Duct static pressure control

Abstract

A method of controlling a variable-air-volume heating, ventilating, and air-conditioning system, includes causing the static pressure in a duct to be maintained at a desired static pressure by controlling the speed of a fan in proportion to the square root of the ratio of a desired duct static pressure setpoint to a measured duct static pressure. A HVAC system employing the method is further included.

1 . A method of adjusting the speed of a supply fan to maintain the static pressure in a supply duct at a desired pressure setpoint in a variable-air-volume heating, ventilating, and air-conditioning system comprised of the supply fan, a variable frequency drive, a static pressure sensor, and a controller coupled to the static pressure sensor and the variable frequency drive, the method comprising:
 varying the speed of the supply fan in proportion to the square root of the ratio of the desired duct static pressure setpoint to the current measured duct static pressure.   
   
   
       2 . The method of  claim 1 , including changing the fan speed to the new calculated speed whenever the actual duct static pressure differs from the duct static pressure setpoint by more than a known amount and the variable frequency drive speed has not been changed for longer than a known current sample time. 
   
   
       3 . The method of  claim 1 , including changing the fan speed to the new calculated speed whenever the fan speed is stabilized. 
   
   
       4 . The method of  claim 3 , including determining fan speed stabilization by multiplying the change in speed expressed as a percent of the maximum variable frequency drive speed by the acceleration/deceleration time for the variable frequency drive and then adding an additional known sample time period. 
   
   
       5 . The method of  claim 4 , including limiting the absolute value of the speed change of the fan to a maximum value. 
   
   
       6 . The method of  claim 5 , including limiting the absolute value of the speed change of the fan to a maximum value is limited to a known maximum value so that timely speed changes can be made when the duct static pressure is relatively far from the desired setpoint. 
   
   
       7 . The method of  claim 6 , including setting the speed to the known maximum value for providing a positive fan speed when the current fan speed is zero. 
   
   
       8 . The method of  claim 1 , including changing the speed of a variable frequency drive more at higher fan speeds than at lower fan speeds so that the pressure change is nearly the same at all speeds for a given set of circumstances. 
   
   
       9 . The method of  claim 1 , including not tuning control parameters of a certain variable frequency drive to maintain a desired duct static pressure. 
   
   
       10 . A method of controlling a variable-air-volume heating, ventilating, and air-conditioning system, comprising:
 causing the static pressure in a duct to be maintained at a desired static pressure by controlling the speed of a fan in proportion to the square root of the ratio of a desired duct static pressure setpoint to a measured duct static pressure.   
   
   
       11 . The method of  claim 10 , including changing the fan speed to the new calculated speed whenever the actual duct static pressure differs from the duct static pressure setpoint by more than a known amount and the variable frequency drive speed has not been changed for longer than a known current sample time. 
   
   
       12 . The method of  claim 10 , including changing the fan speed to the new calculated speed whenever the fan speed is stabilized. 
   
   
       13 . The method of  claim 12 , including determining fan speed stabilization by multiplying the change in speed expressed as a percent of the maximum variable frequency drive speed by the acceleration/deceleration time for the variable frequency drive and then adding an additional known sample time period. 
   
   
       14 . The method of  claim 10 , including limiting the absolute value of the speed change of the fan to a maximum value. 
   
   
       15 . The method of  claim 14 , including limiting the absolute value of the speed change of the fan to a maximum value is limited to a known maximum value so that timely speed changes can be made when the duct static pressure is relatively far from the desired setpoint. 
   
   
       16 . The method of  claim 15 , including setting the speed to the known maximum value for providing a positive fan speed when the current fan speed is zero. 
   
   
       17 . A HVAC system, comprising:
 a method of control, the method causing the static pressure in a duct to be maintained at a desired static pressure by controlling the speed of a fan in proportion to the square root of the ratio of a desired duct static pressure setpoint to a measured duct static pressure.   
   
   
       18 . The HVAC system of  claim 17 , the method including changing the fan speed to the new calculated speed whenever the actual duct static pressure differs from the duct static pressure setpoint by more than a known amount and the variable frequency drive speed has not been changed for longer than a known current sample time. 
   
   
       19 . The HVAC system of  claim 17 , the method including changing the speed of a variable frequency drive more at higher fan speeds than at lower fan speeds so that the pressure change is nearly the same at all speeds for a given set of circumstances. 
   
   
       20 . The HVAC system of  claim 19 , the method including not tuning control parameters of a certain variable frequency drive to maintain a desired duct static pressure.