Centrifugal roof fans are essentially variable flow constant pressure devices. When the speed is constant, the theoretical pressure-flow curve of a centrifugal fan should be a straight line. Due to internal losses, the actual characteristic curve is curved. The pressure generated in a centrifugal fan is significantly affected by changes in inlet temperature or density. For a given air volume, there is a corresponding power-flow characteristic curve for a given pressure and flow characteristic curve. When the blower operates at a constant speed, the required power increases with the decrease in inlet temperature for a given flow rate.