Posts Tagged ‘steam tables’

Enthalpy Values in the Absence of a Condenser

Tuesday, November 26th, 2013

      Last time we learned that the amount of useful work, W, that a steam turbine performs is calculated by taking the difference between the enthalpy of the steam entering and then leaving the turbine.   And in an earlier blog we learned that a vacuum is created in the condenser when condensate is formed.    This vacuum acts to lower the pressure of turbine exhaust, and in so doing also lowers the enthalpy of the exhaust steam.   Putting these facts together we are able to generate data which demonstrates how the condenser increases the amount of work produced by the turbine.

      To better gauge the effects of a condenser, let’s look at the differences between its being present and not present.   Let’s first take a look at how much work is produced by a steam turbine without a condenser.

Steam Turbine Engineering Expert Witness

      The steam entering the turbine inlet has a pressure of 2000 pounds per square inch (PSI) and a temperature of 1000°F.   Knowing these turbine inlet conditions, we can go to the steam tables in any thermodynamics book to find the enthalpy, h1.   Titles such as Fundamentals of Classical Thermodynamics by Gordon J. Van Wylen and Richard E. Sonntag list enthalpy values over a wide range of temperatures and pressures.   For our example this volume tells us that,

h1 = 1474 BTU/lb

where BTU stands for British Thermal Units, a unit of measurement used to quantify the energy contained within steam or water, in our case the water to steam cycle inside a power plant.   Technically speaking, a BTU is the amount of heat energy required to raise the temperature of one pound of water by one degree Fahrenheit.   The term lb should be a familiar one, it’s the standard abbreviation used for pound, so enthalpy is the measurement of the amount of energy per pound of steam flowing through, in this case, the turbine.

      Since there is no condenser attached to the steam turbine’s exhaust in our illustration, the turbine discharges its spent steam into the surrounding atmosphere.   The atmosphere in our scenario exists at 14.7 PSI because our power plant happens to be at sea level.   Knowing these facts, the steam tables inform us that the value of the exhausted steam’s enthalpy, h2, is:

h2 = 1015 BTU/lb

      Combining the two equations we are able to calculate the useful work the turbine is able to perform as:

W = h1h2 = 1474 BTU/lb – 1015 BTU/lb = 459 BTU/lb

      This equation tells us that for every pound of steam flowing through it, the turbine converts 459 BTUs of the steam’s heat energy into mechanical energy to run the electrical generator.

      Next week we’ll connect a condenser to the steam turbine to see how its efficiency can be improved.