Abstract:
An air conditioning system having a supply fan for supplying air to a supply duct; a fan motor for driving the supply fan; and a controller for controlling a speed of the fan motor, the controller operating the fan motor at a first speed in a first mode, a second speed lower than the first speed in a second mode and a third speed lower than the second speed in a ventilation mode.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter disclosed herein generally relates to air conditioning systems, and in particular relates to an air conditioning system having a ventilation mode. 
         [0002]    Air conditioning systems are used to provide heating, cooling and ventilation to buildings. As buildings become more insulated due to energy efficiency demands, operating the air conditioning system in a ventilation mode of operation occurs more commonly, to introduce fresh, outdoor air to the building. Improvements in the energy efficiency of air condition system ventilation modes would be well received in the art. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0003]    One embodiment is an air conditioning system having a supply fan for supplying air to a supply duct; a fan motor for driving the supply fan; and a controller for controlling a speed of the fan motor, the controller operating the fan motor at a first speed in a first mode, a second speed lower than the first speed in a second mode and a third speed lower than the second speed in a ventilation mode. 
         [0004]    Another embodiment is a method of controlling an air conditioning system having supply fan for supplying air to a supply duct and a fan motor for driving the supply fan, the method comprising: determining the commanded mode of operation; and controlling a speed of the fan motor, the controlling including operating the fan motor at a first speed in a first mode, operating the fan motor at a second speed lower than the first speed in a second mode and operating the fan motor at a third speed lower than the second speed in a ventilation mode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  depicts an exemplary air conditioning unit. 
           [0006]      FIG. 2  is a flowchart of a control process in an exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0007]    Shown in  FIG. 1  is a typical packaged rooftop air conditioner having a condenser section  11 , an evaporator section  12  and an economizer section  13 . The condenser section  11  includes a compressor  14  for receiving refrigerant vapor from the evaporator section  12  and compressing the vapor before it is condensed. Also included in the condenser section  11  is a condenser coil  16  and a condenser fan  17  for passing ambient air through the condenser coil  16 . 
         [0008]    The evaporator section  12  includes a supply fan  18  which is driven by a fan motor  19 . The fan motor  19  is adapted to operate at variable speeds to meet the cooling/heating requirements of the system and provide ventilation as described in further detail herein. A heater  23  is placed in a downstream position from the supply fan  18 . 
         [0009]    Leading into the evaporator section  12  from the economizer section  13  is a cooling coil  21  and its associated filters  22 . In operation, the supply fan  18  draws air in through the filter  22  and the cooling coil  21  where it is cooled by refrigerant passing through the cooling coil  21 . The cooled air then passes to the supply air duct  24  from which it is distributed within the building. Alternatively, in the heating mode, the air is passed from the supply fan  18  through the heater  23  where it is heated prior to being passed into the supply air duct  24 . 
         [0010]    Included within the economizer section  13  is an outside air intake vent  26 , an exhaust air vent  27  and its associated exhaust fan  28 , and an economizer damper  29 . The economizer damper  29  includes an inlet air damper  30  and a return air damper  35 . The inlet air damper  30  and a return air damper  35  may be linked so that the dampers move in concert. Inlet air damper  30  and a return air damper  35  include an actuator responsive to position commands from controller  32  to adjust the position of the dampers. As described in further detail herein, inlet air damper  30  and a return air damper  35  are adjustable by controller  32  to selectively mix an amount of outside air coming in the outside air intake vent  26  with the portion of the return air that is flowing into the economizer section  13  from the return air duct  31 . Another portion of the return air is caused to pass out the exhaust air vent by the exhaust fan  28 . 
         [0011]    A controller  32  receives commands signals from thermostat  40  and controls fan speed  33  of fan motor  19  and damper position  34  of the inlet air damper  30  and a return air damper  35 . Controller  32  may be a microprocessor-based device that executes computer program code to perform the functions described herein. 
         [0012]    In embodiments of the invention, fan  18  is driven in a ventilation mode to reduce power consumption and still meet desired ventilation thresholds.  FIG. 2  is a flowchart of a control process in an exemplary embodiment. The process begins at  100  with thermostat  40  indicating a mode for operation, and optionally a desired air flow rate. If the mode is high heating or high cooling, at  102  flow proceeds to  104  where controller  32  commands fan motor  19  to run at a first speed. This first speed corresponds to a high air flow mode, which may be 2000 cubic feet per minute (CFM). If the mode is low heating or low cooling, at 106 flow proceeds to 108 where controller  32  commands fan motor  19  to run at a second speed slower than the first speed. This second speed corresponds to a low air flow mode, which may be 1340 cubic feet per minute (CFM). 
         [0013]    If the mode is ventilation, at  110  flow proceeds to  112  to where controller  32  commands fan motor  19  to run at a third speed slower than the second speed. This third speed corresponds to a ventilation air flow mode, which may be  130  cubic feet per minute (CFM) in exemplary installations. The third speed of fan motor  19  will vary depending upon the ventilation demand of the space supplied by supply air duct  24 . For example, the third speed of the fan motor  19  may be set based on regulations from industry entities (e.g., ASHREA 90.1) to meet certain minimum ventilation levels or ventilation demands of the space supplied by supply air duct  24 . The installer may program the third speed of fan motor  19  as a fraction of the first speed or second speed. Further, the third speed of fan motor  19  may be programmed into controller  32  by an installer when the system is installed. This allows the ventilation demands of the space to be determined before the third speed is set. 
         [0014]    At  114 , controller  32  adjusts positions of the inlet air damper  30  and a return air damper  35 . In ventilation mode, controller  32  opens inlet air damper  30  and closes return air damper  30  such that so that the airflow that is being circulated through the space is close to being 100% outdoor air. Return air from return duct  31  is drawn through exhaust air vent  27  and by exhaust fan  28 . At  116 , controller  32  monitors temperature of the space (e.g., via thermostat  40 ) to determine if the space has become too warm or cold, in response to the influx of outdoor air. 
         [0015]    In exemplary embodiments, fan motor  19  is a three speed motor that operates at the first speed, second speed and third speed based on commands from controller  32 . Controller  32  and fan motor  19  may use a variety of techniques to achieve the three speeds including using an electronically commutated motor (ECM), pulse width modulation (PWM) of drive signals, variable frequency drive (VFD), etc. 
         [0016]    Embodiments provide a third speed for the fan motor to gain additional energy savings. This third fan speed is utilized whenever the unit is ventilating (the space is not requiring either heating or cooling). As the third speed is lower than the first and second speeds, less power is needed to operate the fan motor  19  at the third speed. The energy savings of this third speed would vary by locale and application, but could be as high a 40% 
         [0017]    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.