FANS

FANS

What is Fan ? Pressure rise up to 2 psig.

Blowers ? 2 to 10 psig

Compressors > 10

Uses of Fans :

1.Exhausting or introducing air or other gases in to ;

(a) Process Reactors

(b) Dryers, Cooling Towers and Kilns

(c)Fan assisted air cooled heat exchangers

-Assisting combustion in furnaces

-Conveying pneumatically

-Ventilating for safety and comfort

Classification :

1.Axial

Tube Axial

Vane Axial

2.CentrifugalRadial

Forward Curved

Backward Curved or inclined

Where axial Low resistance large quantities at low pressure.

Where centrifugal requiring greater head, high frictional resistances

Characteristics of Centrifugal Fan

Radial Blade :

-Performs well in many applications ranging from pneumatic conveying to exhausting processor air or gas in high resistance system.

-High static pressure with low capacity.

-Not for ventilation.

Typical impeller types.

(a) Open type is general purpose self cleaning.

(b)Type closed on one side with stringy materials.

(c)Rim type good for severe duties.

(d)Back plate good draft but not suitable for chunky or fibrous materials.

Forward Curved Type :

(1)Imparts greater velocity to the air than backward inclined type.(forced draft types)

(2)Runs at slower speeds which makes it suitable for process equipment requiring long shaft.

Backward curved and Backward inclined types :

-To develop much of energy directly as pressure this makes efficient ventilators.(induced draft types)

-Operate at medium speed.

-Broad pressure volume capabilities.

-Develop less velocity head than forward of same size.

-Easy to control small variations in system volume result in small variations of air pressure.

Characteristics of Axial Fans

Tube Axial

:Wide range of volumes at medium

Fans pressure,Propeller, enclosed in cylinder.

Helical OR SCREW LIKE MOTION AIR

DISCHARGE PATTERN

VANE AXIAL FANS:AIR guide vanes on discharge side

straight line air discharge

Turbine is reduced improves efficiency and pressure characteristics.

-Can develop pressure 20 inch of water.

-Non-overloading

-Available with adjustable fan-blade pitch which provides performance variation.

-Direction connection of the fan wheel to the motor shaft.

Airfoil Centrifugal Fans

(1)To increase their stability, efficiency and PErformance.

Generally quite, do not pulsate because less turbulance.

Tubular Centrifugal Fans :

-Enclosed in a duct

-Steeply rising pressure over a wide range of capacity

-Non-overloading these fans are good.

(a)General building ventilation and air conditioning as well as for frame removal, humidifying, drying, motor cooling and supplying combustion air.

Axial Versus Centrifugal

(1)Centrifugal fans are easier to control.

(2)More robust in construction.

(3)Less noisy than axial.

(4)Their efficiency does not fall-off rapidly of off-design conditions.

Inlet boxes (devices area to the air 90 degree at the fan inlet) without impairing the pressure or efficiency.

COMPARISON OF AXIAL AND CENTRIFUGAL FANS

Appearances

Axial fan smaller and is part of the duct.

Capacity Axial have more capacity per unit weight.

Construction Centrifugal is more complex and require larger shafts and bearings.

First CostAxial fan is less expensive.

Installation Axial fan is easier to install.

Maintenance Impeller is costlier than axial propeller.

Nose LevelAxial is noisy.

OverloadingAxial fan gets overloaded when shutoff.

Power(1) Axial more. Vane axial are more efficient than tube axial.

(2) Backward inclined more efficient than radial or forward inclined.

Pressure CapacityAxial fans can handle 8 to 9 inch water gage,

Centrifugals can handle 60 inch >

Reverse FlowAxial fan change from supply to exhaust when rotating reverse.

Space Axial more compact.

APPLICATION EXAMPLES

(1) Abrasive ServiceCentrifugal more sturdier.

(2) Corrosive FumesTube axial fan suitable.

(3) Dust Collection SystemRadial Centrifugal

(4) Heat RecoveryAxial Fan

(5) High TemparatureAxial Fan with belt drive

(6) High VolumesAxial Fan at low pressure

(7) Low VolumesAt high pressure

(8) Root Ventilator The axial

(9) Sticky Particles Radial