Components of Gas Turbine Power Plant
The main components of plants are:
1. Air Compressor
2. Intercooler and Regenerator as an auxiliary
3. Combustion Chamber
4. Gas Turbine
5. Alternator
6. Starting motor
1. Air Compressor
● A compressor is a device which pressurizes a gas. It is a critical component of the gas turbine plant since it consumes a large percentage of the turbine energy and plays a significant role in determining the overall performance and efficiency of the unit. Compressors are categorized, by their principle of operation and the mass flow and pressure requirements.
● The high flow rate of air through the turbine and relatively moderate pressure necessitate the use of rotary compressor. The air at atmospheric pressure is drawn by the compressor through the filter which removes the dust from the air. The rotatory blades of the compressor push the air between stationary blades o raise its pressure. Thus air at high pressure is available at the output of the compressor
● The types of compressor commonly used are:
1. Centrifugal compressor and
2. Axial flow compressor or Centrifugal compressor
(a) Axial Flow Compressor
● Axial flow compressors are also continuous flow devices and are therefore extensively used in gas turbines for high flow applications. These are invariably multi-stage devices achieving pressure ratios as high as 40:1, through a series of small increases in each stage, resulting in very high efficiency levels.
● An axial flow compressor is similar to that of an axial flow turbine with moving blades on the rotor shaft and then diffusing it with a row of fixed blades arranged around the stator
● There by converting the velocity increase obtained by the action of the rotating blades to a pressure increase, Axial flow compressor consists of a rotor and a casing or stator
● Moving blades are mounted on the rotor and fixed blade on the inner side of casing known as stationary blades.
● Air enters the blades axially and also leaves them in the axial direction.
● The type of compressor is designed according to reaction principle.
● The air from atmosphere enters axially and due to rotion of moving blade causes centrifugal action passes increases velocity and then passes through fixed blade. Thus rise in pressure through the stage (rotor and stator) to both blade rows, moving as well as fixed and air leaves them in the axial direction.
● A single stage compressor does not give appreciable pressure ratio. Axial flow compressor is mostly multi stage compressor. Pressure ratio which can be developed per stage of an axial flow compressor is 1.2.
● Axial flow compressor gives preference over centrifugal compressor in the application of aircraft and industrial gas turbine power plant because of higher efficiency, less frontal area and capable of producing higher pressure ratio on a single shaft by increasing number of stages
● The axial flow compressor run at lower speeds, high weight, higher starting torque and sensitive to any deposite formation on blades and complicated compared with centrifugal compressor
The advantages with using these compressors are:
1. They are most suitable for multi-staging operations
2. They give higher efficiencies at high pressure ratios
3. Higher mass flow with smaller frontal areas.
The disadvantages associated with axial flow compressors are:
1. Their efficiency decreases with reduction in engine size on account of the high percentage of blade-tip leakage in smaller compressors.
2. Narrow range of stable operation between surging and choking limits makes part load operations difficult.
(b) Centrifugal compressors:
● Centrifugal compressors are continuous flow devices and are therefore widely used in gas turbine applications requiring medium pressure and flow. They possess the relative advantages of smooth operation, higher tolerance to process fluctuations and higher operational reliability.
● A majority of the centrifugal compressors in use (such as those in small turbines). produce pressure ratios in the range of 1.2:1 to 4.5:1 in a single stage
● It mainly consist of impeller and diffuser. The impeller consist of an impeller disc and impeller vanes, attached on the impeller disc radially forming radial diverging passage.
● The impeller rotates with the shaft at high speed and gir is drawn into the impeller eye in an axial direction. The air is flown radially outwards through the impeller passages due to centrifugal force, and kinetic energy is imported to the air with some static pressure rise.
● The remaining pressure rise is obtained in the diffuser. The diffuser which is stationary, consist of a number of fixed diverging passages. The air leaves the impeller tip with high velocity and enters the diffuser.
● The diffuser in which reduces the high velocity thus by diffusion process or deceleration of air in the diffuser, kinetic energy is converted into pressure energy.
● The flow from the diffuser is collected in a spiral passage from which it is discharged from the compressor
● Single stage compressor can develop a pressure ratio 4:1 pressure ratio as high as 10:1 can be developed with the help of multi-stage centrifugal compressor.
● The impeller may be single or double sided.
● The compressor might have single inlet or double inlet. In a double inlet impeller it will be having eye on both side of diffuser passage.
Advantages of centrifugal compressors:
1. They are very durable
2. They are less chance to damage by foreign objects.
3. They are cheaper to manufacture and have low maintenance requirements.
4. They are less sensitive to fouling.
Disadvantages of centrifugal compressors :
1. They have very large diameters when compared to other compressors
2. With reduction in size, they become less efficient
3. They experience loss in efficiency with increase in pressure ratio
4. Pressure ratios in these compressors rarely exceed 4.5: 1
2. Regenerator and intercooler :
● Regenerator is a device which recovers heat from the exhaust gas turbines to increase the cycle efficiency. The exhaust is passed through the regenerator before wasting to atmosphere. They utilize the heat generated by the exhaust gases, which is otherwise wasted by transferring it to the combustor incoming air, resulting in reduced fuel consumption.
● A regenerator consists of a nest of tubes contained in a shell. The compressed air from the compressor passes through the tubes on its way to the combustion chamber. In this way compressor air is heated by the hot exhaust
● A schematic of a typical regenerator unit used in gas turbines. Air entering the regenerator air inlet absorbs heat from the hot exhaust gases before leaving via the air outlet to enter the combustor. By utilizing the concept of regeneration in gas turbines, a reduction in fuel consumed to the tune of almost 30 % can be achieved.
● The working gus leaving the turbine after expansion is cooled in (heat exchanger) intercooler with help of surrounding and supplied to the compressor to repeat cycle of operation
3. Combustion Chamber :
Air leaving the compressor enters the combustor through the diffuser zone. The diffuser zone acts as a transition zone between the compressor discharge and the combustor inlet. Here the compressor discharge air is diffused such that its velocity reduces. This is done in order to minimize pressure loss which in turn is a function of the squared velocity.
Design of a good combustion chamber should achieve the following:
1. Ensure stable and efficient operation over a wide operating range
2. Ensure uniform temperature distribution for greater efficiency in operation
3. Complete release of chemical energy in the space provided for efficient combustion
4. Pressure drop across the combustion chamber should be minimum
5. High combustion efficiency giving greater reliability in operation and low emission levels
● The combustion chambers is divided into three distinct zones, A recirculation zone where the fuel is partially burnt und evaporated, in order to prepare it for full combustion in the burning zone.A burning zone where the fuel is completely burnt to form products of combustion andA dilution one where the products of combustion in the form of the hot burnt gases are mixed with the morning air, in order to attain a suitable turbine inlet temperature
● In an open cycle gas turbine plants combustion can be arranged to take place in one or two large cylindrical can type combustion chamber with ducting to convey hot gases to turbine
● The air at high pressure from the compressor is led to the combustion chamber through the regenerator. In the combustion chamber oil is injected through nozzle at high pressure ensure atomisation of oil and its thorough mixing with air.
● The heat is added to the air by burning of oil, initiated by an electrical spark and once the fuel start burning the flame is required to be stabilized.
● A pilot or re-circulated zone is created in main flow to establish a stable flame which helps to sustain continuous combustion. The common methods of flame stabilization are by swirl flow and by bluff body. The result is that the chamber attains a very high temperature. The combustion gases are suitably cooled and then delivered to gas turbine.
● The combustion chamber is designed in two types swirl type of combustion chamber and Tubular-itinular combustion chamber
● In the swirl type of combustion chamber, the air from compressor is supplied to the combustion chamber. 15% to 20% of the air enters into prinury zone to provide Air-From fuel ratio of about 15:1 and remainder air is passed through the casing.
● The high velocity air compressor is reduced in the diffuses section
● Fuel is sprayed through fuel nozzle into air stream, it gives self piloting flame in the air stream. Some lowering mixture in primary zone is recirculated back on the incoming air & fuel.
● It is done by introducing primary air through twisted radial vanes called swirl vanes.
● It result into a vortex motion along the arses combustion chambers. At the distance away, the free vortex created by swirl vane dies down and pressure became uniform. The flame travel forward and mixed with secondary air passed through perforation in the tube at velocity 30 - 60 m/s. It helps for complete combustion.
Tubular-annular combustion chamber :
● Tubular-annular combustion chamber of gas-turbine engine has annular space formed by casing which accommodates fire tubes with burning zone and mixing Zone, fuel feed system and igniter. Annular space and front part of each fire tube are made hermetically sealed. Fuel feed system is made in form of nozzle to deliver ordinary wiler into casing space and nozzle provided with cauter to deliver mixture of water solution of electrolyte and fuel into each fire tube. Igniter is made in form of are electrodes with possibility of striking and blowing out of arc, dissociating, ionization, igniting and converting into plasma vapors of mixture passing through zone of arc burning said electrodes being installed in each fire tube. One of electrodes is made for displacement at striking of arc, and the other is made conical and provided with conical channel.
● Effect : improved economy and ecological characteristics
4. Gas Turbine
The expander section of gas turbines can be one of two basic types. They are:
i. The axial-flow turbine and
ii. The radial-in flow turbine, but axial-flow turbines are the most efficient.
Axial-flow turbine :
● Axial-flow turbines are the most widely used in gas turbine units. They are highly efficient in most operational ranges. The flow in these turbines is in the axial direction, both at the entrance and exit sections Like their compressor counterparts, they usually consist of more than one stage. There are two types of axial-flow turbines, The Impulseturbine and The Reaction turbine.
● In the Impulse type axial-flow turbine, the gases or working fluid enters the rotor at very high velocity, because the entire enthalpy drop takes place in the nozzle, whereas in the case of reaction turbines, the enthalpy drop is divided between the nozzle and the rotor.
● Gas turbine are also axial flow types. The basic requirements of a turbine are light weight, high efficiency, reliability in operation and long working life. The products of combustion consisting of a mixture of gases at high temperature and pressure are passed to the gas turbine. These gases in passing over the turbine blades expand and thus do the mechanical work. The temperature of the exhaust gases from the turbine is about 900° F.
● Large work output can be obtained per stage with high blade speeds when blades are designed to sustain high stresses. More stages are always preferred on gas turbine power plants because it helps to reduce stress on the blade and increases overall life of the turbine. Cooling of gas turbine blade is essential for long life as it is continuously subjected to high temperature gases.
5. Alternator
The gas turbine is coupled into the alternator. The alternator converts the mechanical energy of the turbine into electrical energy. The output of the alternator is given to the bus-bars through transformers, isolators and circuit breakers.
6. Starting Motor
Before starting the turbine, compressor has to be started. For this purpose, an electric motor is mounted on the same shaft as that of the turbine. The motor is energized by the batteries. Once the unit starts, a part of the mechanical powerof the turbine drives the compressor and there is no need of the motor as turbine power is utilized by compressor
