There are two types of steam turbines. First of Non-Condensing (Back-pressure) Steam Turbine and the other is Extraction Steam Turbine .
The primary type of turbine used for central power generation is the condensing turbine. These power-only utility turbines exhaust directly to condensers that maintain vacuum conditions at the discharge of the turbine. An array of tubes, cooled by river, lake, or cooling tower water,
condenses the steam into (liquid) water. The cooling water condenses the steam turbine exhaust steam in the condenser creating the condenser vacuum. As a small amount of air leaks into the system when it is below atmospheric pressure, a relatively small compressor removes noncondensable gases from the condenser. Non-condensable gases include both air and a small amount of the corrosion byproduct of the water-iron reaction, hydrogen.
The condensing turbine processes result in maximum power and electrical generation efficiency from the steam supply and boiler fuel. The power output of condensing turbines is sensitive to ambient conditions.
CHP applications use two types of steam turbines: non-condensing and extraction.
Non-Condensing (Back-pressure) Turbine
Figure shows the non-condensing turbine (also referred to as a back-pressure turbine) exhausts its entire flow of steam to the industrial process or facility steam mains at conditions close to the process heat requirements.
Usually, the steam sent into the mains is not much above saturation temperature.3 The term “back-pressure” 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 extraction turbine has opening(s) in its casing for extraction of a portion of the steam at some intermediate pressure before condensing the remaining steam. Figure illustrates the
The steam extraction pressure may or may not be automatically regulated. Regulated extraction permits more steam to flow through the turbine to generate additional electricity during periods of low thermal demand by the CHP system. In utility type steam turbines, there may be several
extraction points, each at a different pressure corresponding to a different temperature. The facility’s specific needs for steam and power over time determine the extent to which steam in an extraction turbine is extracted for use in the process.
In large, often complex, industrial plants, additional steam may be admitted (flows into the casing and increases the flow in the steam path) to the steam turbine. Often this happens when using multiple boilers at different pressure, because of their historical existence. These steam
turbines are referred to as admission turbines. At steam extraction and admission locations there are usually steam flow control valves that add to the steam and control system cost.
Numerous mechanical design features increase efficiency, provide for operation over a range of conditions, simplify manufacture and repair, and achieve other practical purposes. The long history of steam turbine use has resulted in a large inventory of steam turbine stage designs. For example, the division of steam acceleration and change in direction of flow varies between competing turbine manufacturers under the identification of impulse and reaction designs.
Manufacturers tailor clients’ design requests by varying the flow area in the stages and the extent to which steam is extracted (removed from the flow path between stages) to accommodate the specification of the client.
When the steam expands through a high-pressure ratio, as in utility and large industrial steam systems, the steam can begin to condense in the turbine when the temperature of the steam drops below the saturation temperature at that pressure. If water drops form in the turbine, blade
erosion occurs from the drops impact on the blades. At this point in the expansion the steam issometimes returned to the boiler and reheated to high temperature and then returned to the turbine for further (safe) expansion. In a few large, high pressure, utility steam systems install
double reheat systems.
With these choices the designer of the steam supply system and the steam turbine have the challenge of creating a system design which delivers the (seasonally varying) power and steam which presents the most favorable business opportunity to the plant owners.
Between the power (only) output of a condensing steam turbine and the power and steam combination of a back-pressure steam turbine essentially any ratio of power to heat output can be supplied. Back-pressure steam turbines can be obtained with a variety of back pressures, further increasing the variability of the power-to-heat ratio.