Patent Publication Number: US-8113270-B2

Title: Tube insert and bi-flow arrangement for a header of a heat pump

Description:
TECHNICAL FIELD 
     This invention relates generally to heat exchangers and, more particularly, to microchannel heat exchangers for use with two-phase refrigerant in a heat pump. 
     BACKGROUND OF THE INVENTION 
     Microchannel heat exchangers are currently designed in a parallel flow configuration, wherein there is a long inlet header that extends the length of the core and feeds multiple parallel tubes that then feed into an outlet header. The diameter of the headers must be larger than the major axis of the microchannel tube. When this parallel flow microchannel heat exchanger operates as an evaporator, two-phase refrigerant is being fed into the inlet header. Since this two-phase refrigerant is a mixture of vapor and liquid, it tends to separate in the inlet header leading to maldistribution within the evaporator (i.e. some tubes are fed mostly vapor instead of a balanced mixture of vapor and liquid), which has a negative effect on the cooling capacity and efficiency of the air conditioner. Because the performance is compromised in this manner, additional surface must be added to the evaporator to match the capacity and efficiency of a comparable round tube, plate fin evaporator. This increases the cost as well. 
     Typically, an inlet header is only fed from one side in what is referred to as a direct feed approach. Such a direct feed approach causes two-phase refrigerant to flow through the entire length of the header, with the vapor and liquid tending to separate out such that some tubes get mostly vapor and others get mostly liquid, thereby resulting in dry surfaces and poor utilization of the heat exchanger. 
     An alternative to the direct feed approach is to use a distributor leading to multiple feeder tubes that feed into baffled sections of the header. This method results in considerable additional expense over the direct feed method as additional hardware such as the distributor/feeder tube assembly must be added as well as the baffles in the header. 
     When particular structures are added to heat exchangers in order to promote uniform flow from the inlet manifold to the microchannels during cooling mode operation, those same structures may interfere with refrigerant flowing in the opposite direction during operation in the heating mode. 
     DISCLOSURE OF THE INVENTION 
     In accordance with one aspect of the invention, the distribution of two-phase refrigerant to the multiple channels of a microchannel heat exchanger in a heat pump can be made more uniform when operating in the cooling mode by the placement of a perforated tube within the inlet header, with the tube being fed refrigerant at its one end and extending substantially the length of the header. The perforations act as distributors to conduct the flow of two-phase refrigerant from the insert tube into the inlet manifold. In this manner, each region of the inlet header will be fed a well-mixed, uniform flow of two-phase refrigerant that then enters the individual channels in a uniform manner. A bi-flow expansion device is provided at the inlet to the perforated tube insert such that during cooling mode operation the refrigerate expansion occurs immediately before entering the perforated tube and during heating mode operation, the expansion device allows the refrigerant to bypass the perforated tube such that the refrigerant flows directly from the manifold to the expansion device. 
     In accordance with another aspect of the invention, the size/shape of the perforations in the tube can be selectively formed in order to obtain optimal distribution. In general, the size of the perforations increases toward the downstream end of the tube. 
     In accordance with another aspect of the invention, the number of perforations in the tube is made equal to the number of channels in the microchannel heat exchanger. That is, the perforations are so placed that there is a perforation located in longitudinal alignment with each of the channels. They may be either axially aligned or radially offset from the axes of their respective channels. 
     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 perspective view of a conventional A-coil in accordance with the prior art. 
         FIG. 2  is a perspective view of a microchannel A-coil in accordance with one embodiment of the invention. 
         FIG. 3  is a longitudinal cross-sectional view of an inlet header thereof. 
         FIGS. 3A and 3B  are alliterative transverse cross-section views thereof. 
         FIG. 4  is a longitudinal cross-section view thereof showing details of the expansion device thereof. 
         FIG. 5  is a cross-sectional view of the expansion valve portion thereof as shown in the cooling mode operation. 
         FIG. 6  is a cross-sectional view thereof as shown in the heating mode operation. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , there is shown a conventional A-coil having a pair of coil slabs  12  and  13  with each having a plurality of refrigerant carrying tubes passing through a plurality of fins which, in turn, are adapted to have air passed therethrough by way of a blower or fan. 
     In practice, liquid refrigerant from a condenser (not shown) passes to an expansion device  14 , with the resulting two-phase refrigerant then passing to a distributor  16  and then to a plurality of connecting lines  17  that carry the two-phase refrigerant into the various circuits of tubes. As the air passes through the slabs  12  and  13  is cooled, the refrigerant is boiled off with the refrigerant vapor then passing to a compressor and then back to the condenser. 
       FIG. 2  shows a microchannel A-coil  18  in accordance with one aspect of the invention, with the A-coil  18  being formed of a pair of microchannel evaporator coils  19  and  21 . Each of the microchannel evaporator coils  19  and  21  have an inlet header  22 , an outlet header  23  and a plurality of microchannels  24  fluidly interconnected therebetween. 
     At the entrance of each inlet header  22  is an expansion device  26 . The liquid refrigerant is introduced from the condenser along line  27  and splits into lines  28  and  29  to feed the expansion devices  26  which, in turn, pass the two-phase refrigerant directly into the inlet headers  22 . The two-phase refrigerant then passes into the individual microchannels  24  and flows to the respective outlet manifolds  21  and  23 , after which the refrigerant vapor passes to the compressor. 
     As will be seen in  FIG. 3 , the inlet header  22  is a hollow cylinder having end walls  31  and  32  and having the plurality of microchannels  24  extending outwardly on one side thereof for conducting the flow of two-phase refrigerant toward the outlet header  23 . Fins  33  are placed between adjacent microchannels  24  for enhancing the heat transfer characteristics of the coils. 
     The tube  34  passes through the end wall  31  and extends substantially the length of the inlet header  22  from an inlet end  37  to a downstream end  38  as shown. The tube  34  may be concentrically located within the inlet header  22  as shown or may be offset from the centerline thereof in order to enhance the ability of the inlet header  22  to provide uniform flow of two-phase refrigerant to the individual channels  24 . A plurality of openings  36  are provided in the tube  34  for conducting the flow of refrigerant from the tube  34  to the inlet header  22  and hence to the individual microchannels  24 . The size and shape of the openings  36  may be selectively varied in order to promote the uniform flow of refrigerant to the individual microchannels  24 . Generally, the size of the openings  36  will increase from the inlet end  37  to the downstream end  38 , for example as illustrated in  FIGS. 5 and 6 . 
     Although the number and location of the openings  36  may be varied as desired, the embodiment as shown in  FIG. 3  provides a single opening  36  for each of the microchannels  24  such that the opening  36  is substantially longitudinally aligned with its respective microchannel  24 . 
     In addition to the possible size and shape of the openings  36  as discussed hereinabove, the angular orientation of the openings  36  with respect to the axes of the microchannels may be varied as desired in order to promote uniform flow distribution. That is, the openings  36  may be axially aligned with the microchannels  24  as shown in  FIG. 3A , or they may be angularly offset in a manner such as shown in  FIG. 3B . Such an angular offset of 90° has been found to be helpful in creating a desired mixing offset such that more uniform flow distribution occurs. 
     In accordance with the present invention the refrigerant is distributed in the liquid phase from the liquid line into an expansion device  39  that expands directly into the inlet end  37  of the perforated tube. In this way, all of the liquid refrigerant is first distributed to the microchannel slabs and then expanded to a two-phase state thus, eliminating the two-phase separation that occurs when expanding prior to distribution as described in respect to the prior art above. Further, there is no pressure drop that is associated with the feeder tubes of the prior art. 
     Referring now to  FIG. 4 , it will be seen that the expansion device  39  of  FIG. 3  is comprised of a bi-flow piston assembly  41  having a body  40  that houses a floating piston  42 , which is adapted to be in one of two extreme positions, depending on the direction of refrigerant flow. That is, for the cooling modes of operation, the heat exchanger is operated as an evaporator coil and the refrigerant flows into the inlet header, whereas during heating operation, the coil is operated as a condenser coil and the refrigerant is flowing from the same header which is now the outlet header of the condenser coil. The features of the piston  42  which allow for this bi-flow relationship are a central opening  43  and a plurality of peripheral flutes  44  as shown in  FIG. 4 . 
     As shown in  FIG. 5 , when the system is operating in a cooling mode, the refrigerant is flowing into the bi-flow piston assembly  41 , and the piston  42  is to the far right with its flutes  44  resting against a shoulder of the body  40 . The refrigerant then passes through the central opening  43  which acts as an expansion device such that two-phase refrigerant than flows into the tube  34  and then to the individual microchannels  24 . 
     In the  FIG. 6  embodiment, the flow of refrigerant is passing from the header and into the bi-flow piston assembly  41 , such that the piston  42  is moved to the far left. In this position, the refrigerant is free to flow from the manifold  22  and between the flutes  44  to pass around a periphery of the piston  42 . While the central opening  43  is still open, there is very little, if any refrigerant in the tube  34  since the refrigerant flow is most likely to travel by way of the least resistant path, directly from the manifold  22  and around the periphery of the piston  42 .