Air handling unit fan control systems and methods

Abstract

A method and system of controlling a supply fan and a return fan of an air handling unit (AHU). The supply fan has an associated design supply airflow, and the return fan has an associated design return airflow. In one embodiment, the method includes calculating an actual supply airflow, generating a supply fan speed set point based on the design supply airflow and the actual supply airflow, and modulating the speed of the supply fan based on the supply fan speed set point. The method also includes calculating an actual return airflow, generating a return airflow set point based on the actual supply airflow, and modulating the speed of the return fan based on a comparison of the return airflow set point and the actual return airflow.

1 . A method of controlling a supply fan having an associated design supply airflow and a return fan having an associated design return airflow, the supply fan and the return fan being associated with an air handling unit, the method comprising: 
 calculating an actual supply airflow;    generating a supply fan speed set point based on the design supply airflow and the actual supply airflow;    modulating the speed of the supply fan based on the supply fan speed set point;    calculating an actual return airflow;    generating a return airflow set point based on the actual supply airflow; and    modulating the speed of the return fan based on a comparison of the return airflow set point and the actual return airflow.    
   
   
       2 . The method of  claim 1 , wherein modulating the speed of the return fan includes stopping the return fan if the return airflow set point is less than approximately 30 percent of the design return airflow.  
   
   
       3 . The method of  claim 1 , wherein modulating the speed of the return fan includes increasing the speed of the return fan if the actual return airflow is less than the return airflow set point.  
   
   
       4 . The method of  claim 1 , wherein modulating the speed of the return fan includes decreasing the speed of the return fan if the actual return airflow is greater than the return airflow set point.  
   
   
       5 . The method of  claim 1 , wherein the actual supply airflow is calculated utilizing supply fan speed data and supply fan power usage data, and wherein the actual return airflow is calculated utilizing return fan speed data and return fan power usage data.  
   
   
       6 . The method of  claim 1 , wherein the actual supply airflow is calculated utilizing supply fan speed data and supply fan pressure data, and wherein the actual return airflow is calculated utilizing return fan speed data and return fan pressure data.  
   
   
       7 . The method of  claim 1 , wherein generating the supply fan speed set point includes calculating a supply fan speed comparison value by multiplying a supply fan design speed by a ratio of the actual supply airflow and the design supply airflow, wherein the supply fan design speed corresponds to an optimal supply fan speed and the design supply airflow corresponds to an optimal supply airflow.  
   
   
       8 . The method of  claim 7 , wherein generating the supply fan speed set point further includes choosing between a minimum supply fan speed and the supply fan speed comparison value.  
   
   
       9 . The method of  claim 8 , wherein modulating the speed of the supply fan includes comparing a preset speed value equivalent to approximately 5 to 15 percent of the supply fan design speed to a difference between the supply fan speed and the supply fan speed set point.  
   
   
       10 . The method of  claim 9 , further comprising maintaining the supply fan speed if the difference between the supply fan speed and the supply fan speed set point is less than the preset speed value.  
   
   
       11 . The method of  claim 9 , further comprising increasing the supply fan speed if the difference between the supply fan speed and the supply fan speed set point is greater than the preset speed value.  
   
   
       12 . The method of  claim 1 , wherein generating the return airflow set point includes calculating an exhaust airflow corresponding to unrecoverable airflow, calculating a loss airflow corresponding to airflow losses within the air handling unit, and subtracting the exhaust airflow and the loss airflow from the actual supply airflow.  
   
   
       13 . A fan speed control system configured to control a supply fan and a return fan associated with an air handling unit, the fan speed control system comprising: 
 a supply fan head sensor configured to measure airflow pressure proximate to the supply fan;    a return fan head sensor configured to measure airflow pressure proximate to the return fan; and    a controller configured to communicate with the supply fan head sensor and the return fan head sensor, to calculate a supply airflow value based at least partially on the air pressure proximate to the supply fan and a return airflow value based at least partially on the air pressure proximate to the return fan, and to modulate the speed of the supply fan and the speed of the return fan based at least partially on the supply airflow value and the return airflow value, respectively.    
   
   
       14 . The fan speed control system of  claim 13 , wherein the controller is further configured to measure the speed of the supply fan and the speed of the return fan, and wherein the respective speeds of the supply fan and the return fan are also used to calculate the supply airflow value and the return airflow value.  
   
   
       15 . The fan speed control system of  14 , further comprising a supply variable frequency drive configured to drive the supply fan and a return variable frequency drive configured to drive the return fan, wherein the supply variable frequency drive and the return variable frequency drive are in communication with the controller.  
   
   
       16 . The fan speed control system of  15 , wherein the controller is further configured to measure a supply fan power value using data from the supply variable frequency drive and a return fan power value using data from the return variable frequency drive, and the respective power values from the supply fan and the return fan are also used to calculate the supply airflow value and the return airflow value.  
   
   
       17 . The fan speed control system of  claim 13 , wherein the controller is further configured to calculate a supply fan speed set point using the supply airflow value, and to modulate the speed of the supply fan based at least partially on a comparison between the supply fan speed and the supply fan speed set point.  
   
   
       18 . The fan speed control system of  claim 13 , wherein the controller is further configured to calculate a return airflow set point using the supply airflow value, and to modulate the speed of the return fan based at least partially on a comparison between the return airflow value and the return airflow set point.  
   
   
       19 . A method of retrofitting an air handling unit with a fan control system, the air handling unit having existing control components, the method comprising: 
 integrating a controller into the air handling unit such that the controller is in communication with a supply fan and a return fan; and    by the controller, modulating the speed of the supply fan based at least partially on a comparison of a calculated supply airflow value and a design supply airflow value, and the speed of the return fan based at least partially on a comparison of a calculated return airflow value and a design return airflow value, wherein the design supply airflow value and the design return airflow value correspond to theoretical maximum efficiency airflow values.    
   
   
       20 . The method of  claim 19 , further comprising disabling the fan control system such that the air handling unit can utilize the existing control components.