Calculating Turbine Efficiency

You know the input pressure and temperature. Another input data you need is the flow for the 10 bar and the 3 bar extractions, input and output vanes. The thing is that the more steam you extract the lower thermal to mechanical efficiency you will have. Anyway...another thing you need to calculate is the enthalpy of the steam at the input vane, at each extraction and at the evacuation of the turbine. Finnaly your input data wlii be: 4 values for flow and 4 values of enthalpy(one pair flow-enthalpy for the input, two pairs for the extractions and another for the exit).The enthalpy is a function of pressure and temperature. it can be read in diagrams or tables.

the formula goes something like this:

D4*(h1-h4)+D3*(h1-h3)+D2*(h1-h2)+P=D1*h1

where:

D4=the flow at the turbine evacoation

D3,D2= flow for each extraction(you call it which)

D1= the input flow of steam

h1to h4 are the corresponding enthalpy for each flow(D1 to D4)

P = the raw mechanical power .

finnaly the turbine efficiency= P/(D1*H1)

i have a two stage steam turbine (single unit). with injection of low pressure steam in mid way......

plz tell me how to calculate its overall efficiency

As in other equipments the efficiency means output devided by input. In case of Turbine the input is total amount of heat in kcal given to turbine ( steam quantity x Enthalpy of the steam) and out put will be Power in KW which needs to be converted in to heat units ( 1 KW = 860 Kcal) Now you devide Output by input you will get your turbine efficiency.

rangacharya

Dear Mr. Rangacharya,

I want to add a correction to your expression.

The steam comes out of Turbine after expanding, and in this process heat energy is converted into Mechanical Energy. The Steam leaving the turbine either at last stage (which will be having vaccuum for condensing turbine) or at bleeding point, WILL HAVE HEAT ENERGY, all the energy is NOT ABSORBED BY TURBINE. Hence the DIFFERENCE between the HEAT in the INLET STEAM and the the HEAT in the OUT LET STEAM IS THE NETT HEAT DROP/Kg. of STEAM. Hence the TOTAL HEAT INPUT to the Turbine for conversion of ENERGY is STEAM FLOW/Hr x NETT HEAT DROP/Kg. of STEAM. Let this value be A

The Turbine Power output in KW is known . In terms of Heat Energy it will be (POWER OUTPUT in KW X 860) K.Cal. Let this value be B. Now the Efficiency is B/A.

The point here is that the NETT HEAT DROP/Kg. of Steam, is the basis, where as you have considered entire Heat in the In-Let Steam is absorbed by the Turbine, which is not so.

In the Exhaust Steam the Latent heat is enormous, transferred to cooling water in the condenser- if it is a condensing turbine and in the case of bled steam, condensing will take place in the Heat Exchanger.

DHAYANANDHAN.S

Dear Friend,

There is NO EMPIRICAL FORMULA for your Question. With the help of the STEAM TABLE the HEAT DROP/Kg. of Steam is to be calculated , before each Extraction and the quantity of Steam flow for next stage is known and then calculate the OUT PUT POWER.

DHAYANANDHAN.S

[Energy flow out]/[Energy flow in]=[Efficiency]

The properties of <...steam...> are widely available and in the public domain: "Thermodynamic Properties of Fluids", "Kempe's Engineers' Yearbook" and "The Chemical Engineer's Handbook" (any edition of any of them), and there are steam tables on the internet that will appear after a brief search.