Refrigerant vapor compression systems are well known in the art. Air conditioners and heat pumps employing refrigerant vapor compression cycles are commonly used for cooling or cooling/heating air supplied to a climate controlled comfort zone within a residence, office building, hospital, school, restaurant or other facility. Refrigerant vapor compression systems are also commonly used for cooling air to provide a refrigerated environment for food items and beverage products within display cases in supermarkets, convenience stores, groceries, cafeterias, restaurants and other food service establishments.
Conventionally, these refrigerant vapor compression systems include a compressor, a condenser, an expansion device, and an evaporator connected in refrigerant flow communication. The aforementioned basic refrigerant system components are interconnected by refrigerant lines in a closed refrigerant circuit and arranged in accord with the vapor compression cycle employed. An expansion device, commonly an expansion valve or a fixed-bore metering device, such as an orifice or a capillary tube, is disposed in the refrigerant line at a location in the refrigerant circuit upstream with respect to refrigerant flow of the evaporator and downstream of the condenser. The expansion device operates to expand the liquid refrigerant passing through the refrigerant line running from the condenser to the evaporator to a lower pressure and temperature. In doing so, a portion of the liquid refrigerant traversing the expansion device expands to vapor. As a result, in conventional refrigerant vapor compression systems of this type, the refrigerant flow entering the evaporator constitutes a two-phase mixture. The particular percentages of liquid refrigerant and vapor refrigerant depend upon the particular expansion device employed and the refrigerant in use, for example R-12, R-22, R-134a, R404A, R-410A, R-407C, ammonia, carbon dioxide or other compressible fluid.
In some refrigerant vapor compression systems, the evaporator is a parallel tube heat exchanger. Such heat exchangers have a plurality of parallel refrigerant flow paths therethrough provided by a plurality of tubes extending in parallel relationship between an inlet header and an outlet header. The inlet header receives the refrigerant flow from the refrigerant circuit and distributes the refrigerant flow amongst the plurality of flow paths through the heat exchanger. The outlet header serves to collect the refrigerant flow as it leaves the respective flow paths and to direct the collected flow back to the refrigerant line for return to the compressor in a single pass heat exchanger or through an additional bank of heat exchange tubes in a multi-pass heat exchanger.
Historically, parallel tube heat exchangers used in such refrigerant vapor compression systems have used round tubes, typically having a diameter of ⅜ inch or 7 millimeters. More recently, flat, rectangular dimension, multi-channel tubes are being used in heat exchangers for refrigerant vapor compression systems. Each multi-channel tube has a plurality of flow channels extending longitudinally in parallel relationship the length of the tube, each channel providing a small flow area refrigerant flow path. Thus, a heat exchanger with multi-channel tubes extending in parallel relationship between the inlet and outlet headers of the heat exchanger will have a relatively large number of small flow area refrigerant flow paths extending between the two headers. In contrast, a parallel tube heat exchanger with conventional round tubes will have a relatively small number of large flow area flow paths extending between the inlet and outlet headers.
A problem associated with heat exchangers having flat, rectangular tubes extending between an inlet header and an outer header versus heat exchangers having round tubes is the connection of the inlet ends of the tubes to the inlet header. Conventionally, the inlet header is an axially elongated cylinder of circular cross-section provided with a plurality of rectangular slots cut in its wall at axially spaced intervals along the length of the header. Each slot is adapted to receive the inlet end of one of the flat, rectangular heat exchange tubes with the inlets to the various flow channels open to the chamber of the header, whereby fluid within the chamber of the inlet header may flow into the multiple flow channels of the various heat exchange tubes opening into the chamber. As the flat, rectangular heat exchange tubes have a lateral dimension significantly greater than the diameter of conventional round tubes, the diameters of the round cylindrical headers associated with conventional flat tube heat exchangers are significantly greater than the diameters of headers associated with round tube heat exchangers for a comparable volumetric fluid flow rate.
Non-uniform distribution, also referred to as maldistibution, of two-phase refrigerant flow is common problem in parallel tube heat exchangers which adversely impacts heat exchanger efficiency. Two-phase maldistribution problems are caused by the difference in density of the vapor phase refrigerant and the liquid phase refrigerant present in the inlet header due to the expansion of the refrigerant as it traversed the upstream expansion device.
One solution to control refrigeration flow distribution through parallel tubes in an evaporative heat exchanger is disclosed in U.S. Pat. No. 6,502,413, Repice et al. In the refrigerant vapor compression system disclosed therein, the high pressure liquid refrigerant from the condenser is partially expanded in a conventional in-line expansion value upstream of the heat exchanger inlet header to a lower pressure, liquid refrigerant. A restriction, such as a simple narrowing in the tube or an internal orifice plate disposed within the tube, is provided in each tube connected to the inlet header downstream of the tube inlet to complete expansion to a low pressure, liquid/vapor refrigerant mixture after entering the tube.
Another solution to control refrigeration flow distribution through parallel tubes in an evaporative heat exchanger is disclosed in Japanese Patent No. JP4080575, Kanzaki et al. In the refrigerant vapor compression system disclosed therein, the high pressure liquid refrigerant from the condenser is also partially expanded in a conventional in-line expansion value to a lower pressure, liquid refrigerant upstream of a distribution chamber of the heat exchanger. A plate having a plurality of orifices therein extends across the chamber. The lower pressure liquid refrigerant expands as it passes through the orifices to a low pressure liquid/vapor mixture downstream of the plate and upstream of the inlets to the respective tubes opening to the chamber.
Japanese Patent No. JP2002022313, Yasushi, discloses a parallel tube heat exchanger wherein refrigerant is supplied to the header through an inlet tube that extends along the axis of the header to terminate short of the end the header whereby the two phase refrigerant flow does not separate as it passes from the inlet tube into an annular channel between the outer surface of the inlet tube and the inside surface of the header. The two phase refrigerant flow thence passes into each of the tubes opening to the annular channel.
Obtaining uniform refrigerant flow distribution amongst the relatively large number of small flow area refrigerant flow paths is even more difficult than it is in conventional round tube heat exchangers and can significantly reduce heat exchanger efficiency. Two-phase maldistribution problems may be exacerbated in inlet headers associated with conventional flat tube heat exchangers due to the lower fluid flow velocities attendant to the larger diameter of such headers. At lower fluid flow velocities, the vapor phase fluid more readily separates from the liquid phase fluid. Thus, rather than being a relatively uniform mixture of vapor phase and liquid phase fluid, the flow within the inlet header will be stratified to a greater degree with a vapor phase component separated from the liquid phase component. As a consequence, the fluid mixture will undesirably be non-uniformly distributed amongst the various tubes, with each tube receiving differing mixtures of vapor phase and liquid phase fluid.
In U.S. Pat. No. 6,688,138, DiFlora discloses a parallel, flat tube heat exchanger having an inlet header formed of an elongated outer cylinder and an elongated inner cylinder disposed eccentrically within the outer cylinder thereby defining a fluid chamber between the inner and outer cylinders. The inlet end of each of the flat, rectangular heat exchange tubes extend through the wall of the outer cylinder to open into the fluid chamber defined between the inner and outer cylinders.
Japanese Patent No. 6241682, Massaki et al., discloses a parallel flow tube heat exchanger for a heat pump wherein the inlet end of each flat, multi-channel tube connecting to the inlet header is crushed to form a partial throttle restriction in each tube just downstream of the tube inlet. Japanese Patent No. JP8233409, Hiroaki et al., discloses a parallel flow tube heat exchanger wherein a plurality of flat, multi-channel tubes connect between a pair of headers, each of which has an interior which decreases in flow area in the direction of refrigerant flow as a means to uniformly distribute refrigerant to the respective tubes.