STEAM TURBINE POWER PLANTS

High pressure and high temperature steam is created in a boiler, furnace, or heat exchanger and moved through a steam turbine generator (STG) that converts the steam’s energy into rotational energy that turns the generator shaft. The steam turbine’s rotating shaft is directly coupled to the generator rotor. The STG shaft speed is tightly controlled for it is directly related to the frequency of the electrical power being produced.

High temperature, high pressure steam is used to turn steam turbines that ultimately turn the generator rotors. Temperatures on the order of 1,000°F and pressures on the order of 2,000 pounds per square inch (psi) are commonly used in large steam power plants. Steam at this pressure and temperature is called super heated steam, sometimes referred to as dry steam.

The steam’s pressure and temperature drop significantly after it is applied across the first stage turbine blades. Turbine blades make up the fan shaped rotor to which steam is directed, thus turning the shaft. The super heated steam is reduced in pressure and temperature after it passes through the turbine.

The reduced steam can be routed through a second stage set of turbine blades where additional steam energy is transferred to the turbine shaft. This second stage equipment is significantly larger than the first stage to allow for additional expansion and energy transformation. In some power plants, the steam following the first stage is redirected back to the boiler where it is reheated and then sent back to the second turbine stage for a more efficient energy transformation.

Once the energy of the steam has been transferred to the turbine shaft, the low temperature and low-pressure steam has basically exhausted its energy and must be fully condensed back to water before it can be recycled. The condensing process of steam back to water is accomplished by a condenser and cooling tower(s). Once the used steam is condensed back to warm water, the boiler feed pump (BFP) pumps the warm water back to the boiler where it is recycled. This is a closed-loop processes. Some water has to be added in the process due to small leaks and evaporation.

The condenser takes cold water from nearby lakes, ponds, rivers, oceans, deep wells, cooling towers, and other water sources and pumps it through pipes in the condenser. The used steam passes through the relatively cold water pipes and causes dripping to occur. The droplets are collected at the base of the condenser (the well) and pumped back to the boiler by the BFP.

The overall steam generation plant efficiency in converting fuel heat energy into mechanical rotation energy and then into electrical energy ranges from 25 to 35%. Although it is a relatively low efficiency system, steam turbine generation is very reliable and is commonly used as base load generation units in large electric power systems. Most of the inefficiency in steam turbine generation plants comes from the loss of heat into the atmosphere in the boiler process.