Steam Turbines

Steam turbines are available in virtually any capacity and are very well suited for most mechanical drive applications. They offer good reliability, low maintenance requirements, and long service life, in addition to providing precise control with variable speed capability and excellent overload capability. While operating speeds in excess of 10,000 rpm are not uncommon, optimal operating speeds for many turbine models range from

3,600 to 5,500 rpm, which is compatible with the speed ranges of many types of equipment, such as centrifugal air and refrigeration compressors. Steam turbines generally require less floor space and foundation than reciprocating engines or gas turbines (with heat recovery), and can therefore be more easily fitted in many applications.

Steam turbine characteristics and operation are discussed in Chapter 11. The distinguishing features of condensing turbines are reasonably good thermal efficiency and a need for both a high-pressure steam system and a condensing system. They usually feature a high mechanical efficiency, multi-stage design. The distinguishing feature of non-condensing (topping cycle) back-pressure turbines is that while simple-cycle thermal efficiency is very low, net thermal efficiency approaches 100% (excluding boiler losses) because almost all unused heat energy (in the form of lower-pressure steam) is passed directly to process. They usually feature a low or moderate mechanical efficiency, single-stage design, but are none-the-less very efficient shaft power producers due to their high net thermal efficiency. Extraction steam turbines are multi-stage machines that combine these elements, with reasonably good thermal efficiency for the portion of the steam that is condensed and high net thermal efficiency for the portion of the steam that is extracted as low-pressure steam for thermal applications.

In facilities with high-pressure steam systems and abundant low-pressure steam loads, topping-cycle applications using back-pressure steam turbines are often highly cost-effective. When low-pressure steam loads are limited, high-efficiency multi-staged full condensing or extraction turbines can be used. Multi-stage back-pressure turbines can also be used under conditions of moderate low-pressure steam loads. Back-pressure and extraction turbines can also be designed to operate under varying intake and discharge pressures.

While shaft power can be extracted from virtually any pressure drop, condensing turbines generally become cost-effective when boiler-generated steam pressures are at least 100 psig (7.9 bar). Steam turbines are sometimes used as part of a mixed system hybrid configuration in which the turbine-driven equipment is run during peak periods and electric driven-units run during off-peak periods. An electric generator can be attached to the turbine shaft along with the driven equipment, allowing two modes of operation.

Multi-stage turbines require more maintenance than turbines with single-stage design. Condensing turbine systems have auxiliary components such as surface condensers and cooling towers that require maintenance. In all cases, key considerations include control over steam

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