This set of Refrigeration Objective Questions & Answers focuses on “Refrigerant Compressors – 2”.

1. What is the work done during an isothermal compression?

a) 2.3 mRT_{1} ln (p_{2} / p_{1})

b) 2.3 mRP_{1} ln (p_{2} / p_{1})

c) 2.3 mRT_{1} ln (p_{1} / p_{2})

d) mRT_{1} ln (p_{2} / p_{1})

View Answer

Explanation: Isothermal process is the process in which temperature remains constant and represented by PV = C. Work done is the area under the curve on PV chart. So, for isothermal process the work done is given by = p

_{1}v

_{1}ln (v

_{1}/ v

_{2}) as by ideal gas equation we can replace PV = mRT and v

_{1}/ v

_{2}by p

_{2}/ p

_{1}which is called as compression ratio.

Work done = mRT

_{1}ln (p

_{2}/ p

_{1}) or 2.3 mRT

_{1}log (p

_{2}/ p

_{1}).

2. What is the work done during a polytropic compression?

a) [n / (n – 1)] mR (T_{2} – T_{1})

b) [n / (n + 1)] mR (T_{2} – T_{1})

c) [n / (n – 1)] mR (T_{2} + T_{1})

d) [n / (n + 1)] mR (T_{2} + T_{1})

View Answer

Explanation: Polytropic process is represented by PV

^{n}= C and where n = polytropic index. Work done is the area under the curve on PV chart. So, for polytropic process the work done by using ideal gas equation PV = mRT and v

_{1}/ v

_{2}by p

_{2}/ p

_{1}which is called as compression ratio.

Work done = [n / (n – 1)] mR (T

_{2}– T

_{1}).

3. What is the work done during an isentropic compression?

a) [n / (n – 1)] mR (T_{2} – T_{1})

b) [n / (n + 1)] mR (T_{2} – T_{1})

c) [n / (n – 1)] mR (T_{2} + T_{1})

d) [n / (n + 1)] mR (T_{2} + T_{1})

View Answer

Explanation: Polytropic process is represented by PV

^{n}= C and where n = polytropic index. Work done is the area under the curve on PV chart. So, for polytropic process the work done by using ideal gas equation PV = mRT and v

_{1}/ v

_{2}by p

_{2}/ p

_{1}which is called as compression ratio.

Work done = [n / (n – 1)] mR (T

_{2}– T

_{1}).

4. What is the expression for optimum intercooler or intermediate pressure P_{2} if the cooling ratio is fixed in a compound compression refrigeration system with intercooling?

a) P_{2} = P_{1} / P_{3}

b) P_{2} = P_{3} / P_{1}

c) P_{2} = √P_{1} x P_{3}

d) P_{2} = ∛P_{1} x P_{3}

View Answer

Explanation: Putting (n / n – 1) = k and dW / dP

_{2}= 0 we get,

T

_{1}P

_{2}

^{k – 1}/ P

_{1}

^{k}= T

_{3}P

_{3}

^{k}/ P

_{2}

^{k + 1}

For the given conditions T

_{1}= T

_{3},

P

_{2}x P

_{2}= P

_{1}x P

_{3}

P

_{2}= √P

_{1}x P

_{3}.

5. Where is the centrifugal compressor mostly suitable?

a) Small displacements and low condensing pressures

b) Small displacements and high condensing pressures

c) Large displacements and low condensing pressures

d) Large displacements and high condensing pressures

View Answer

Explanation: Centrifugal compressor is the type of compressor in which in the simplest form consists of an impeller to which a number of curved vanes are fitted symmetrically. As per the construction, the impeller rotates in an airtight volute casing with inlet and outlet points, and for better performance as per the flow of refrigerant, it is very suitable for larger displacements and low condensing pressures to give the optimum refrigeration.

6. Where is the reciprocating compressor mostly suitable?

a) Small displacements and low condensing pressures

b) Small displacements and high condensing pressures

c) Large displacements and low condensing pressures

d) Large displacements and high condensing pressures

View Answer

Explanation: Reciprocating compressor is the type of compressor in which vapor refrigerant is compressed by the back and forth motion of the piston. As per the construction of piston and cylinder and for better performance as per the flow of refrigerant it is very suitable for small displacements and high condensing pressures to give the optimum refrigeration.

7. What is the value of swept volume?

If, D = 10 mm

L = 6 mm

a) 188.49

b) 94.2477

c) 47.1238

d) 471.23

View Answer

Explanation: Swept volume is the volume covered by the piston in the cylinder by back and forth motion. As we know, Volume = Area x Length

Here, D is diameter so for a circle with diameter D area = π D

^{2}/ 4

So, swept volume = π D

^{2}/ 4 x L = π D

^{2}L / 4 = π x 10 x 10 x 6 / 4 = 471.23.

8. What is the value of the clearance factor?

If, V_{C} = 0.137 and V_{P} = 0.034

a) 0.00465

b) 0.0465

c) 4.029

d) 0.2481

View Answer

Explanation: Piston displacement volume is the volume covered by the piston by back and forth motion in the cylinder and clearance volume is the extra space in the cylinder which is kept for easy circulation of the refrigerant. Clearance factor is the ratio of clearance volume to the piston displacement volume or swept volume.

Clearance factor = V

_{C}/ V

_{P}= 0.137 / 0.034 = 4.029.

9. What is the value of volumetric efficiency?

V_{C} = 0.445, V_{S} = 0.843 and V_{P} = 1.087

a) 1.289

b) 0.5278

c) 1.894

d) 0.775

View Answer

Explanation: Volumetric efficiency is the ratio of the compressor capacity or the suction volume to the piston displacement volume. So, volumetric efficiency is represented as V

_{S}/ V

_{P}. A good compressor has a volumetric efficiency of around 70 – 80 %.

V

_{S}/ V

_{P}= 0.843 / 1.087 = 0.775 = 77.5 %.

10. The main disadvantage of centrifugal compressors is surging.

a) False

b) True

View Answer

Explanation: When refrigeration load decreases due to the reversal flow of refrigerant from compressor to the evaporator and causes stress conditions in the compressor, this phenomenon is called surging which occurs in these types of compressors and is one of the disadvantages.

**Sanfoundry Global Education & Learning Series – Refrigeration & Air Conditioning.**

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