Increase Turbine Efficiency by Increasing Vacuum in Condenser

[RohitRmB]

By increasing vacuum in the condensor the efficiency of the turbine increases, also the rankine graph shows this!
But my question is, why does this happen?
how can efficiency of turbine can increase just by increasing vacuum in the condensor?
in reality it does increase, so i want to know what actually causes this increase in efficiency.
Plz bare with my english, my first language is not english

 

[SteamKing]

Increasing the vacuum in the condenser allows the turbine to do more work, because the steam is allowed to expand to a lower absolute pressure. The practical limit in lowering the pressure in the condenser is the point at which a significant amount of moisture begins to form in the final stages of the turbine. More moisture means that erosion of the turbine blades can occur, which could lead to damage to the turbine.

 

[RohitRmB]

this is evident from the pressure tempreture graph, but i want to know what actually happens within the turbine that increases the work output,
Turbine has fixed number of moving and stationary blades, when steam passes through these blades, turbine rotates because of the impulsive and reactive action of the steam on the blades.
Once a turbine is designed and manufactured then the number of blades are fixed, the angles in the blades and there shape is fixed. so the pressure drop of the steam between the inlet and exhaust of the turbine is fixed.
So how can it be that just by increasing vacuum in the condensor will lead to more work output.
When vacuum is increased in the condensor, what are the actual physical changes in the steam condition in the turbine, because ultimately work output is nothing but the work done by the steam on the turbine blades, so if the work output increases that means there must be some changes happening with the steam??
Plz help me.
And Plz bare with my english, my first language is not english

 

[RohitRmB]

I just mean that by increasing vacuum in the condensor there must be some changes in the condition of steam that in working in the turbine so as to increase the work output

 

[turbo]

Work in a turbine is performed by steam that drops in pressure stage-to-stage. Using condensers at the exit allows a much wider range of pressures. In addition, there could be steam ejectors to remove more vapor from the condenser and improve the vacuum in the condenser further.

 

[rollingstein]

Maybe this? As you reduce Pexit (by applying a vacuum) Texit reduces. Hence Enthalpy of the outlet stream reduces.

At constant input enthalpy, a reduction in outlet Enthalpy must appear as an increase in the shaft work of the turbine.

My 2 cents. I could be wrong.

 

[SteamKing]

The efficiency of the cycle is greater when there is a greater difference in the high and low temperatures of the cycle. Like I said, with a lower pressure in the condenser, more expansion of the steam in the turbine is possible, and more work is done by the turbine.

 

[jim hardy]

The key to making that little bit of thermodynamics seem natural is this:

Instead of thinking of yourself as the turbine, which doesn't really care what is exhaust pressure*,
imagine yourself instead to be a pound of steam making your way through the turbine.
As you pass through each stage of the turbine you give up some of your energy to the turbine blades.

When you reach the last stage, how much energy can you give up to the turbine blades?
Well, you can give just enough to place you at the turbine exhaust pressure & temperature, and no more than that.
So the lower the exhaust pressure, the more of your energy you can give to the those last few stages of turbine blades.

So that's what a vacuum condenser does - let's your turbine extract more energy from the steam as it expands and cools on its way out of the turbine.
But that's not something done by the turbine, it's done to the turbine.
For some reason we state it as if the turbine were responsible.

*In reality the turbine may very well care about exhaust pressure for that determines the steam density surrounding those last stage blading. But that's a mechanical problem not thermodynamic one.

 

[moogull]

And Plz bare with my english, my first language is not english

Just chiming in here with nothing really that relevant to the problem at hand. Your english is actually fine!

 

[jennymathew]

It helps maintain that vacuum by removing non condensable gasses from the condenser. Exhausting a turbine into a vacuum will dramatically improve the over all efficiency of the turbine. The lower exhaust temps and pressures allow for more energy to be extracted from the steam by the turbine which gives an over all improvement to plant (turbine)efficiency.