Vapor and Gas Refrigeration

Question 1
Consider an ideal vapour compression refrigeration cycle working on R-134a refrigerant. The COP of the cycle is 10 and the refrigeration capacity is 150 kJ/kg. The heat rejected by the refrigerant in the condenser is _______kJ/kg (round off to the nearest integer).
A
125
B
185
C
215
D
165
GATE ME 2021 SET-2   Refrigeration and Air-conditioning
Question 1 Explanation: 
\begin{aligned} \mathrm{RE} &=150 \mathrm{~kJ} / \mathrm{kg} \\ \mathrm{COP} &=10 \\ \mathrm{COP} &=\frac{Q_{L}}{W_{i n}}\\ \Rightarrow\qquad W_{\text {in }} &=15 \mathrm{~kJ} / \mathrm{kg} \\ Q_{\mathrm{H}}=Q_{\mathrm{C}} &=Q_{L}+W_{\text {in }} \\ &=150+15 \\ &=165 \mathrm{~kJ} / \mathrm{kg} \end{aligned}

Question 2
Consider an ideal vapour compression refrigeration cycle. If the throttling process is replaced by an isentropic expansion process, keeping all the other processes unchanged, which one of the following statements is true for the modified cycle?
A
Coefficient of performance is higher than that of the original cycle.
B
Coefficient of performance is lower than that of the original cycle.
C
Coefficient of performance is the same as that of the original cycle.
D
Refrigerating effect is lower than that of the original cycle.
GATE ME 2019 SET-1   Refrigeration and Air-conditioning
Question 2 Explanation: 
Due to isentropic expansion instead of throttling
1. R.E increases
2. Work input reduces
\therefore \mathrm{COP}=\frac{\mathrm{R} \cdot \mathrm{E}}{\mathrm{W}_{\mathrm{in}}}

Question 3
A standard vapor compression refrigeration cycle operating with a condensing temperature of 35^{\circ}C and an evaporating temperature of -10^{\circ}C develops 15 kW of cooling. The p-h diagram shows the enthalpies at various states. If the isentropic efficiency of the compressor is 0.75, the magnitude of compressor power (in kW) is _________ (correct to two decimal places).
A
8
B
10
C
12
D
6
GATE ME 2018 SET-2   Refrigeration and Air-conditioning
Question 3 Explanation: 
\begin{aligned} R C &=15 \mathrm{kW} \\ R C &=\dot{m} \times\left(h_{1}-h_{4}\right) \\ 15 &=\dot{m} \times(400-250) \\ \dot{m} &=0.1 \mathrm{kg} / \mathrm{sec} \\ \omega_{\text {isentropic }} &=\left(h_{2}-h_{1}\right)=(475-400)=75 \mathrm{kJ} / \mathrm{kg}\\ \eta_{C} &=\frac{\omega_{\text {isentropic }}}{\omega_{\text {actual }}} \\ \omega_{\text {actual }} &=\frac{75}{0.75}=100 \mathrm{kJ} / \mathrm{kg} \\ \omega_{\text {actual }} &=\dot{m} \times \omega_{\text {actual }}=0.1 \times 100 \\ P_{\dot{m}} &=10 \mathrm{kW} \end{aligned}
Question 4
In the vapour compression cycle shown in the figure, the evaporating and condensing temperatures are 260 K and 310 K, respectively. The compressor takes in liquid-vapour mixture (state 1) and isentropically compresses it to a dry saturated vapour condition (state 2). The specific heat of the liquid refrigerant is 4.8 kJ/kg-K and may be treated as constant. The enthalpy of evaporation for the refrigerant at 310 K is 1054 kJ/kg.

The difference between the enthalpies at state points 1 and 0 (in kJ/kg) is ____________
A
1000.21kJ/kg
B
1103.51kJ/kg
C
1212.90kJ/kg
D
1443.78kJ/kg
GATE ME 2016 SET-3   Refrigeration and Air-conditioning
Question 4 Explanation: 
Given data:
\begin{aligned} T_{e} &=260 \mathrm{K} \\ T_{c} &=310 \mathrm{K} \\ c_{p l} &=4.8 \mathrm{kJ} / \mathrm{kgK} \\ h_{fg} &=1054 \mathrm{kJ} / \mathrm{kg} \text { at } T_{c}=310 \mathrm{K} \end{aligned}

Taking reference temperature T , at which entropy is s_{T}
\begin{aligned} \therefore \quad s_{3}-s_{T} &=c \ln \frac{T_{3}}{T} \\ s_{3} &=s_{T}+c \ln \frac{T_{3}}{T}=s_{T}+4.8 \ln \frac{310}{T} \\ \text { Similarly, } s_{0} &=s_{T}+c \ln \frac{260}{T}=s_{T}+4.8 \ln \frac{260}{T} \\ \therefore \quad s_{2}-s_{3} &=\frac{h_{f g}}{T}=\frac{1054}{310} \\ s_{2} &=s_{3}+\frac{1054}{310} \end{aligned}
Substitute the value of s_{3} in above equation, we get
\begin{aligned} s_{2} &=s_{T}+4.8 \ln \frac{310}{T}+\frac{1054}{310} \\ s_{2} &=s_{1} \\ s_{1} &=s_{T}+4.8 \ln \frac{310}{T}+\frac{1054}{310} \\ h_{1}-h_{0} &=T_{e}\left[s_{1}-s_{0}\right] \\ =& 260 \times\left[s_{T}+4.8 \ln \frac{310}{T}+\frac{1054}{310}\right.\\ &\left.-s_{T}-4.8 \ln \frac{260}{T}\right] \\ &=260\left[4.8 \ln \left(\frac{310}{T} \times \frac{T}{260}\right)+\frac{1054}{310}\right] \\ &1103.51\mathrm{kJ}/\mathrm{kg} \end{aligned}
Question 5
A refrigerator uses R-134a as its refrigerant and operates on an ideal vapour-compression refrigeration cycle between 0.14 MPa and 0.8 MPa. If the mass flow rate of the refrigerant is 0.05 kg/s, the rate of heat rejection to the environment is _________ kW.
Given data:
At P = 0.14 MPa, h =236.04 kJ/kg, s =0.9322 kJ/kg-K
At P = 0.8 MPa, h = 272.05kJ/kg (superheated vapour)
At P = 0.8 MPa, h = 93.42 kJ/kg (saturated liquid)
A
3.43 kW
B
8.93 kW
C
2.45 kW
D
4.56 kW
GATE ME 2016 SET-2   Refrigeration and Air-conditioning
Question 5 Explanation: 
Given data:
\begin{aligned} m&=0.05 \mathrm{kg} / \mathrm{s}\\ \text{At }\quad p_{e}&=0.14 \mathrm{MPa} \\ h_{1}&=236.04 \mathrm{kJ} / \mathrm{kg} \\ s_{1}&=0.9322 \mathrm{kJ} / \mathrm{kg} \mathrm{K}\\ \text{At }\quad p_{c}&=0.8 \mathrm{MPa} \\ h_{3}&=h_{4}=h_{f}=93.42 \mathrm{kJ} / \mathrm{kg} \end{aligned}

h_{2}=272.02 \mathrm{kJ} / \mathrm{kg}
Rate of heat rejection to the environment,
\begin{aligned} Q_{2-3} &=m\left[h_{2}-h_{3}\right]=0.05 \times[275.02-93.42] \\ &=8.93 \mathrm{kW} \end{aligned}
Question 6
Refrigerant vapor enters into the compressor of a standard vapor compression cycle at -10^{\circ} (h = 402 kJ/kg) and leaves the compressor at 50^{\circ}(h = 432 kJ/kg). It leaves the condenser at 30^{\circ} (h = 237 kJ/kg). The COP of the cycle is ________
A
9.5
B
2.5
C
6.5
D
5.5
GATE ME 2015 SET-3   Refrigeration and Air-conditioning
Question 6 Explanation: 
Given data:
\begin{array}{l} h_{1}=402 \mathrm{kJ} / \mathrm{kg}, \quad h_{2}=432 \mathrm{kJ} / \mathrm{kg} \\ h_{3}=h_{4}=237 \mathrm{kJ} / \mathrm{kg} \end{array}

\begin{aligned} \mathrm{COP} &=\frac{h_{1}-h_{4}}{h_{2}-h_{1}}=\frac{402-237}{432-402}=\frac{165}{30} \\ &=5.5 \end{aligned}
Question 7
Which one of the following is a CFC refrigerant?
A
R744
B
R290
C
R502
D
R718
GATE ME 2014 SET-1   Refrigeration and Air-conditioning
Question 7 Explanation: 
\begin{array}{l} \mathrm{R} 744-\mathrm{CO}_{2} \\ \mathrm{R} 290-\mathrm{C}_{3} \mathrm{H}_{8}(\text { Propane }) \\ \mathrm{R} 502-\mathrm{CHClF}_{3}+\mathrm{CCIF}_{2} \mathrm{CF}_{3} \\ \mathrm{R} 718-\text { Water } \end{array}
Question 8
A refrigerator operates between 120kPa and 800kPa in an ideal vapor compression cycle with R - 134 a as the refrigerant. The refrigerant enters the compressor as saturated vapor and leaves the condenser as saturated liquid. The mass flow rate of the refrigerant is 0.2 kg/s. properties for R-134 a are as follows.


The power required for the compressor in kW is
A
5.94
B
1.83
C
7.9
D
39.5
GATE ME 2012   Refrigeration and Air-conditioning
Question 8 Explanation: 
Power required for the compressor,
\begin{aligned} P &=m\left(h_{2}-h_{1}\right)=0.2(276.45-237) \\ &=7.89 \mathrm{kW} \approx 7.9 \mathrm{kW} \end{aligned}
Question 9
A refrigerator operates between 120kPa and 800kPa in an ideal vapor compression cycle with R - 134 a as the refrigerant. The refrigerant enters the compressor as saturated vapor and leaves the condenser as saturated liquid. The mass flow rate of the refrigerant is 0.2 kg/s. properties for R-134 a are as follows.


The rate at which heat is extracted, in kJ/s from the refrigerated space is
A
28.3
B
42.9
C
34.4
D
14.6
GATE ME 2012   Refrigeration and Air-conditioning
Question 9 Explanation: 
From given R-134a tables, we get
\begin{aligned} h_{1}&=237 \mathrm{kJ} / \mathrm{kg} \\ h_{2}&=276.45 \mathrm{kJ} / \mathrm{kg} \\ h_{3}&=h_{4}=95.5 \mathrm{kJ} / \mathrm{kg} \end{aligned}

Mass flow rate: m=0.2 \mathrm{kg} / \mathrm{s}
Rate of heat extracted:
\begin{aligned} Q_{4-1} &=m\left(h_{1}-h_{4}\right)=0.2(237-95.5) \\ &=28.3 \mathrm{kW} \end{aligned}
Question 10
In an ideal vapour compression refrigeration cycle, the specific enthalpy of refrigerant (in kJ/kg) at the following states is given as:
Inlet of condenser: 283
Exit of condenser: 116
Exit of evaporator: 232
The COP of this cycle is
A
2.27
B
2.75
C
3.27
D
3.75
GATE ME 2009   Refrigeration and Air-conditioning
Question 10 Explanation: 
Given data:
\begin{aligned} h_{2} &=283 \mathrm{kJ} / \mathrm{kg} \\ h_{3} &=116 \mathrm{kJ} / \mathrm{kg} \\ &=h_{4} \\ h_{1} &=232 \mathrm{kJ} / \mathrm{kg} \end{aligned}

Coefficient of performance,
\begin{aligned} \mathrm{COP} &=\frac{\text { Refrigerating effect: Re }}{\text { Work input: } w} \\ \text { where } \mathrm{Re} &=h_{1}-h_{4}=232-116 \\ &=116 \mathrm{kJ} / \mathrm{kg}\\ \text { and } \quad w &=h_{2}-h_{1}=283-232 \\ &=51 \mathrm{kJ} / \mathrm{kg} \\ \therefore \quad \mathrm{COP} &=\frac{116}{51}=2.27 \end{aligned}
There are 10 questions to complete.

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