Classification of Gas Turbines

 CLASSIFICATION OF GAS TURBINES

Gas turbines are classified as follows:

1. Open Cycle Gas Turbine and

2. Closed Cycle Gas Turbine

The basic difference between the open cycle and closed cycle is the course of flow of the working fluid in the cycle of operation. If the flow of the working fluid comes from the atmosphere and is returned back to the atmosphere in each cycle of operation, then the gas turbine is set to work on Open Cycle. Instead, if the flow of the working fluid is confined within the cycle path and used again and again, the gas turbine works on Closed Cycle.

 

1. OPEN CYCLE GAS TURBINE

Description

 Fig. 1 shows an Open Cycle Gas Turbine. It consists of a compressor, combustion chamber and gas turbine. The compressor, turbine and the generator are coupled coaxially. The combustion chamber is connected between the compressor and the turbine. The compressor is driven by the turbine.

Working Principle

Fig. 2 shows the block diagram of an Open Cycle Gas Turbine. Atmospheric air is drawn in and passed through the air filter. It then flows into the compressor and is compressed.

The compressed air is heated by directly burning the fuel (natural gas or coal gas or kerosene or gasoline) in the air at constant pressure in the combustion chamber. Then, the high pressure hot gases expand in the turbine and mechanical power is developed.

Note that major percentage of power developed (66%) by the gas turbine is used to run the compressor. Remaining power is available as useful output.

The working fluid, i.e., air and fuel, must be replaced continuously as they are exhausted into the atmosphere. Thus, the entire flow comes from the atmosphere and is returned to the atmosphere. Hence, it is called Open Cycle. Since the air enters into the compressor and gases coming out of the turbine are exhausted into the atmosphere, the working fluid must be replaced continuously.

Advantages of Open Cycle Gas Turbine

 1. Fuels: Almost any hydro-carbon fuel from high octane gasoline to heavy diesel oil can be used in the combustion chamber.

 2. Warm-up Time: Once the turbine is brought up to the rated speed by the starting motor and the fuel is ignited, the gas turbine will be accelerated from cold start to full load without warm-up time.

 3. Low Weight and Size: The weight in kg/kW developed is less than the closed cycle. Hence, it is widely used.

 4. Less Space: Open cycle plant occupies less space.

5. Peak Load Plant: The stipulation of a quick start and frequent take-up of load are the points in favour of open cycle plant, when the plant is used as the peak load plant.

6. Cooling Medium: Open cycle plant, except those having an intercooler, does not require cooling water. Hence, the plant is independent of cooling medium.

Disadvantages of Open Cycle Gas Turbine

1. Less Part Load Efficiency: Part load efficiency decreases rapidly, as considerable percentage of power developed by the turbine is used to drive the compressor.

2. Sensitiveness: Open cycle plant is sensitive to changes in the atmospheric air temperature, pressure and humidity.

 

2. CLOSED CYCLE GAS TURBINE

Working Principle

See Fig. 3. In this, the fuel is not burnt directly in the air. The compressed air from the compressor is heated in a heat exchanger (Air Heater) by an externally fired combustion chamber using coal or oil. Thus, the working fluid is not contaminated by the products of combustion and is constantly re-circulated.

Then, the high pressure hot gases expand passing through the turbine and the pressure is reduced. The thermal energy is converted into mechanical power.

The exhaust gas is then cooled in a Cooler to its original temperature before passing into the compressor again. The cooler is used to minimize the compressor work.

The Main difference between the open and closed cycles is that the working fluid is continuously replaced in open cycle whereas it is used again and again in a closed cycle.

Advantages of Closed Cycle Gas Turbine

 1. No Erosion of Turbine Blades: The closed cycle avoids erosion of the turbine blades due to the contaminated gases. Therefore, its efficiency is not affected.

 2. No Air Filtering: The need for filtration of the incoming atmospheric air is a severe problem in the open cycle plant. This problem is completely eliminated in closed cycle.

 3. Inferior Oil or Solid Fuel: As indirect heating is used in the closed cycle plant, an inferior oil or solid fuel can be used in the combustion chamber. These fuels can be used more economically, because these are available in abundance.

 4. Low Maintenance Cost: The maintenance cost of closed cycle plant is low.

Disadvantages of Closed Cycle Gas Turbine

 1. Requirement of Cooling Water: The system is dependent of external means, as considerable quantity of cooling water is required in the cooler.

2. Response to Load Variations. The response to the load variations is poor compared to the open cycle plant.

3. Big Heat Exchangers: The closed cycle plant requires very big heat exchangers, since the heating of the working fluid is done indirectly. The space required for the heat exchanger is considerably large. The full heat of the fuel is also not used in this plant.