.

3. A heat exchanger as set forth in claim 1 wherein said first and second passes are fluidly connected by a return bend.

4. A heat exchanger as set forth in claim 1 wherein said tubes are substantially orthogonal to said manifold.

5. A heat exchanger as set forth in claim 1 wherein said inducer nozzle is disposed at one end of said first pass.

6. A heat exchanger as set forth in claim 5 wherein said inducer nozzle is co-axially orientated with respect to said first pass.

7. A heat exchanger as set forth in claim 1 wherein said manifold is a unitary structure with a longitudinally extending divider disposed therein to define upper and lower chambers.

8. A heat exchanger as set forth in claim 7 wherein said divider does not extend the full length of the manifold but leaves gaps at each end thereof between the ends of the divider and the respective ends of the manifold.