Sunday, May 13, 2012
Posted by Ankit Chugh on 3:55 AM
The gas turbine obtains its power by utilizing the energy of a jet of burnt gases and air, the velocity of the jet being absorbed as it flows over several rings of moving blades which are fixed to a common shaft. It thus resembles a steam turbine and its blades are designed in the same manner. The gas turbine requires an air compressor, which is usually driven off its own shafting; this absorbs a considerable proportion of the power produced and thus lowers the overall efficiency.
Gas turbines have been constructed to work on the following fuels: Coal gas, producer gas, blast furnace gas, oil and pulverised coal. They consist of two main types: -
1) Continuous combustion or constant pressure type.
2) The explosion or constant volume type.
Constant pressure turbine: In this type the fuel is burned at constant pressure and the cycle used is the same as the Joule cycle. The turbine is of the reaction type using oil fuel and is fitted with an axial flow air compressor.
The total air supply is drawn from the surrounding atmosphere by the compressor and is compressed to the combustion pressure It is then forced to the combustion chamber. Part of this air is used as combustion air for the oil, which enters the burner. The reminder is forced through the annular space between the wall of the combustion chamber and the burner jacket. The air receives heat from the burner jacket and also mixes with the products of combustion from the burner; this raises the temperature and volume of the air.
The use of a very large quantity of air, in excess of the combustion air, prevents the temperature of the mixtures from reaching values, which are too high for the metal of the rotor blades; it also prevents the burner from becoming too hot.
The high-pressures mixture of air and exhaust gases now enters the turbine and flows through the blade rings. Whilst passing over the blades the gas is continuously expanding, its pressure energy being converted into kinetic energy which, in turn, is absorbed by the rotor. On leaving the turbine the spent gases pass away to exhaust. The speed of the turbine is governed by controlling the fuel oil supply and by the safety valve, which releases some of the mixture if the pressure becomes too high.
During operation ‘ab’air is drawn into the compressor and is compressed adiabatically. Operation ’bc’ represents the burning of the oil at constant pressure, and ‘cd’ is adiabatic expansion of the gases through the turbine. Operation ‘da’ represents the exhausting and cooling of the gases at constant pressure.
It will be seen that the equation for the air standard efficiency of this cycle is the same as that of the Otto cycle.
Heat supplied = heat absorbed during the operation ‘bc’ = Kp (Tc- Tb)
Heat rejected = heat rejected during operation da. = Kp (Td-Ta)
One of the chief difficulties in the design of the gas turbine is that the metal of the blading is unable to withstand the high combustion temperatures of the gases. This difficulty is partly overcome by supplying a large quantity of excess air, by this means combustion temperature of 36000 0 F. are reduced to 1000° F. when entering the blades.
i) The high power to weight ratio and the jet propulsion of the gas turbine makes it suitable for aircraft over the piston engines. The low weight, lightness of the rotating parts and high power by high internal pressure and temperatures are important characteristics for aircraft engines.
ii) They are used in marine applications.
iii) They are used in Electricity generation.
iv) Gas turbines are extensively used in the oil and gas industry which can afford to run them at relatively low efficiency. A major use is pumping oil and gas through pipelines over long distances through deserts, sea and other remote places. Pumping stations used to boost the pressures are run on gas turbines which use the pumped oil or gas and can run without supervision. Gas turbines are also used extensively on offshore oil drilling rigs. There, they generate electricity, supply shaft power for pumping duties and may heat the rig with their exhaust gas.
v) Electrical load variations may be smoothened by gas turbines.
Simplest form of combustion turbine plant with reaction type gas
The turbine and it’s connecting piping is important and critical since these lines are always working at elevated temperatures and high pressures. Special care is required in installing this line for the safe operation of the plant.
Tagged as : Layout Engineering