Classification of Turbines – steam supply and exhaust conditions
Steam turbines may be classified into different categories depending on their purpose and working pressures. The industrial usage of a turbine influences the initial and final conditions of steam. For any steam turbine to operate, a pressure difference must exist between the steam supply and the exhaust.
This classification includes:
Condensing steam turbines are most commonly found in thermal power plants. In a condensing steam turbine, the maximum amount of energy is extracted from the steam, because there is very high enthalpy difference between the initial (e.g. 6MPa; 275°C; x = 1) and final (e.g. 0.008MPa; 41.5°C; x = 0.9) conditions of steam. This is achieved by passing the exhaust steam into a condenser (called a surface condenser), which condenses the exhaust steam from the low-pressure stages of the main turbine (decreases the temperature and pressure of exhausted steam). The exhausted steam is condensed by passing over tubes containing water from the cooling system.
The goal of maintaining the lowest practical turbine exhaust pressure is a primary reason for including the condenser in a thermal power plant. The condenser provides a vacuum that maximizes the energy extracted from the steam, resulting in a significant increase in net work and thermal efficiency. But also this parameter (condenser pressure) has its engineering limits:
- Decreasing the turbine exhaust pressure decreases the vapor quality (or dryness fraction). At some point the expansion must be ended to avoid damages that could be caused to blades of steam turbine by low quality steam.
- Decreasing the turbine exhaust pressure significantly increases the specific volume of exhausted steam, which requires huge blades in last rows of low-pressure stage of the steam turbine.
In a typical condensing steam turbine, the exhausted steam condenses in the condenser and it is at a pressure well below atmospheric (absolute pressure of 0.008 MPa, which corresponds to 41.5°C). This steam is in a partially condensed state (point F), typically of a quality near 90%. Note that, the pressure inside the condenser is also dependent on the ambient atmospheric conditions:
- air temperature, pressure and humidity in case of cooling into the atmosphere
- water temperature and the flow rate in case of cooling into a river or sea
An increase in the ambient temperature causes a proportional increase in pressure of exhausted steam (ΔT = 14°C is usually a constant) hence the thermal efficiency of the power conversion system decreases. In other words, the electrical output of a power plant may vary with ambient conditions, while the thermal power remains constant.
The pressure inside condenser is given by the ambient air temperature (i.e. temperature of water in the cooling system) and by steam ejectors or vacuum pumps, which pull the gases (non-condensibles) from the surface condenser and eject them to the atmosphere.
The lowest feasible condenser pressure is the saturation pressure corresponding to the ambient temperature (e.g. absolute pressure of 0.008 MPa, which corresponds to 41.5°C). Note that, there is always a temperature difference between (around ΔT = 14°C) the condenser temperature and the ambient temperature, which originates from finite size and efficiency of condensers.
Back-pressure steam turbines or non-condensing turbines are most widely used for process steam applications. Steam is a principle energy source for many industrial processes. The popularity of process steam as an energy source stems from its many advantages, which include:
- high heat capacity,
- transportability
- low toxicity
The process steam can be produced by back-pressure steam turbines, which also generates mechanical work (or electrical energy). Back-pressure turbines expand the live steam supplied by the boiler to the pressure at which the steam is required for the process. The exhaust pressure is controlled by a regulating valve to suit the needs of the process steam pressure. Back-pressure turbines are commonly found at refineries, district heating units, pulp and paper plants, and desalination facilities where large amounts of low pressure process steam are needed. The electric power generated by the back-pressure turbine is directly proportional to the amount of process steam required.
Reheat turbines are also used almost exclusively in thermal power plants. All turbines, that have high-pressure turbine and low-pressure turbines use a steam reheat between these stages. Reheat allows to deliver more of the heat at a temperature close to the peak of the cycle (i.e. thermal efficiency increases). This requires the addition of another type of heat exchanger called a reheater. The use of the reheater involves splitting the turbine, i.e. use of a multistage turbine with a reheater. It was observed that more than two stages of reheating are unnecessary, since the next stage increases the cycle efficiency only half as much as the preceding stage.
High pressure and low pressure stages of the turbine are usually on the same shaft to drive a common generator, but they have separate cases. With a reheater, the flow is extracted after a partial expansion (point D), run back through the heat exchanger to heat it back up to the peak temperature (point E), and then passed to the low-pressure turbine. The expansion is then completed in the low-pressure turbine from point E to point F.
The steam must be reheated or superheated in order to avoid damages that could be caused to blades of steam turbine by low quality steam. High content of water droplets can cause the rapid impingement and erosion of the blades which occurs when condensed water is blasted onto the blades. To prevent this, condensate drains are installed in the steam piping leading to the turbine. The reheater heats the steam (point D) and then the steam is directed to the low-pressure stage of steam turbine, where expands (point E to F). The exhausted steam is at a pressure well below atmospheric, and, as can be seen from the picture, the steam is in a partially condensed state (point F), typically of a quality near 90%, but it is much higher vapor quality, than that it would be without reheat. Accordingly, superheating also tends to alleviate the problem of low vapor quality at the turbine exhaust.
Almost all large steam turbines use the heat regeneration (i.e. they are extraction turbines), since it reduces the amount of fuel that must be added in the boiler. The reduction in the heat added can be done by transferring heat (partially expanded steam) from certain sections of the steam turbine, which is normally well above the ambient temperature, to the feedwater. Note that, most of energy contained in the steam is in the form of latent heat of vaporization. Extraction flows may be controlled with a valve, or left uncontrolled.
For example, most of nuclear power plants operates a single-shaft turbine-generator that consists of one multi-stage HP turbine with 3 or 4 self-regulating extraction lines and three parallel multi-stage LP turbines with 3 or 4 self-regulating extraction lines.
The high pressure feedwater heaters are usually heated by extraction steam from the high pressure turbine, HP, whereas the low-pressure feedwater heaters are usually heated by extraction steam from the low pressure turbine, LP. Both are usually self-regulating. It means that the greater the flow of feedwater the greater the rate of heat absorption from the steam and the greater the flow of extraction steam.
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