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How to calculate turbine efficiency

Updated February 21, 2017

Turbines, both steam and gas (internal combustion) versions, create large amounts of shaft horsepower in a small physical package, especially compared to piston engines. Steam turbines are by far the most common way to drive electrical power generators in utility companies because of their reliability and high efficiency, especially compared to internal combustion methods of producing electrical power. Calculating turbine efficiency is possible with knowledge of how much power the turbine produces and how much energy it needs to produce it.

Define the steam turbine calculation. In this example, a 1.2-megawatt-hour power generator in a sugar mill is driven by a steam turbine that receives 23796 Kilogram per hour of superheated steam at 204 Kilogram per square inch absolute (psia) pressure with a temperature of 291 degrees Celsius. The steam exits the turbine saturated at 30 psia (about 15-psi/gauge) and used elsewhere in the plant. If the electrical generator has a conversion efficiency of 82 per cent, you can calculate the turbine efficiency.

Calculate the actual power used by the generator to produce the 1.2 megawatt-hours of electricity: 1.2 megawatt-hours divided by 0.82 (82 per cent) generator efficiency = 1.4634 megawatt-hours of power.

Convert the to shaft horsepower for the turbine: 1.4634 megawatt-hours / (0.746-kilowatt-hours/horsepower X 1,000 kilowatt-hours/megawatt-hours) = 1,962 horsepower.

Convert the 1,962 horsepower produced by the turbine into BTU/hour. Multiplying 1,962.45 by 2,545 BTU/Hour results in a total of 4,993,323 BTU/hour consumed by the turbine.

Look up the enthalpy values of the entering and exiting steam. The superheated steam has an enthalpy of 1275.42 BTU/pound. The exiting steam has an enthalpy of 1164.1 BTU/pound. The net value of power supplied to the turbine by the steam is 111.32 BTU/pound of steam.

Calculate the theoretical pounds of superheated steam at the above conditions to supply 4,993,323 BTU/hour by dividing by the 111.32 BTU/pound of net steam enthalpy consumed in the turbine. This works out to 4,993,323 / 111.32 = 44,38808 Kilogram/hour of steam.

Calculate steam turbine efficiency by dividing the theoretical pounds per hour of steam by the steam actually consumed. This is 44,38808 Kilogram (theoretical) / 23796 Kilogram actual = 0.855 or 85.5 per cent operating efficiency.

Tip

Use steam turbines instead of internal combustion engines on large scale applications for greater efficiency.

Warning

All steam turbine designs should be checked by a professional before implementation.

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