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An inlet header (22) of a microchannel heat pump heat exchanger has a tube (34) disposed therein and extending substantially the length of the inlet header (22), with the tube (34) having a plurality of openings (36) therein. During cooling mode operation, refrigerant is caused to flow into an open

An inlet header (22) of a microchannel heat pump heat exchanger has a tube (34) disposed therein and extending substantially the length of the inlet header (22), with the tube (34) having a plurality of openings (36) therein. During cooling mode operation, refrigerant is caused to flow into an open end of the tube (34) and along its length to thereby flow from the plurality of openings (36) into the inlet header (22) prior to entering the microchannels (24) to thereby provide a uniform flow of two-phase refrigerant thereto. A bi-flow expansion device (41) placed at the inlet end of the tube (34) allows for the expansion of liquid refrigerant into the tube (34) during periods in which the heat exchanger operates as an evaporator and allows the refrigerant to flow directly from the header (22) and around the tube (34) during periods in which the heat exchanger operates as a condenser coil.

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1. A parallel flow heat exchanger arrangement for a heat pump comprising: a header defining in a cooling mode an inlet header, said inlet header having an inlet opening for conducting the flow of fluid into said inlet header and a plurality of outlet openings for conducting the flow of fluid from sa

1. A parallel flow heat exchanger arrangement for a heat pump comprising: a header defining in a cooling mode an inlet header, said inlet header having an inlet opening for conducting the flow of fluid into said inlet header and a plurality of outlet openings for conducting the flow of fluid from said inlet header;a plurality of channels aligned in a substantially parallel relationship and fluidly connected to said plurality of outlet openings for conducting the flow of fluid from said inlet header;a tube disposed within said inlet header and being fluidly connected at an inlet end to said inlet opening, said tube extending substantially the length of said inlet header and a having a plurality of openings formed therein for conducting the flow of refrigerant from said tube to said inlet header; and,a bi-flow piston assembly disposed at the inlet end of said tube, said piston assembly having a floating piston being adapted to selectively operate in response to the flow of refrigerant in a first position in a cooling mode condition to expand liquid refrigerant to a two-phase condition prior to entering said tube or in a second position in a heating mode condition to permit the flow of refrigerant directly from said header to said piston assembly without passing through said tube. 2. The parallel flow heat exchanger as set forth in claim 1 wherein said plurality of openings includes openings of different sizes. 3. The parallel flow heat exchanger as set forth in claim 2 wherein said differently sized openings are generally larger toward a downstream end of said tube. 4. The parallel flow heat exchanger as set forth in claim 1 wherein a number of said plurality of openings is substantially equal to the number of said plurality of channels. 5. The parallel flow heat exchanger as set forth in claim 4 wherein said plurality of openings have their respective axes aligned with the respective axes of said plurality of channels. 6. The parallel flow heat exchanger as set forth in claim 4 wherein said plurality of openings have their axes aligned substantially normal to the respective axes of said plurality of channels. 7. The parallel flow heat exchanger as set forth in claim 1 wherein said heat exchanger comprises an A-coil and includes: a second header defining in a cooling mode condition a second inlet header, said second inlet header having an inlet opening for conducting the flow of fluid into said second inlet header and a second plurality of outlet openings for conducting the flow of fluid from said second inlet header;a second plurality of channels aligned in substantial parallel relationship and fluidly connected to said second plurality of outlet openings for conducting the flow of fluid from said second inlet header;a second tube disposed within second inlet header and being fluidly connected at an inlet end to an inlet opening, said second tube extending substantially the length of said second inlet header and having a second plurality of openings formed therein for conducting the flow of refrigerant from said second tube to said second inlet header; and,a second bi-flow piston assembly disposed at the inlet end of said second tube, said second piston assembly having a floating piston being adapted to selectively operate in response to the flow of refrigerant in a first position in a cooling mode condition to expand liquid refrigerant to a two-phase condition prior to its entering said tube or in a second position in a heating mode condition to permit the flow of refrigerant directly from said second header to said piston assembly without passing through said second tube. 8. A method of promoting uniform refrigerant flow from a header of a heat pump heat exchanger defining an inlet header during a cooling mode of operation to a plurality of parallel minichannels fluidly connected thereto, comprising the steps of: forming a tube with an inlet end, a downstream end and a plurality of openings therebetween;mounting said tube within said inlet header such that it extends substantially the length of said inlet header; to allow refrigerant to flow into said inlet end and through said tube and out of said plurality of openings into said inlet header prior to flowing into said plurality of parallel minichannels; andproviding an piston assembly disposed at said inlet end of said tube, said piston assembly having a floating piston being adapted to operate in response to the flow of refrigerant in a first position during cooling mode operation to expand liquid refrigerant to a two-phase condition prior to entering said inlet header and to operate in a second position during heating mode operation to cause the refrigerant to flow directly from said header to said piston assembly without passing through said tube. 9. The method as set forth in claim 8 wherein said plurality of openings include openings of different sizes. 10. The method as set forth in claim 9 wherein said differently sized openings are generally larger toward a downstream end of said tube. 11. The method as set forth in claim 8 wherein the number of said plurality of openings is substantially equal to the number of said plurality of channels. 12. The method as set forth in claim 11 wherein said plurality of openings have their respective axes aligned with the respective axes of said plurality of channels. 13. The method as set forth in claim 11 wherein said plurality of openings have their axes aligned substantially normal to the respective axes of said plurality of channels. 14. The method as set forth in claim 8 wherein said heat exchanger comprises an A-coil and said method further includes: providing a second header defining a second inlet header during a cooling mode of operation having an inlet opening for conducting the flow of fluid into said second inlet header and a second plurality of outlet openings for conducting the flow of fluid from said second inlet header;providing a second plurality of channels aligned in substantial parallel relationship and fluidly connected to said second plurality of outlet openings for conducting the flow of fluid from said second inlet header;providing a second tube having an inlet end, a downstream end and a second plurality of openings therebetweendisposing said second tube within said second inlet header and being fluidly connected at the inlet end to said inlet opening, said second tube extending substantially the length of said second inlet header and having a second plurality of openings formed therein for conducting the flow of refrigerant from said second tube to said second inlet header; andproviding a second piston assembly at the inlet end of said second tube, said second piston assembly having a floating piston being adapted to operate in response to the flow of refrigerant in a first position during cooling mode operation to expand liquid refrigerant to a two-phase condition prior to its entering said second tube and to operate in a second position during heating mode operation to cause the refrigerant to flow directly from said header to said expansion device without passing through said tube. 15. A parallel flow heat exchanger arrangement for a heat pump comprising: a header defining in a cooling mode an inlet header, said inlet header having an inlet opening for conducting the flow of fluid into said inlet header and a plurality of outlet openings for conducting the flow of fluid from said inlet header;a plurality of channels aligned in a substantially parallel relationship and fluidly connected to said plurality of outlet openings for conducting the flow of fluid from said inlet header;a tube disposed within said inlet header and being fluidly connected at an inlet end to said inlet opening, said tube extending substantially the length of said inlet header and a having a plurality of openings formed therein for conducting the flow of refrigerant from said tube to said inlet header;a bi-flow piston assembly having a body housing a floating piston, said floating piston adapted to be selectively positioned in response to refrigerant flow in a first position in a cooling mode condition and in a second position in a heating mode condition, said floating piston having a central opening, the central opening functioning as an expansion device in the cooling mode condition. 16. The parallel flow heat exchanger as set forth in claim 15 wherein said floating piston further includes a plurality of peripheral flutes defining flow passages around the periphery of said floating piston through which refrigerant is free to flow to pass from said header around said floating piston in a heating mode condition.