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Extraction Rates for Steam Turbines

A steam turbine converts thermal energy from high pressure steam into rotary motion. This rotary motion is the turbine action and can be used to generate electricity. The steam turbine is highly efficient. An extraction type steam turbine uses high pressure steam to turn the blades in the turbine. The power that can be generated is proportional to the pressure drop across the turbine. Extraction type steam turbines are multi-staged to allow for different pressure steam flows to be extracted for process applications. The more steam that is extracted from the turbine, the less power is produced.

Instructions

- 1
  Determine the desired power output of a typical cogeneration power plant. For instance, the plant could be designed for an optimum output of 5 MW (megawatts) of power (P).
- 2
  Determine the maximum steam pressure that can be generated by the plant̵7;s boilers. For instance, assume the boiler feeding the steam turbines can generate 500 psi (pounds per square inch) of steam pressure and 1,000 pounds per hour of steam flow (m). This provides a steam enthalpy (measure of heat) of 1204 Btu/pound (hs). The corresponding condensed liquid water has an enthalpy of 499.5 Btu/pound (hw).
- 3
  Determine the optimum energy flow rate (Q) using the equation Q= m x (hs - hw) / P. So, for an optimum rate with no extracted steam, the flow rate is 1000 x (1204 - 499.5) / 5 which is 140,900 Btu/MW.
- 4
  Determine the extraction rate of steam if the energy flow rate is reduced to 100,000 Btu/MW. This is done by rearranging the formula to be m = Q x P / (hs - hw) or 100,000 x 5 / (1204-499.5) which is 709.7 pounds per hour steam. Therefore, the extraction rate to produce 100,000 Btu/MW is almost 710 pounds per hour.

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