Gas turbines are used mainly for electric power generation and also in jet engines of aircrafts and in turbochargers of internal combustion (IC) engines.
It has limited application in marine engines. Gas turbines have the unique advantage of using any type of fuel, i.e. solid, liquid or gas.
Advantages of Gas Turbine:
- The absence of reciprocating and rubbing members which reduces the vibration and balancing problems
- High reliability
- Low lubricating oil consumption and
- High power to weight ratio
CLASSIFICATION OF GAS TURBINES:
Gas turbines are classified according to three factors,
- Combustion process
- Path of working substance
- Action of combustion gases in turbine
So, from this figure there are:
Two types of gas turbine:
- Open cycle gas turbine
- Closed cycle gas turbine
Open cycle gas turbine:
-Working on Joule cycle or Brayton cycle
-Air is compressed in a rotary compressor and passed into a combustion chamber where fuel is burnt, the products of combustion are made to impinge over rings of turbine blades with high velocity and work is produced. After the work done by the combustion products, rest are given to the atmosphere.
-60% of work produced is used to drive the compressor and rest is available as useful power
-For starting purpose, it is first motored to minimum speed, called coming in speed, before the fuel is turned on
-About 5% power output by the motor is used to start the turbine
To improve the turbine performance intercooler, heat exchanger and reheat cycles are used
with simple gas turbine cycle.
Working of an Open Cycle Single Stage Gas Turbine
A simple open cycle gas turbine is represented in Fig below.
It consists of a compressor, a combustion chamber and a turbine.
The compressor and turbine co by a common shaft with a suitable flange.
Air from the atmosphere is taken and compressed to a pressure ratio ranging from 2-8 before assing to the combustion chamber where the fuel is injected.
The fuel burns and the temperature is raised at constant pressure. Then, it passes to the turbine where it expands to its original pressure before being exhausted to atmosphere. Fig below.
Methods to Improve the Thermal Efficiency of a Single Stage Gas Turbine
- By using a regenerator to heat the compressed air before entering the combustion chamber as in Fig below thereby making use of the heat in the exhaust gases before leaving to the atmosphere.
2. By using a multistage compressor with intercooling to reduce the work of compression.
3. By using a multistage turbine to reduce the temperature of exhaust gases before leaving the turbine.
Components of Open Cycle Gas Turbine:
- Burner or Combustion Chamber
The main features of open-cycle gas turbines include the following.
It is applicable for combined cycles
Long life and reliability is high
It is used to generate electric power as well as in aircraft propulsion
Open cycle gas turbine efficiency is high thermal up to 44%.
The back work ratio is high up to 50% as compared with less percent within steam power plants.
The start-up time for the turbine is fast like 2 mins as compared with steam-propulsion systems because it takes four hours.
Advantages of Open Cycle Gas Turbine:
The gas used in this turbine can be any type. For instance, helium and the combination of helium & carbon dioxide can give high efficiency, so it is
The blades of the turbine are not contaminated through the combustion products.
This gas turbine working doesn’t depend on the pressure of the atmospheric, so therefore any type of force can be used so that specific plant output can be improved. Once the pressure is applied to the turbine, the size of components used in this can be decreased.
The turbine regulation is very simple
used in nuclear plants.
Simplicity, less weight, and cost
Low thermal stresses can be present at different loads due to the stable temperatures.
Disadvantages of Open Cycle Gas Turbine:
An exchanger with fast heat is required whenever high forces are applied to the input of the compressor.
In this type of turbine, the combustion chamber is more efficient and is applicable to a larger volume of air.
The results of poor heat transfer and poor combustion efficiency are mainly due to the heat exchanger based on the indirect type.
The part-load efficiency of these turbines is poor.
They are sensitive.