Backward Inclined Centrifugal Fans

Blade geometry

Blade form	Straight blade, inclined away from the direction of rotation
Exit blade angle	Below 90°
Number of blades	10 to 12
Air leaving the impeller	At a velocity less than the blade tip speed. Deep blades give efficient expansion within the blade passages.

Where it is used

• Process and ventilation duties where a single-thickness blade is preferred
• Corrosive or mildly erosive airstreams where a hollow aerofoil blade is not acceptable
• Lined or stainless steel construction where the blade must be solid plate

A solid flat blade can be lined or built in thicker plate for corrosive or erosive service. For heavy dust the straight radial type is the usual choice.

Applications selected with this blade

Application	What it does	Type usually selected
ID Fan	Sits at the outlet of a boiler, kiln or furnace and draws the flue gas through the plant to the stack. It handles hot, dust-laden gas at large volume, so it is sized at the hot condition and checked at cold start.	Backward curved, backward inclined and radial tipped
FD Fan	Supplies combustion air to a boiler or furnace, pushing ambient air through the air heater and into the windbox against the furnace draft. Clean air, so the blade is chosen for efficiency.	Backward curved and backward inclined
Ventilation Fan	Moves air through a plant or building for fresh air supply, heat removal and fume dilution where the system is ducted. Low pressure, long hours, so efficiency and noise set the selection.	Backward inclined and forward curved
SA Fan	Supplies the secondary combustion air that completes burning above the grate or around the burner. On bagasse and biomass boilers it also carries the over-fire air that controls the fuel bed.	Backward curved and backward inclined
Hot Air Fan	Circulates or supplies air at elevated temperature to a dryer, oven or kiln. Selected at the hot density with the shaft and bearing arrangement built for continuous hot running.	Backward curved and backward inclined
Fume Exhaust System	Hoods, ducting and an exhaust fan that capture fume at source and deliver it to a scrubber, bag filter or stack. Fan material follows the fume: stainless steel, PP-FRP or rubber lined where it is corrosive.	Backward inclined and radial tipped
Dust Collector Fan	The fan behind a bag filter, cyclone or multiclone on a dust extraction system. Placed on the clean side where possible so the blade can be chosen for efficiency.	Backward inclined and radial tipped
HVAC	Supply, return and exhaust fans for heating, ventilation and air conditioning of plant and office space. Low pressure, so efficiency and noise govern.	Axial, backward inclined and forward curved

Industries

Drive and construction

Offered direct mounted on the motor shaft, direct coupled, or belt driven. Material of construction follows the medium and temperature: mild steel, stainless steel, PP-FRP lined, MS FRP lined or MS rubber lined.

Testing and balancing

Performance tested as per IS 4894, BS 848 Part I, or as per customer standards. Rotors are statically and dynamically balanced to grade G 6.3 or G 2.5 as per ISO 21940-11.

Information needed to select or quote

1. Duty point. Volume flow and static pressure at the fan inlet, with the gas density and temperature or the conditions to derive it.
2. Operating conditions. Site altitude, normal operating temperature and continuous or intermittent duty.
3. Gas stream. Dust loading and particle size, corrosive species, moisture, upset temperature and any tendency of the material to deposit on the impeller.
4. Mechanical. Rotation viewed from the drive end, discharge orientation and motor power.