Patent Publication Number: US-8528358-B2

Title: Refrigerant vapor injection for distribution improvement in parallel flow heat exchanger manifolds

Description:
This application is a U.S. National Phase application of PCT Application No. PCT/US2006/047966 filed Dec. 15, 2006. 
     BACKGROUND OF THE INVENTION 
     This application relates to a parallel flow heat exchanger, wherein vapor refrigerant from an upstream location is utilized to provide additional momentum in driving liquid phase refrigerant along a manifold to improve refrigerant distribution among parallel tubes that are in fluid communication with this manifold, and thus enhance the heat exchanger and overall refrigerant system performance. 
     Refrigerant systems utilize a refrigerant to condition a secondary fluid, such as air, delivered to a climate controlled space. In a basic refrigerant system, the refrigerant is compressed in a compressor, and flows downstream to a condenser, where heat is typically rejected from the refrigerant to ambient environment, during heat transfer interaction with this ambient environment. Then refrigerant flows through an expansion device, where it is expanded to a lower pressure and temperature, and to an evaporator, where during heat transfer interaction with a secondary fluid (e.g., indoor air), the refrigerant is evaporated and typically superheated, while cooling and often dehumidifying this secondary fluid. 
     In recent years, much interest and design effort has been focused on the efficient operation of the heat exchangers (condenser and evaporator) in the refrigerant systems. One relatively recent advancement in the heat exchanger technology is the development and application of parallel flow, or so-called microchannel or minichannel, heat exchangers (these two terms will be used interchangeably throughout the text), as the condensers and evaporators. 
     These heat exchangers are provided with a plurality of parallel heat transfer tubes, typically of a non-round shape, among which refrigerant is distributed and flown in a parallel manner. The heat transfer tubes are orientated generally substantially perpendicular to a refrigerant flow direction in the inlet, intermediate and outlet manifolds that are in flow communication with the heat transfer tubes. The primary reasons for the employment of the parallel flow heat exchangers, which usually have aluminum furnace-brazed construction, are related to their superior performance, high degree of compactness, structural rigidity and enhanced resistance to corrosion. 
     When utilized in condenser applications, these heat exchangers are normally designed for a multi-pass configuration, typically with a plurality of parallel heat transfer tubes within each refrigerant pass, in order to obtain superior performance by balancing and optimizing heat transfer and pressure drop characteristics. In such designs, the refrigerant that enters an inlet manifold (or so-called inlet header) travels through a first multi-tube pass across a width of the condenser to an opposed, typically intermediate, manifold. The refrigerant collected in a first intermediate manifold reverses its direction, is distributed among the heat transfer tubes in the second pass and flows to a second intermediate manifold. This flow pattern can be repeated for a number of times, to achieve optimum condenser performance, until the refrigerant reaches an outlet manifold (or so-called outlet header). Typically, the individual manifolds are of a cylindrical shape (although other shapes are also known in the art) and are represented by different chambers separated by partitions within the same manifold construction assembly. 
     Heat transfer corrugated and typically louvered fins are placed between the heat transfer tubes for outside heat transfer enhancement and construction rigidity. These fins are typically attached to the heat transfer tubes during a furnace braze operation. Furthermore, each heat transfer tube preferably contains a plurality of relatively small parallel channels for in-tube heat transfer augmentation and structural rigidity. 
     However, there have been some obstacles to the use of the parallel flow heat exchangers in a refrigerant system. In particular, a problem, known as refrigerant maldistribution, typically occurs in the microchannel heat exchanger manifolds when the two-phase flow enters the manifold. A vapor phase of the two-phase flow has significantly different properties, moves at different velocities and is subjected to different effects of internal and external forces than a liquid phase. This causes the vapor phase to separate from the liquid phase and flow independently. The separation of the vapor phase from the liquid phase has raised challenges, such as refrigerant maldistribution in parallel flow heat exchangers. This phenomenon takes place due to unequal pressure drop inside the channels and in the inlet and outlet manifolds, as well as poor manifold and distribution system design. In the manifolds, the difference in length of refrigerant paths, phase separation and gravity are the primary factors responsible for maldistribution. Inside the heat exchanger channels, variations in the heat transfer rate, airflow distribution, manufacturing tolerances, and gravity are the dominant factors. Furthermore, a recent trend of heat exchanger performance enhancement promoted miniaturization of its channels, which in turn negatively impacted refrigerant distribution. Since it is extremely difficult to control all these factors, along with the complexity and inefficiency of the proposed techniques or prohibitively high cost of the solutions, many of the previous attempts to manage refrigerant distribution, have failed. 
     On the other hand, refrigerant maldistribution may causes significant heat exchanger and overall system performance degradation over a wide range of operating conditions. Therefore, it would be desirable to reduce or eliminate refrigerant maldistribution in parallel flow heat exchangers. 
     SUMMARY OF THE INVENTION 
     In a disclosed embodiment of this invention, refrigerant vapor is tapped from an upstream location, and directed into a location in a parallel flow heat exchanger intermediate manifold where two-phase refrigerant flow is present, and a liquid phase is likely to separate from a vapor phase and accumulate, causing refrigerant maldistribution in the downstream heat transfer tubes that are in fluid communication with this intermediate manifold. The refrigerant vapor from an upstream location has a higher velocity and enough momentum to create predominantly homogeneous flow conditions, while mixing, atomizing and redistributing the initially separated two-phase refrigerant in the intermediate manifold. 
     In one embodiment the vapor refrigerant is tapped from a line connecting a compressor to the parallel flow heat exchanger. 
     In another embodiment, the predominantly vapor or homogeneous two-phase refrigerant is tapped from a location in an upstream manifold and redirected to a location in a downstream manifold. 
     In further features, the flow of the refrigerant vapor may be pulsed or periodically modulated to enhance the refrigerant distribution effects. Also, multiple taps may be utilized to tap a portion of refrigerant from the same manifold and redirect it to different downstream manifolds. On the other hand, a portion of refrigerant from different upstream manifolds may be delivered to the same downstream manifold. 
     Furthermore, the disclosed invention can be implemented in parallel flow heat exchanger installations functioning as condensers as well as evaporators. 
     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  shows a refrigerant system incorporating the present invention. 
         FIG. 2A  is a first schematic of a heat exchanger incorporating the present invention. 
         FIG. 2B  shows a cross-sectional view of a heat exchanger tube. 
         FIG. 3A  is a second schematic of a heat exchanger incorporating the present invention. 
         FIG. 3B  shows another schematic. 
         FIG. 4  shows yet another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A basic refrigerant system  20  is illustrated in  FIG. 1  and includes a compressor  22  delivering refrigerant into a discharge line  23  heading to a condenser  24 . The condenser  24  is a parallel flow heat exchanger, and in one disclosed embodiment is a microchannel heat exchanger. The heat is transferred in the condenser  24  from the refrigerant to a secondary loop fluid, such as air. The high pressure, but desuperheated, condensed and typically cooled, refrigerant passes into a liquid line  25  downstream of the condenser  24  and through an expansion device  26 , where it is expanded to a lower pressure and temperature. Downstream of the expansion device  26 , refrigerant flows through an evaporator  28  and back to the compressor  22 . Although a basic refrigerant system  20  is shown in  FIG. 1 , it is well understood by a person ordinarily skilled in the art that many options and features may be incorporated into a refrigerant system design. All these refrigerant system configurations are well within the scope and can equally benefit from the invention. 
     As shown in  FIG. 2A , the condenser  24  has a manifold structure  30  that consists of multiple chambers  30 A,  30 B and  30 C. An inlet manifold chamber  30 A receives the refrigerant, typically in a vapor phase, from the discharge line  23 . The refrigerant flows into a first bank of parallel heat transfer tubes  32 , and then across the condenser core to a chamber  34 A of an intermediate manifold structure  34 . It should be noted that in practice there may be more or less refrigerant passes than the four illustrated passes  32 ,  36 ,  38 , and  40 . Further, it should be understood that, although for simplicity purposes each refrigerant pass is represented by a single heat transfer tube, typically there are many heat transfer tubes within each pass amongst which refrigerant is distributed while flowing within the pass, and, in the condenser applications, a number of the heat transfer tubes within each bank typically decreases in a downstream direction with respect to a refrigerant flow. For instance, there could be 12 heat transfer tubes in the first bank, 8 heat transfer tubes in a second bank, 5 heat transfer tubes in a third bank and only 2 heat transfer tubes in the last forth bank. A separator plate  42  is placed within the manifold  34  to separate the chamber  34 A from a chamber  34 B positioned within the same manifold structure  34 . 
     As shown in the  FIG. 2A  at this location, the refrigerant is starting to condense while flowing through the first pass along the tubes  32  (due to heat transfer interaction with a secondary fluid) and is in a two-phase thermodynamic state, although typically with a relatively small liquid amount in a two-phase mixture. Also, at this location, liquid phase may be starting to separate from the vapor refrigerant, as shown by  35 , since liquid and vapor phases have different thermophysical properties and are affected differently by external forces such as gravity and momentum sheer. Separation of liquid and vapor phases may create maldistribution conditions, while the refrigerant flows from a chamber  34 A of the intermediate manifold structure  34  back across the core of the condenser  24  through a second bank of parallel heat transfer tubes  36  into a chamber  30 B of the manifold structure  30 . 
     Since, in many cases, a somewhat insignificant amount of liquid refrigerant is accumulated within the chamber  34 A, refrigerant maldistribution does not have a profound effect on the performance of the condenser  24  yet, and no special measures may be required (although, in some cases, special design provisions may be implemented). The refrigerant in the second bank of heat transfer tubes  36  is flowing in generally parallel (although counterflow) direction to the refrigerant flow in the first bank of heat transfer tubes  32 . As shown in the  FIG. 2A , a separator plate  42  prevents refrigerant mixing or direct flow communication between the manifold chambers  30 A and  30 B. In the chamber  30 B, the refrigerant is also in a two-phase thermodynamic state but containing lower vapor quality and potentially promoting the conditions for liquid refrigerant accumulation, as shown at  144 , at the bottom of the chamber  30 B. 
     In such circumstances, vapor refrigerant will predominantly flow into the upper portion of the heat transfer tubes of the third pass  38  with liquid refrigerant flowing through the lower portion of the third bank  38  of heat transfer tubes. Therefore, refrigerant maldistribution may have a profound effect on performance of the condenser  24 . 
     The refrigerant flows from the intermediate chamber  30 B of the manifold structure  30  into a third bank of parallel heat transfer tubes  38  generally positioned in parallel arrangement to the first and second banks of heat transfer tubes  32  and  36 , across the condenser  24  and into an intermediate chamber  34 B of the manifold structure  34 . The liquid refrigerant level in the manifold chamber  34 B, as shown at  244 , is even higher than in the chambers  34 A and  30 B. 
     The refrigerant flowing through the chamber  34 B has even lower vapor quality and potentially creating similar maldistribution conditions for the fourth (and last) bank of heat transfer tubes  40 . Again, a separator plate  42  positioned between the chambers  30 B and  30 C ensures the refrigerant flow in the desired downstream direction without short-circuiting or bypass. From the chamber  30 C, the liquid refrigerant exits condenser  24  through the liquid line  25 . As known, corrugated, and typically louvered, fins  33  are located between and attached to the heat transfer tubes (typically during a furnace brazing process) to extend the heat transfer surface and improve structural rigidity of the condenser  24 . 
     As shown in  FIG. 2B , the heat transfer tubes within the tube banks  32 ,  36 ,  38 , and  40  may consist of a plurality of parallel channels  100  separated by walls  101 . The  FIG. 2B  is cross-sectional view of the heat transfer tubes shown in  FIG. 2A . The channels  100  allow for enhanced heat transfer characteristics and assist in improved structural rigidity. The cross-section of the channels  100  may take different forms, and although illustrated as a rectangular in  FIG. 2B , may be, for instance, of triangular, trapezoidal or circular configurations. 
     In the present invention, refrigerant is tapped from the discharge line  23  into a line  46  and directed to a location  47 , that may or may not be directly associated with the separator plate  42  dividing the chambers  30 B and  30 C, where a significant amount of accumulated liquid refrigerant  144  is expected (e.g., due to separation under gravity force). This high pressure compressed refrigerant vapor will tend to mix (creating more homogeneous conditions) and redistribute the liquid refrigerant phase amongst the third bank of the heat transfer tubes  38  in more uniform manner. 
     Similarly, another line  48  may be directed to a location  49 , providing favorable conditions for more uniform distribution of the liquid refrigerant phase  244  within the manifold chamber  34 B and amongst the forth bank of the heat transfer tubes  40 . Valves  50  associated with a control  10  may be placed on the lines  46  and/or  48  to allow the flow of this discharge gas to be pulsed, modulated or completely shutdown. In this manner, a refrigerant system designer can achieve precise control over the desired amount of bypassed high pressure refrigerant vapor, which can be tailored, for instance, to specific operating conditions, to provide uniform distribution of liquid and vapor refrigerant phases amongst the heat transfer tubes. 
     It should be understood that the liquid levels  35 ,  144  and  244  may be somewhat exaggerated to illustrate the concept of the present invention as well as may vary with operating and environmental conditions. 
     Also, as shown in  FIG. 2A , perforated screen plates  44 , may be utilized in conjunctions with the bypass lines  46  and  48  and placed within the manifold chambers  30 B and  34 B to prevent droplets of liquid interfering with the refrigerant flow exiting an upstream bank of heat transfer tubes. Therefore, performance degradation of the condenser coil  24  due to refrigerant maldistribution will be minimal or entirely eliminated. 
       FIG. 3A  shows another embodiment  124  wherein the parallel flow heat exchanger construction is similar to the heat exchanger shown in  FIG. 2A . However, a portion of the refrigerant vapor is tapped at a point  136  from a location in the chamber  34 A of the intermediate manifold structure  34  upstream of a point  138  in the chamber  30 B of the manifold structure  30 , where a small portion of the refrigerant vapor is redirected from the chamber  34 A to the chamber  30 B to improve refrigerant distribution in the chamber  30 B and amongst the heat transfer tubes in the bank  38 . Similarly, a small portion of the refrigerant vapor tapped from a point  140  in the chamber  30 B of the manifold structure  30  can be utilized to improve distribution in the chamber  34 C and the heat transfer tubes in the bank  40 , and is directed to a point  142  within the chamber  34 C. 
     The multiple taps in  FIGS. 2A and 3A  deliver a small portion of predominantly vapor refrigerant to different locations within the condenser.  FIG. 3B  shows separate taps  346  and  348 , which deliver still relatively small amounts of predominantly vapor refrigerant form separate locations within the condenser to a common location  350 , such as one of the intermediate manifold chambers, having certain amount of accumulated liquid refrigerant  344 , in order to assist in uniform distribution of this liquid refrigerant among the heat transfer tubes fluidly connected to this manifold chamber and positioned downstream in relation to refrigerant flow. Similarly, the small amounts of predominantly vapor refrigerant may be delivered from the same upstream location to different downstream locations to improve distribution of two-phase refrigerant at those downstream locations. 
       FIG. 4  shows yet another embodiment  220 , where there is no refrigerant re-routing is taking place, and instead the mixing between the vapor and liquid phases is accomplished by pulsing the main refrigerant flow through the parallel flow heat exchanger. The pulsing of the main flow is accomplished by periodically changing the size of the opening of the flow control device, such as electronically controlled expansion valve  226 . When the refrigerant flow through the expansion valve  226  is throttled (the opening of the valve is decreased in size), pressure in the condenser  224  is built up, and when the expansion valve  226  is opened wider, the pressure in the condenser  224  is reduced. The varying pressure in the condenser  224  will result in fluctuating refrigerant velocities in the condenser, which in turn will enhance the uniform refrigerant distribution effects by providing mixing of liquid and vapor phases. 
     The pulsing of the main refrigerant can also be accomplished by using, for example, a flow control device installed between the evaporator and compressor. In this case, the function of such flow control device can be combined with a function of so-called suction modulation valve (SMV)  228  that is often installed in refrigeration units to selectively reduce the unit capacity by throttling the flow at the compressor suction to control the amount of refrigerant reaching the compressor. A smaller amount of opening through the SMV valve allows less refrigerant to be delivered to the compressor. The SMV  228  can be rapidly cycled (opened and closed) to generate pulses of refrigerant through the condenser  224 , with the pulsing refrigerant flow in turn enhancing the mixing of liquid and vapor refrigerant phases in the condenser  224  in a similar fashion as it was accomplished by the electronic expansion valve  226 . Both, an electronic expansion valve and a suction modulation valve, can be utilized individually or in combination with each other and controlled by a controller  200  that would selectively open and close these valves to enhance the mixing of the vapor and liquid refrigerant phases. The suction modulation valve  228  can be substituted, for example, by a solenoid valve which would cycle between open and closed position (some limited amount of flow still might be permitted through the valve in its closed position to prevent compressor suction approaching deep vacuum). Further, it has to be understood that other location for such flow control devices are feasible within the refrigerant system. Analogously, for instance, a valve located on the discharge refrigerant line or liquid refrigerant line can perform the same function and may be controlled in a similar manner. 
     In summary, the present invention utilizes a small portion of predominantly vapor refrigerant from an upstream location, such as a discharge line or upstream manifold, and redirects this refrigerant to a location within a parallel flow heat exchanger, such as an intermediate manifold, downstream along the refrigerant path, where the vapor and liquid phase separation is likely to occur. This high pressure vapor refrigerant allows for better mixing and promotes homogeneous conditions for a two-phase refrigerant, such that maldistribution is appreciably reduced or eliminated for a refrigerant entering a downstream bank of heat transfer tubes positioned generally in a parallel arrangement. 
     While the main focus of the invention is on the condenser applications, refrigerant system evaporators can also benefit from the invention. In the case of an evaporator, a small portion of refrigerant vapor would be redirected to an inlet or intermediate manifolds from any number of a higher pressure locations within the refrigerant system, such as a discharge line, condenser manifolds, etc. The flow pulsing, though illustrated for the condenser heat exchangers, can be used in a similar fashion as described above to enhance refrigerant distribution in the evaporator heat exchangers. While the invention is disclosed for parallel flow heat exchangers, it does have applications for other heat exchanger types, for instance, for the heat exchangers having intermediate manifolds in the condenser applications. Also, the four-pass heat exchangers of  FIGS. 2A and 3A  are purely exemplary, and a heat exchanger with any number of passes can equally benefit from the present invention. Also, the manifold constructions  30  and  34  encompassing a number of chambers may have many different design shapes and configurations. Also, the manifold chambers may not necessarily be positioned within the same manifold construction. Lastly, the separator plates  42  can be replaced by check valves or solenoid valves. 
     Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in the 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.