Abstract:
Various control methods are disclosed for removing moisture from the external surfaces of an evaporator in a refrigerant system to avoid moisture entering a conditioned space. In one embodiment, the evaporator fan is driven in a reverse direction, and the air is guided to the outdoor environment. In other embodiments, a supplemental exhaust fan is utilized in conjunction with the evaporator fan. Also, a reheat circuit, hot gas bypass circuit, or specific features of a heat pump unit may be utilized to more efficiently perform the moisture removal.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This application relates to the control of a refrigerant system, and in particular, to the control of indoor fan operation to prevent moisture being re-evaporated from evaporator external surfaces and then being delivered by indoor airflow into a conditioned environment, when a refrigerant compressor is shut down or during system startup. 
         [0002]    Refrigerant systems are utilized to condition the air being delivered into an indoor environment. As an example, an air conditioning system or a heat pump is utilized to cool and dehumidify or heat air being delivered into the environment to be conditioned. 
         [0003]    In recent years, significant attention has been paid to indoor air quality issues. In particular, precise control of the indoor relative humidity within the comfort zone has been the subject of an increased scrutiny. In part, this desired humidity control is attributed to prevention of mold, bacteria and fungus formation and growth. 
         [0004]    As known, refrigerant systems operate at part-load conditions for most of their design life. Thus, the system operates in a start-stop mode quite frequently to satisfy the demanded sensible and latent capacity requirements, when all other means of system unloading are already exhausted. When the system is operating in a cooling mode, an evaporator that cools and dehumidifies the air being delivered into the indoor environment has cold external surfaces. Moisture forms on the cold external surfaces of the evaporator heat exchanger, while the cooled and dehumidified air flows through the heat exchanger and into the conditioned space. This moisture is removed from the air stream and continuously drained into a drain pan. When the system is shut down, there is often a significant amount of moisture accumulated on the evaporator external surfaces. As the evaporator is gradually warming up, this moisture re-evaporates and is re-introduced into the indoor airstream and consequently into the conditioned environment, since in many application cases, the indoor fan has to operate continuously to comply with legislation and regulation requirements. 
         [0005]    Even with the indoor fan shut down simultaneously with other system components, such as a compressor, at system startup, a burst of moist air will often be supplied to the indoor environment causing undesired high humidity fluctuations and consequent occupant discomfort. Additionally, this moisture accumulated on the evaporator external surfaces will promote mold, bacteria and fungus formation and growth. It has become an industry practice to treat external evaporator surfaces with anti-microbial compounds, or employ UV lights to prevent growth of microorganisms. These measures are associated with design complexities and additional costs. 
         [0006]    Thus, it would be desirable to provide a solution to the problems mentioned above that does not have the drawbacks of the prior art. 
       SUMMARY OF THE INVENTION 
       [0007]    In a disclosed embodiment of this invention, a motor for driving the fan that blows air over the evaporator has a rotation direction reversal feature. Many of three-phase motors are already capable of phase reversal (when the phases are reversed the motor turns in the opposite direction). At a compressor shutdown, the fan is run in reverse for a short period of time, and air flows over the evaporator in an opposite direction. As the moisture is driven off the evaporator external surfaces, this moisture-loaded air is preferably disposed into the outdoor environment. In one embodiment, an airside economizer controlling the appropriate percentages of air mixture from a return duct and from an outdoor environment closes off the flow from the return duct. All of the air that removes the moisture from the gradually warming evaporator is thus delivered to the outside environment. Heat generated by the indoor fan assists in faster moisture re-evaporation and removal from external evaporator surfaces. 
         [0008]    In a second embodiment, a supplemental exhaust fan, which in many cases is already incorporated into the system design, assists the main indoor fan in driving air over the evaporator coil in the reverse direction, while fresh air intake may be closed. It has to be noted that, in this embodiment, the return duct may be blocked by a damper and the indoor fan may be shut down completely. In the latter case, the indoor fan does not need to be equipped with the rotation direction reversal feature. 
         [0009]    In yet another embodiment, a system equipped with a variable volume temperature (VVT) feature, and having a bypass duct, may utilize the main indoor fan and the exhaust fan to flow air over the evaporator in forward direction to remove moisture. The air would then flow through the bypass duct and then to the outdoor environment. In this embodiment, the air may be repeatedly recycled through the evaporator for a short period of time by the main indoor fan and, when a majority of moisture is removed from the evaporator and accumulated in the re-circulating air, the exhaust fan is turned on for a brief period of time to dump this moist air to the outdoor environment. In this embodiment, the main indoor fan does not have to be equipped with the rotation direction reversal feature as well. 
         [0010]    In yet another embodiment, the refrigerant system has a reheat circuit, which is selectively run for a short period of time before the shutdown. In this case, not only indoor fan heat but also the heat from the reheat coil can be utilized to promote faster moisture re-evaporation and removal from the evaporator external surfaces. Analogously, if the refrigerant system is a heat pump, it can be run in a heating mode for a short period of time during the moisture removal process described above. Further, a hot gas by-pass circuit, as known in the industry, can be employed to bypass high pressure refrigerant from the compressor discharge region into the evaporator inlet. In this case, the hot gas bypass circuit can be utilized to assist in moisture re-evaporation and removal by providing additional preheating. 
         [0011]    In all embodiments, the moisture removal process can be terminated by a timer or by a sensor such as a humidity sensor, a dew point sensor, a sensor measuring pressure drop across the evaporator, an evaporator surface temperature sensor, an air temperature sensor or an enthalpy sensor. In all cases, the system resumes normal operation after moisture removal is completed, either in an active cooling mode or in air circulation mode. 
         [0012]    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 
         [0013]      FIG. 1  is a schematic view of the system incorporating the present invention. 
           [0014]      FIG. 2  shows the control operation of the present invention. 
           [0015]      FIG. 3  shows another embodiment. 
           [0016]      FIG. 4  shows yet another embodiment. 
           [0017]      FIG. 5  shows yet another embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    A refrigerant system  20  is illustrated in  FIG. 1 , and serves to provide conditioned air to an environment  22 , such as a building. A thermostat  24  within the building allows a user to demand a particular temperature level as known. A control for the refrigerant system  20  thus operates the refrigerant system to achieve the demanded conditions. A closed-loop refrigerant circuit  26  includes a compressor  28  compressing refrigerant and delivering it to an outdoor heat exchanger or condenser  30 . From the condenser, the refrigerant passes through an expansion device  32 , and then to an indoor heat exchanger or evaporator  34 . An indoor fan  36  is associated with the evaporator  34 , and drives air over the evaporator  34 . As is known, a return duct  38  serves as a conduit for air delivered by the fan  36  from the indoor space  22 , and over the evaporator  34  to be conditioned. This air is then delivered to a supply duct  40  to be returned into the conditioned space  22 . An airside economizer  44  allows appropriate mixture amounts of outside air from an outdoor opening  42  and re-circulated indoor air from the return duct  38  to be delivered over the evaporator  34 . As is known, the economizer  44  is also controlled by the control for the refrigerant system  26  to comply with specified requirements. 
         [0019]    As mentioned above, when the cooling demands within the conditioned space  22  are met and all available means of system capacity unloading are exhausted, the refrigerant system operates in a start-stop mode. During shutdown periods, moisture accumulated on the evaporator  34  external surfaces re-evaporates into the airstream and makes its way into the conditioned space, which, as mentioned above, is undesirable. 
         [0020]    One embodiment of the present invention is illustrated in  FIG. 2 . As shown in  FIG. 2 , the airside economizer  44  is moved to a position where the airflow through the return duct  38  is blocked and airflow to the outdoor opening  42  is opened. The motor for the fan  36  is a reversible fan motor. For a short period of time, the motor is driven in the reverse direction to the flow of  FIG. 1 , and air is pulled through the supply duct  40  and over the evaporator  34 . This air removes moisture from the evaporator  34  external surfaces and is disposed into an outdoor environment through the outdoor opening  42 . The operation in this manner removes the moisture at the refrigerant system compressor shutdowns. Heat generated by the indoor fan assists in faster moisture re-evaporation and removal from external evaporator surfaces. Preferably, such a step is taken soon after the shutdown, in case of continuous air circulation requirement, or before the next startup. This operation should continue for as long as certain criteria for the moisture removal are satisfied. Such criteria for the moisture removal process termination can be associated with a timer or a sensor such as a humidity sensor, a dew point sensor, a sensor measuring pressure drop across the evaporator, an evaporator surface temperature sensor, an air temperature sensor or an enthalpy sensor. The system resumes normal operation after moisture removal is completed, either in an active cooling mode (when a call is issued by a thermostat) or in an air circulation mode. 
         [0021]      FIG. 3  shows another embodiment, wherein a supplemental exhaust fan  48  associated with the return duct  38 , and in many cases already incorporated into the system design, assists the main indoor fan  36  in driving air over the evaporator in the reverse direction, while the fresh air intake may be closed. Further, if desired, the return duct  38  may be blocked by a damper, and the main indoor fan  36  may be shut down completely. In the latter case, the main indoor fan  36  does not need to be equipped with the rotation direction reversal feature. 
         [0022]      FIG. 4  shows another embodiment wherein the refrigerant system  20  is equipped with a variable volume temperature (VVT) feature and there is a bypass duct  52  between the return duct  38  and the supply duct  40 . A damper  50  associated with the supply duct  40  is closed and a damper  54  associated with the return duct  38  is closed as well. The main indoor fan  36  is operated in the conventional forward,  FIG. 1  direction and does not need to be reversible. When operated, the supplemental exhaust fan  48  receives the airflow from the bypass duct  52 , and delivers that air to the outdoor environment. The main indoor fan  36 , operating in a forward direction, drives air over the evaporator  34  external surfaces to remove the accumulated moisture. In this embodiment, the air is repeatedly recycled through the evaporator for a short period of time by the main indoor fan  36  and, when a majority of moisture is removed from the evaporator  34  and accumulated in the re-circulating air, the exhaust fan is turned on, for a brief period of time, to dump this moist air to the outdoor environment. During such communication with the outdoor environment, the main indoor fan  36  may not need to be operating. 
         [0023]      FIG. 5  shows another embodiment  60 . Embodiment  60  is similar to the  FIG. 2  embodiment, however, a reheat circuit is incorporated in the refrigerant system design. As known, for example, a three-way valve  62  would selectively bypass refrigerant to a reheat coil  61 , and return the refrigerant to a point  64  in the main refrigerant circuit. Reheat circuits can tap and return at least a portion of refrigerant to any number of locations within a main refrigerant circuit, and the disclosed locations are merely shown as one example. As known, reheat circuits typically serve to reheat the indoor air downstream of the evaporator (where the air was cooled and dehumidified), in case there is a dehumidification demand (humidistat call) and no significant cooling demand (no thermostat call) in the conditioned space. However, in this invention, the reheat coil  61  serves to further facilitate moisture removal process from external surfaces of the evaporator  34 . In the embodiment  60 , before the refrigerant compressor is shutdown, the refrigerant system is operated in the reheat mode, for a short period of time, to allow the reheat coil to warm up to its conventional operating temperature. When the refrigerant compressor  28  is shutdown and the indoor fan  36  is operated in reverse, not only the indoor fan heat but also the heat from the reheat coil  61  is utilized to warm up air flowing over the evaporator  34  to promote faster moisture re-evaporation and removal. 
         [0024]    Analogously, if the refrigerant system is a heat pump, it can be run in a heating mode, for a short period of time, during moisture removal process to allow the indoor heat exchanger (serving as a condenser in the heating mode of operation) to warm up and facilitate the moisture removal process during indoor airflow reversal, as described above. It has to be noted that the refrigerant system can be operated in a heating mode, for a short period of time, prior to the refrigerant compressor shutdown with the indoor fan  36  turned off. This allows the indoor heat exchanger to warm up faster. When the desired temperature is reached, the indoor fan is operated in reverse, as described above, during the moisture removal process. In the same manner, hot gas bypass to the evaporator inlet can be utilized to assist in moisture re-evaporation and removal. 
         [0025]    It is understood that although single-circuit configurations have been disclosed, the benefits of the invention are applicable to multi-circuit system arrangements. 
         [0026]    Although preferred embodiments of this invention have 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.