Mechanical (Rotary) Common Questions -4

EPC-School 2012

M for Mechanical  

Topic - Fans

1. What are fans, blowers and what are exausters?

2. What is the difference between ID and FD fans and what are its advantages and disadvantages?

3. What is the principle of operation of axial fan?

4. What are the capacity control methods for centrifugal fans?

5. What is the effect of change in density of gas handled at constant volume flow rate?

6. What are the sources of efficiency losses in a fan?

7. How is the drive arrangement for a CT fan?

8. What does the fan noise depend on?

9. Define lift and drag forces on an aerofoil?

10. State five applications of fans in Process Plants.

Answers to Above Questions-

1. Fans, blowers are machines which push gas into a system whereas exhausters are machines which suck gas from system

2. ID (induced draft) fans suck flue gases from top of stack/column and FD (forced draft) fans are push air into a heater, furnace etc

    FD fans helps better combustion of fuel and it has more efficiency as it handles cooler gases

    ID fans helps in recovering heat from flue gases and it is less efficient than FD fans, reduces stack height

3. Aerodynamic shape of blades generate force on the blades and since blades are stationary, they exert reaction force and push the gas forward

4. Centrifugal fans: variable speed drive

5. If density increases, power consumption increases at constant volume flow rate

6. Tip clearances, clearance between hub and blade (recirculation), improper design causing flow recirculation, high vibrations

7. Motor drive via gearbox

8. Tip speed, blade aerofoil design, misalignment, number of blades

9. Lift is the force acting on the blade perpendicular to blade and drag is the force acting on the blade in the direction opposite to the rotation of the blade

10. Stacks (ID fan), Furnaces (FD fan), Cooling towers (CT fans), Fin fan, HVAC rooms