What is the difference between forward and backward curve fan and what are the applications?

1) Forward-curved blades curve in the direction of the fan wheel's rotation on the other hand Backward-curved blades,curve against the direction of the fan wheel's rotation.

2)  Forward-curved blades provide a low noise level comparitive to Backward-curved blades because Backward curved blades have high speed relatively to Forward curved blades for same capacity range

3) Forward-curved blades are good and optimised solution for  small air flow with a high increase in static pressure. On other side Backward curved blades are good for high pressure, medium flow applications.

My conclusion :- Forward curved fans are an excellent solution for ducted fan applications with  Low pressures-resulting in low first cost & energy, compact size and low sound.

As per ASHRAE Systems and Equipment 2008

* Backward curved fan:

- Efficiency only slightly less than airfoil fan. Ten to 16 single-thickness blades curved or inclined away from direction of rotation. Deep blades allow efficient expansion within blade passages. Air leaves impeller at velocity less than tip speed.

- Efficiency slightly lower than airfoil fan Highest efficiencies occur at 50 to 60% of wide-open volume. This volume also has good pressure characteristics. Power reaches maximum near peak efficiency and becomes lower, or self-limiting, toward free delivery.

- Heating, ventilating, and air-conditioning applications as airfoil fan. Used in some industrial applications where environment may corrode or erode airfoil blade.

* Forward curved fan:

- Flatter pressure curve and lower efficiency than the airfoil, backward-curved, and backward-inclined.

Do not rate fan in the pressure curve dip to the left of peak pressure. Power rises continually toward free delivery. Motor selection must take this into account.

- Pressure curve less steep than that of backward-curved fans. Curve dips to left of peak pressure.

Highest efficiency to right of peak pressure at 40 to 50% of wide-open volume. Rate fan to right of peak pressure. Account for power curve, which rises continually toward free delivery, when selecting motor.

- Primarily for low-pressure HVAC applications, such as residential furnaces, central station units, and

packaged air conditioners.

Backward curved centrifugal fan is generally used in exhaust system and farwered curved centrifugal fan is used in supply system of air conditioning.