Abstract:
A heating system includes a refrigerant boiler including a heat source for heating a refrigerant from a liquid state to a vapor state, a boiler outlet and a boiler inlet; a heat exchanger in fluid communication with the refrigerant boiler, the heat exchanger including a upper manifold having a heat exchanger inlet coupled to the boiler outlet, a lower manifold having a heat exchanger outlet coupled to the boiler inlet and a plurality of tubes connecting the upper manifold and the lower manifold, wherein refrigerant passes from the upper manifold to the lower manifold via gravity; and a fan moving air over the heat exchanger to define supply air for a space to be heated.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a non-provisional patent application which claims the benefit of U.S. provisional patent application 61/561,309 filed Nov. 18, 2011, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]    Embodiments of the invention relate generally to air conditioning systems, and in particular to an air heating system using a refrigerant boiler. 
         [0003]    Packaged rooftop air conditioning systems are used in the art for air conditioning (e.g., heating or cooling) of a building. Existing gas heat technology in use for most packaged equipment utilizes tubular gas heat exchangers with an induced draft combustion system. One downside of such designs is that the heat exchangers must be located on the discharge side of the fan, are very sensitive to airflow and system configuration changes and very expensive and time consuming to qualify. The combustion module also requires significant space that results in larger unit sizes than required for the electric heat option. For outdoor weatherized applications, the technology is currently limited to non-condensing furnaces (&lt;81% efficiency) due to added air side pressure drop, corrosion issues and disposal of the condensate. In current packaged rooftops, a direct gas heat exchanger system is used where gas is burned inside a tubular or similar heat exchanger located in the indoor airflow leaving the supply fan. The designs are very cost effective, but once again, are very time-consuming to qualify and require extensive testing for each unit size and airflow configuration. As such, improvements in air heating systems would be well received in the art. 
       BRIEF DESCRIPTION OF THE INVENTION  
       [0004]    According to an exemplary embodiment of the present invention a heating system includes a refrigerant boiler including a heat source for heating a refrigerant from a liquid state to a vapor state, a boiler outlet and a boiler inlet; a heat exchanger in fluid communication with the refrigerant boiler, the heat exchanger including a upper manifold having a heat exchanger inlet coupled to the boiler outlet, a lower manifold having a heat exchanger outlet coupled to the boiler inlet and a plurality of tubes connecting the upper manifold and the lower manifold, wherein refrigerant passes from the upper manifold to the lower manifold via gravity; and a fan moving air over the heat exchanger to define supply air for a space to be heated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0005]    The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0006]      FIGS. 1-8  depict heating systems in exemplary embodiments; and 
           [0007]      FIGS. 9-11  depict heat exchangers in exemplary embodiments. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0008]      FIG. 1  depicts a heating system  10  in an exemplary embodiment. Heating system  10  may be used as part of a packaged rooftop air conditioning system. Heating system  10  includes a boiler  12  for boiling a refrigerant to change state of the refrigerant from liquid to gas. The refrigerant used may be any known type refrigerant, such as R134a. Boiler  12  may utilize a gas heater (e.g., inshot burner), electric heater, infrared heater, etc. to apply heat to a refrigerant in a coil within boiler  12 . Boiler  12  heats the refrigerant from a liquid to a vapor. Pressure created by the boiling refrigerant and thermo-siphon action are used to circulate the vapor refrigerant from a boiler outlet through a check valve  14  to a heat exchanger  16 , also referred to as a condenser. At heat exchanger  16 , the vapor refrigerant condenses and heat is released. Return air (which may include a mix of outside air) is directed over the heat exchanger  16  by a fan  18 . Air passing over heat exchanger  16  is heated and provided as supply air to a spaced to be heated. Liquid refrigerant from heat exchanger  16  flows through a pressure regulator  20  back to an inlet of boiler  12 , to continue the cycle. As discussed in further detail herein, heat exchanger  16  is a vertically mounted, gravity operated heat exchanger allowing refrigerant to flow back to boiler  12  via gravity. The system configuration in  FIG. 1  allows the heat exchanger  16  to be located in any position in the unit and allows for more creative and compact designs. System  10  may use 2L class semi-flammable refrigerants, due to the relatively lower temperatures used to boil the refrigerant in boiler  12 . 
         [0009]      FIG. 2  depicts a heating system in an alternate embodiment. Typical refrigerant boiler installations rely on a dedicated pump to move refrigerant through the system. As described above, boiler  12  changes refrigerant from a liquid state to a vapor state. The vapor refrigerant is provided to heat exchanger  16  to condense and release heat. The embodiment of  FIG. 2  uses valves  24  and  26  to control the flow of refrigerant through the system. Valves  24  and  26  may be solenoid valves opened and closed under commands from controller  28 . In an initial state, valves  24  and  26  are both closed, which traps the refrigerant in boiler  12 . As more heat is added to the trapped refrigerant by combustion, refrigerant temperature and pressure increase. Controller  28  monitors pressure and/or temperature in boiler  12  via sensors. When the refrigerant pressure in the trapped volume has increased to a specified value, the downstream valve  26  is opened and high pressure refrigerant is expelled into the system propelling the refrigerant toward the heat exchanger  16 . Shortly after the downstream valve  26  is opened, controller  28  opens upstream valve  24  to let the refrigerant be returned back into boiler  12 . After both valves  24  and  26  are open for a predetermined period of time, controller  28  closes both valves  24  and  26  and the cycle repeats. The timing of opening/closing of the valves can be controlled based on temperature and/or pressure measurements in the boiler  12 . This timing can be set at the predetermined interval at the factory or it can be adjusted in the field based on the operating conditions. The embodiment of  FIG. 2  eliminates the need for a dedicated pump to pump the refrigerant through the system. 
         [0010]      FIG. 3  depicts a heating system in an alternate embodiment. One challenge in operation of refrigerant boiler systems is control of the system refrigerant charge. It is known that the amount of refrigerant needed for most efficient system operation varies with respect to the refrigerant boiler operating condition. If there is too little refrigerant in the system, then the system may not perform efficiently because there is not enough refrigerant circulating through the system to provide an effective level of heating. If there is too much refrigerant in the system, then significant parasitic flow pressure losses might be present, causing the system performance to deteriorate. Since different operating conditions require different amounts of refrigerant for the most efficient operation, it is beneficial to adjust the amount of the circulating refrigerant based on the operating condition. Further, the required heating capacity of a heating system varies appreciably and strongly depends on environmental and operational conditions as well as heating demands in the climate-controlled space. Therefore, the refrigerant charge in the heating closed-loop circuit of the system needs to be adjusted accordingly. 
         [0011]    The embodiment of  FIG. 3  includes an accumulator  32  to manage the refrigerant charge. Accumulator  32  is positioned, for instance, between the outlet of heat exchanger  16  and the inlet of boiler  12 . A check valve  30  is positioned upstream of accumulator  32  so that accumulator  32  is positioned on the low-pressure side of the refrigerant path. Accumulator  32  may be located in other positions, such as on a branch line and valved on and off when it is required. 
         [0012]      FIG. 4  depicts a heating system in an alternate embodiment for managing refrigerant charge. The system of  FIG. 4  includes a receiver  36  located between the outlet of heat exchanger  16  and the inlet of boiler  12 . A check valve  38  is positioned downstream of the receiver  36 , so that receiver  36  is positioned on the high-pressure side of the refrigerant path. If the system has too much refrigerant, then the excess refrigerant would be stored in receiver  36  and not be circulated through the system. Since the excess refrigerant is stored in receiver  36 , then the refrigerant boiler system can be operated more efficiently without experiencing extra parasitic pressure losses. The size of receiver  36  can be selected based on the maximum variations of the circulating refrigerant in the system. 
         [0013]      FIG. 5  depicts a heating system in an alternate embodiment. It is desirable to improve efficiency of the refrigerant boiler, especially since the flue gas exiting the refrigerant boiler still has high temperature, and its heating potential is essentially wasted. It is known from the gas furnace experience that a condensing furnace would have a much higher efficiency. The embodiment of  FIG. 5  uses a boiler  42  having two heat exchanger sections  44  and  46  arranged in a counterflow manner, with respect to the flue gas flow, shown by arrows labeled X. A first heat exchanger section  44  is positioned closer to a burner  52  and second heat exchanger section  46  is positioned farther from the burner  52  than first heat exchanger section  44 . The second heat exchanger section  46  serves as a condensation section of the heat exchanger, where condensation from the flue gas forms on the second heat exchanger section  46 . A tray  48  is used to collect condensation and a condensation drain  50  directs a flue gas condensate from tray  48  away from the unit. 
         [0014]      FIG. 6  is an alternate version of the embodiment of  FIG. 5 , in which the first heat exchanger section  44  and second heat exchanger section  46  are represented by two separate heat exchangers. In this embodiment, the efficiency of the refrigerant boiler  42  can be improved even further by placing a liquid-vapor separator  54  in between the two heat exchanger sections  46  and  44 . The upper portion of the liquid-vapor separator  54  (i.e., the part containing vapor) is coupled to the refrigerant path downstream of refrigerant boiler  42  which is coupled to the inlet of heat exchanger  16 . The lower portion of the liquid-vapor separator  54  (i.e., the part containing liquid) is coupled to an inlet of the first heat exchanger section  44 . Condensate drain  50  directs a flue gas condensate from tray  48  in second heat exchanger section  46  away from the unit. 
         [0015]      FIG. 7  depicts a heating system in an alternate embodiment. One phenomenon associated with refrigerant boiler  12  is referred to as cold blow. Cold blow occurs when the mass flow of air blowing over the heat exchanger  16  is excessively high, which results in less than desirable preheating of the air as it passes over the condenser coils. However, if the amount of air blowing over the condenser is too low, then there is not enough heating capacity generated to heat the environment. Therefore, the refrigerant boiler design should prevent cold blow while at the same time delivering a sufficient amount of heated air. 
         [0016]    The embodiment of  FIG. 7  addresses the effects of cold blow through the use of a variable speed condenser fan  62  controlled by controller  64 . If it is determined that the cold blow is present, then fan  62  is slowed down to increase the amount of the air as it passes over the coil of condenser  16 . Fan  62  may be implemented using a variable frequency fan controlled by variable frequency drive (VFD) signal from controller  64 . Alternatively, condenser fan  62  can be a two speed fan. When the cold blow is present, the fan is switched to a lower speed motor operation. The fan speed can be controlled by controller  62  based on the temperature of the air passing over the coil as detected by temperature sensor  66  that provides a temperature signal to controller  64 . If the temperature of the return air is below a certain threshold, then the fan speed is slowed until the temperature reaches the acceptable value.  FIG. 7  also depicts a pump  68  that may be used to circulate refrigerant through the system. 
         [0017]      FIG. 8  depicts a heating system in an alternate embodiment. The embodiment of  FIG. 8  provides control of refrigerant boiler  12  through a number of sensors and a controller  110 . Temperature sensor  112  and pressure sensor  114  monitor temperature and pressure of vapor refrigerant exiting refrigerant boiler  12 , and provide a temperature signal and pressure signal to controller  110 . A refrigerant level sensor  116  senses the level of refrigerant in boiler  12  and provides a refrigerant level signal to controller  110 . Controller  110  controls boiler  12  by controlling heat generated by burner  52  and/or by controlling flue gas fan  118 . 
         [0018]    The output of burner  52  may be controlled in a number of ways. Burner  52  may be a multi-stage burner having a burner stage valve  120  electrically controlled by controller  110 . Controller  110  opens burner stage valve  120  to increase the heat output of burner  52  by effectively adding another burner stage. Conversely, controller  110  closes burner stage valve  120  to decrease heat output of burner  52 . Burner stage valve  120  may also be placed in a position between open and closed, providing variable fuel flow to the additional burner stage. 
         [0019]    Fuel (e.g., gas) flow to burner  52  may also be controlled by metering the flow of fuel to burner  52 . Controller  110  controls a fuel flow control device  122  to affect the flow of fuel to burner  52 . Fuel flow control device  122  is electronically controlled by controller  110 . Fuel flow control device  122  may be a valve that can be opened, closed, or positioned in any number of positions between open and closed. Fuel flow control device  122  may also implement more complex metering functions, such as modulating fuel flow or pulsating fuel flow to burner  52  in response to control signals from controller  110 . 
         [0020]    The flow of flue gas over the heat exchanger in boiler  12  is controlled through flue gas fan  118 . Control of flue gas fan  118  may be implemented in a number of ways. In one embodiment, flue gas fan  118  may be implemented using a variable frequency fan controlled by variable frequency drive (VFD) signal from controller  110 . Alternatively, flue gas fan  118  may be a two speed fan electronically controlled by controller  110 . Alternatively, multiple flue gas fans may be used, with controller  110  turning individual fans on and off to achieve a desired flue gas flow over the heat exchanger in boiler  12 . 
         [0021]    In operation, controller  110  receives the temperature signal, pressure signal and refrigerant level signal from sensors  112 ,  114  and  116 , respectively. Controller  110  then controls the heat at burner  52  and flue gas flow as described above to maintain the temperature and pressure of vapor refrigerant exiting boiler  12  and the refrigerant level in boiler  12  within acceptable operational ranges. 
         [0022]      FIG. 9  depicts condenser  16  in an exemplary embodiment. Condenser  12  includes an inlet  71  to an upper manifold  70  for receiving vapor refrigerant from boiler  12 . The vapor refrigerant flows to a plurality of vertical tubes  72 , condenses in the vertical tubes and travels by gravity down vertical tubes  72 . A lower manifold  74  collects the liquid refrigerant, which flows by gravity through a trap  76  to an outlet  78  and back to the boiler  12 . The vertical tube condenser utilizes gravity to help circulate refrigerant throughout the system. Hence, a circulation pump is not required. The trap  76  insures that the refrigerant flow will be in the preferred direction to maximize refrigerant flow. The trap  76  is located at the exit of the lower manifold  74  and provides a barrier for vapor refrigerant from entering heat exchanger  16  via the lower manifold  74 . This resistance results in forcing the vapor refrigerant to enter through the top manifold  70 , hence resulting in an orderly progression of the refrigerant through heat exchanger  16  as it condenses. 
         [0023]      FIG. 10  depicts a condenser  16  in an alternate embodiment. In the embodiment of  FIG. 10 , the condenser  16  is constructed similar to that in  FIG. 9 , with the exception that a single pipe  80  carries both vapor refrigerant to inlet  71  and liquid refrigerant from outlet  78 . The one-pipe system allows both the refrigerant vapor and liquid condensate to travel in the same pipe, thus eliminating the need for a separate condensate line and a separate vapor line. The liquid refrigerant will generally cling to the pipe walls, thus not interfering with the flow of the vapor refrigerant, which flows in the pipe center. Hence, the system piping can be much simpler, saving material costs, and reducing the likelihood of system leaks. This also eliminates the need for a check valve in the system to manage the refrigerant flow in the correct direction, as there is only one pipe in the system, and hence only one direction for flow. 
         [0024]      FIG. 11  depicts a condenser  16  in an alternate embodiment. In the embodiment of  FIG. 11  the condenser  16  is constructed similar to that in  FIG. 10 , with the exception that the pipe  80  includes an internal tube  90 , that is connected to outlet  78 . The one-pipe system allows both the refrigerant vapor and liquid condensate to travel in the same pipe, thus eliminating the need for a separate condensate line and a separate vapor line. For a portion of the piping  80 , internal tube  90  separates the vapor and fluid flows, thus eliminating any interference the opposing flows may have upon each other. This also eliminates the need for a check valve in the system to manage the refrigerant flow in the correct direction. With the one-pipe system, the vapor and liquid are allowed to flow in the same pipe. However, the liquid will be routed through a separate internal passage within a larger pipe required for the vapor flow. Hence, the flow of the condensate can be better managed as to not interfere with the vapor flow (or vise-versa). This reduces likelihood of system leak. This also eliminates the need for a check valve in the system to manage the refrigerant flow in the correct direction, as there is only one pipe in the system, and hence only one direction for flow. 
         [0025]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.