Abstract:
A rooftop air conditioner having a variable speed fan and an economizer also includes a control feature for adjusting the minimum damper position in response to the speed of the fan during predetermined conditions in order to maintain a desired volume of fresh air from the economizer.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates generally to variable air volume air conditioning systems and, more particularly, to a system for operating the economizer damper therein to ensure sufficient outside air is introduced in order to meet minimum ventilation code requirements. 
       BACKGROUND OF THE INVENTION 
       [0002]    In a typical climate control system of a building, a central thermostat controls the temperature of air in the building. A building central heating/cooling unit forces a constant amount of heated or cooled air to various points in the building through duct work comprising a supply air duct, a plurality of zone ducts and a return air duct. 
         [0003]    In variable air volume (VAV) systems the space temperature is controlled by varying the airflow delivered to the space. This is done by means of a variable speed fan controlled by a variable frequency inverter, the position of inlet dampers to the fan or other airflow control devices. For larger or by varying VAV systems, individual zone dampers are used to control the zone cfm and the supply fan speed is controlled to provide a constant supply duct static pressure. On smaller VAV systems used in single zone applications the supply cfm is directly controlled on the basis of a space temperature sensor. 
         [0004]    When outdoor temperatures are lower than the indoor temperature, many units are equipped with outside and return air dampers that are modulated to control the use of outside air to provide free cooling to the space. These damper systems are typically called economizers. The economizer controls the flow of outside air through the inlet air duct and return air through the return air duct and using the inlet air damper and return air damper with the respective positions being controlled such that air temperature to the space is maintained. 
         [0005]    Because of air quality control needs in a building, the industry has established standards (i.e. ASHRAE 62.1) to ensure that a certain amount of fresh air (i.e. 15 cfm per person) is brought in through the economizer. For that purpose, a minimum open position is established such that the economizer damper is never fully closed but can only be closed to a predetermined minimum position. It has been recognized that with a variable indoor flow rate as with a VAV system and other zoning systems, the fixed minimum damper position would result in insufficient ventilation air when the indoor air flow rate is decreased below the design full load airflow. To solve this problem the industry sometimes uses expensive airflow sensors that measure the actual amount of outside air that is coming through the outside air inlet and then uses the value to readjust the economizer damper minimum position accordingly to ensure that the minimum ventilation air is delivered even through the supply airflow is varying to meet the cooling load. For example for a 20 ton unit the required full load cfm might be 8,000 cfm. For 20% outside air the ventilating air would be 1,600. As per the previous discussions this might be the amount of air that is obtained with a damper position of 16%. As the building load decreases the supply fan will be slowed to provide the amount of air to cool the space. If the supply air flow reduces to 4,000 cfm then at the 16% damper position the outside air would also reduce in proportion to the supply air and the amount of outside air would reduce to around 1,000 cfm and the proper supply air would not be delivered. For the units equipped with the outside air cfm sensor the unit controls would readjust the economizer minimum damper position to a larger position where the required 2,000 cfm of air would be delivered. Depending on the characteristics of the dampers and the building ductwork this might be at a damper position of around 45%. 
         [0006]    What is needed is an inexpensive and effective approach to ensure that sufficient outside air will be brought in, particularly during periods of time when the variable air volume fan is slowed because of reduced building cooling requirements. 
       DISCLOSURE OF THE INVENTION 
       [0007]    Briefly, in accordance with one aspect of the invention, the minimum damper position does not remain fixed but is varied to ensure that sufficient outside air is brought in even during periods in which the VAV fan speed is reduced. 
         [0008]    By another aspect of the invention, the minimum damper position is varied as a function of the fan speed such that, generally, as the fan speed is reduced, the minimum damper position is adjusted to a further opened position. 
         [0009]    By another aspect of the invention, the minimum damper position is varied in a non-linear relationship with the variable fan speed to compensate for the typical non-linearity of the economizer dampers. 
         [0010]    In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the spirit and scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic illustration of a rooftop air conditioning system with the present invention incorporated therein. 
           [0012]      FIG. 2  is a graphic illustration of the percentage of outside air in relationship to the damper position of the economizer. 
           [0013]      FIG. 3  is a graphic illustration of the economizer damper position in relationship to the fan speed of the system. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    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 . 
         [0015]    The evaporator section  12  includes a supply fan  18  which is driven by a fan motor  19 . The fan motor is adapted to operate at variable speeds to meet the cooling/heating requirements of the system. One manner of providing the variable speed is by the use of an inverter for providing variable frequency power to the fan motor  19 . 
         [0016]    Leading into the evaporator section  12  from the economizer section  13  is a cooling coil  21  and its associated filters  22 . A heater  23  is placed in a downstream position from the supply fan  18 . 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 . 
         [0017]    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 linked return air damper  35 , which are adjustable 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 . 
         [0018]    It will be understood that, although the economizer damper may, to a degree, be placed in further opened positions, as the speed of the fan motor  19  is reduced, less air, and therefore less outside air, is drawn into the evaporator section  12 . Provision is therefore made to ensure that sufficient outside air is available to meet the requirements of the system. This is accomplished by way of a control  32  which senses fan speed  33  and controls the damper position  34  in response thereto in a manner to be described hereinafter. 
         [0019]    The first step is to define the characteristic curve for the damper position vs. the % outside air by a series of points. Typically curves defining the damper position vs. % outside are not linear. Depending on how non-linear the curve is, a number of points are required to define the curve. A typical curve is shown in  FIG. 2 . In this example, the curve is defined by four (4) points as shown. Point A represents a damper position of 68% open, resulting in 80% outside air. Similarly, point B, a 47% position with 60% outside air, point C, a 39% position with 40% outside air, and point D, a 14% position with 20% outside air. It is also known that the dampers will provide 0% cfm at the 0% position and 100% cfm at the 100% position. Between these points one can then use straight line interpolation to determine the damper position required to maintain the percentage of outside air. 
         [0020]    With the characteristic curve now defined in the unit control software one can then use the curve to adjust the outside air percentage required as a function of the indoor fan speed. 
         [0021]    We know that for a given damper position the amount of outside air delivered will vary just about directly with the fan speed on inverter driven fans due to the characteristics of the fan. Therefore we can take the fan speed and use it to adjust the percent outside air that we want to deliver to the space. For example if the VAV controls only require the indoor fan to run at 50% speed, then we know we would have to provide 40% outsider air to maintain the same absolute amount of outside air that would have been deliver at 100% fan speed with 20% outside air. So using the curve we would adjust the dampers open from the 100% fan speed point of 14% to the 40% point damper position which is 29%. In between the defined points, a straight line interpolation can be used to solve for the damper position to deliver the required percentage of outside air and cfm. 
         [0022]    Using the characteristic curve for the damper position vs. outside air percentage, one can then relate the indoor fan speed to the damper setting as shown in Table I as follows: 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 Fan 
                 Supply 
                 Outside 
                 % Outside 
                 Damper 
               
               
                 Speed 
                 CFM 
                 Air CFM 
                 Air 
                 Position 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 100%  
                 10000 
                 2000 
                 20% 
                 14% 
               
               
                 75% 
                 7500 
                 2000 
                 27% 
                 20% 
               
               
                 50% 
                 5000 
                 2000 
                 40% 
                 39% 
               
               
                 25% 
                 2500 
                 2000 
                 80% 
                 68% 
               
               
                 20% 
                 2000 
                 2000 
                 100%  
                 100%  
               
               
                   
               
             
          
         
       
     
         [0023]    One knows that the supply cfm is going to vary linearly with the fan speed so in order to maintain constant outside air cfm, one can use the control  32  to maintain the proper damper position  34  on the basis of fan speed as set forth in  FIG. 3  so as to maintain constant outside air. The control  32  thus includes look-up tables relating the damper position to percent of outside air and the damper position to the fan speed as shown in  FIGS. 2 and 3 , respectively. 
         [0024]    While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawings, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.