CFM Calculation for Exhaust Fan

CFM Calculation for Exhaust Fan The amount of air depends on some other factors as well, such as the diameter and shape of the blades, speed at which the blades turn (revolutions per minute or rpm), horsepower (hp) of the motor, and overall fan design. These combined factors establish the air moving capacity of a fan. Fan capacity is measured in terms of the cubic feet, and again, this is how CFM (cubic feet per minute is determined.) Air speed (feet per minute) X area (square feet) = CFM It is the amount of air a fan moves. Let’s take an Example: Motor 2 HP, Dia of fan 24″, 1400 rpm, 50hz/220v. Let the efficency of the motor is typically 75 to 90 %. You may find more on the motor tag. Here note that, The efficiency of the fan may be 70% to 90 %, lower for simple sheet metal blades, higher for twisted blade or blade integral with the hub and small tip clearance. 60% if the fan is not ducted. Hence, e.g. 2 hp x 0.75 x 0.6 = 0.9 HP is transfered to the air Energy balance: 0.9 x 735 (W per HP) = pi/4 x (Dia x 0.0254)^2*AirDensity/2 x Velocity^3 Hence Velocity = {(0.9 x 735)/[3.14/2 x (24 x 0.0254)^2*1.25/2]}^0.33 So CFM = Velocity x 3.14/4 x (24 x 0.0254) in cubic meters For cubic feet you multiply by 1/(0.305)^3 which is about 30. Example 2: Calculate the flow rate(CFM) of the blower from the given motor @ fan RPM? And what is the air velocity produced at input and also at output? Motor Spec: motor RPM = 1470rpm

3 phase induction motor Type Y 180L-4 415V, 40 Amps Power = 30HP / 22kW Frequency = 50Hz diameter of motor pulley = 250mm Blower spec : diameter of fan pulley = 200mm Fan diameter = 710mm No. of blades = 10 blades Thickness of blades = 3 mm Type of fan = Centrifugal Fan (Type BC) Efficiency = 85% Solve this question yourself to explore your brain!!!!