open cycle gas turbine power plant

A simple cycle gas turbine consists of a (1) compressor, (2) combustion chamber and (3) gas turbine.

In the open cycle gas turbine, ambient air enters at the compressor and after the compression of air, fuel is burned in the air itself to raise it to high temperature and then product of combustion are passed on to the turbine for expansion. After delivering the work combustion products are finally rejected to atmosphere. In the open cycle the working medium is continuously replaced by fresh air and fuel. It works on the joule cycle and Brayton cycle.

The air is sucked in by the compressor from the atmosphere through the filter which removes the dust from the air. The rotary blades of the compressor push the air between the stationary blades to raise its pressure to 4-5 atmosphere. Hence the air is available at high pressure at output of the compressor. Then high pressure air passes through combustion chamber, in which heat added to the air at constant pressure by burning the fuel and raises temperature (about 1650) of working medium. This high temperature must be brought down to the level so that the thermal stresses in the turbine blades do not become excessive. This is achieved by allowing the reminder air to enter the combustion chamber at downstream to mix and cool down the combustion gases. The products of combustion comprising of mixture of gases at high pressure and temperature are passes through the gas turbine. These gases in passing over the turbine blades expand and thus result in motion of rotor and finally discharged to the atmosphere at the temperature about 540.

Advantage of open cycle:

         I.            Simplicity: There are only few rotating parts as turbine, compressor and gear train driving the auxilliaries. Hence problem of vibration and lubrication is not so severe. The ignition system is also simple compared to closed cycle.

       II.            Flexibility: since different processes within the cycle take part in separate components, a great variety in the arrangement of the system is possible.

     III.            Low weight and size: The weight in kg. per KW developed is less.

    IV.            Independent system: Open cycle gas turbine power plant, except those having intercooler, does not require cooling water. There fore the plant is independent of cooling medium and becomes self-contained.

      V.            Fuels: Almost any hydrocarbon fuel from high octane gasoline to heavy diesel oils including some solid fuels can be burned in the combustion chamber.

    VI.            Warm-up time: After the turbine has been brought up to speed by the starting motor and the fuel ignited the gas turbine will accelerate from cold start to full load without a warm up time. This is particularly important in stand by emergency plants.

Disadvantage of open cycle:

            I.            Part load performance: The part load efficiency of the open cycle plant decreases rapidly as the considerable percentage of power developed by the turbine, is used to drive the compressor. Also, the system is sensitive to the changes in components efficiency.

          II.            Sensitivity: Since system sensitive to the component efficiency, particularly that of compressor. The efficiency of compressor is affected by change in the atmospheric conditions such as temperature ad humidity of air at the inlet and foreign matter contained in the air.

        III.            High air rate: The simple open cycle gas turbine has a very high air rate as compared to other prime movers. However, the air rate may be lowered by intercooling and reheating.

       IV.            Erosion and corrosion: The working fluid is mixture of air and fuel. Since air contains dirt being deposited on the compressor blades. Due to carbon and other foreign deposits from combustion in the combustion chamber, turbine and regenerator, it is necessary that the dust should be prevented from entering into the compressor in order to minimized erosion and depositions on the blades and passages of the compressor and turbine.

         V.            In the simple open cycle, the turbine exhaust is discharged into atmosphere. Since turbine exhaust contain large amount of heat resulting in loss of heat.