# Fans and Blowers | Interview QnA

Fans and Blowers in this blog we will read Interview questions of Fans and Blowers which are probability of asking in Technical interview in chemical industry.

 Fans and Blowers | Interview QnA

### Interview Questions from this blog

1. What are the main parameters to be observed during its performance test of a Fan?
2. What is dynamic pressure?
3. Give the formula to calculate volumetric flow by using Dynamic pressure and area
4. What is the relationship between static pressure, dynamic pressure and total pressure?
5. List at least three different flow control methods adopted in a fan system?
6. Give the formula to evaluate the velocity by using dynamic pressure for air system?
7. What are the factors affecting fan performance?
8. What are the various instruments used for evaluation of air flow in fans?
9. Calculate the density of the air at the measurement temperature of 32°C
10. Definitions - Static pressure, Fan shaft power, Fan efficiency, Static Fan efficiency %.

### 1. What are the main parameters to be observed during its performance test of a Fan?

• Static pressure
• Dynamic pressure
• Power input to the motor
• Cross section area of the duct
• Temperature
• Fan RPM
 Centrifugal Fan

### 2. What is dynamic pressure?

Dynamic pressure is the rise in static pressure which occurs when air moving with specified velocity at a point is bought to rest without loss of mechanical energy. It is known as dynamic pressure.

### 3. Give the formula to calculate volumetric flow by using Dynamic pressure and area

Where
• A= Area of cross section of duct, m2
• Cp =  The picot tube coefficient ( Take manufacturer's value or assume 0.85)
• P = The average velocity pressure measured using picot tube and inclined manometer by taking number of points over the entire cross section of the duct mmWC

### 4. State relationship between static dynamic and total pressure?

Total pressure = Static pressure + Dynamic pressure

### 5. How to control flow of air in Centrifugal fan?

The three important

1. Variable speed drive
2. Change in pulley size
3. Inlet guide vane control
 Blower

### 7. What are the factors affecting fan performance?

The factors affecting fan performance are
1. Type of fan used for a particular application
3. Air flow delivered by fan
4. System pressure
5. Ducting dimensions
6. Pressure drop

1. Pitot tube
2. Anemometer

### 10. Definitions

#### Static pressure :

The absolute pressure at a point minus the reference atmospheric pressure

#### Fan shaft power :

The mechanical power supplied to the fan shaft.

#### Fan efficiency :

The air power static divided by impeller power

Static Fan efficiency % = [Volume in m3/ sec * Total static pressure mmWC] / 102 * Power input to the shaftin kW

### 11. What is surging in compressors and how it can be prevented?

• In centrifugal and axial compressors when the suction volume falls below a certain limit i.e. when the compressor is not getting enough volume of gas to compress, the gas from discharge side will flow back to the compressor( which would reduce the speed of compressor )and join together with the suction side to increase the volume input.
• Thereafter the speed of compressor picks and it come to normal operation. But meanwhile such revere in flow will cross huge pressure gradient across the compressor and vibration thus may damage the impeller. This momentary reverse flow lapses for very short time
• To prevent surging we need to install an anti-surging system. The anti-surging system shall have a mechanism to measure the suction flow and discharge flow time to time along with a control valve. When the suction volume falls below a certain set value a part of discharge gas will be directed towards the suction to maintain the minimum suction volume.
• As normally compression leads to increase in temperature the gas recycle sis taken after the “after cooler” to ensure the discharge temperature is within control.