Calculate Turbine Efficiency - How To Improve Steam Turbine Efficiency?

☛ HOW TO CALCULATE STEAM TURBINE EFFICIENCY?

In this page, at first we will discuss the steam turnine efficiency and then see how to improve the turbine efficiency with the help of some modern methods.Efficiency of steam turbine is mainly three types.As per blades movement and steam supply,steam turbine efficiency is different types.Also read working principle of impulse and reaction steam turbine

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1. Diagram Efficiency of Steam Turbine or Blading Efficiency of Steam Turbine:-

Diagram efficiency of steam turbine or blading efficiency of steam turbine is the ratio of work done on the blades to the energy supplied to the blades.The quantities used in diagram efficiency is directly related to the velocity diagram of steam turbine.

Let
V₁ = Absolute velocity of inlet steam in m/s, m = Mass of steam supplied in kg/s,
V_r1 = Velocity of steam relative to moving blades at entrance, (see velocity diagram of steam turbine),
V_r2 = Velocity of steam relative to moving blades at exit,

So, energy supplied to the blade/sec,

As we all know that the work done on the blades per second,

So,Diagram or blading efficiency equation of steam turbine is,

2. Nozzle Efficiency Of Steam Turbine:-

Nozzle efficiency of steam turbine is the ratio of energy supplied to the blades per kg of steam to the total energy supplied per stage per kg of steam.

The energy supplied to the blades per kg of steam

So,Nozzle Efficiency equation of steam turbine is,

3. Gross or Stage Efficiency Of Steam Turbine:-

Gross efficiency of steam turbine or stage efficiency of steam turbine is the ratio of the work done on the blades per kg of steam to the total energy supplied per stage per kg of steam.

calculation of gross or stage efficiency of steam turbine is

,

Let, h₁ = Enthalpy or total heat of steam before expansion through the nozzle in kJ/kg of steam, h₂ = Enthalpy or total heat of steam after expansion through the nozzle in kJ/kg of steam,

Enthalpy or heat drop in the nozzle ring of an impulse wheel,

Total energy supplied per stage = 1000 h_d J/kg of steam

Work done on the blade per kg of steam,

Gross or stage efficiency

,

We know that the Gross efficiency is the multiplication of blading efficiency and stage efficiency,

All the above, steam turbine efficiency formula are important for both impulse and reaction turbine.

HOW TO IMPROVE STEAM TURBINE EFFICIENCY?

The performance of steam turbine efficiency can be discussed with the help of its internal efficiency characteristics. Some modern methods are used to improve the turbine efficiency. These methods are very easy and interesting too.Now we will discuss how to improve steam turbine efficiency as well as the power developed by the turbines. Though there are so many methods to improve the Steam turbine efficiency but we only discuss the popular three methods.These three methods are used in modern steam turbines to improve their efficiency.The methods are-

   1.Reheating of steam turbine

   2. Regenerative feed heating of steam turbine

   3.Binary vapour cycle of steam turbine

1. Reheating cycle of steam turbine

In this system,the steam's ability to work may be slightly increased by reheating the steam during its passing through the turbine.Steam first enters into the turbine at superheated state.Now steam's pressure and temperature is improved by efficiency of Rankine cycle.This will increase its expansion ratio and steam has become wet condition at the end of expansion. Now wet steam is exhausted from that the turbine and it is not good for turbine.It reduces erosion and internal loss of the turbine. This problem may be solved by reheating the steam at a constant pressure by the flue gases until it is again in the superheated state and use for turbine. The superheated steam is now taken back into the turbine to complete its expansion through the turbine. At a certain limit, steam's performance will be slightly increased with the help of this process.we will show this the h-s diagram of reheat cycle.The block diagram of reheating of steam is shown fig.

Advantages of reheating cycle the steam turbine

The rating of steam in a turbine has the following advantages:

1.It increases the efficiency of the turbine.

2.It increases the work done through the turbine.

3.It reduces erosion and Internal loss of the turbine as because of increase in dryness fraction of steam at exhaust.

In reheat cycle,superheated steam enters into the turbine at a point A,shown in the picture.After that it expands isentropically shows in this picture vertical line AB.Now steam becomes wet condition and again it is heated by constant pressure and temperature shown at a point C.Again the steam expands isentropically and shows next stage of the turbine showing in the picture by vertical line CD.Here is the h-s diagram of reheat cycle-

Let,
h_A = Enthalpy or total heat of steam at a point A.
h_B,h_C,h_D = Corresponding values at B,C and D.
h_fd = Enthalpy or senseble heat of water at D.

We know that the total heat supplied to the steam is the summation of total heat at a point A and the heat supplied during the reheating between B and C.

So, total het supplied =Total heat at A + Heat suppliedbetween B and C

We also know that the work done,

And efficiency,

2. Regenerative feed heating cycle of steam turbine

The dry saturated steam is coming from the boiler and enters into the turbine at higher temperature.In the turbine it expands isentropically to a lower temperature as the same as Rankine and Carnot cycle. As we all know that the heat is not added at higher temperature in the Rankin cycle and Rankin cycle is less efficient from the Carnot cycle.Now the steam is coming from the turbine and condensed in the condenser. So a large amount of heat is rejected from the condensers shown in the area DEFG.This heat is now pumped back and circulated around the turbine casing in the opposite direction to the steam flow in the turbine. This hot steam is now entering into the boiler shown in the fig E to A.This type of steam heating is called Regenerative Heating. But due to heat loss, steam turbines expansions is no more isentropic and it flows the path BC, which is exactly parallel to EA.

Heat transferred to the liquid India EAGF is equal to the heat transferred from the steam represents BPQC. At a constant temperature, heat is supplied to the working fluid and heat is rejected from the working fluid shown by the curve AB and CE. This also represented by the area AbPG and CQFE.CQFE is equal to RPGD and it The presence the heat rejected in the Carnot cycle. That's why the efficiency of ideal regenerative cycle is equal to the efficiency of Carnot cycle.But Practically ideal Regenerative cycle of steam turbine is not possible.Because,

Liquid feed water can't supply the necessary heat to the steam turbine and moisture content is increased for the heat transfer.

Since ideal regenerative cycle of steam turbine is not possible so some sort of advantages we can get from the bleeding of the steam.

WHAT IS BLEEDING?

In regenerative heating, some steam is carried out from the turbine at certain points during its expansion. This team is used to the feed water in the feed water heater increasing its temperature and then supplying to the boiler is known as bleeding.Using this process, there is a slight increase in efficiency but there is also a decrease in the horsepower developed.

3. Binary vapour cycle

Binary paper plans, Mercury is used to increasing the temperature of the plant without increasing its pressure. Actually, maximum possible efficiency of any steam Engine is given by the equation:

In the above equation T₁ is the higher temperature at which heat is absorbed and T₂ is the lower temperature at which heat is rejected. So if we increase the plant efficiency then we must increase the T₁ temperature because T₂ is fixed by the atmospheric condition. In a steam plant if T₁ is increased correspondingly some pressure is also increased.That's why Mercury vapour is used because it has the temperature range at low pressure.The critical temperature of mercury vapour is 588.4 c at a critical pressure of 21 bar.

In the above picture, liquid mercury is coming from a heater to boiler,where it is evaporated. After that it flows and expands in the mercury turbine at low pressure limit. Now Mercury exhausts to the Mercury condenser steam boiler where its latent heat is given out to the hot feed water. The Mark Curry is then returned to the mercury liquid heater and complete the cycle.

In the above shows T-S diagram of bindery vapour cycle.ABCD represents the Mercury cycle and 1-2-3-4-5 line shows the steam cycle.

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