Question

Question No.5: With help of T-s diagram, explain the effects of variables on efficiency of the Rankine cycle

Question No.6: State various methods to improve efficiency of Brayton cycle. With suitable diagrams, explain any two of them

Answer 1

Question No.5

The variable which affect the efficiency of rankine cyle are

1.condensor pressure 2. Superheating of steam 3. boiler pressure

This is explained in comparison with the simple Rankine cycle (Fig 1)

1. Refer fig 2 By lowering the condensor pressure, the temperature at which the heat is rejected also lowered. Thus the amount of heat transfer increases (that can be see in the figure - shaded portion ) . Thus the effect is increase in the thermal efficiency of the cycle.  

2. Refer fig 3 Super heating the steam from the boiler increases the temperature of steam without changing the boiler pressure. This will add the heat energy to the steam and increases the thermal efficiency. This dcreases the moisture content in the steam during the expansion.  

3. Refer fig 4 , by increasing the operating pressure of boiler, the temperature at which boiling takes place also increases. This increases the heat added to the steam and increases the thermal efficiency of the cycle.

Question No.6

The various methods to improve efficiency of Brayton cycle are

1. Regeneration 2. Increasing the pressure ratio 3. Inter cooling 4. Reheating

Intercooling

Refer figure 6

Inter cooling can be done with staged compression. The compression is divided in to two stages. Air is compressed in the first stage , is cooled to initial temperature in the intercooler, then compressed further in the second stage. 1-2'-5-6 is the ideal cycle without intercooling. 1-2-3-4-5-6 is the cycle with intercooling. The cycle 2-3-4-2' is added to the basic cycle. We can see that there is more work capacity (included area is more), there is more heat supply also. So the efficiency of the cycle can be increased by intercooling.

Reheat

Refer fig 7

In this the expansion in the turbine is done in two stages. In between reheat is provided (4-5-6-4' is added to the basic cycle due to reheat). The work capacity and heat supply also increases. The efficiency can be increased.