Open-Cycle Gas Turbine Plant

A simple open-cycle gas turbine consists of a compressor, a combustion chamber and a turbine, as shown in the figure below. The compressor takes in ambient fresh air and raises its pressure. Heat is added to the air in the combustion chamber by burning the fuel and raising its temperature.

The heated gases from the combustion chamber are then passed to the turbine, which expands and performs mechanical work. Some of the power the turbine develops is used to drive the compressor and other accessories, and the remaining is used for power generation. Fresh air enters the compressor, and gases from the turbine are exhausted into the atmosphere. The working medium needs to be replaced continuously. This type of cycle is known as an open cycle gas turbine plant and is mainly used in most gas turbine power plants as it has many inherent advantages.

Advantages:

1. Warm-up time: Once the starting motor brings the turbine up to the rated speed and the fuel is ignited, the gas turbine will be accelerated from a cold start to full load without warm-up time.
2. Low weight and size: The weight in kg per kW developed is less.
3. Fuels: Almost any hydrocarbon fuel, from high-octane gasoline to heavy diesel oils, can be used in the combustion chamber.
4. Open-cycle plants occupy less space compared to closed-cycle plants.
5. The stipulation of a quick start and frequent load take-up are the points in favor of an open cycle plant when the plant is used as a peak load plant.
6. Component or auxiliary refinements can be varied in open-cycle gas turbine plants to improve thermal efficiency. They can give the most economical overall cost for the plant load factors and other operating conditions.
7. Open-cycle gas turbine power plants, except those having an intercooler, do not need cooling water. Therefore, the plant is independent of the cooling medium and becomes self-contained.

Disadvantages:

1. The part load efficiency of the open cycle gas turbine plant decreases rapidly as the considerable percentage of power developed by the turbine is used for driving the compressor.
2. The system is sensitive to the component efficiency, mainly the compressor. The open-cycle gas turbine plant is sensitive to atmospheric air temperature, pressure and humidity changes.
3. The open-cycle plant has a high air rate compared to the closed-cycle plants. Therefore, it increases heat loss in the exhaust gases and extensive diameter duct work is needed.
4. The dust must be prevented from entering the compressor to decrease erosion and depositions on the blades and passages of the compressor and turbine. So it damages their profile. The deposition of the carbon and ash content on the turbine blades is not desirable as it reduces the overall efficiency of the open-cycle gas turbine plant.