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B Chap 4 - Prob Sugg

C 5. (15 Marks Total) A.

B) Exemples d'examen f

B) Final Fall 2013 - MCG.

: Final Fall 2013 - Solut .

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A

8 December 2013

Page 2 of 6

4. (17 marks total) The sketch shows a standard

refrigeration cycle using R134a as the working fluid. The

QH

valve is an adiabatic throttling process; the compressor is

adiabatic and reversible. The evaporator is a heat exchanger.

condenser

The following states are given

expansion

W

valve

1: saturated liquid at T, = 39.3 .C

3: saturated vapour at T3 = -10 C

evaporator

compressor

3

(a) (2 mark) State the pressures at point 2 and 4 and sketch

I Qc

the processes for the valve, evaporator and compressor on

a T-s diagram.

(b) (3 marks) Calculate the state (temperature or quality, whichever is appropriate) at the outlet of

the expansion valve, point 2.

(c) (2 marks) Calculate the heat transfer in the evaporator in kJ/kg.

(d) (5 marks) Determine the compressor work in kJ/kg and the compressor outlet temperature T4.

(e) (3 marks) Calculate the coefficient of performance of the cycle and compare it to that of a Carnot

cycle working between a hot reservoir at TH = T, and a cold reservoir at Tc = T3 .

(f) (2 marks) If the the refrigerant mass flow is 0.1 kg/s, determine the diameter of pipe required at

point 3 if the flow velocity entering the compressor is to be V, = 10 m/s.

5. (6 marks total) Steam in a cylinder fitted with a frictionless piston expands in a reversible

isothermal process from an initial state of P, = 1000 kPa with a quality x, = 0.95 to a final pressure

of 300 kPa. Calculate the work done and the heat transfer in kJ/kg.

6. (12 marks total) An insulated container holds two volumes of air

separated by a partition. Volume A initially contains 0.1 m' at PAI =

300 kPa and TAI = 120 C, while volume B initially contains 0.1 m3

at PBI = 100 kPa and TB1 = 150 C. The partition is then removed and

the two volumes mix until a uniform state is achieved.

B

(a) (6 marks) Determine the final temperature and pressure.

(b) (6 marks) Determine the change in the entropy of the system and

of the surroundings, assuming that the surroundings are at 20 C.

+

What does this change indicate about the process?

You may assume that air is an ideal gas with constant specific heats.

Total marks for this paper: 60

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