1. LECTURE UNIT NO. 9
Fans and Blowers
A fan is a machine used to apply power to a gas to increase energy content thereby causing it to flow or
move.
A blower is a fan used to force air under pressure which means resistance to gas flow is imposed upon
discharge.
An exhauster is a fan used to withdraw air under suction, that is, the resistance to gas flow is imposed
primarily upon the inlet.
Functions of Fans
1. To move air or gasses through distribution systems and apparatus required for conditioning of
buildings.
2. For drying and cooling.
3. For pneumatic conveying.
4. For dust collection, separation and exhaust.
5. For mine and tunnel ventilation.
6. For forced and induced draft of steam generating units.
Basic Elements in Fan Design
1. Wheel or impeller
- the rotating member.
2. Housing
- stationary member provided with an intake opening (inlet) and a discharge opening (outlet).
Basic Difference According to the American Society of Mechanical Engineers (ASME)
1. Fan – if the pressure rise is equal to or below 1 psig.
2. Blower – if the pressure rise is between 50 to 1 psig.
3. Compressor – if the pressure rise is above 50 psig.
Factors Affecting Fan Selection
1. Quantity of gas (or air) to be moved per unit time.
2. Estimated resistance and expected variations.
3. Amount of noise permitted.
4. Space available for the fan.
5. Economic implications.
Fan Control
Regulation of fan capacity or varying the volume delivered
1. Damper control
Simplest method. Dampering is a throttling action applied to the gas flow.
2. Inlet vane control
Series of adjustable position vanes at fan inlet to control air entering the wheel.
3. Variable speed control
Controls the speed of the motor.

2. Total Air Power
Fan Capacity
Q=Av
Energy Equation
From Law of Conservation of Energy
Basic Assumption:
1. Considering inlet and discharge static pressure
2. Considering inlet and discharge velocities
3. Constant temperature
[Ein = Eout]
PEs + KEs + Us + Wfs + Air Power = PEd + KEd + Ud + Wfd
Us, Ud = 0, since change in temperature is minimal.
ΔPE = 0
Let:
Hv = velocity head / velocity pressure head
Hs = static head / static pressure head
Ht = total head / total fan pressure head
Ht = Hv + Hs
Static Air Power
Ps = Q γa H s
Static Pressure Head
Relationship:
γa Hs = γw hw

3. Fan Efficiency
- Total Fan Mechanical Efficiency
- Static Fan Efficiency
- Motor Fan Efficiency
Fan Affinity Laws
a. Variable Speed (Constant fan size, constant density)
b. Variable Density (Constant fan size, constant speed)
c. Variable Density and Speed (Constant fan size)
Problems:
1. A steam generator has two f-d fan with a capacity of 150 000 cfm. Static pressure is 10.5 in. of
water (a.) based on a fan input of 345 hp each, calculate static efficiency. (b.) Calculate the
capacity, head and brake power requirements for a speed increase of 15%.
2. Measurements made on newly installed air handling system were N = 1200 rpm fan speed. 2.4
m3/s capacity, static pressure of 3 mm Hg and approximately 1.8 kW of power is supplied to the
motor, measurements taken at standard conditions. Eventually the system will operate at much
higher air temperature that will result into new reading for static pressure of 2.5 mm Hg. Assuming
fan speed remains constant, determine the new power required and temperature of the air.
3. The mass flow rate of air inside a duct system is approximately 6 kg/s at 20°C. The power required
is 5.5 kW at fan speed of 1200 rpm. Operating conditions changed such that mass flow rate is
maintained the same. Determine the fan operating speed and power requirement if the
temperature reading is 45°C.
4. A fan is driven by a variable speed motor. The fan capacity ranges up to 4 m 3/s and gives a static
pressure of 14 cm of water at 1700 rpm. If the static efficiency is 70%, what is the maximum
speed at which fan can operate without overloading a 11 hp motor.