# Thermal Engineering Questions and Answers – Centrifugal & Reciprocating Pumps

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This set of Thermal Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Centrifugal & Reciprocating Pumps”.

1. Which hydraulic machine convert mechanical energy into pressure energy?
a) Pump
b) Refrigerator
c) Heat engine
d) Turbine

Explanation: Pump is a hydraulic machine which convert mechanical energy into hydraulic energy or pressure energy. Pump is a device which transfer the fluid from suction head to delivery head.

2. Which of the following is a positive displacement pump?
a) Centrifugal pump
b) Diffuser pump
c) Axial pump
d) Reciprocating pump

Explanation: Reciprocating pump and rotary pumps are positive displacement pumps. Centrifugal pump, diffuser pump and axial pump are Dynamic pumps.

3. Which of the following is a dynamic pump?
a) Centrifugal pump
b) Rotary pump
c) Multiple rotor
d) Reciprocating pump

Explanation: Reciprocating pump and rotary pumps are positive displacement pumps. Centrifugal pump, diffuser pump and axial pump are Dynamic pumps.

4. Dynamic pumps are also known as ______
a) Positive displacement pumps
b) Non-positive displacement pumps
c) Reciprocating pump
d) Rotary pump

Explanation: Dynamic pumps are also known as non-positive displacement pumps or roto-dynamic pumps. Reciprocating pump and rotary pumps are positive displacement pumps.

5. Hydraulic machine convert mechanical energy into hydraulic energy by means of centrifugal force acting on the fluid ______
a) Centrifugal pump
b) Rotary pump
c) Multiple rotor
d) Reciprocating pump

Explanation: Pump is a hydraulic machine which convert mechanical energy into hydraulic energy or pressure energy. In centrifugal pump mechanical energy is converted into hydraulic energy by means of centrifugal force.

6. The rise in pressure head at any point of the impeller is proportional to ______
a) $$\frac{u^3}{2g}$$
b) $$\frac{u^2}{2g}$$
c) $$\frac{u^3}{3g}$$
d) $$\frac{u^2}{g}$$

Explanation: From Bernoulli equation $$\frac{∆p}{ρ}+\frac{∆u^2}{2g}+∆z=0$$
Here, u1 = 0 and ∆z = 0
$$\frac{∆p}{ρ} = \frac{u^2}{2g}$$

7. Which of the following is not an advantage of centrifugal pump?
a) Low initial cost
b) High efficiency
c) Difficult installation
d) Easy maintenance

Explanation: Advantages of centrifugal pump are:

• Low initial cost
• Easy maintenance
• Easy installation
• High efficiency
• Uniform and continuous discharge

8. When the speed of the centrifugal pump is doubled, the power required to drive the pump will be _____
a) Increase 8 times
b) Increase 4 times
c) Double
d) Remain the same

Explanation: PαN3 so P2=8P1

9. Why an air vessel is used in a reciprocating pump?
a) To obtain a continuous supply of water at uniform rate
c) To increase the delivery head
d) To reduce cavitation

Explanation: Air vessel are fitted to delivery side of reciprocating pump. An air vessel is used in a reciprocating pump to obtain a continuous supply of water at uniform rate and to reduce power consumption.

10. Negative slip occurs in reciprocating pumps, when delivery pipe is ______
a) Long and suction pipe is short and pump is running at low speed
b) Long and suction pipe is short and pump is running at high speed
c) Short and suction pipe is long and pump is running at low speed
d) Short and suction pipe is long and pump is running at high speed

Explanation: Negative slip occurs in reciprocating pumps, when delivery pipe is long and suction pipe is short. Also, pump should be running at low speed.

11. Why is a minimum of net positive suction head (NPSH) required for a hydraulic pump?
a) To prevent cavitation
b) To increase discharge
d) To increase efficiency

Explanation: NPSH is the difference of absolute pressure head at the inlet to the pump and the vapour pressure head corresponding to temperature of the liquid pumped plus the velocity head. The NPSH should always be positive to keep away the cavitation.

12. For a given centrifugal pump _____
a) The discharge varies directly as the speed
b) The head varies inversely as the speed
c) The power varies as the square of the speed
d) The discharge varies as the square of the speed

Explanation: The dimensionless number for centrifugal pump is $$\frac{Q}{ND^3}$$