21-01-2013, 03:52 PM
FANS & BLOWERS
FANS.ppt (Size: 1.17 MB / Downloads: 149)
Fans
Fans generally add only a small amount of pressure to a gas.
Fluid compressibility can be ignored in the calculations.
They pull air from the atmosphere and discharge to a space that is slightly above atmospheric pressure..
Blowers
Blowers impart significant positive pressure to gases.
In these devices it is necessary to take account of change in density with pressure and also the heat evolved by work (P dV) done on the gas.
System Resistance
Sum of static pressure losses in system
Configuration of ducts, pickups, elbows
Pressure drop across equipment
Increases with square of air volume
Long narrow ducts, many bends: more resistance
Large ducts, few bends: less resistance
Static pressure
Static pressure is the potential energy put into the system by the fan. It is given up to friction in the ducts and at the duct inlet as it is converted to velocity pressure.
At the inlet to the duct, the static pressure produces an area of low pressure.
Velocity pressure
Velocity pressure is the pressure along the line of the flow that results from the air flowing through the duct. The velocity pressure is used to calculate air velocity.
Total pressure
Total pressure is the sum of the static and velocity pressure.
Centrifugal Fans – Forward curved
Advantages
Large air volumes against low pressure
Relative small size
Low noise level
Disadvantages
Not high pressure / harsh service
Difficult to adjust fan output
Careful driver selection
Low energy efficiency 55-65%
Centrifugal Blowers
Gear-driven impeller that accelerates air
Single and multi-stage blowers
Operate at 0.35-0.70 kg/cm2 pressure
Airflow drops if system pressure rises