Steam and Gas Turbines

Steam turbines and gas turbines are rotating machines that extract energy from pressurized steam and/or from combustion gases. There are two basic types of steam turbines: impulse and reaction. Impulse turbines are used for smaller power-generation applications. Reaction turbines are used for industrial-scale power generation. Different types of gas turbines are also available. Examples include combined-cycle gas turbines and air-cooled gas turbines. Often, suppliers designate steam and gas turbines as small, medium, or heavy-duty.

Steam turbines have two cycles: non-condensing and condensing. During the non-condensing cycle, high-pressure steam enters the turbine and is exhausted to a process heater. Because steam turbines absorb most of the heat before the condensate returns to the boiler, cycle efficiency is relatively high. During the condensing cycle, however, the exhaust steam is brought to a level lower than atmospheric pressure. The cycle efficiency is low because the cooling water absorbs most of the heat. Typically, condensing cycles are used to send low-pressure process exhaust steam through a multi-stage steam turbine.

Steam turbines and gas turbines differ in terms of product specifications. Steam turbines differ in terms of inlet steam pressure and temperature, exhaust steam pressure, pump power, and pump speed. Speed control is either manual or automatic. Pressure, temperature, and the cleanliness of the cooling water are also important considerations when selecting steam turbines. As a rule, off-normal steam affects both the steam path and the turbine’s ability to generate power. Low-power conditions may create high-exhaust temperatures and affect the method of lubrication.

Gas turbines have an upstream compressor that is coupled to a downstream rotary engine. The combustion chamber is located in-between. Typically, gas turbines carry parameters such as output, heat rate, pressure ratio, mass flow, speed, and exhaust temperature. With gas turbines, the combustor adds energy to the gas stream. There, air is mixed with fuel and ignited. Combustion increases the temperature, velocity and volume of the gas flow, which is forced through a nozzle over the turbine’s blades. This spins the turbine and powers the compressor. The energy that is extracted as shaft power, compressed air and thrust is used to power airplanes, generators, marine vessels, and motorized vehicles.

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