Patent Publication Number: US-2009229282-A1

Title: Parallel-flow evaporators with liquid trap for providing better flow distribution

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a parallel-flow evaporator wherein a liquid trap is positioned upstream of an inlet manifold to provide better flow distribution among parallel channels, improved heat transfer and enhanced system reliability. 
     Refrigerant systems are utilized to control the temperature and humidity of air in various indoor environments to be conditioned. In a typical refrigerant system operating in the cooling mode, a refrigerant is compressed in a compressor and delivered to a condenser (or an outdoor heat exchanger in this case). In the condenser, heat is exchanged between outside ambient air and the refrigerant. From the condenser, the refrigerant passes to an expansion device, at which the refrigerant is expanded to a lower pressure and temperature, and then to an evaporator (or an indoor heat exchanger if the system operates in the cooling mode). In the evaporator, heat is exchanged between the refrigerant and the indoor air, to condition the indoor air. When the refrigerant system is operating in the cooling mode, the evaporator cools and typically dehumidifies the air that is being supplied to the indoor environment. 
     One type of evaporator that could be utilized in refrigerant systems is a parallel-flow evaporator. Such evaporators have several parallel channels for communicating refrigerant between an inlet manifold and an outlet manifold. Each channel typically has numerous parallel internal paths of various cross-sectional shape separated by internal walls. Corrugated fins are disposed in between the channels for heat transfer enhancement and structural rigidity. Usually, the channels, manifolds and fins are constructed from similar materials such as aluminum and are attached to each other by furnace brazing. Recently, parallel-flow evaporators have attracted a lot of attention and interest in the air-conditioning field due to their superior performance, compactness, rigid construction, and enhanced resistance to corrosion. However, one concern with parallel-flow evaporators is maldistribution of the refrigerant among their channels. The maldistribution problem in the parallel-flow evaporators is typically caused by the liquid phase separating from the vapor in the inlet manifold due to gravity combined with insufficient refrigerant velocity, and thus manifests itself in unequal amounts of vapor and liquid refrigerant passing through the evaporator channels. Additional phenomena effecting maldistribution can be attributed to different distances the refrigerant must flow to reach various channels and to exit them, unequal pressure impedances and variations in the heat transfer rates between the channels, etc. 
     Known parallel-flow evaporators typically have inlet and outlet manifolds that are cylindrical in shape. The channels are typically made of identical aluminum extrusions that form flat tubes. As the two-phase refrigerant enters the inlet manifold, the vapor phase is often separated from the liquid phase. Since the two phases will move independently from each other after separation, the problem of refrigerant maldistribution often arises. 
     When such maldistribution occurs, the heat exchanger performance drops significantly, frequently resulting in liquid refrigerant leaving the outlet manifold. This liquid refrigerant can cause serious reliability problems and permanent compressor damage. Obviously, this is undesirable. 
     SUMMARY OF THE INVENTION 
     In a disclosed embodiment of this invention, a parallel-flow evaporator is provided with a liquid trap upstream of its inlet manifold. In this manner, should the refrigerant be moving at a speed such that the liquid phase will not separate from the vapor phase, it can flow through the trap, into the manifold, and into the evaporator channels in a generally equal distribution. However, should the refrigerant be moving at reduced speed, such that separation of liquid is likely to occur, then the liquid will tend to separate and accumulate in the liquid trap. As the liquid accumulates in the liquid trap, the flow cross-sectional area for the remainder of the refrigerant will become smaller. Since the flow cross-sectional area becomes smaller, then the refrigerant velocity will increase, creating a jetting effect that will carry droplets of liquid into the inlet manifold and will limit further phase separation. This phenomenon will be self-regulating, to ensure that an adequate refrigerant velocity will be maintained such that the refrigerant liquid will tend not to separate from the vapor. 
     In one embodiment, rather than having a single u-shaped trap, a serpentine path provides by a number of such u-shaped structures is utilized. 
     In another disclosed embodiment, the refrigerant system is provided with an economizer circuit, and the liquid trap is utilized on a line directing the tapped two-phase refrigerant mixture into the economizer heat exchanger. This embodiment will provide the benefit and function as with regard to the first disclosed embodiment. 
     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of an evaporator incorporating the present invention. 
         FIG. 2  shows the  FIG. 1  evaporator in a different flow condition. 
         FIG. 3  shows another embodiment. 
         FIG. 4  shows yet another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A refrigerant system  20  is illustrated in  FIG. 1  having a parallel-flow evaporator  22 . As is known, refrigerant moves from the evaporator  22  downstream to a compressor  24 , a condenser  26 , through an expansion device  28 , and back to the evaporator  22 . The refrigerant leaving the expansion device  28  is in a mixed vapor and liquid state. The evaporator  22  has a plurality of parallel channels  32  spaced along an inlet manifold  34 . The channels  32  and inlet manifold  34  are in fluid communication with each other. Further, the channels  32  are similarly positioned and communicated with an outlet manifold  35 . Fins  30  are disposed between the channels  32 . The channels  32 , fins  30 , inlet manifold  34 , and outlet manifold  35  are typically attached to each other by furnace brazing. As is known, air is passed over the fins  30  and channels  32  to be conditioned. Due to heat transfer interaction with air supplied to a conditioned space, refrigerant evaporates inside the channels  32 . 
     As mentioned above, should the velocity of the refrigerant approaching the inlet manifold  34  be insufficiently low, it may cause liquid refrigerant to separate from the vapor. This can result in a poor distribution of the two refrigerant phases among the channels  32 . As shown in  FIG. 1 , the refrigerant is moving at an adequate velocity, and little or no separation of refrigerant phases occurs. 
     A tube  36  leading into the inlet manifold  34  is positioned downstream of a liquid trap  38 . As illustrated, the liquid trap  38  generally extends vertically in a u-shape. Thus, any liquid that tends to separate will collect in the liquid trap  38 . 
     As shown in  FIG. 2 , the refrigerant velocity is insufficiently low in comparison to the  FIG. 1  condition to prevent phase separation, and a certain quantity of liquid refrigerant  40  has collected in the trap  38 . As a result, the cross-sectional area  42  remaining for the flow of refrigerant decreases significantly. This in turn increases the velocity of the refrigerant passing to the inlet manifold  34 . As the velocity of the refrigerant flow increases, the vapor refrigerant will tend to carry its liquid phase to the channels  32  in a homogeneous manner to ensure generally equal distribution. In effect, a jetting zone is created to increase velocity and limit additional phase separation. Thus, by including the liquid trap  38  upstream of the header  34 , the present invention self-regulates the velocity of the refrigerant and ensures that other than the initial separation of a small quantity of liquid refrigerant  40 , the remaining liquid refrigerant will tend not to separate form the vapor phase resulting in homogeneous flow conditions in the inlet manifold  34 . Of course, the inlet manifold  34  should be of an appropriate cross-sectional area and length to sustain this flow homogeneity. Also, the liquid trap  38  should be positioned in close proximity to the inlet manifold  34 . Preferably, the liquid trap  38  should be located within 5 inches from the entrance to the inlet manifold  34  and extend vertically beneath it. Consequently, the evaporator performance is improved. This will also result in no liquid refrigerant in the evaporator outlet manifold  35  and system reliability enhancement. 
     While this invention is disclosed in a conventional evaporator, other heat exchangers, for instance economizer heat exchangers (or so-called brazed plate heat exchangers) also performing an evaporator function, may equally benefit from this invention. 
     Further, although the liquid trap  38  is shown in its simplest configuration, other arrangements (such as multiple u-shape segments connected together, local flow impedances, etc.) are also feasible. 
     Another embodiment  100  shown in  FIG. 3  has a plurality of serial u-shaped traps  102  upstream of the portion  104  leading into the inlet manifold  34 . Each liquid trap  102  can collect small amount of liquid refrigerant, increasing velocity of the vapor phase and promoting homogeneous conditions at the entrance of the inlet manifold  34 . 
     Another refrigerant system embodiment  110  is illustrated in  FIG. 4 . In this embodiment, a compressor  112  delivers a compressed refrigerant to a condenser  114 . A line  116  is tapped off of a main refrigerant flow line  126 , and passed through an economizer expansion device  118 . A liquid trap  120  regulates the refrigerant passing through an inlet  122 , to an economizer heat exchanger  124 . The liquid trap  120  will provide the function and will operate as described with regard to the  FIG. 1  and  FIG. 2  embodiments. It should be understood that the economizer heat exchanger  124  is structured to have adjacent channels such that heat is exchanged between the refrigerant in the tap line  116  and the refrigerant in the main flow line  126 . The main flow line  126  delivers refrigerant to an outlet  128  and passes it through a main expansion device  130  to an evaporator  132 . The present invention can utilize the liquid trap with both the economizer heat exchanger  124 , and the evaporator  132 . The refrigerant returns from the evaporator  132  back to the compressor  112 . A line  134  downstream of the economizer heat exchanger  124  returns the tapped refrigerant back to an intermediate compression point in the compressor  112 . 
     It has to be pointed out that although all inlet manifolds are shown in a horizontal configuration, the maldistribution phenomenon is more pronounced in a vertical orientation. In such circumstances, the benefits of the present invention become even more pronounced. 
     Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.