Question

Consider the Rankine power cycle using vapor and liquid.

a: Explain what kinds of problems the Carnot Vapor Cycle has.

b: Draw a T-s diagram for the ideal Rankine cycle and label each process explicitly.

c: There is a method to superheat steam to high temperature in order to improve the efficiency of the ideal Rankine cycle. Explain its advantages and disadvantages explicitly.

d: Name the gas power cycle which uses the same four processes in its cycle and draw the T-s diagram and the P-v diagram for the gas power cycle.

Answer 1

a) Carnot cycle has following problems-   

Dryness fraction at lower stages of turbine expansion will be less if expansion take place along 3'-4'

It is not possible to control condensation process in such a manner to get the desired quality of steam as given by state 1'

It is not possible to design a pump which can handle liquid vapour mixture phase

Pump work is more

It is not possible to add heat at constant temperature and with pressure continuosly decreasing.

b) 1234 is rankine cycle

c) The main advantage of superheating steam is to increase the efficiency and dryness fraction after expansion of steam from turbine. The mean temperature of heat addition increase so efficiency increase. Net work also increase.

The disadvantage is that heat required increases. Also maximum temperature increases so the material cost increases. Specific volume of steam increase so size of boiler increase.

d) The cycle use in gas power cycle which is exactly similar to rankine cycle is Brayton cycle which has all four same processes as that rankine cycle.

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