Q5. Carbon Dioxide is compressed reversibly in a cylinder according to the law pVn = C.
The initial pressure, volume and temperature are 1.02 bar, 0.084 m3 and 30ºC respectively.
The final pressure and temperature are 5.1 bar and 123ºC respectively.
(a) Sketch the p-V and T-s diagrams.
(b) Calculate EACH of the following:
(i) The index of compression
(ii) The work transfer
(iii) The heat transfer
(iv) The change in entropy.
Note: For CO2 γ = 1.33, Relative atomic masses C = 12, O = 16
The universal constant Ro = 8.3145 kJ/kmolK
Q5. An air standard dual combustion cycle operates with a minimum pressure and temperature of 1 bar and 300 K respectively. The volume compression ratio is 23:1.
The maximum pressure is 100 bar and the maximum temperature is 1800 K.
(a) Sketch the cycle on p-V and T-s diagrams.
(i) The temperatures and pressures at the cardinal points in the cycle
(ii) The specific work output
(iii) The cycle efficiency.
Note: For air γ = 1.4 and R = 0.287 kJ/kg K
Q6. A vapour compression refrigeration cycle operates between evaporation and condensation temperatures -15ºC and 35ºC respectively.
The refrigerant enters the compressor as a dry saturated vapour and the expansion valve as saturated liquid. Compression is regarded as isentropic.
Two refrigerants R134a and Ammonia are to be considered.
(a) Sketch the cycle on p-h and T-s diagrams.
(b) Determine EACH of the following:
(i) The specific work and compressor discharge temperature when using R134a
(ii) The specific work and compressor discharge temperature when using Ammonia
(iii) Which system requires the least work for the same cooling load.
Q5. Wet steam at a pressure of 60 bar flows through a steel pipe of inside diameter of 34 mm and wall thickness 5 mm. It is covered with insulation 25 mm thick. The surrounding air temperature is 27ºC and the thermal resistance of the steam film may be ignored.
Determine EACH of the following:
(a) The rate of heat loss per metre run of pipe
(b) The outside surface temperature of the insulation
(c) The percentage reductions in the rate of heat transfer if the insulation thickness is increased by 5 mm.
Note: The thermal conductivity of the steel is 52 W/m K.
The thermal conductivity of the insulation is 0.05 W/m K.
The outside surface coefficient is 14 W/m2K
Q4. At the beginning of compression in a single stage, single acting reciprocating air compressor, the air is at a pressure of 0.98 bar and a temperature of 24ºC. When the delivery valve opens, the pressure is 5.5 bar and the temperature is 145ºC. The cylinder has a diameter of 270 mm and the stroke is 300 mm. The clearance Volume is 5% of the swept volume and the compressor runs at a speed of 500 rev/min.
(a) Sketch the process on a p-V diagram.
(ii) The volumetric efficiency
(iii) The indicated power.
(c) Explain how the polytropic index influences the quantity of air delivered.
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