Determine Thermal Efficiency, Work ratio, Pressure ratio of Gas Turbine Power Plant

I will solve Some problems of simple gas turbine power plant with the students during this power plant engineering lecture. This lecture on Gas Turbine Power Plant includes:
1) Calculation of thermal efficiency in gas turbine power plant
2) Calculation of work ratio of gas turbine power plant
3) Calculation of pressure ratio in gas turbine power plant

Condition of Maximum Work done in Brayton Cycle, Maximum Efficiency, work Ratio and Air Rate for simple gas Turbine

 

In this lecture on Power Plant engineering you will Learn: (1) Condition of maximum work (2) Joule cycle Maximum Thermal Efficiency derivation (3) Work Ratio of Gas Turbine: It is the ratio of net work produced by the plant to the turbine work. The work ratio represents the amount of net work generated by the gas turbine plant per unit turbine work output. The work ratio increases with an increase in turbine inlet temperature, a decrease in compressor inlet temperature, and a decrease in the pressure ratio of the cycle. the compressor inlet temperature is always atmospheric temperature particularly in the open cycle and turbine inlet temperature is limited by metallurgical consideration. The highest temperature ever used for a gas turbine power plant is about 1000 k. (4) Air rate: The size of the gas turbine plant is dependent upon the rate of air flow in relation to net output work. Compact gas turbine plant has a lower value of air rate. Air rate is expressed as kg of air per kWhr.
(1) Condition of maximum work
(2) Joule cycle Maximum Thermal Efficiency derivation
(3) Work Ratio of Gas Turbine: It is the ratio of net work produced by the plant to the turbine work. The work ratio represents the amount of net work generated by the gas turbine plant per unit turbine work output. The work ratio increases with an increase in turbine inlet temperature, a decrease in compressor inlet temperature, and a decrease in the pressure ratio of the cycle. the compressor inlet temperature is always atmospheric temperature particularly in the open cycle and turbine inlet temperature is limited by metallurgical consideration. The highest temperature ever used for a gas turbine power plant is about 1000 k. (4) Air rate: The size of the gas turbine plant is dependent upon the rate of air flow in relation to net output work. Compact gas turbine plant has a lower value of air rate. Air rate is expressed as kg of air per kWhr.

Ideal Brayton cycle efficiency for gas turbine with P V and T S diagram

The ideal cycle for gas turbine is Brayton or jule cycle. The Brayton cycle has two constant pressure and two isentropic processes. In this video, i have explained PV and TS diagram of Brayton cycle. This video will show you the Analysis of Brayton cycle, and at the end of the video, you will get the equation to find the thermal efficiency of a simple gas turbine power plant. The analysis of Brayton is carried out with the following assumptions: 1) Compression and expansion are isentropic. 2) Pressure losses in the system are neglected. 3) Heat losses in the system are neglected. 4) specific heat of working fluid is taken constant throughout the cycle. Considering the 1kg of working fluid.

Gas Turbine Engine Fuel

The gas turbine engine can be operated with gas, liquid and solid fuels. However, this statement is not true for all gas turbine engine. The selection of fuel for gas turbine is dependent on type of cycle, size and length of engine and application of gas turbine engine. The most important requirement of a gas turbine fuel are 
(1) composition should provide proper combustion and rapid burning,
(2) the product of combustion should contain a minimum of mater which might get deposited on the turbine blades or the heat exchanger,
(3) highest heating value per kg or m^3 volume specially for aircraft engine.


The modern gas turbine are normally operated on the following fuels:
a) Gaseous fuels: Natural gas, Blast furnace gas, Producer gas, Sewage gas, etc.…
b) Liquid fuels: Kerosene, Gasoline, Residual oils, etc.…
c) Solid Fuels: Coal


The solid fuel (Coal) is normally used in closed cycle gas turbine. The main problem with using solid fuel in the case of open cycle plans is of reducing the level of flyash in the gases leaving the combustion chamber so that the blades of the turbines are not eroded. For this, it is necessary to ensure proper cleaning of the combustion gases from fine ash and dust. Before the gas enters the turbine, it is purified with help of the various flyash collectors. The use of coal as a fuel of open cycle gas turbine is under the development. Coal is burnt in two methods. In the first method, the coal is completely or partially gasified and fuel gas produced is supplied to gas turbine combustor. In other method, pressurized bubbling or circulating fluidized bed, where fuel gas, after it is adequately filtered, expands in the gas turbine. Coal is usually considered as a gas turbine fuel in combined cycle power plant.

Liquid oil like kerosene, gasoline, etc. are easily used in a gas turbine combustor. Residual oils can be used as fuel in gas turbine. If the viscosity of the oil is high, some heating arrangement required.

What are the pros and cons of using closed cycle gas turbines?

As we know that in open cycle gas turbine power plant, the fuel is mixed with air in the combustion chamber and the combustion gases are expanded in the gas turbine which causes erosion and corrosion of turbine blades and therefore it is necessary to use fuel superior quality in the combustion chamber in order to minimize erosion and corrosion. This negative effect overcome in case closed gas turbine power plant. In the closed gas turbine power plant, the same air or working fluid is circulated over and over again. The working medium is not mixed with fuel, but it is heated by the burning of fuel in a separate supply of air in the combustion chamber and the transferring this heat to the working fluid which passes through tubes fitted in the combustion chamber. The working fluid does not come in to direct contact with products of combustion. The other disadvantage of open cycle gas turbine gas turbine plant is that the turbine exhaust is discharged into the atmosphere resulting in rejection of heat of exhaust gases to the atmosphere. In case of closed cycle these heat are recovered in a heat exchanger or recooler.

The working fluid (air or any other suitable gas such as helium, argon, hydrogen, and neon) coming out from compressor is heated in the heat exchanger (heater) by an external source at constant pressure. The high temperature and high pressure air coming out from the external heater is passed through the gas turbine. The working fluid coming out from the turbine is cooled to its original temperature in the heat exchanger (cooler) using external heating source before passing in the compressor. In the closed cycle, the working fluid is continuously circulated through compressor, cooler, heater and turbine without its change of phase, the required heat addition and rejection taken place in the heater and cooler respectively.

The performance characteristics, component elements and analysis for open cycle apply equally as well as to the closed cycle gas turbine.

Advantages of closed cycle over open cycle:
1) In the closed cycle gas turbine, the working medium is heated externally and the fuel does not mix with it, hence any inexpensive solid fuel such as coal can be used to heat the working fluid. Also need for filtration of the incoming air is completely eliminated.
2) They are kept free from product of combustion. Hence absence of corrosion and abrasion of the interiors of turbines and compressors extends the life of the plant and maintains the efficiency of the plant constant throughout its life.
3) The working medium is at relatively high internal pressure. So, specific volume is less and the dimensions of the compressors and turbines can be reduced and the maximum unit capacity can be increased. 
4) A working medium such as helium, argon, hydrogen, neon may be used which has very good physical properties compared to those of air. For example thermal conductivity of hydrogen is about 6.8 times that of air and therefore require smaller heat exchanger.
5) The closed cycle can be operated with highest efficiencies in comparison to open cycle plants at an equal initial temperature of working fluid.
6) The power output at constant speed can be varied by adding or substracting the working fluid and thus altering the weight of the charge. This gives improved part load efficiency as compared to open cycle gas turbine.
7) High heat transfer can be possible

Disadvantages of closed cycle:
1) In closed cycle gas turbine considerable quantity of cooling water is required.
2) Heat addition to the working medium takes place through heat exchanger, hence full heat of fuel fired is not utilized.
3) Additional equipments such as externally fired heater, cooler are required, the plant and its operation makes additional complexity.

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.

Classification of Gas Turbine Power Plant

The gas turbine can be classified as follows:

1)      According to types of combustion process

                                 I.            Constant volume or explosion type gas turbine: This type of Gas turbine works on Atkinson cycle in which combustion of air fuel mixture is takes place at constant volume, hence air and fuel mixture should be isolated from compressor. This is possible by valves in the combustion chamber, resulting in an intermittent combustion which inherently impairs smooth running of machine. This type of turbine has better thermal efficiency than a constant pressure cycle gas turbine. The main disadvantages of this type of gas turbine is that complexity in mechanical system and pressure difference and velocity of hot gases are not constant, so turbine speed fluctuates. The constant volume combustion type gas turbine is not popular in practical use. The constant volume gas turbines are absolute in use.

                               II.            Constant pressure or continuous combustion type gas turbine: This type of gas turbine works on Brayton cycle in which combustion of air fuel mixture is takes place at constant pressure.

2)      According to direction of flow

                                 I.            Axial flow gas turbine

                               II.            Radial flow gas turbine.

3)      According to action of expanding gases

                                 I.            Impulse gas turbine

                               II.            Impulse- Reaction gas turbine

4)      According to path of working substance

                                 I.            Open cycle gas turbine plants: 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 a high temperature and then product of combustion is passed on to the turbine for expansion and which after delivering the work are finally ejected to atmosphere. In the open cycle the working medium is continuously replaced by fresh air and fuel.

                               II.            Closed cycle gas turbine plants: In the closed cycle gas turbine power plant, the same air or the working fluid is circulated over and over again. The working medium is not mixed with fuel, but it is heated by the burning of fuel in separate supply of air in the combustion chamber and transferring this heat to the working fluid which passes through tubes fitted in the combustion chamber. The working fluid does not come into direct constant with product of combustion.

                             III.            Semi-closed cycle gas turbine plants: Semi-closed cycle gas turbine plant is combination of open and closed cycle, in which some part of working fluid is recirculated to the plant and another part of working fluid flows into and from the atmospheric air.

5)      According to thermodynamic cycle

                                 I.            Simple cycle

                               II.            Simple cycle with intercooling: in which the air is cooled between stages of compression.

                             III.            Simple cycle with regeneration: in which the air after compression is heated with help of exhaust gases coming from turbine.

                            IV.            Simple cycle with reheating: in which combustion products (gases) after part of expansion in high pressure turbine is reheated in second combustion chamber and then it is expanded in the low pressure turbine.

                              V.            Simple cycle with intercooling, regeneration and reheating.

6)      According to shaft configuration

                                 I.            Single shaft engine: - Examples: Turbojet engine, Turboshaft engine.

                               II.            Multi-shaft engine: - Example: Two shaft engine with power turbine, Turbojet engine, industrial slit shaft engine, Twin-spool engine.

7)      According to applications

                                 I.            Power or industrial gas turbines

                               II.            Aviation or aircraft turbines

What are the different uses for gas turbines?

if you want to know, What are the different uses for gas turbines then you came to right place because this gas turbine power plant lecture is about applications of gas turbine. The continued developments in gas turbine materials toward increasing the turbine inlet temperature and improving the components efficiency over the past decade has enlarged the spectrum of gas turbine applications to area such as power generation, utility industry, space, marine, automotive propulsion etc.. The main reasons of such a wide popularity being employed by the gas turbine is its advantages of simplicity, high power/weight ratio, smooth running, less maintenance, multi-fuel capability for combined cycle and reliability. The gas turbine applications can be divided in the following categories: 1) For electric power generation: The gas turbine are very popular for electric power generation because of the ability of starting and brought up-to full load quickly less cost of installation and maintenance. - Majority of the gas turbine power plants are used for peak load service with other types of power plants(steam and hydro power plants) - A large number of gas turbine power plants is also used as stand by power plants in hydro electric power plants. - Gas turbine power plants are also used as base load power plants where fuel oil or natural gas are cheap and easily available, water supply is scarce and load factor is very low(15-18%) - Now a day gas turbine plants are used to operate as combustion plants with steam plants, called as combined cycle power plant. 2) For jet propulsion engine: 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 the turbo propeller engines they may drive the propeller in addition to the compressor. 3) For supercharging: A small gas turbine run by the hot exhaust gases which is used to run supercharger(compressor) for aviation gasoline engines and for heavy duty diesel engines. 4) For marine field: A gas turbine can be also used for propulsion of ships or power generation on the ships. 5) Railway and road transport: Gas turbine can be also used for rail propulsion and high speeds road vehicles like racing cars. 6) Industry: Gas turbine are also employed for industrial applications like blast of air for blast furnace in steel industries, oil and other chemical industries. Gas turbine is also used to supply preheated combustion air, hot gases for an industrial process in petrochemical industries.

working principle of gas turbine power plant

working principle of gas turbine power plant :

in a gas turbine first air is obtained from the atmosphere and compressed in an air compressor. this high prsuure air is then pased into the combustion chamber, where it is heated due to  combustion of fuel. The product of combustion(hot gases) of high pressure and temperature passes through the passages formed by the stationary and rotating blades of gas turbine. Ajet of hot gases is made to flow over ring of blades imparting rotary motion to the shaft of turbine. A large part of power developed by the turbine rotor is consumed for a driving a compressor which supplies air under pressure of combustion chamber, while remaining power is utilized for doing the external work.

Gas turbine engines derive their power from burning fuel in a combustion chamber and using the fast flowing combustion gases to drive a turbine in much the same way as the high pressure steam drives a steam turbine. A simple gas turbine is comprised of three main sections a compressor, a combustor, and a power turbine. The gas-turbine operates on the principle of the Brayton cycle, where compressed air is mixed with fuel, and burned under constant pressure conditions. The resulting hot gas is allowed to expand through a turbine to perform work.

In an ideal gas turbine, gases undergo four thermodynamic processes: an isentropic compression, an isobaric (constant pressure) combustion, an isentropic expansion and heat rejection. Together, these make up the Brayton cycle.

In a real gas turbine, mechanical energy is changed irreversibly (due to internal friction and turbulence) into pressure and thermal energy when the gas is compressed (in either a centrifugal or axial compressor). Heat is added in the combustion chamber and the specific volume of the gas increases, accompanied by a slight loss in pressure. During expansion through the stator and rotor passages in the turbine, irreversible energy transformation once again occurs. Fresh air is taken in, in place of the heat rejection.

introduction to gas turbine

A gas turbine is most satisfactory way of producing large power in compact unit. In gas turbine power plant, a gas turbine is used as the prime mover to produce mechanical energy. In gas turbine power plant, the working medium is either a mixture of combustion products(air and fuel) or heated air at a certain pressure and a higher initial temperature. The gas turbine obtains its power by utilizing the energy of burnt gases and air which are at high pressure and pressure by expanding through several rings of fixed or moving blades.


The simple gas turbine consists of

1) compressor,
2) combustion chamber and
3) turbine.


The basic on which a gas turbine works is similar to internal combustion engine. In both cases air is made to enter the prime mover which compresses and  then heated by combustion process. Hence pressure and temperature of working fluid is increased. This high temperature and prssure working fluid is then expanded in prime mover. The expanded products are discharged through exhust. However in the case of internal combustion engine is reciprocating machine in which charge induces in to engine cylinder intermittently. In case of gas turbine, flow of working fluid through the rotary machine(gas turbine) is continuous and smooth. with respect to design, a gas turbine is quite similar to steam turbine. In the turbine blading working gas expands and the heat energy is converted first in to the kinetic energy and then to the rotary motion of the turbine shaft. However in the seam power plant the product of combustion do not form the working medium. These are utilized to produced an intermediate fluid(steam) which is expanded in the steam turbine.