Either axial flow, centrifugal, or plug fans may be chosen as supply air fans for straight-through flow applications. In factory-fabricated units, more than one centrifugal fan may be tied to the same shaft. If headroom permits, a single-inlet fan should be chosen when air enters at right angles to the flow of air through the equipment. This permits a direct flow of air from the fan wheel into the supply duct without abrupt change in direction and loss in efficiency. It also permits a more gradual transition from the fan to the duct and increases the static regain in the velocity pressure conversion. To minimize inlet losses, the distance between the casing walls and the fan inlet should be at least the diameter of the fan wheel. With a single-inlet fan, the length of the transition section should be at least half the width or height of the casing, whichever is longer. If fans blow through the equipment, the air distribution through the downstream components needs analyzing, and baffles should be used to ensure uniform air distribution.
