Main Condenser – Steam Condenser
The main steam condenser
(MC) system is designed to condense
the exhaust steam from the main turbine and provide a heat sink for the turbine bypass system. The exhausted steam from the LP turbines is condensed by passing over tubes containing water from the cooling system. There is a main condenser unit under each LP turbine
, usually below the turbine with its axis perpendicular to the turbine axis. Since nuclear power plants usually contain also an auxiliary condenser (e.g. to condense steam from steam driven feedwater pumps), engineers use the term “main condenser
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 pressure inside condenser determines the overall power output of the power conversion system. Thermal power plants are usually equipped with so called “surface condensers”.
The condensed steam (now called condensate) is collected in the condenser’s hotwell. Condenser’s hotwell provides also a water storage capacity, which is required for operational purposes such as feedwater makeup. The condensate (saturated or slightly subcooled liquid) is delivered to the condensate pump and then pumped by condensate pumps to the deaerator through feedwater heating system. The condensate pumps increase the pressure usually to about p = 1-2 MPa. There are usually four one-third-capacity centrifugal condensate pumps with common suction and discharge headers. Three pumps are normally in operation with one in the backup.
Isobaric heat rejection (in a heat exchanger) – In this phase the cycle completes by a constant-pressure process in which heat is rejected from the partially condensed steam. There is heat transfer from the vapor to cooling water flowing in a cooling circuit. The vapor condenses and the temperature of the cooling water increases. The net heat rejected is given by Qre = H4 - H1
Parameters of the Main Condenser
The condenser must maintain a sufficient low vacuum in order to increase the power plant efficiency. The vacuum pumps maintain a sufficient vacuum in the condenser by extracting air and uncondensed gases. 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. Since neither the condenser is 100% efficient heat exchanger, there is always a temperature difference between the saturation temperature (secondary side) and the temperature of the coolant in the cooling system. Moreover, there is a design inefficiciency, which decreases the overall efficiency of the turbine. Ideally the steam exhausted into the condenser would have no subcooling
. But real condensers are designed to subcool the liquid by a few degrees of Celsius in order to avoid the suction cavitation
in the condensate pumps. But, this subcooling increases the inefficiency of the cycle, because more energy is needed to reheat the water.
Decreasing the turbine exhaust pressure increases the net work per cycle but also decreses the vapor quality of outlet steam.
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 wet 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.
To maintain the parameters inside the condenser (0.008 MPa and 41.5 °C), the cooling water from cooling system must be sufficiently cold and there cannot be large temperature difference between the outlet and inlet water temperauter, hence the flowrate through the cooling system must be very high. The flowrate through the cooling system (with wet cooling towers) may be up to 100 000 m3/h (27.7 m3/s). The condenser inlet water may have about 22°C (strongly depending on ambient conditions), while the condenser outlet may have about 25°C. The sea water cooling systems operate at higher flowrates, for example, 130 000 m3/h.