This set of Design of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Design of Fan”.

1. What is the formula for the fundamental relationship for the design of ventilation system?

a) head of air inside the machine = hydrodynamic resistance * volume of air passing2

b) head of air inside the machine = hydrodynamic resistance + volume of air passing2

c) head of air inside the machine = hydrodynamic resistance – volume of air passing2

d) head of air inside the machine = hydrodynamic resistance / volume of air passing2

View Answer

Explanation: First the hydrodynamic resistance is calculated along with the volume of air passing. On substitution in the formula gives the head of air inside the machine.

2. What is the formula for the total head produced?

a) total head produced = ∑ coefficient of hydrodynamic resistance + volume of air passing per second2

b) total head produced = ∑ coefficient of hydrodynamic resistance – volume of air passing per second2

c) total head produced = ∑ coefficient of hydrodynamic resistance * volume of air passing per second2

d) total head produced = ∑ coefficient of hydrodynamic resistance / volume of air passing per second2

View Answer

Explanation: The various coefficient of hydrodynamic resistance is calculated along with the volume of air passing per second. On substituting the various values and on addition gives the total head produced.

3. What are the ventilating parts in the ventilating circuits?

a) sharp or projecting inlet edges

b) inlet corners

c) variations in cross-sections of air paths

d) sharp or projecting inlet edges, inlet corners, variations in cross-sections of air paths

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Explanation: There are various ventilation parts provided in the ventilation circuits. They are sharp or projecting inlet edges, inlet corners, variations in cross-sections of air paths.

4.What is the range of the coefficients of hydrodynamic resistances for the protruding edges at inlet?

a) 40-50 * 10^{-3}

b) 40-60 * 10^{-3}

c) 30-50 * 10^{-3}

d) 30-40 * 10^{-3}

View Answer

Explanation: The coefficients of hydrodynamic resistances are used in the calculation of the total head. For protruding edges the range is about 40-60 * 10

^{-3}.

5. What is the range of the coefficients of hydrodynamic resistances for the rectangular edges at inlet?

a) 10-20 * 10^{-3}

b) 30 * 10^{-3}

c) 20-30 * 10^{-3}

d) 20-25 * 10^{-3}

View Answer

Explanation: The coefficients of hydrodynamic resistances are used in the calculation of the total head. For rectangular edges the range is about 30 * 10

^{-3}.

6. What is the range of the coefficients of hydrodynamic resistances for the rounded edges at inlet?

a) 12-20 * 10^{-3}

b) 10-20 * 10^{-3}

c) 15-20 * 10^{-3}

d) 12-30 * 10^{-3}

View Answer

Explanation: The coefficients of hydrodynamic resistances are used in the calculation of the total head. For rounded edges the range is about 12-20 * 10

^{-3}.

7. What factor/factors are required to evaluate the hydrodynamic resistance?

a) area of cross section

b) hydrodynamic coefficients

c) area of cross section or hydrodynamic coefficients

d) area of cross section and hydrodynamic coefficients

View Answer

Explanation: The hydrodynamic resistance is calculated from the hydrodynamic coefficients. It is also calculated from the area of cross section.

8. How many data are required for the design of fan?

a) 3

b) 4

c) 5

d) 6

View Answer

Explanation: There are 3 data required in the design of fan. They are outside diameter of fan, volume of air, hydrodynamic resistance.

9. How many steps are required in the design of the fan?

a) 7

b) 8

c) 9

d) 6

View Answer

Explanation: There are 7 steps involved in the design of fan. They are maximum air passing per second, peripheral speed of fan, width of fan, static head of fan under rated duty, inside diameter of fan, number of blades, power input of fan.

10. What is the formula for the volume of air?

a) volume of air = 0.9 * losses in kW / difference of air temperature at inlet and outlet

b) volume of air = 0.9 * losses in kW * difference of air temperature at inlet and outlet

c) volume of air = 0.9 / losses in kW * difference of air temperature at inlet and outlet

d) volume of air = 1 / 0.9 * losses in kW * difference of air temperature at inlet and outlet

View Answer

Explanation: The losses in kW is calculated along with the difference of air temperatures at inlet and outlet. On substitution the volume of air can be obtained.

11. What is the range of the difference of air temperature at inlet and outlet?

a) 11-15^{0}C

b) 10-13^{0}C

c) 12-16^{0}C

d) 14-18^{0}C

View Answer

Explanation: The difference of air temperatures at inlet and outlet has a minimum value of 12

^{0}C. The difference of air temperatures at inlet and outlet has a maximum value of 16

^{0}C.

12. What is the formula for the area of outlet opening?

a) area of outlet opening = maximum air passing per second / 0.42 * peripheral speed

b) area of outlet opening = maximum air passing per second * 0.42 * peripheral speed

c) area of outlet opening = maximum air passing per second * 0.42 / peripheral speed

d) area of outlet opening = 1/maximum air passing per second * 0.42 * peripheral speed

View Answer

Explanation: The maximum air passage per second along with the peripheral speed is calculated. On substitution the area of outlet opening is obtained.

13. What is the formula of the width of fan?

a) width of fan = area of outlet opening * 2.88 * outside diameter */ coefficient of utilization

b) width of fan = 1/ area of outlet opening * 2.88 * outside diameter * coefficient of utilization

c) width of fan = area of outlet opening * 2.88 * outside diameter * coefficient of utilization

d) width of fan = area of outlet opening / 2.88 * outside diameter * coefficient of utilization

View Answer

Explanation: The area of outlet opening and the outside diameter is calculated. After fixing the coefficient of utilization and on substituting the width of fan is obtained.

14. What is the formula for the number of blades?

a) number of blades = 3.14 * outside diameter * (1.25 – 1.5)* width of fan

b) number of blades = 3.14 / outside diameter * (1.25 – 1.5)* width of fan

c) number of blades = 3.14 * outside diameter / (1.25 – 1.5)* width of fan

d) number of blades = 3.14 * outside diameter * (1.25 – 1.5) / width of fan

View Answer

Explanation: The outside diameter and the width of fan is first calculated. Then the range is fixed according to the diameter and on substitution gives the number of blades.

15. What is the formula for the maximum air passing per second at maximum efficiency ?

a) maximum air passing = 2 * volume of air passing per second

b) maximum air passing = volume of air passing per second

c) maximum air passing = 2 / volume of air passing per second

d) maximum air passing = volume of air passing per second / 2

View Answer

Explanation: The volume of air passing per second is first calculated and multiplying it by 2 gives the maximum air passing per second. The maximum air passing per second is used in the calculation of the area of the outlet opening.

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