Q15. Ideal gas in a cylinder is heated at constant volume from a temperature of 600°C to a temperature of 1000°C, and then further heated at constant pressure from 1000°C to 1300°C. The change in specific entropy in the first process is 0.283 kJ/kg K. After being heated, the gas expands isentropically to fifteen times its initial volume and a temperature of 306°C.
(a) Sketch the processes on p-V and T-s diagrams.
(b) Determine EACH of the following:
(i) The values of Cv, Cp and γ for the gas
(ii) The change in specific entropy during the constant pressure process
(iii) The work transfer per kg of gas during isentropic expansion.
Q4. A regenerative steam power cycle operates between pressures of 50 bar and 0.08 bar. The maximum temperature is 450°C. The optimum mass of steam is bled for feed heating at a pressure of 5 bar. A direct mixing feed heater is used.
(a) Sketch the T-s diagram for the cycle.
(b) Determine for the cycle EACH of the following:
(i) the specific work output (taking account of feed pump work);
(ii) the thermal efficiency.
Note: Expansion in the turbine and compression in the feed pumps are isentropic.
Q16. Sea water is to be used to cool engine cooling water in a single pass shell and tube heat exchanger. The cooling water is to enter the tubes at a temperature of 80°C and to be cooled to 30°C. The flow rate of cooling water will be 2 kg/s. The sea water will enter at a temperature of 12°C and its flow rate will be 12.5 kg/s. The specific heat capacities of both cooling water and sea water may be taken as 4.2 kJ/kg K. The overall heat transfer coefficient is expected to be 3000 W/m2 K, bared on the outside surface area of the tubes. The tube outside diameter is to be 50 mm. Determine EACH of the following:
(a) the outlet temperature of the sea water
(b) the logarithmic mean temperature difference for EACH of the following cases:
(i) counter flow
(ii) parallel flow
(c) the total length of tubing required for EACH of the following cases:
(ii) parallel flow.
Q16. At the beginning of compression in a single stage, single acting reciprocating air compressor, the air is at a pressure of 1.01 bar and a temperature of 28°C. The delivery valve opens at a pressure of 6.2 bar. The delivery temperature is 175°C. The bore diameter and stroke length are 0.25 m and 0.32 m respectively. The clearance volume is 4% of the swept volume and the compressor runs at 500 rev/min.
(a) Sketch the p-V diagram.
(i) the index of compression
(ii) the volumetric efficiency
(iii) the indicated work per kg of air
(iv) the free air capacity in m3/min, given that free air conditions are 1.03 bar and 25°C.
Note: For air, R = 0.287KJ/kg K.
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