Parallel Flow Heat Exchangers Incorporating Porous Inserts

Abstract

A parallel flow (minichannel or microchannel) evaporator includes a porous member inserted at the entrance of the evaporator channels which provides refrigerant expansion and pressure drop controls resulting in the elimination of refrigerant maldistribution and prevention of potential compressor flooding.

Description

CROSS-REFERENCE TO RELATED APPLICATION[0001]Reference is made to and this application claims priority from and the benefit of U.S. Provisional Application Ser. No. 60 / 649,425, filed Feb. 2, 2005, and entitled PARALLEL FLOW EVAPORATOR INCORPORATING POROUS CHANNEL INSERTS, which application is incorporated herein in its entirety by reference.BACKGROUND OF THE INVENTION[0002]This invention relates generally to air conditioning, heat pump and refrigeration systems and, more particularly, to parallel flow evaporators thereof.[0003]A definition of a so-called parallel flow heat exchanger is widely used in the air conditioning and refrigeration industry 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 t...

AI Technical Summary

Benefits of technology

[0011]The objective of the present invention is to introduce a pressure drop control for the parallel flow (microchannel or minichannel) evaporator that will essentially equalize pressure drop through the heat exchanger circuits and therefore eliminate refrigerant maldistribution and the problems associated with it. Further, it is the objective of the present invention to provide refrigerant expansion at the entrance of each channel, thus eliminating a predominantly two-phase flow in the inlet manifold, which is one of the main causes for refrigerant maldistribution. It has been found that the introduction of a porous media inserted in each parallel flow evaporator channel, or at the entrance of each parallel flow evaporator channel, accomplishes these objectives. For instance, these porous media inserts can be brazed in each channel during furnace brazing of the entire heat exchanger, chemically bonded or mechanically fixed in place. Furthermore, these inserts can be used as primary (and the only) expansion devices for low-cost applications or as secondary expansion devices, in case precise superheat control is required and a thermostatic expansion valve (TXV) or an electronic expansion valve (EXV) is employed as a primary expansion device.

[0012]Any suitable porous insert which accomplishes the above objectives may be used. Suitable and inexpensive porous inserts may be made of sintered metal, compressed metal, such as steel wool, specialty designed porous ceramics, etc. When inexpensive porous media insert is placed in each channel of the parallel flow evaporator, or at the entrance of each parallel flow evaporator channel, it represents a major resistance to the refrigerant flow within the evaporator. In such circumstances, the main pressure drop region will be across these inserts and the variations in the pressure drop in the channels or in the manifolds of the parallel flow evaporators will play a minor (insignificant) role. Further, since refrigerant expansion is taking place at the entrance to each channel, a predominantly single-phase liquid refrigerant is flown through the inlet manifold, especially in the case when the porous inserts are utilized as the primary and the only expansion devices. Hence, uniform refrigerant distribution is achieved, evaporator and system performance is enhanced and, at the same time, precise superheat control is not lost (whenever required). Furthermore, low extra cost for the proposed method makes this invention very attractive.

Problems solved by technology

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.

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.

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.

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.

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.

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.

Moreover, maldistribution phenomenon may cause the two-phase (zero superheat) conditions at the exit of some channels, promoting potential flooding at the compressor suction that may quickly translate into the compressor damage.

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