1. What is the formula for COP of an ideal vapour absorption refrigeration system?
Qg = the heat given to the refrigerant in the generator
Qe= the heat absorbed by the refrigerant in the evaporator
a) \(\frac{Q_e}{Q_g}\)
b) \(\frac{Q_g}{Q_e}\)
c) Qg × Qe
d) Qg – Qe
Explanation: The COP of an ideal vapour absorption refrigeration system is the ratio of the heat given to the refrigerant in the generator and the heat absorbed by the refrigerant in the evaporator.
COP = \(\frac{Q_e}{Q_g}\).
2. An ideal vapour absorption refrigeration system may be regarded as a combination of _________
a) Carnot engine and Carnot pump
b) Carnot refrigerant and Carnot pump
c) Carnot engine and Carnot refrigerant
d) Carnot engine alone
Explanation: An ideal vapour absorption refrigeration system may be regarded as a combination of Carnot engine and Carnot refrigerant.
COP = \(\frac{Q_e}{Q_g}\).
3. Find the COP of a vapour absorption refrigeration system using the following data.
Qg = 6600
Qe = 2100
a) 0.45
b) 0.65
c) 0.31
d) 0.35
Explanation: The COP of an ideal vapour absorption refrigeration system is
COP = \(\frac{Q_e}{Q_g}\)
COP = \(\frac{2100}{6600}\) = 0.31.
4. Find the COP of a vapour absorption refrigeration system using the following data.
Te = 268
Tg = 393.2
Tc = 303
a) 1.53
b) 1.76
c) 1.66
d) 1.98
Explanation: The COP of an ideal vapour absorption refrigeration system is
COP = \(\frac{Q_e}{Q_g}\)
= \(\frac{T_e}{T_c – T_e} × \frac{T_g-T_c}{T_g}\)
= \((\frac{268}{303-268})*(\frac{393.2-303}{393.2})\)
= 1.76.
5. The expression \(\frac{T_e}{T_c – T_e}\) in the formula for COP i.e. \(\frac{T_e}{T_c – T_e} × \frac{T_g-T_c}{T_g}\) refers to __________
Te and Tc are the temperature limits for Carnot refrigerator
Tg and Tc are the temperature limits for Carnot engine
a) COP of Carnot engine
b) COP of Carnot refrigerator
c) COP of Carnot pump
d) COP of Bell-Coleman cycle
Explanation: An ideal vapour absorption refrigeration system may be regarded as a combination of Carnot engine and Carnot refrigerant. Hence the expression \(\frac{T_e}{T_c – T_e}\) refers to the COP of Carnot refrigerator and the second expression \(\frac{T_g}{T_c – T_g}\) refers to Carnot engine
6. In case the heat is discharged at different temperatures in condenser and absorber then the formula for COP would be?
Ta = Temperature at which heat is discharged in the absorber
Te and Tc are the temperature limits for Carnot refrigerator
Tg and Tc are the temperature limits for Carnot engine
a) COP = \(\frac{T_e}{T_c – T_e} × \frac{T_g-T_c}{T_g}\)
b) COP = \(\frac{T_e}{T_c – T_e} × \frac{T_g-T_c}{T_g} × T_a\)
c) COP = \(\frac{T_e}{T_c – T_e} × \frac{T_g-T_c}{T_g} ± T_a\)
d) COP = \(\frac{T_e}{T_c – T_e} × \frac{T_g-T_c}{T_g}\)
Explanation: In case the heat is discharged at different temperatures in condenser and absorber then the formula for COP would be
COP = \(\frac{T_e}{T_c – T_e} × \frac{T_g-T_a}{T_g}\).
7. Find the COP of an ideal vapour absorption refrigeration system using the following data.
Qg = 1880
Qe = 1045
a) 0.47
b) 0.65
c) 0.69
d) 0.55
Explanation: The COP of an ideal vapour absorption refrigeration system is
COP = \(\frac{Q_e}{Q_g}\)
COP = \(\frac{1045}{1880}\) = 0.555.
8. What is the fundamental equation of the C.O.P. of Ideal VAR system?
a) Heat absorbed in the evaporator / Work done by a pump
b) Heat absorbed in the evaporator / Heat supplied in the generator
c) Heat absorbed in the evaporator / Work done by compressor + Heat supplied in the generator
d) Heat absorbed in the evaporator / Work done by pump + Heat supplied in the generator
Explanation: VAR uses the generator to do work as well as pump to pressurize the liquid to the generator. So, total work done is the summation of work done by a pump and generator. Refrigeration effect is the heat absorbed in the evaporator.
So, C.O.P. = Refrigeration effect / Total work done
= Heat absorbed in the evaporator / Work done by pump + Heat supplied in the generator.
9. What is the relation between QC, QG, and QE if they represent heat given to the refrigerant in the generator, heat discharging to atmosphere from condenser to absorber and heat absorbed by the refrigerant in the evaporator?
a) QC + QG = QE
b) QC = QG + QE
c) QC + QE = QG
d) QC = QG – QE
Explanation: According to the first law of thermodynamics, heat discharged to the atmosphere or cooling water from the condenser to absorber is equal to the heat given to the refrigerant in a generator, heat absorbed by the refrigerant in the evaporator and heat added to the refrigerant due to pump work. Heat added due to pump work is neglected due to minimal effect.
So, QC = QG + QE
10. What is the equation of VAR system in terms of entropy?
a) QC + QE / TC = QG / TG + QE / TE
b) QG + QC / TC = QG / TG + QE / TE
c) QG + QE / TC = QG / TG + QE / TE
d) QG + QE / TC = QC / TC + QE / TE
Explanation: As vapour absorption system is considered as a perfectly reversible system,
Hence the initial entropy of the system should be equal to the entropy of the system after some change in its conditions.
As entropy is heat absorbed divided by the temperature.
From the first law of thermodynamics, QC = QG + QE
So, the equation in terms of entropy QC / TC = QG / TG + QE / TE
Modified as, QG + QE / TC = QG / TG + QE / TE