A simple Cycle Gas turbine follows the Brayton cycle. In many aircrafts gas turbine engines are used in their simple forms as the aircraft is needed to be light. While gas turbines which are used in land or marine application can be equipped with additional parts to increase the efficiency. Modifications that are usually seen in Gas Turbine cycle plants are regeneration, reheating and intercooling.
Lets have a look at the diagram of an open cycle and close cycle gas turbine –
open cycle gas turbine
It is clearly visible that in open cycle GT the exhaust gas is not been used in any other purpose. It is discharged to atmosphere. But in closed cycle gas turbine the exhaust gas is used in an heat exchanger as a result the working fluid is reusable. It is also a clean cycle as the working fluid is recirculated.
closed cycle gas turbine
Lets have a look at the T-s and P-V diagram of an ideal Brayton Cycle or Gas Turbine cylce .
The simple gas turbine cycle consists of two isentropic and two isobaric processes. But there are some deviations in the actual cycle of gas turbine in comparing to the ideal Brayton cycle. The following diagram will focus on the deviations. The deviations are mainly due to the irreversibilities.
Regeneration process involves the installation of a heat exchanger in the gas turbine cycle. The heat-exchanger is also known as the recuperator. This heat exchanger is used to extract the heat from the exhaust gas . This exhaust gas is used to heat the compressed air. This compressed and pre-heated air then enters the combustors. When the heat exchanger is well designed , the effectiveness is high and pressure drops are minimal. And when these heat exchangers are used an improvement in the efficiency is noticed. Regenerated Gas turbines can improve the efficiency more than 5 % . Regenerated Gas Turbine work even more effectively in the improved part load applications.
In Intercooling a heat exchanger is used to cool the compressor gases at the time of compression process. When the compressor involves the high and low pressure unit in it, the intercooler could be installed between them to cool down the flow. This cooling process will decrease the work needed for the compression in the high pressure unit. The cooling fluid can be water , air. In marine gas turbines the sea water is used to cool the fluid. It is observed that a successful implementation of the intercooler can improve the gas turbine output.
Gas turbine with regeneration, reheating and intercooling
Reheating is applied in a gas turbine in such a way that it increases the turbine work without increasing the compressor work or melting the turbine materials. When a gas turbine plant has a high pressure and low pressure turbine a reheater can be applied successfully. Reheating can improve the efficiency up to 3 % . A reheater is generally is a combustor which reheat the flow between the high and low pressure turbines.
In jet engines an afterburner is used to reheat. It is attached at the exhaust of the turbine. As a result the thrust is increased. But it takes a lot of fuel to increase the thrust.