Answer:
a. The COP of the cycle is 2.725 b. The COP of the cycle is 2.886
Step-by-step explanation:
Working fluid = R-134a
Evaporator pressure P1 = P4 = 200 kPa
Condenser presser P2 = P3 = 1400 kPa
Isentropic efficiency of the compressor ηc = 0.88
Mass flow rate to compressor m = 0.025kg/s
Sub cooled temperature T3’ = 4.4 C
a. State 1
Obtain the saturation temperature at evaporator pressure. Since, the refrigerant enters the compressor in super heated state
Obtain the saturation temperature from the super heated refrigerant R-134a table at P1 = 200kPa and T(sat) = -10.1 C
Calculate the temperature at state 1. As the refrigerant super heated by 10.1 C when it leaves the evaporator.
T1 = (-10.1) + 10.1 = 0 C
Obtain the specific enthalpy and specific entropy at state 1 from the table at T1 = 0 C and P1 = 200 kPa, which is, h1 = 253.05 kJ/kg and s1 = 0.9698 kJ/kg.K
State 2
Obtain the ideal specific enthalpy and saturation temperature at state 1 from refrigerant R-134a table at P2 = 1400 kPa and s1 = s2 = 0.9698kj/kg.K
Using the interpolation
h(2s) = 285.47 + (0.09698 – 0.9389) (297.10 – 285.47)/(0.9733 – 0.9389)
h(2s) = 295.91 kJ/kg
T(sat@1400kPa) = 52.40 C
State 3 and State 4
Calculate the temperature at state 3
T3 = T(sat@1400kPa) – T3
= 52.40 – 4.4 = 48 C
Obtain the specific enthalpy from the saturated refrigerant R -134a temperature table at T3 = 48 C, which is, h3 = hf = 120.39 kJ/kg
Since state 3 to state 4 is the throttling process so enthalpy remains constant
h4 = h3 = 120.39 kJ/kg
Calculate the actual enthalpy at state 2. Consider the Isentropic efficacy of the compressor
ηc = (h(2s) – h1)/(h2 – h1)
0.88 = (295.91) – (253.05)/h2 – (253.05)
h2 = 301.75 kJ/kg
Calculate the cooling effect or the amount of heat removed in evaporator
Q(L) = m (h1 – h4)
= (0.0025) (253.05 – 120.39)
= 3.317 kW
Therefore, the rate of cooling provided by the evaporator is 3.317 kW
Calculate the power input
W(in) = m (h2 – h1)
= (0.025) (301.75 – 253.05)
= 1.217 kW
Therefore, the power input to the compressor is 1.21 kW
Calculate the Coefficient of Performance
COP = Q(L)/W(in)
= 3.317/1.217
= 2.725
Therefore, the COP of the cycle is 2.725
b. Ideal vapor compression refrigeration cycle
State 1
Since the refrigerant enters the compressor is superheated state. So, obtain the following properties from the superheated refrigerant R-134a at P1 = 200 kPa
X1 = 1, h1 = 244.46kJ/kg, s1 = 0.9377 kJ/kg.K
State 2
Obtain the following properties from the superheated R-134a table at P2 = 1400kPa, which is s1 = s2 = 0.9377kJ/kg.K
Using the interpolation
h2 = 276.12 + (0.9377 – 0.9105) (285.47 – 276.12)/(0.9389 – 0.9105)
= 285.08kJ/kg
State 3
From the saturated refrigerant R-134a, pressure table, at p3 = 1400kPa and x3 = 0
H3 = hg = 127.22 kJ/kg
Since state 3 to state 4 is the throttling process so enthalpy remains constant
H4 = h3 = 127.22 kJ/kg
(hg should be hf because in ideal case it is a should exist as a liquid in state 3)
Calculate the amount of heat removed in evaporator
Q(L) = m (h1 – h4)
= (0.025) (244.46 – 127.22)
= 2.931 kW
Therefore, the rate of cooling provided by the evaporator is 2.931 kW
Calculate the power input to the compressor
W(H) = m (h2 – h1)
= (0.025) (285.08 – 244.46)
= 1.016 kW
Therefore, the power input to the compressor is 1.016 kW
Calculate the COP of the ideal refrigeration cycle
COP = Q(L)/W(in)
= 2.931/1.016 = 2.886
Therefore, the COP of the cycle is 2.886