Differences between backpressure, extraction, and condensing steam turbines

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Differences between back-pressure, extraction, and condensing steam turbines 1. Back-pressure steam turbine A back-pressure steam turbine is a type of turbine in which the exhaust steam from the turbine is used to heat end-users. Its exhaust pressure (back pressure) is higher than atmospheric pressure. Back-pressure steam turbines have a high exhaust pressure, fewer stages in the flow passage, and a simple structure; moreover, they do not require large condensers or cooling water systems, resulting in a compact unit with lower costs. When its exhaust steam is used for heating, the thermal energy can be fully utilized. However, in this case the power of the turbine is directly related to the amount of steam required for heating; as a result, it is not possible to meet both the heating demands and the fluctuations in electrical (or mechanical) power demands simultaneously. This is the limitation of back-pressure turbines when used for heating. The main feature of this type of unit is its good economic efficiency under design conditions, along with significant energy-saving effects. Additionally, it has a simple structure, low investment costs, and reliable operation. The main drawback is that the power generation amount depends on the heat supply, and it cannot be adjusted independently to meet the needs of both heat users and electricity users simultaneously. Therefore, back-pressure steam turbines are mostly used in corporate captive power plants with a stable thermal load throughout the year, or in regional thermal power plants with a steady base thermal load. 2. Extraction-backpressure steam turbine: An extraction-backpressure steam turbine is a type of turbine that extracts some steam from the intermediate stages of the turbine to supply heat users who require a higher pressure level, while simultaneously maintaining a certain backpressure in the exhaust steam to serve heat users who need a lower pressure level. The economics of this type of unit are similar to those of back-pressure units; it performs well under design conditions, but has poor adaptability to load changes. 3. Extraction-condensing steam turbine: An extraction-condensing steam turbine is a type of condensing steam turbine that extracts a portion of the steam from within the turbine to supply heat to end users. Suction-condensing steam turbines draw steam at a certain pressure from the intermediate stages of the turbine to supply it to heat users; they are generally divided into single-suction and double-suction types. Among them, a double-suction steam turbine can supply steam at two different pressures to heat users. The main feature of this type of unit is that when the steam demand from the heat users suddenly decreases, the excess steam can continue to expand and generate electricity through the stages downstream of the turbine extraction point. The advantage of this type of unit is its high flexibility, allowing it to meet both thermal and electrical load requirements over a wide range. Therefore, it is used in regional thermal power plants where load variations are large and frequent. Its disadvantage is that its thermal efficiency is lower than that of back-pressure units, it has more auxiliary equipment, is more expensive, and its system is more complex. Back-pressure units do not have condensers, while condensing steam turbines generally have extraction pipes located after the governor, which are used for industrial customers. Another portion of the steam continues to do work, and the waste steam that has completed its function is discharged into the condenser, where it is cooled and condensed into water. This condensed water is then pumped to the deaerator using a condensate pump, and after deoxygenation, it is supplied as water for use in the boiler. There’s a big difference between the two! For the condensation type, since there is still vacuum present, attention must also be paid to the vacuum condition during monitoring. The backpressure exhaust is above atmospheric pressure. Incidentally, let’s briefly explain the function of the condenser equipment: it creates and maintains a high vacuum at the turbine exhaust outlet, allowing the steam to expand to very low pressures within the turbine. This increases the available heat energy drop of the steam, enabling the turbine to convert more thermal energy into mechanical work and thus improving thermal efficiency. It also recovers the condensed water from the turbine exhaust. 4. Summary: In back-pressure turbines, all of the exhaust steam is used for heating; although less electricity is generated, the overall energy utilization efficiency of such turbines can reach 70–85%, making them the turbines with the best energy utilization efficiency. The energy utilization rate of condensing steam turbine systems is currently only up to 45%. Back-pressure steam turbines are generally suitable only for units of 50MW or less. The main reason is the limitation imposed by the steam distribution network; typically, steam can be transported over a distance of 4 km within such a network, while hot water can be transported over 10 km. As a result, larger units cannot be utilized. For seasonal heating units, a steam extraction condensing type is generally used. The current **industrial policy is that full-condensing steam turbines are not allowed for units below 300MW (except in coal gangue power plants or circulating fluidized bed plants); pure-condensing steam turbines are generally used in units of 600MW and above.