Types of Fans and Blowers
Fan and blowers
1. Centrifugal
a) Forward blade
b) Backward blade
c) Radial blade
2. Axial flow
a) Propellerb) Tube
c) Axial
Centrifugal Fans:
● A centrifugal fan have an impeller or wheel mounted in a scroll type of casing.
● The impeller is turned either by direct drive or by an electric motor employing pulleys or belt.
● The centrifugal force is created by the rotating impeller moves the air outward along the blade channels.
● The outward moving air streams are combined by the scroll into a single large air stream. This air stream leaves the fan through discharge outlet.
● These are classified in three types:
(1) Forward curved blade
(2) Backward curved blade
(3) Radial or straight blade
1) Forward Curved Blades Centrifugal Fan :
● Since the blades are very shallow in depth these for the diameter of the housing (casing) air-inlet opening more nearly approaches to that of the impeller. This promotes a smooth flow of air into the rotating blade which increase efficiency and reduce its noise.
● The capacity of air delivered is more than backward flow and radial flow. This type of blade and operate at low speed. The blade on impeller is 24 to 64.
2) Backward Curved Blade Centrifugal Fan:
● These blades must be operated at much higher speed of rotation than forward curved blade. The tip of the blade is curved in backward direction.
● They are used in high capacity at constant speed with small changes in the power requirement.
● The number of blades on the impeller usually 10 to 16.
3) Radial or Straight Blades Centrifugal Fan :
● The radial blades are have simple in construction.
● The blades run straight out from the central hub. Some fans have heavy steel blades with high structural strength. The number of blades on impellers have more than 8 or 10.
● These fan provides very high pressure at high speeds.
Axial Flow Fans :
When air flow parallel to the axis of impeller (rotor) is called axial flow fan. These are classified into three groups :
a) Propeller Fan :
● This type of fan consist of a propeller or disc type wheel which operates within a mounting ring.
● The design of the ring surrounding the wheel is important because it prevents the air discharged from being drawn backward into the wheel around its periphery.
● The propeller fans are used only when the resistant to air movement is small.
● They are useful for the ventilation of laboratories, bath room, etc.
● It consists of a propeller wheel housed in a simple cylinder.
● The wheel may be driven externally either from electric motor within the cylinder directly connected to a shaft of may be driven through belt arrangement from the motor mounted outside the housing.
● The discharge follows spiral path.
● They are more efficient than propeller fan
c) Vane Type Axial Fan :
● These fan combines a tube axial fan wheel mounted in a cylinder within a set of air guide vane. This fan eliminate spiral flow of discharge air and reduce turbulence of flow.
● The efficiency of operation and the pressure characterized are better than tube axial fan.
● The axial flow fans are never used for duct air conditioning system because they are incapable of developing high pressure
● These fans are particularly used for handling large volume of air at relatively low pressure.
Total Pressure Develop by a Fan :
● The total pressure develop by a fan is the sum of static pressure at fan outlet and velocity pressure at fan outlet.
● In actual system, the fan provided with suction duct, filter, coil, damper at the entry of the duct so the total pressure at the fan inlet is equal to the total frictional resistance and it is always negative also it is less than static pressure at the fan inlet.
Fan air power:
The power output of a fan is expressed as air power and represent the work done by the fan
Pat = g×Q×PT×Kc / 60 Watts
Where,
Q = Total air inlet in m.cu/min.
PT = Fan total pressure in mm of water.
PT = PSF + PVF
PSF = Static pressure of fan.
PVF = Velocity pressure of fan.
Kc = Coefficient of compressibility.
