In this article we will discuss about gas turbines:- 1. Introduction to Gas Turbines 2. Types of Gas Turbines 3. Fuels Used 4. Types of Compressors 5. Use of Heat Exchanger 6. Gas Turbine with Intercooling and Reheating 7. Advantages 8. Applications.
Contents:
- Introduction to Gas Turbines
- Types of Gas Turbines
- Fuels Used in Gas Turbines
- Types of Compressors Used in Gas Turbines
- Use of Heat Exchanger in Gas Turbines
- Gas Turbine with Intercooling and Reheating
- Advantages of Gas Turbines
- Applications of Gas Turbines
1. Introduction to Gas Turbines:
The gas turbine is a prime mover which develops power by burning of fuel. The gas turbines are axial flow machines which convert the heat from combusted fuel into the trust power or shaft power. The hot gases undergo momentum changes when they flow through the passages formed by the stationery and rotating blades.
ADVERTISEMENTS:
A jet of hot gases and air mixture is made to flow over rings of moving blades mounted on a shaft and in doing so the velocity of the jet decreases. Its kinetic energy is absorbed by the rings of blades imparting rotary motion to the shaft. A larger part of the power developed by the turbine rotor is consumed for driving a compressor which supplies air under pressure to a combustion chamber, while the remaining power is utilized for doing the external work.
The basic principle on which a gas turbine works is similar to internal combustion reciprocating engine. In both the cases, air is made to enter the prime mover which compresses, air and it is heated by the combustion process and thus raising the pressure still further expanded and finally the expanded products are discharged through the exhaust.
However in the case of internal combustion reciprocating engine the charge is induced into the engine cylinder intermittently. On the other hand the flow of working fluid in a gas turbine is continuous and smooth except in constant volume or explosion type of gas turbine which is not commonly used now.
The fuel used for combustion in a gas turbine plant may be oil, coal gas, producer gas, blast furnace gas and even pulverized coal or peat.
2. Types of Gas Turbines:
The gas turbines can be divided into two main types:
(1) Constant volume or explosion type
(2) Constant pressure or continuous combustion type
ADVERTISEMENTS:
(1) Constant Volume or Explosion Type:
The combustion of fuel in this case takes place at constant volume.
Fig. 15-3 shows the sketch of gas turbine of constant volume type.
The air drawn from the atmosphere is compressed in the compressor to a pressure of 15 N/cm2 to 35 N/cm2 and forced into the combustion chamber through the valves. The oil which is stored in the fuel tank is pumped into the combustion chamber through the injector by means of a fuel injection pump.
ADVERTISEMENTS:
Afterwards the injection is stopped and the oil is ignited in presence of air by the spark plug. The ignition takes place at constant volume resulting in an explosion and during this the pressure suddenly rises to about 10 N/cm2 to 145 N/cm2.
This opens up the exit valve through which the products of combustion flow. In passing through the exit valve the pressure of the gaseous products of combustion drops but there is considerable increase in velocity.
As the high velocity gases flow over the rings of moving blades of the turbine their velocity drops imparting a rotary motion to the impulse turbine rotor shaft. The exit valve remains open until the products of combustion from the combustion chamber are exhausted.
ADVERTISEMENTS:
The rotor blades, valves and combustion chamber are cooled by water circulating in the jackets provided, as only a small quantity of excess air is supplied. This type of gas turbine suffers from some disadvantages. The speed of rotation of the turbine shaft is not continuous it is rather intermittent.
A number of valves operating automatically, they are required for the combustion chamber. Besides oil, other fuels that have so far been used in this type of turbine are blast furnace gas and pulverised coal.
The constant volume or explosion type of turbine works on the Atkinson cycle. It has now become outdated and has been superseded by the constant pressure type.
(2) Constant Pressure or Continuous Combustion Type Turbine:
It is called continuously combustion type gas turbine. It works on the Joule cycle or Braytron cycle. Fig. 15-4 shows a plant employing this type of turbine. Atmospheric air is drawn and compressed by the compressor which is usually driven by the turbine rotor shaft to a pressure of 15 N/cm2 to 40 N/cm2.
The high temperature and pressure air flows through the annular space between the walls of combustion and mixing chamber. This hot air then meets and mixes with the hot gases from the burner. Thus it is seen that a quantity of air quite in excess of that required for the combustion of oil in the combustion chamber is supplied by the compressor.
This is because the products of combustion from the burner are at a very high temperature and are likely to damage the blades and other components of the turbine. The supply of excess air lowers the temperature considerably. It has been observed that by this excess air the temperature of the products of combustion may be lowered to about 600°C to 800°C.
As this mixture of hot gases and the excess air passes over the various rings of moving blades of the turbine it expands continuously and in the process the pressure drops giving rise to an equivalent increase in kinetic energy.
The turbine drives the compressor and the generator. The part of the turbine output which is used to drive the compressor is called the “Back Work”. The net power available out of a gas turbine is equal to total power developed by the turbine minus power required to drive the compressor (i.e., back work).
The compressor consumes about 75% of the power developed by the turbine, only the remaining 25% being made available to the plant for driving the generator.
The constant pressure gas turbine plants are further sub-classified as open cycle and closed cycle plants.
3. Fuels Used in Gas Turbines:
The gas turbine can use fuels in the combustion chamber of three different varieties:
(1) Liquid fuels
(2) Gaseous fuels
(3) Solid fuels.
The liquid fuels used for gas turbine are diesel oil, furnace oil, high speed diesel oil etc. The advantages of liquid fuels are easy handling, less space for storage and injection of fuel is easily possible and can be controlled. Methanol can be used as fuel for gas turbine.
The gaseous fuels which are used are Liquefied Petroleum Gas (LPG), Compressed Natural Gas (CNG). The advantages of the gaseous fuels are that they are easily mixed with air, less pollution and corrosion or erosion problems for the blades. They also use blast furnace gas which has calorific value of 2.82 MJ/m3.
The liquid and gaseous fuels are preferred in gas turbine because:
(I) They have high calorific value.
(II) The volume and weight of fuel required to satisfy the power output is relatively low.
(III) These fuels are easily burnt within the combustion system.
(IV) They are relatively clean burning fuels.
The solid fuels can be used as fuel for gas turbine such as pulverized coal or saw dust. The pulverised coal may also be used as a fuel in gas turbine plant. But it may contain as much as 5-10 percent of its weight of dirt in the form of ash.
In case, the ash particles are allowed to pass through the turbine the latter’s blades are likely to be blocked and even damaged. The use of high volatile and low ash content solid fuel provides advantage that they can be easily burned. The pressurising fluidized bed combustor is used for burning of pulverised fuel.
The experiments have been conducted on the use of coal/water slurry which provides advantage of water being used as fuel by breaking the molecules of water and obtaining high calorific value of hydrogen fuel. But the research work is being conducted on coal/water as slurry fuels.
4. Types of Compressors Used in Gas Turbines:
The compressors are used in the gas turbine plant for compression of the medium.
The compressor used in a gas turbine plant is usually of rotary type and may be classified as:
1. Centrifugal Compressors:
The principal feature of a centrifugal compressor is illustrated in fig. 15-18. The rotating member known as the impeller consists of a large number of blades and is mounted on the compressor shaft inside the stationary casing. As the impeller rotates, the pressure in the region falls and hence the air enters through the eye and flows radially outwards through the impeller blades as shown in fig. 15-18.
Both the velocity and pressure increase as the air flows through the impeller blades. Then this air- enters and flows through the convergent passages formed by the diffuser blades where by its velocity decreases but the pressure increases still further.
In other words during this part of the flow the velocity energy is converted into pressure energy. Finally, this high pressure air escapes into the compressor delivery. Thus it is seen that in this type of compressor the air enters in an axial direction but leaves radially. The arrangement is known as single stage compression and is suitable only for small pressure ratios say upto 4:1.
In order to have high pressure ratios there are numbers of such compressors are arranged in series.
2. Axial Compressor:
In this type of compressor which is now more commonly used, the air flows in an axial direction right from the intake to the delivery. The working principle is illustrated in fig. 15-19. The stator which encloses the rotor both of which are provided with rings of blades.
As the air enters in the direction shown, it flows through the alternately arranged stator and rotor blade rings. While flowing through each pair of blade rings formed up of one rotor blade ring and one stator blade ring the air gets compressed successively. The air is finally delivered in the direction as shown.
3. Vane Type:
It consists of drum on which a number of vanes are mounted in such a manner that they can slide inside or outside against the spring force. They all the time remain in contact with the inner surface of the supercharger body. The space between the body and the drum decreases from the inlet to the outlet. The medium trapped between any two vanes at inlet goes on decreasing in volume and increasing in pressure as it reaches the outlet.
4. Root Air Blower:
The compressor consists of two rotors of epicycloid shape. Each rotor is fixed to a shaft by a key. The two shafts are connected together by means of gears of equal size. The two rotors revolve at the same speed. It works just like the gear pump where the mixture at the outlet side is at a high pressure.
These compressor are simpler in construction and requires less maintenance. It has comparatively longer life. It works even at lower speed. The centrifugal compressor has poor working characteristics at lower speeds. Vane type compressor has problems of wear of vane tips.
5. Use of Heat Exchanger in Gas Turbines:
In fig. 15-22 shows the simplest of a gas turbine plant. Now the exhaust gases leave the turbine at a sufficiently high temperature (about 430°C) much higher than that of the air discharged from the compressor. Hence a large quantity of heat is wasted by being lost to the atmosphere.
A large part of this heat can be recovered by introducing a heat exchanger or regenerator as shown in fig. 15-22 in which the exhaust gases from the turbine heat up the compressed air before the latter goes to the combustion chamber. In this case, the fuel required per unit mass of air will be less giving a higher overall efficiency of the plant. Different sizes of heat exchangers are suitable for different purposes. For aviation purposes only a small heat exchanger if any, is used.
In large stationary service of the gas turbine plant where the weight or spaces are less important, heat exchangers used are quite large in size. Medium sized units are used in locomotive or marine services.
Effectiveness of Heat Exchanger:
By using a heat exchanger the air from the compressor can be heated from T3 to T2, while the hot exhaust gases are cooled thereby from T4 to T1 (= T 5). But in actual practice, the air from the compressor is heated only upto some point 4′, while the hot exhaust gases from the turbine are cooled to 5′. The effectiveness of the heat exchanger is defined as the ratio of the actual heat transferred to the heat which can be transferred under ideal conditions. In other words Effectiveness of the heat exchanger-
6. Gas Turbine with
Intercooling and Reheating:
Fig. 15-26 shows the layout of the plant in which the compression of air takes place in two stages. An intercooler is employed when high pressure ratios are involved to cool the air from the low pressure compressor before entry into the high pressure compressor, thus reducing the overall power required for compression. Though the intercooler reduces the compression work, there is some pressure loss. But even then it contributes towards overall economy.
Fig. 15-27 shows the gas turbine with expansion in two stages as in HP and LP turbine and the gases exhausted from HP turbine are reheated to same temperature before expansion or less and then sent to the LP turbine.
The air fuel ratio in a gas turbine is quite high. Therefore, the products of combustion are exhausted after expansion in the high pressure turbine which are still rich in oxygen and are subjected to combustion once again in the second combustion chamber just before the air.
After reheating is same as that before entry into the high pressure turbine. The compressors may be driven by power from the high pressure turbine (fig. 15-28) or low pressure turbine.
Fig. 15-28 shows the gas turbine power plant with intercooler, reheater and heat exchanger for heat recovery. It should be noted that though by employing heat exchanger, intercooler and reheaters, there is some gain in the overall efficiency of the gas turbine plant this gain is at the expense of the increased weight and cost.
Sometimes two turbines are connected in tandem (fig. 15-29) in which the compressed air after passing through the heat exchanger is split into two parts each part passing through a separate combustion chamber immediately before entry into each of the two turbines.
7. Advantages of Gas Turbines
:
Advantages of Gas Turbines over Reciprocating Internal Combustion Engines:
The gas turbine has number of advantages over the reciprocating internal combustion engine.
The advantages are:
(1) For the same output the weight per power developed by the gas turbine is about one third that of reciprocating piston type internal combustion engine. That is one of the reasons that the turbojet engines are being more commonly used in the aircrafts.
(2) The fuel and lubrication costs are lower.
(3) Even a low grade fuel can be used. In some cases even coal and peat have been burnt.
(4) The output from a gas turbine increases, if the inlet temperature of the working fluid decreases. At high altitudes where the temperature of air is quite low the output of the aviation gas turbine can be well maintained. On the other hand the output of the internal combustion engine at high altitude would decrease.
(5) In winter night especially in the cold regions, some provision has to be made to prevent the freezing in I. C. engines using water cooling when the temperature of the atmosphere fall to sub-zero values. For this purpose an anti-freeze is generally used. Since a gas turbine does not use any water, there is no danger of freezing. On the other hand, the gas turbine works better if the temperature of the atmosphere falls.
(6) The vibrations are very less.
(7) There are less fluctuations of energy.
(8) It has high speed operation.
(9) It has low power to weight ratio.
(10) There is smooth operation.
8. Applications of Gas Turbines:
The gas turbines have wide applications:
(1) Supercharging:
The gas turbines are used for supercharging. A small gas turbine run by the hot exhaust gases which drives the compressor for aviation gasoline engines and for heavy duty diesel engines.
(2) Turbojet and Turbo Propeller Engines:
Every turbojet and turbo propeller engine has a gas turbine. The turbine supplies power only to drive the air compressor in the turbojet engines, while in turbo propelled engines they may drive the propeller in addition to the compressor. The expansion of gases may take place in only one turbine or in a set of low pressure and high pressure turbine. The temperatures at which such turbines are to be operated range from 800°C to 1000°C.
(3) Marine Field:
The gas turbine can also be used in the marine field. These are used for propulsion of ships or power generation on the ship.
(4) Railway:
The gas turbines can be also be used for rail propulsion.
(5) Road Transport:
The gas turbines are used for heavy duty armored vehicles to cruise at high speeds.
(6) Electric Power Generation:
The gas turbines are very popular for electric power generation because of the ability of starting and brought upto full load quickly and less cost in installation and maintenance. As compared to steam power station, the gas turbine power station requires much less water for a particular efficiency.
(7) Industry:
Gas turbines are also employed for industrial purposes, e.g., blast of air for blast furnaces in steel industry, oil and other chemical industries.