Patent Publication Number: US-2015083378-A1

Title: System and method for distributing refrigerant to a parallel flow heat exchanger using refrigerant injectors

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority to U.S. Provisional Patent Application No. 61/881,193 filed Sep. 23, 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 refrigerant distribution system that employs refrigerant injectors for independently metering single phase refrigerant into parallel refrigerant flow paths of a heat exchanger. 
     2. Description of Related Art 
     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, the 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 branches are at less than their maximum capacity. So the capacity of the heat exchanger is sacrificed because all 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 two phase flow of refrigerant into the parallel flow paths of the heat exchanger. It is envisioned that the 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 refrigerant injector operatively associated with an inlet to each refrigerant path of the heat exchanger for independently metering single phase refrigerant into each of the parallel refrigerant paths. 
     Preferably, the refrigerant injectors are adapted and configured to meter equal amounts of refrigerant into the refrigerant paths of the heat exchanger. This ensures even parallel refrigerant mass flow distribution throughout the branches of the heat exchanger. In the case of a compact boiler, uneven distribution is less susceptible to amplification from boiling heat transfer and viscosity variation of the hot side fluid. Furthermore, all refrigerant paths will be equally utilized and essentially reach the full boiling limit simultaneously. 
     Each of the refrigerant injectors is in fluid communication with a source of pressurized refrigerant. More particularly, a distribution manifold delivers single phase refrigerant to each of the refrigerant injectors from a source of pressurized refrigerant. The source of pressurized refrigerant could be a compressor or a blow down tank depending upon the application or operating environment. The heat exchanger further includes means for controlling the flow of single phase refrigerant through the refrigerant injectors. 
     The step of independently injecting single phase refrigerant into each of the refrigerant flow paths of the heat exchanger involves metering equal amounts of refrigerant into each refrigerant flow path. The method further includes the steps of distributing single phase refrigerant to each of the refrigerant injectors, and controlling the flow of single phase refrigerant through each of the refrigerant injectors. 
     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 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 injectors for independently metering single phase refrigerant to each of 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. Plate and fin type heat exchangers are often made of aluminum alloys, which provide high heat transfer efficiency, while reducing the weight of the equipment. 
     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 . 
     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, two phase flow with concurrent heat transfer and fluid viscosity variations can lead to instabilities that limit its performance. 
     Referring now to  FIG. 2 , there is illustrated a compact plate and fin type heat exchanger constructed in accordance with preferred embodiment of the subject invention and designated generally by reference numeral  110 . Preferably, the compact heat exchanger  110  of the subject invention is configured as a boiler or evaporator. Heat exchanger  110  includes a plurality of alternating refrigerant and hot fluid layers  112 ,  114  that are arranged in a parallel configuration. A refrigerant injector  118  is operatively associated with an inlet opening of each refrigerant layer  112  of the heat exchanger  110 . The refrigerant injectors  118  are configured to meter equal amounts of refrigerant into each parallel refrigerant layer  112  of heat exchanger  110 . By distributing refrigerant mass flow evenly throughout the branches of the heat exchanger, performance is maximized and operational instability is reduced. 
     An electronic controller  122  associated with the refrigerant injectors  118  controls the flow of single phase refrigerant to each of the refrigerant injectors  118 . Two phase flow will result downstream of the injectors With individual flow control through the refrigerant injectors  118 , single paths can be turned off in cases of less than maximum heat load and for fault accommodation. Furthermore, those skilled in the art will readily appreciate that individual flow path control provides the capability for superheat control for each path, with appropriate feedback sensors. 
     A distribution manifold  116  delivers single phase refrigerant to each of the refrigerant injectors  118  from a source of pressurized refrigerant  120 . The source of pressurized refrigerant  120  could be a compressor or a blow down tank depending upon the application or operating environment. Those skilled in the art will readily appreciate that by using independent refrigerant injection in heat exchanger  110 , there is no need to place a thermal expansion valve between the distribution manifold  116  and the source of pressurized refrigerant  120 . Instead, the injectors  118  control the hot fluid outlet temperature and if desired, the amount of superheating at the refrigerant outlet of the heat exchanger  110 . 
     With each refrigerant flow path  112  of the heat exchanger  110  receiving an even share of the total flow through the heat exchanger, the maximum capacity of the heat exchanger can be realized. Moreover, balanced refrigerant flow to the parallel refrigerant flow paths  112  results maximum useable capacity for the 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 provides a more robust design that is insensitive to effects of gravity or other body accelerations. 
     The subject invention is further 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 associating a refrigerant injector  118  with each refrigerant flow path  112  of the heat exchanger  110 , and independently injecting single phase refrigerant into each of the refrigerant flow paths  112  of the heat exchanger  110  to maximize the capacity of the heat exchanger  110 . 
     The step of independently injecting single phase refrigerant into each of the refrigerant flow paths  112  of the heat exchanger  110  involves metering equal amounts of refrigerant into each refrigerant flow path  112 . The method further includes the steps of distributing single phase refrigerant to each of the refrigerant injectors  118 , and controlling the flow of single phase refrigerant through each of the refrigerant injectors  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.