Usually, the steam sent into the mains is not much above saturation temperature. The term “back-pressure” for back pressure turbines refers to turbines that exhaust steam at atmospheric pressures and above. The specific CHP application establishes the discharge pressure. 50, 150, and 250 psig are the most typical pressure levels for steam distribution systems. District heating systems most often use the lower pressures, and industrial processes use the higher pressures.
Industrial processes often include further expansion for mechanical drives, using small steam turbines for driving heavy equipment that runs continuously for long periods. Power generation capability reduces significantly when steam is used at appreciable pressure rather than being expanded to vacuum in a condenser. Discharging steam into a steam distribution system at 150 psig can sacrifice slightly more than half the power that could be generated when the inlet steam conditions are 750 psig and 800°F, typical of small steam turbine systems. The steam turbine operates on basic principles of thermodynamics using the part of the Rankine cycle.
Superheated vapor (or dry saturated vapor, depending on application) enters the back pressure turbines, after it having exited the boiler, at high temperature and high pressure. The high heat/pressure steam is converted into kinetic energy using a nozzle (a fixed nozzle in an impulse type turbine or the fixed blades in a reaction type turbine). Once the steam has exited the nozzle it is moving at high velocity and is sent to the blades of the turbine. A force is created on the blades due to the pressure of the vapor on the blades causing them to move. A generator or other such device can be placed on the shaft, and the energy that was in the vapor can now be stored and used.
The gas exits the back pressure turbine as a saturated vapor (or liquid-vapor mix depending on application) at a lower temperature and pressure than it entered with and is sent to the condenser to be cooled. If we look at the first law we can find an equation comparing the rate at which work is developed per unit mass. To maximize turbine efficiency the steam in the back pressure turbine is expanded, doing work, in a number of stages. These stages are characterized by how the energy is extracted from them and are known as either impulse or reaction turbines. Most steam turbines use a mixture of the reaction and impulse designs: each stage behaves as either one or the other, but the overall turbine uses both. Typically, higher pressure sections are reaction type and lower pressure stages are impulse type.
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