Refrigeration Multiple Choice Questions on “Multiple Evaporators and Compressor System – 1”.

1. Multiple evaporator system is used to maintain different temperatures at different points.

a) False

b) True

Answer: b

Clarification: Multiple evaporator system is used to maintain different temperatures at a different point as per the specific requirements. For example, fresh fruits and fresh vegetables need to be stored in optimum temperatures and frozen products, dairy products to be stored at different optimum temperatures; for such cases, multiple evaporator system is beneficial.

2. Vegetables, Fruits, Frozen products can be maintained at the same temperatures and humidity.

a) False

b) True

Answer: a

Clarification: Multiple evaporator system is used to maintain different temperatures at a different point as per the specific requirements. For example, fresh fruits and fresh vegetables need to be stored in optimum temperatures and frozen products, dairy products to be stored at different optimum temperatures; for such cases, multiple evaporator system is beneficial. Hence, these different products cannot be maintained at the same temperatures and humidity; otherwise, it will lead to ruining of the products.

3. Which of the following system is used for maintaining at the same temperatures when food products kept in different compartments?

a) Evaporator at the same temperature with a single compressor

b) Evaporator at the same temperature with multiple compressors

c) Evaporator at different temperature with a single compressor

d) Evaporator at different temperature with multiple compressors

Answer: a

Clarification: For the given case, Multiple evaporators at the same temperatures with a single compressor and expansion valve is used. Though food products are kept in different compartments, the temperatures to be maintained is the same. Hence, this arrangement gives the effective output.

4. What do P_{C} and P_{E} represent?

a) Evaporator and expansion pressure

b) Compression and expansion pressure

c) Condenser and evaporator pressure

d) Intercooling pressure

Answer: c

Clarification: P_{C} is for condenser pressure. The pressure at which latent heat is rejected from the refrigerant and converting vapor form to liquid form. P_{E} is for evaporator pressure. The pressure at which latent heat of vaporization is absorbed from the medium and phase change from liquid to vapor is carried out. Attaining and maintaining these pressures is essential for the smooth operation of the cycle.

5. What does process 1-2 represent?

a) Evaporation

b) Compression

c) Condensation

d) Expansion

Answer: b

Clarification: Process 1-2 represent compression. This type of compression is isentropic compression, which is carried out in the compressor, and pressure is enhanced from P_{E} to P_{C}.

6. What does process 2-3 represent?

a) Evaporation

b) Compression

c) Condensation

d) Expansion

Answer: c

Clarification: Process 2-3 represent condensation. This process results in the phase change of refrigerant from superheated vapor to liquid form at the condenser pressure.

7. What does process 2-3 represent?

a) Evaporation

b) Compression

c) Condensation

d) Expansion

Answer: d

Clarification: Process 3-4 represent expansion. This process results in expanding the liquid refrigerant from condenser pressure to evaporative pressure.

8. What does process 4-1 represent?

a) Evaporation

b) Compression

c) Condensation

d) Expansion

Answer: a

Clarification: Process 4-1 represent evaporation. This process results in a phase change of refrigerant from liquid to vapor form at the evaporator pressure. Multiple evaporators at the same temperatures are used. Evaporation process gives the refrigeration effect.

9. What is the total mass of refrigerant flowing through the compressor if the masses flowing through each evaporator are m_{1}, m_{2}, and m_{3}?

a) m_{1} – m_{2} – m_{3}

b) m_{1} + m_{2} – m_{3}

c) m_{1} – m_{2} + m_{3}

d) m_{1} + m_{2} + m_{3}

Answer: d

Clarification: m_{1} is the mass flowing through EP_{1} i.e. first evaporator, which is given by 210 Q_{1} / h_{1} – h_{4}

m_{2} is the mass flowing through EP_{2} i.e., the second evaporator, which is given by 210 Q_{2} / h_{1} – h_{4}

m_{3} is the mass flowing through EP_{3} i.e., the third evaporator, which is given by 210 Q_{3} / h_{1} – h_{4}

As the evaporators are connected in such a way that masses get converged and then enter into the compressor.

So, mass flowing through the compressor will be the summation of masses flowing through each evaporator.

Mass of refrigerant flowing through compressor, m = m_{1} + m_{2} + m_{3}.

10. What is the value of work done in the following arrangement?

a) m_{1} – m_{2} – m_{3} (h_{2} – h_{1})

b) m_{1} + m_{2} + m_{3} (h_{2} – h_{1})

c) m_{1} + m_{2} – m_{3} (h_{2} – h_{1})

d) m_{1} + m_{2} + m_{3} (h_{4} – h_{1})

Answer: b

Clarification: m_{1} is the mass flowing through EP_{1} i.e. first evaporator, which is given by 210 Q_{1} / h_{1} – h_{4}

m_{2} is the mass flowing through EP_{2} i.e., the second evaporator, which is given by 210 Q_{2} / h_{1} – h_{4}

m_{3} is the mass flowing through EP_{3} i.e., the third evaporator, which is given by 210 Q_{3} / h_{1} – h_{4}

As the evaporators are connected in such a way that masses get converged and then enter into the compressor.

So, mass flowing through the compressor will be the summation of masses flowing through each evaporator.

Mass of refrigerant flowing through compressor, m = m_{1} + m_{2} + m_{3}

The enthalpy change across the compressor is h_{1} and h_{2}.

Work done = mass of refrigerant flowing through the compressor x enthalpy change across the compressor.

= m_{1} + m_{2} + m_{3} (h_{2} – h_{1}).

11. What is the value of the refrigeration effect in the following arrangement?

a) m_{1} – m_{2} – m_{3} (h_{2} – h_{1})

b) m_{1} + m_{2} + m_{3} (h_{4} – h_{1})

c) m_{1} + m_{2} + m_{3} (h_{1} – h_{4})

d) m_{1} + m_{2} – m_{3} (h_{4} – h_{1})

Answer: c

Clarification: m_{1} is the mass flowing through EP_{1} i.e. first evaporator, which is given by 210 Q_{1} / h_{1} – h_{4}

m_{2} is the mass flowing through EP_{2} i.e., the second evaporator, which is given by 210 Q_{2} / h_{1} – h_{4}

m_{3} is the mass flowing through EP_{3} i.e., the third evaporator, which is given by 210 Q_{3} / h_{1} – h_{4}

As the evaporators are connected in such a way that masses get converged and then enter into the compressor.

Total mass of refrigerant flowing through the evaporator, m = m_{1} + m_{2} + m_{3}

The enthalpy change across the evaporator is h_{1} and h_{4}.

Total Refrigeration effect = Total mass of refrigerant flowing through the evaporator x enthalpy change across the evaporator

= m_{1} + m_{2}+ m_{3} (h_{1} – h_{4}).

12. What is the value of C.O.P. in the following arrangement?

a) (h_{1} – h_{4}) / (h_{2} – h_{1})

b) (h_{1} – h_{4}) / (h_{4} – h_{1})

c) (h_{1} – h_{2}) / (h_{4} – h_{1})

d) (h_{2} – h_{3}) / (h_{2} – h_{1})

Answer: a

Clarification: m_{1} is the mass flowing through EP_{1} i.e. first evaporator, which is given by 210 Q_{1} / h_{1} – h_{4}

m_{2} is the mass flowing through EP_{2} i.e., the second evaporator, which is given by 210 Q_{2} / h_{1} – h_{4}

m_{3} is the mass flowing through EP_{3} i.e., the third evaporator, which is given by 210 Q_{3} / h_{1} – h_{4}

As the evaporators are connected in such a way that masses get converged and then enter into the compressor.

Total mass of refrigerant flowing through the evaporator, m = m_{1} + m_{2} + m_{3}

The enthalpy change across the evaporator is h_{1} and h_{4}.

Total Refrigeration effect = Total mass of refrigerant flowing through the evaporator x enthalpy change across the evaporator

= m_{1} + m_{2} + m_{3} (h_{1} – h_{4})

So, mass flowing through the compressor will be the summation of masses flowing through each evaporator.

Mass of refrigerant flowing through compressor, m = m_{1} + m_{2} + m_{3}

The enthalpy change across the compressor is h_{1} and h_{2}.

Work done = mass of refrigerant flowing through the compressor x enthalpy change across the compressor.

= m_{1} + m_{2} + m_{3} (h_{2} – h_{1})

So, C.O.P. = Total Refrigeration effect / Work done

= (m_{1} + m_{2} + m_{3}) (h_{1} – h_{4}) / (m_{1} + m_{2} + m_{3}) (h_{2} – h_{1})

= (h_{1} – h_{4}) / (h_{2} – h_{1}).

13. What is the value of C.O.P. in the following arrangement?

If the values of enthalpy at point 1, 2 and 3 are 184, 210 and 75 kJ/kg and masses are m_{1} = 13 kg/min, m_{2} = 15 kg/min and m_{3} = 28.085 kg/min

a) 6.23

b) 1.88

c) 4.19

d) 0.2386

Answer: c

Clarification: Given: h_{1} = 184 kJ/kg, h_{2} = 210 kJ/kg and h_{3} = h_{4} = 75 kJ/kg

Though masses are given but for work done and refrigeration effect, summation is same. So, it is a redundant data.

C.O.P. = R.E. / Work done

= (h_{1} – h_{4}) / (h_{2} – h_{1})

= (184 – 75) / (210 – 184)

= 109 / 26

= 4.19.

14. What is the value of mass flowing through the compressor in the following arrangement?

If the values of enthalpy at point 1, 2 and 3 are 184, 210 and 84 kJ/kg and refrigeration capacities are 10, 15 and 20 TR respectively.

a) 94.5 kg/min

b) 32.6 kg/min

c) 49.5 kg/s

d) 94.5 kg/s

Answer: a

Clarification: Given: h_{1} = 184 kJ/kg, h_{2} = 210 kJ/kg and h_{3} = h_{4} = 84 kJ/kg

Q_{1} = 10 TR, Q_{2} = 15 TR and Q_{3} = 20 TR

As we know, m = 210 Q / (h_{1} – h_{4})

m_{1} is the mass flowing through EP_{1} i.e. first evaporator, which is given by 210 Q_{1} / h_{1} – h_{4},

m_{1} = 210 x 10 / (184 – 84) = 2100 / 100 = 21 kg/min

m_{2} is the mass flowing through EP_{2} i.e. second evaporator, which is given by 210 Q_{2} / h_{1} – h_{4},

m_{2} = 210 x 15 / (184 – 84) = 3150 / 100 = 31.5 kg/min

m_{3} is the mass flowing through EP_{3} i.e., the third evaporator, which is given by 210 Q_{3} / h_{1} – h_{4},

m_{3} = 210 x 20 / (184 – 84) = 4200 / 100 = 42 kg/min

So, mass flowing through the compressor will be the summation of masses flowing through each evaporator.

Mass of refrigerant flowing through compressor, m = m_{1} + m_{2} + m_{3}

m = 21 + 31.5 + 42 = 94.5 kg/min.

15. What is the value of Refrigeration effect in the following arrangement?

If the values of enthalpy at point 1, 2 and 3 are 170, 240 and 107 kJ/kg and masses are m_{1} = 13 kg/min, m_{2} = 15 kg/min and m_{3} = 19 kg/min

a) 3000 kJ/min

b) 2961 kW

c) 2861 kW

d) 2961 kJ/min

Answer: d

Clarification: Given: h_{1} = 170 kJ/kg, h_{2} = 240 kJ/kg and h_{3} = h_{4} = 107 kJ/kg

Total mass of refrigerant flowing through the evaporator, m = m_{1} + m_{2} + m_{3}

Total mass flowing through the evaporator = m_{1} + m_{2} + m_{3} = 13 + 15 + 19 = 47 kg/min

The enthalpy change across the evaporator is h_{1} and h_{4}.

Total Refrigeration effect = Total mass of refrigerant flowing through the evaporator x enthalpy change across the evaporator

= m_{1} + m_{2} + m_{3} (h_{1} – h_{4})

Refrigeration effect = 47 (170 – 107) = 47 x 63 = 2961 kJ/min.

16. What is the power required to run the following arrangement?

If the values of enthalpy at point 1, 2 and 3 are 193, 258 and 56 kJ/kg and masses are m_{1} = 11 kg/min, m_{2} = 21 kg/min and m_{3} = 28 kg/min.

a) 65 kJ/min

b) 65 kW

c) 3900 kW

d) 4000 kJ/min

Answer: b

Clarification: Given: h_{1} = 193 kJ/kg, h_{2} = 258 kJ/kg and h_{3} = h_{4} = 56 kJ/kg

As, the evaporators are connected in such a way that masses get converged and then enter into the compressor.

Total mass of refrigerant flowing through the evaporator, m = m_{1} + m_{2} + m_{3}

Total mass flowing through the compressor = m_{1} + m_{2} + m_{3} = 11 + 21 + 28 = 60 kg/min

The enthalpy change across the compressor is h_{1} and h_{2}.

Work done = Total mass of refrigerant flowing through the compressor x enthalpy change across the compressor.

= m_{1} + m_{2} + m_{3} (h_{2} – h_{1})

Work done = 60 (258 – 193) = 60 x 65 = 3900 kJ/min

Power = Work done / 60 = 3900 / 60 = 65 kW.