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.
