Patent Publication Number: US-2015075215-A1

Title: System and method for distributing refrigerant to parallel flow paths of a heat exchanger using flow restrictors

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority to U.S. Provisional Patent Application No. 61/879,364 filed Sep. 18, 2013 which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject invention relates to a refrigerant distribution system, and more particularly, to a pressurized refrigerant distribution manifold configured to divide single phase refrigerant equally among parallel refrigerant paths of a heat exchanger. 
     2. Description of Related Art 
     The subject invention can be used to enhance performance of a two phase heat exchanger with parallel refrigerant flow paths. A plate fin heat exchanger is used herein as an example, because enhanced performance is coincident with size and weight optimization. 
     A plate-fin heat exchanger is a type of heat exchanger design that uses plates and finned chambers to transfer heat between two fluids. It is often categorized as a compact heat exchanger to emphasize its relatively high heat transfer surface area to volume ratio. The plate-fin heat exchanger is widely used in many industries, including the aerospace industry due its compact size and lightweight properties. 
     A plate-fin heat exchanger is typically made of layers of corrugated sheets separated by flat metal plates, to create a series of finned chambers. Separate hot and cold fluid streams flow through alternating layers of the heat exchanger and are enclosed at the edges by side bars. 
     Heat is transferred from one stream through the fin interface to the separator plate and through the next set of fins into the adjacent fluid. The fins also serve to increase the structural integrity of the heat exchanger and allow it to withstand high pressures while providing an extended surface area for heat transfer. 
     Refrigerant mass flow is often metered to the inlet line of the heat exchanger with a thermal expansion valve. There is typically a pressure drop as the refrigerant flows through the thermal expansion valve. Often, flashing will result in the formation of a two phase flow between the thermal expansion valve and the heat exchanger. Inlet plumbing, geometry and gravity often contribute to variable two phase flow patterns entering the heat exchanger. 
     When this situation arises, it becomes difficult to divide the two phase fluid and gas mixture so the mass flow is equally distributed to the parallel flow paths of the heat exchanger. As a result, individual branches of the heat exchanger will not perform equal amounts of heat transfer. Consequently, the maximum capacity of the heat exchanger will be determined when one branch reaches its maximum refrigerant flow and the other layers are at less than maximum capacity. So the capacity of the heat exchanger is sacrificed because all of the layers were not used to their maximum capacity. Furthermore, uneven mass flow between the parallel flow paths often results in uneven heat transfer, which can exacerbate poor flow distribution. 
     It would be beneficial therefore, to provide a refrigerant distribution system that is adapted and configured to preclude the distribution of two phase flow into parallel refrigerant flow paths of a compact heat exchanger. 
     SUMMARY OF THE INVENTION 
     The subject invention is directed to a new and useful heat exchanger that is adapted and configured to prevent flow maldistribution of a two phase flow of refrigerant into the parallel flow paths of the heat exchanger. It is envisioned that compact heat exchanger of the subject invention could be configured as an evaporator or a boiler. 
     The compact heat exchanger of the subject invention includes a plurality of parallel refrigerant paths and a distributor for delivering refrigerant to the plurality of parallel refrigerant paths. The distributor defines or otherwise provides a flow restrictor at an entrance opening to each refrigerant path. The flow restrictors are adapted and configured to keep the distributor pressurized and thereby maintain single phase refrigerant between the valve and the restrictors. 
     More particularly, the flow restrictors keep the distributor pressurized to a value that is greater than the saturation pressure of the refrigerant, thereby preventing the refrigerant from undergoing a phase change from a liquid to a vapor. Ideally, the flow restrictors would choke the flow to each layer, making restrictor flow back pressure insensitive. Each restrictor flow would be determined by the same downstream pressure, which is the saturation pressure. 
     Preferably, the flow restrictor at the entrance or opening to each refrigerant path is configured as an orifice. The orifice is dimensioned to maintain single phase flow between the valve and the orifice. It is envisioned that the orifices of the distributor are uniformly dimensioned and configured so as to ensure even distribution to the parallel refrigerant paths of the heat exchanger. Ideally, the orifices would provide choked flow to be back- pressure insensitive. This reduces the feedback from flow maldistribution to heat transfer maldistribution to increased flow maldistribution. 
     A thermal expansion valve is located upstream from the distributor for metering refrigerant flow to the distributor, and a source of pressurized refrigerant is located upstream from the thermal expansion valve. The source of pressurized refrigerant could be a compressor or a blow down tank, depending upon the application or operating environment. The distributor also includes an inlet opening for receiving pressurized refrigerant from the thermal expansion valve. 
     The subject invention is also directed to a plate and fin type heat exchanger that includes a plurality of alternating refrigerant and hot fluid layers arranged in a parallel configuration, and a distribution manifold for delivering refrigerant to the plurality of parallel refrigerant layers. The distribution manifold defines or otherwise provides an orifice at an entrance opening to each refrigerant layer. The orifices are dimensioned to keep the distribution manifold pressurized at a value greater than the saturation pressure of the refrigerant and thereby maintain single phase refrigerant flow into each of the parallel refrigerant layers. The orifices can also provide a means for producing choked flow, so that flow rate is not affected by variable pressures of the individual refrigerant layers. 
     The subject invention is also directed to a method of distributing refrigerant in a heat exchanger having a plurality of parallel refrigerant flow paths. The method includes the steps of pressurizing a refrigerant distributor upstream from each of the refrigerant flow paths of the heat exchanger to divide the refrigerant as single phase fluid. The refrigerant distributor is pressurized to a value greater than the saturation pressure of the refrigerant. 
     The method also includes the step of evenly distributing the single phase fluid to each of the refrigerant flow paths of the heat exchanger to maximize the capacity of the heat exchanger. The method further includes the step of metering the flow of refrigerant with choked flow to make the individual layer mass flow rates insensitive to the backpressure of the individual layers. 
     These and other features of the subject invention and the manner in which it is employed will become more readily apparent to those having ordinary skill in the art from the following enabling description of the preferred embodiments of the subject invention taken in conjunction with the several drawings described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that those skilled in the art to which the subject invention appertains will readily understand how to make and use the refrigerant distribution system of the subject invention without undue experimentation, preferred embodiments thereof will be described in detail herein below with reference to certain figures, wherein: 
         FIG. 1  is a schematic rendering of a prior art compact heat exchanger which includes a distribution manifold that enables the delivery of two phase refrigerant into the refrigerant flow paths of the heat exchanger; and 
         FIG. 2  is a schematic rendering of a compact heat exchanger constructed in accordance with the subject invention, which includes a distribution manifold that enables the equal distribution of single phase refrigerant into the refrigerant flow paths of the heat exchanger. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to the drawings wherein like reference numerals identify similar structural features or aspects of the subject invention, there is illustrated in  FIG. 1  a prior art compact heat exchanger designated generally by reference numeral  10 . Compact heat exchanger  10  is preferably a plate and fin type heat exchanger that includes a plurality of alternating refrigerant and hot fluid layers  12 ,  14  arranged in a parallel configuration. 
     There are several basic types of fins that are used in compact heat exchangers. These include: plain fins, which refer to simple straight-finned triangular or rectangular designs; herringbone fins, where the fins are placed sideways to provide a zigzag path; and serrated and perforated fins which refer to cuts and perforations in the fins to augment flow distribution and improve heat transfer. 
     With continuing reference to  FIG. 1 , a distribution manifold  16  is operatively associated with the compact heat exchanger  10  for delivering refrigerant to the plurality of parallel refrigerant layers  12  of the heat exchanger  10 . A thermal expansion valve  18  is located upstream from the distribution manifold  16  for metering refrigerant flow to the distribution manifold  16 . More particularly, the thermal expansion valve  18  controls the amount of superheating that occurs at the outlet of the heat exchanger  10  and/or the hot fluid outlet temperature. 
     A source of high pressure refrigerant  20  is located upstream from the expansion valve  18 . The source of pressurized refrigerant  20  could be a compressor or a blow down tank depending upon the application with which the heat exchanger is employed. 
     In prior art compact heat exchangers of this type, as the pressure drops across the thermal expansion valve  18 , some of the refrigerant flashes from a liquid phase to a vapor phase. This results in two phase flow from the valve  18  through the distribution manifold  16  and into the refrigerant flow paths  12 , which is not an ideal mode of operation. Indeed, poorly distributed two phase flow with concurrent heat transfer and fluid viscosity variations can lead to instabilities that jeopardize the structural integrity of the heat exchanger and limit its performance. 
     Referring now to  FIG. 2 , there is illustrated a compact plate and fin type heat exchanger constructed in accordance with a preferred embodiment of the subject invention and designated generally by reference numeral  110 . Preferably, the heat exchanger  110  of the subject invention is configured as an evaporator. However, those skilled in the art will readily appreciate that the compact heat exchanger  110  of the subject invention could be configured as a condenser or a boiler. Those skilled in the art will also appreciate that the compact heat exchanger  110  of the subject invention could be constructed with fins that are designed as plain, herringbone, serrated or perforated. 
     Heat exchanger  110  includes a plurality of alternating refrigerant and hot fluid layers  112 ,  114  arranged in a parallel configuration. A distribution manifold  116  is operatively associated or otherwise in fluid communication with the refrigerant layers  112  of heat exchanger  110 . More particularly, the distribution manifold  116  delivers refrigerant to the plurality of parallel refrigerant layers  112  of heat exchanger  110 . The distribution manifold  116  defines or otherwise provides a flow restrictor, and more preferably, an orifice  122  at an entrance or opening to each refrigerant layer  112 . The orifices  122  are dimensioned to restrict flow and thereby keep the distribution manifold  116  pressurized at a predetermined level. This ensures that single phase refrigerant is divided equally into each of the parallel refrigerant layers  112  of the heat exchanger  110 . 
     An expansion valve  118  is located upstream from the distributor  116  for metering refrigerant flow to an inlet opening of the distribution manifold  116 . A source of pressurized refrigerant  120  is located upstream from the expansion valve  118 . The source of pressurized refrigerant  120  could be a compressor or a blow down tank. 
     The plural orifices  122  of the distribution manifold  116  maintain the pressure in the distribution manifold  116  at a value that is greater than the saturation pressure of the refrigerant (i.e., the pressure that the refrigerant will undergo a phase change from liquid to vapor or vapor to liquid). Pressurizing the distribution manifold  116  to a value greater than the saturation pressure of the refrigerant results in uniform single phase fluid flow distribution to the heat exchanger  110 . The single phase flow is homogeneous and at a low velocity, so the manifold shape and gravity have minimal effect on the flow split. 
     In essence, the high pressure refrigerant feeding the flow restrictors or orifices  122  results in a choked flow, which is insensitive to back pressure. This results in an even distribution of the refrigerant to each refrigerant flow path  112 . With each refrigerant flow path  112  of the heat exchanger  110  receiving an even share of the total refrigerant flow, the maximum capacity of the heat exchanger can be realized. 
     Moreover, balanced refrigerant flow to the parallel refrigerant flow paths  112  results in maximum useable capacity for a given size heat exchanger  110 , minimizing the need to oversize the heat exchanger in the design phase, which is often the preferred design choice. Consistent even flow distribution enables a more robust design that is insensitive to effects of gravity or other body accelerations. 
     Because the flow through each restrictor or orifice  122  will be choked flow, and back pressure sensitivity is eliminated, there can be a better correlation between the expansion valve command signals and actual mass flow. This can be used for various control schemes and fault isolation logic, as would be readily appreciated by those having ordinary skill in the art. 
     The subject invention is also directed to a method of distributing refrigerant in a heat exchanger  110  having a plurality of parallel refrigerant flow paths  112 . The method includes the steps of pressurizing a refrigerant distributor  116  upstream from each of the refrigerant flow paths  112  of the heat exchanger  110  to maintain the refrigerant as single phase fluid. Preferably, the refrigerant distributor  116  is pressurized to a value greater than the saturation pressure of the refrigerant. 
     The method also includes the step of evenly distributing the single phase fluid to each of the refrigerant flow paths  112  of the heat exchanger  110  to maximize the capacity of the heat exchanger. The method further includes the step of metering the flow of refrigerant to the distributor through an expansion valve  118 . 
     While the subject invention has been shown and described with reference to preferred embodiments, those skilled in the art will readily appreciate that various changes and/or modifications may be made thereto without departing from the spirit and scope of the subject invention as defined by the appended claims.