In this article we will discuss about:- 1. Meaning of Gas Turbine 2. Classification of Gas Turbines 3. Working of Simple, Constant Pressure, Open-Cycle Gas Turbine 4. Work Ratio of Gas Turbine 5. Concept of Maximum Pressure Ratio and Optimum Pressure Ratio 6. Actual Gas-Turbine Cycle with Fluid Friction 7. Simple Closed-Cycle Gas Turbine and Few Other Details.
Contents:
- Meaning of Gas Turbine
- Classification of Gas Turbines
- Working of Simple, Constant Pressure, Open-Cycle Gas Turbine
- Work Ratio of Gas Turbine
- Concept of Maximum Pressure Ratio and Optimum Pressure Ratio in Gas Turbine
- Actual Gas-Turbine Cycle with Fluid Friction
- Simple Closed-Cycle Gas Turbine
- Semi-Closed Cycle Gas Turbine
- Advantages and Disadvantages of Gas Turbine over Steam Turbine
- Advantages and Disadvantages of Gas Turbine over IC Engines
1. Meaning of Gas Turbine:
A gas turbine is a turbo-machine and basically similar to steam turbine regarding its working principle.
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The first turbine to produce useful work was probably a windmill, where no compression and no combustion exist. The gas turbine as we know today include a compression process and a heat addition or combustion process. Joule and BrayIon independently proposed the cycle that is the ideal prototype of the actual unit. In 1906, a unit that produced net-power had been built.
There were two main reasons or obstacles to be overcome as revealed by thermodynamic analysis.
To produce and deliver a sizeable amount of power, it is noted that:
i. The temperature at the beginning of expansion must be high and the lack of materials to withstand these high temperatures.
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ii. The compressor and turbine must operate at a high efficiency.
iii. For high efficiency, the lack of knowledge of aerodynamic and thermodynamics of flow mechanism.
Metallurgical developments are raising the highest permissible temperatures of about 800°C to 1100°C.
A better knowledge of aerodynamics has been responsible for improving the efficiency of both the centrifugal and axial compressors.
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The gas turbines are also driven by the exhaust of internal combustion engines. Such turbines are used for supercharging IC Engines.
2. Classification of Gas Turbines
:
Basically, gas turbines are classified according to three main events.
These are:
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1. Combustion process
2. Path of working substance
3. Action of combustion gases in turbine.
3. Working of Simple, Constant Pressure, Open-Cycle Gas Turbine:
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The schematic diagram of a simple, Constant Pressure Cycle is shown in Fig. 34.1 as given below:
This gas turbine works on Joule or Brayton cycle. Constant Pressure Cycle is Joule cycle. Generally, standard cycle for gas turbine working on constant pressure cycle is Joule cycle and here mass of fuel is generally neglected.
In Brayton cycle analysis, mass of fuel is-taken into consideration.
Many times, Joules cycle and Brayton cycle are taken as identical.
Air continuously enters the compressor from atmosphere at P1, v1 and T1. This state is shown on diagram by 1. The compressor may be of axial or centrifugal type. Air is compressed isentropically to state 2 given by P2, v2, T2.
This process of compression is shown by 1-2 on P-V and T-S diagrams, as shown in Fig. 34.2 (a) and (b).
After compression, air enters the combustion chamber in the open-cycle. Some of this air goes around the outside of the combustor proper and some supplies oxygen for burning the fuel which is continuously injected as shown in Fig. 34.3. Both these streams mix together so that the temperature of the gases formed is within the limits required for the gas turbine inlet.
Here the combustion is continuous and takes place at constant pressure. This is shown by 2-3 in Fig. 34.2 (a) and (b). Temperature of the gases increases and the temperature of these gases entering the turbine is the maximum temperature that can be sustained by the turbine blades. This maximum temperature that is acceptable to the blade material is known as ceiling temperature.
Gas at the state 3 expands isentropically through the turbine and this expansion is represented by 3-4 as shown in Fig. 34.2. This expansion is complete expansion upto atmospheric pressure as against an expansion in an IC Engine and is an incomplete expansion.
After the expansion, gases will be exhausted—in open cycle—to the atmosphere in condition 4. During this process, the heat is rejected to the atmosphere. This rejection of heat takes place at constant pressure. This process is represented by 4-1 on P-v and T-S diagrams of Fig. 34.2. Thus the cycle is complete.
You will note that all the processes are continuous.
Thus we say that:
1- 2 is isentropic compression.
2- 3 is constant pressure heat addition
3- 4 is isentropic expansion and
4- 1 is constant pressure heat rejection.
Generally, turbine is coupled to compressor and generator. Turbine supplies power required to drive the compressor and the remaining power to generator or to the aircraft (jet or propeller or both).
It is observed that 60-70 % of the power developed by the turbine is absorbed by the compressor and the remaining power is used to generate power or power required to operate the aircraft.
4. Work Ratio
of Gas Turbine:
To know the extent to which the work developed by the turbine is required or supplied to run the compressor, another ratio called Work Ratio is introduced in the study. Work ratio is defined as the ratio of the network (Wt – Wc) to the turbine work developed (W). It is denoted by WR.
5. Concept of Maximum Pressure Ratio and Optimum Pressure Ratio
in Gas Turbine:
We are having, now, two quantities that will be considered. One is the net work (Wnet) and the work ratio (WR).
Network = Wt -Wc
is also a function rp — the pressure ratio and temperature ratio.
Thus we note that the efficiency of a theoretical cycle is a function of pressure ratio while the performance of the cycle is a function of both the pressure ratio and temperature ratio.
Consider now the different cycles with different rp and the same maximum cycle temperature and the same initial temperature and pressure.
Consider a new-cycle 1- 2″- 3″- 4″- 1(Fig. 34.5) where the pressure is increased such that the temperature is very near the ceiling temperature T3. In this case, heat added is less. This heat addition will be zero in case the temperature after compression is same as ceiling temperature. Thus in the limit when T2″ = T3″ – Tmax, no heat being added in the combustion chamber and the cycle plots as a single vertical line and no network is available (Wt = Wc). The maximum pressure ratio can be obtained equating net output equal to zero.
We will now consider another case where the lowest possible pressure is considered. Consider another cycle 1- 2′-3′-4′-1 where some low pressure ratio is considered, and the thermal efficiency will be very poor. In this cycle, it will be noticed that the heat added in the combustion chamber is very large as compared with the work obtained.
Also average temperature at which heat is added and rejected is very small. In the limit, the pressure ratio will be one (1) and the heat addition and rejection lines are same on T-S diagram and network is gain zero.
Between the extreme values of pressure ratios of rp= 1 and rp = (rp)max, there lies an optimum value for which the specific output, the net output per unit mass flow is maximum. Thus taking T1 and T3 as constant temperatures, differentiate expression for Wnet with respect to rp and equating it to zero, we get an optimum pressure ratio (rp)opt. Consider, therefore, a cycle 1-2-3-4-1, Fig. 34.5.
6. Actual Gas-Turbine Cycle with Fluid Friction
:
The efficiency of an actual cycle is the actual network delivered, divided by the actual energy chargeable against the cycle. However, since three or more different pieces of equipment are needed in order for the cycle to operate, and since the efficiency of each is significant, it is customary to study each separately.
As steady flow machines with ΔK = 0, the works of the fluid are adiabatic flow are:
Generally we use symbol mf to designate the mass of fuel used. Its units are defined by the context, usually either mf kg fuel/kg air or mf kg fuel/unit of work say kg fuel/kWh. In this case it is generally called the Specific Fuel Consumption.
Similarly, symbol ma is used to designate the mass of air used. Its units are defined by the context, usually either ma kg air/kg fuel or ma kg air/unit of work say kg air/kWh.
∴ Air rate of an engine or turbine is given by the ratio.
7. Simple Closed-Cycle Gas Turbine
:
As per the methods of classification of gas turbine, one fundamental classification of gas turbine cycles is the path of the working substance. If this working substance is confined within the plant, the gas turbine is said to work on a closed cycle.
In this case, after expanding through the turbine, gas leaves the turbine at the state 4 and passes on to the cooler where gas is cooled to the initial state 1. Generally for stationary closed-cycle gas turbine plant will have water as the cooling medium in the cooler. Gas from the cooler is again taken to the compressor and the cycle is repeated.
The T-S diagram for this cycle is shown in Fig. 34.23 and is same as that for open cycle.
In ideal cycle, compression and expansion are isentropic processes while heat addition and heat rejection is at constant pressure. All types of loses are assumed to be absent and hence pressure drops when passing through the heat exchangers are absent.
Simple closed-cycle gas turbines can work with modifications like intercooling, reheating and regeneration and the numerical analysis is exactly same as that for open cycle gas turbines.
These closed-cycle gas turbine plants are highly suitable for nuclear and high output power stations. The efficiency of such plant is more than 30 %.
Compressor is a steady flow machine and hence the work done on the working substance is given by-
As before, we observe from this relation and conclude that the cycle efficiency is a function of (rp) pressure ratio only while the work ratio is a function of pressure ratio and the temperature ratio of the cycle.
Advantages and Disadvantages of Closed Cycle Gas Turbine:
(a) Advantages:
1. The working of this cycle is not dependent on the atmospheric pressure and therefore any pressure can be used so that the specific output of the plant can be increased. By using high pressure, the sizes of the components are reduced.
2. We can use any gas with favourable properties. Helium and helium-carbon dioxide mixture gives high efficiencies and smaller components. It is highly suitable for nuclear plants.
3. When helium is used, then other alternative materials can be used as no oxidation occurs with these inert gases.
4. The turbine blades are not fouled by the products of combustion.
5. Regulation of the closed cycle gas turbine is simpler.
6. Low thermal stresses exist because of the constant temperatures at all loads.
7. As the working substance or fluid does not come in contact with the fuel or products of combustion, very low quality fuel can be used. Any kind or type of the fuel – solid, liquid, and gases and even nuclear can be used.
(b) Disadvantages:
1. Strong heat exchanger will be required if the high pressures are utilized at the inlet of the compressor.
2. Poor combustion efficiency and poor heat transfer results because of the indirect type of heat exchanger.
3. Because air heater and coolers are used the cost of the plant increases. Also the total plant becomes heavy. Due to all these factors, the cost of the plant is more than that of open type.
4. A coolant is required for pre-cooler and is a disadvantage as compared to an open cycle.
8. Semi-Closed Cycle
Gas Turbine:
As in an internal combustion engine, there is a combination of constant volume heat addition and constant pressure heat addition process, in gas turbine practice, there is a combination of open-cycle gas turbine and closed cycle gas turbine. In this case, the cycle is called as Semi-Closed cycle.
In semi-closed cycle, there are two turbines one of which is a closed-cycle gas turbine and the other is an open-cycle gas turbine. Closed cycle gas turbine drives the compressor while an open cycle gas turbine is driving a power generator. Figure 34.24 shows the schematic diagram of a semi-closed cycle gas turbine plant.
The different components of the semi-closed cycle gas turbine shown in Fig. 34.24 are as follows:
C1 — Low Pressure compressor (LP)
C2 — High Pressure compressor (HP)
TC — Compressor turbine
CC — Combustion chamber
G — Generator
PT — Power turbine
m2 — Air to LP compressor (from atmosphere)
m1 — Air/gas from TC admitted to HP compressor after pre-coder.
In the semi-closed cycle gas turbine, some part of the total fluid is confined to the plant and another part of the fluid is taken from the atmosphere. With reference to the Fig. 34.24, m1 is the mass of the fluid confined to the closed cycle while m2 is the fluid following an open cycle. The closed-cycle part is a high pressure system and hence the parts are smaller than those for open-cycle parts.
It is observed that the closed-cycle plant gives a better part load performance as compared to open-cycle system. Similarly the air-heater is smaller than that of closed cycle plant.
Figure 34.25 shows the T-S diagram for the schematic of the semi- closed cycle gas turbine.
Different processes shown on the T-S diagram are as follows:
1-2 Isentropic compression in LP stage.
1- 2′ Actual compression in LP stage.
2- 3 Intercooling in intercooler.
3- 4 Isentropic compression in HP stage.
3-4′ Actual compression in HP stage.
4′-5 Heating of air in combustion chamber or heater
5-6 Isentropic expansion in compressor turbine.
5-6′ Actual expansion in compressor turbine
5-7 Isentropic expansion in Power turbine
5-7′ Actual expansion of gases in Power turbine.
An illustrative example given below shows the calculations for semi-closed cycle gas turbine plant.
Illustrative Example: (Semi-Closed Cycle)
Following is the data given for a semi-closed cycle gas turbine unit. The plant producers 2000 kW:
1. LP Compressor – Inlet 1 bar and 17°C
Outlet 2 bar
2. Perfect intercooling
HP compressor outlet 4 bar.
3. Compressor turbine inlet temperature 800°C
4. Power turbine inlet at 4 bar and 850°C.
Determine the following-
(i) Temperature of the power turbine exhaust.
(ii) The ratio of mass flow of air in both circuits.
(iii) The overall efficiency.
(iv) Power required to drive the compressor.
Assume:
9. Advantages and Disadvantages of Gas Turbine over Steam Turbine
:
Advantages of gas turbines are:
1. No feed-water supply system is required.
2. No need of condensing plant
3. No boiler is required.
4. There are less components to be designed.
5. Maintenance cost is low.
6. It is quick to start and hence there are no banking loss. This is very much required for aviation, locomotive and stand-by or peak load power plants.
7. It has lower specific weight and size. For aviation this is important.
8. Pressures are less.
9. This is an independent system except closed cycle gas-turbine.
10. Useful for mobile vehicles.
11. Cost-wise it is good (Lower cost).
Disadvantages of gas turbines are:
1. The part load efficiency is lower.
2. The efficiency is lower.
3. The power developed is less than steam turbine because the compressor consumes a large power from the power developed by the turbine.
4. For marine application, it is not suitable because air rate is high and the air is containing salts which are harmful to compressors and turbines.
10. Advantages and Disadvantages of Gas Turbine over IC Engines
:
The advantages are:
1. Well balanced because reciprocating masses are not present.
2. Better mechanical efficiency. This is because there are no sliding parts where friction power is present. Only bearing loss is present.
3. Greater capacity.
4. Specific weight is lower and hence the size is smaller for the same capacity.
5. Cheaper fuel can be used. Even solid fuels like pulverized coal can be used.
6. It requires less lubrication.
7. It has higher operating speed.
8. Smokeless exhaust.
9. Comparatively it works silently because of continuous exhaust.
10. Can be used in jet propulsion system because of the continuous exhaust.
11. Modifications in the Joules or Brayton simple cycle are possible, namely, reheating, regenerative and intercooling arrangements can be added to increase output and efficiency.
12. Lower inlet temperature of air which increases the efficiency of the system.
The disadvantages are:
1. It has poor part load efficiency.
2. High air rate, and open cycle gas turbine is not suitable for marine application.
3. It is very much sensitive to compressor and turbine efficiencies.