Centrifugal fan is essentially a variable flow and constant pressure device. When the speed is constant , the theoretical pressure - flow curve of the centrifugal fan should be a straight line . Due to internal losses , the actual characteristic curve is curved. The pressure generated in the centrifugal fan is greatly affected by changes in the temperature or density of the intake air . For a given intake air volume , the highest intake air temperature (the lowest air density ) produces the lowest pressure . For a given pressure and flow characteristic curve, there is a power and flow characteristic curve . When the blower operates at a constant speed When running, for a given flow rate , the required power increases as the intake air temperature decreases .