Pressure Drop
The efficiency of a steam turbine is determined by the pressure drop through the turbine. The higher the pressure drop, the higher the efficiency. So when we design a steam turbine system, our engineers will attempt to specify the highest pressure that a boiler can economically produce, and also the lowest exhaust pressure that the condenser can deliver. This way, we maximize the efficiency of the steam turbine.

Steam Design Pressure
Typically, power boilers are designed with operating pressures of: 600 psig, 900 psig, 1250 psig, 1500 psig and higher. Occaisionally, a specific application will require a lower pressure design. These steam turbines are usually much smaller than their high pressure cousins, and are not very efficient. For example, some geothermal applications are designed with pressures as low as 20 psig, however, these steam turbines have very low efficiencies.

Extracted Steam
In almost all steam turbines, some of the working steam is removed (or 'extracted') at various points along the turbine. This is high quality steam that can be used within the power plant for heating purposes, or sold to external customers. Typical uses for extraction steam are: chemical production, paper milling, other industrial processes, cooking food or space heating in offices, universities ets.

Condensing/Extraction Turbine
Turbines with extraction points are called "Condensing" or "Extraction" turbines. These turbines are commonly used in cogeneration and combined cycle plants.

Back-pressure Turbine
There are also turbines that use much more of the steam for power production. These "Back-pressure Turbines" only partially drop the inlet pressure, producing electricity and relatively high quality process steam. The process steam is removed at the back of the turbine (hence, the name 'back-pressure'), making this a 'bottoming' cycle.

Specifications
Almost all steam turbines are custom manufactured to a given specificification. Our engineers determine the following main design factors: steam flow into the turbine, temperature and pressure of the steam, location and pressures of the extraction points, and condensing pressure (usually in 2-3 inches of mercury).

Operating Speed
In North America (a 60Hz system) small turbines operate at relatively high speeds (5,000 - 8,000 rpm), while large turbines (found at utilities, nuclear power plants etc) operate at 3,600 rpm.

Cost Estimate
Back-pressure turbines are relatively inexpensive at about $100-$150/kW. Condensing turbines are more expensive - about $200-$250/kW. This is because the low pressure part of the turbine (the back-end) is much larger and more inefficient.

Efficiencies
Large steam turbines produce most of the power generated in the world today. Utilities use steam turbines with a high inlet pressure, re-heat cycles, and low condenser pressure. These turbines can reach efficiencies as high as 40%. Smaller turbines (used in industry and by independent power producers) operate at about 32% efficiency.

Back-pressure turbines only convert a portion of the available inlet steam, leading to a relatively low efficiency of about 15-20%. However, since the exhuast steam is useful, the overall cycle efficiency (electricity + steam) of a back-pressure steam turbine can be very high - in the range of 60-80% efficient.

Overall Efficiencies
There are a number of other loses that can reduce the overall efficiency of a steam turbine power plant. To name a couple:

• Fuel Combustion - Some of the fuel's energy (about 15% - 25%) will be lost when converting the raw fuel into steam. Feedwater and stack temperatures are two factors that can exagerate this loss.
• Plant Service Loads - In the power plant, there are many pumps, fans and motors that need electricity to operate. This 'parasitic' load can be about 6-10% of the plant capacity.

When all of the various losses and parastic loads are taken into account, a steam turbine power plant can convert a unit of fuel (oil, gas, coal) into electricity with an efficiency of about 28-32%.