Power Plant: Numericals on Actual Brayton Cycle, L-5

In competitive examinations like GATE and ESE, numerical solving ability is an important key factor. In objective type paper, quick understanding of the numerical is essential while in conventional papers, step by step solving is needed. Here in this lesson, I have discussed some numericals on actual Brayton Cycle, the aim is to explain the approach to the question. To increase the numerical solving ability, you will need to solve as much questions as possible, solve previous years competitive examination papers for variety of questions.

Numericals on Actual Brayton Cycle

Power Plant: Perfect Intercooling and Numericals , L-8

Similar to perfect reheating conditions for which we have seen the maximum work output in the Brayton Cycle. In case of intercooling also, we can cool down the compressed working fluid at an intermediate pressure up to the ambient atmospheric temperature, that will be the case of perfect intercooling. In the case of perfect intecooling the conditions for minimum work input is discussed in this lesson.

Along with this I have discussed a numerical on use of combined reheating, intercooling and regeneration in the Brayton Cycle.

Perfect Intercooling and Numericals

Power Plant: Reheating and Intercooling in gas turbine, L-7

We have seen that with reheating in Brayton Cycle, the net work output of the cycle increases. Due to metallurgical conditions the maximum temperature in the cycle is fixed, so the perfect reheating will be if we reheat the exhaust from the first stage turbine again to the maximum possible temperature. In the case of perfect reheating the conditions for the maximum work output is discussed in this lesson.

To minimize the compressor work, intercooling is used between the different stages of compressor. In intercooler, a heat exchanger, the compressed working fluid after the compression in the fist stage is cooled, which helps in decreasing the volume and the temperature of the working fluid. The working fluid is again compressed in the second stage of the compressor. Intercooling in the Brayton Cycle decrease the compression work but at the same time it increases the heat supply and the scope of regeneration in the cycle.

Perfect Reheating and Intercooling in gas turbine

Power Plant: Regeneration and Reheating in gas turbine, L-6

Regeneration is a process in which high temperature turbine exhaust is used to increase the temperature of compressed air before entering in the combustion chamber. It is achieved by placing a regenerator i.e. a heat exchanger after compressor and before the combustion chamber as shown below.

Brayton Cycle with Regeneration

In Reheating, turbine exhaust is reheated in another combustion chamber and then further expanded in another turbine. By use of reheating in the gas turbine, net work output of the cycle increase at the cost of efficiency, but at the same time scope of regeneration increases. therefore reheating is generally coupled with regeneration.

Brayton cycle with reheating

Regeneration and reheating in gas turbine

Power Plant: Actual Brayton Cycle, L-4

The processes involved in ideal brayton cycle are not practically possible because of the internal irreversibility and complexity of the system. There will be some pressure drop in pipes of heat exchangers, so heat addition and heat rejection can not be possible at constant pressure and work done by turbine and compressor work are not isentropic as well. Here we will see the later case, losses due to pressure drop are considerably small so we will neglect them. The lesson describes the isentropic efficiency of compressor and turbine and how does the efficiency of actual brayton cycle less than ideal braytoin cycle. It also explains the work ratio and back work ratio.

Power Plant: Effect of pressure ratio, L-3

We have already seen that the efficiency of brayton cycle increases as the pressure ratio increases (in L-2), but that comes with the cost of decrease in the net work output, that ultimately increases the size of the plant for the same power output. Here net work output has more importance than the cycle efficiency. In this lesson we will derive the optimum pressure ratio for maximum work output and the maximum work output. We will also see the maximum possible pressure ratio, its relation with the optimum pressure ratio and variation of net work output with pressure ratio.

Power Plant: Efficiency of Ideal Brayton Cycle, L-2

This is the second lesson in the series of brayton cycle which explains the pressure-volume and temperature-entropy diagram of ideal open and closed brayton cycle. Efficiency for ideal brayton cycle is derived by using basic laws of thermodynamics and we will also see the parameters on which efficiency of the cycle depends upon and efficiency variation with these parameters.