Saturation
In thermodynamics, the term saturation defines a condition in which a mixture of vapor and liquid can exist together at a given temperature and pressure. The temperature at which vaporization (boiling) occurs for a given pressure is called the saturation temperature or boiling point. The pressure at which vaporization (boiling) occurs for a given temperature is called the saturation pressure.
When the vapor quality is equal to 0, it is referred to as the saturated liquid state (single-phase). On the other hand, when the vapor quality is equal to 1, it is referred to as the saturated vapor state or dry steam (single-phase). Between these two states, we talk about vapor-liquid mixture or wet steam (two-phase mixture). At constant pressure, an addition of energy does not changes the temperature of the mixture, but the vapor quality and specific volume changes.
See also: Steam Tables.
For example: In the pressurizer of pressurized water reactors, the saturation temperature is about 350°C, but this corresponds to the pressure of 16,4 MPa, which has to be maintained in the primary circuit. For a pure substance, there is a definite relationship between saturation pressure and saturation temperature. The higher the pressure, the higher the saturation temperature. The graphical representation of this relationship between temperature and pressure at saturated conditions is called the vapor pressure curve, and it can be seen in the phase diagram of water. This diagram is shown in the figure.
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As seen from the phase diagram of water, in the two-phase regions (e.g.,, on the border of vapor/liquid phases), specifying temperature alone will set the pressure, and specifying pressure will set the temperature.
- The saturation vapor curve separates the two-phase state and the superheated vapor state in the T-s diagram.
- The saturated liquid curve separates the subcooled liquid state and the two-phase state in the T-s diagram.
A pressurizer is a component of a pressurized water reactor. Pressure in the primary circuit of PWRs is maintained by a pressurizer, a separate vessel connected to the primary circuit (hot leg), and partially filled with water heated to the saturation temperature (boiling point) for the desired pressure by submerged electrical heaters. The temperature in the pressurizer can be maintained at 350 °C (662 °F), which gives a subcooling margin (the difference between the pressurizer temperature and the highest temperature in the reactor core) of 30 °C. Subcooling margin is a very important safety parameter of PWRs since the boiling in the reactor core must be excluded. The basic design of the pressurized water reactor includes such a requirement that the coolant (water) in the reactor coolant system must not boil. To achieve this, the coolant in the reactor coolant system is maintained at a pressure sufficiently high that boiling does not occur at the coolant temperatures experienced while the plant is operating or in an analyzed transient.
Functions
Pressure in the pressurizer is controlled by varying the temperature of the coolant in the pressurizer. For these purposes, two systems are installed. Water spray system and electrical heaters system. The volume of the pressurizer (tens of cubic meters) is filled with water on saturation parameters and steam. The water spray system (relatively cool water – from the cold leg) can decrease the pressure in the vessel by condensing the steam on water droplets sprayed in the vessel. On the other hand, the submerged electrical heaters are designed to increase the pressure by evaporating the water in the vessel. Water pressure in a closed system tracks water temperature directly; as the temperature rises, the pressure goes up.
Steam generators are heat exchangers that convert feedwater into steam from heat produced in a nuclear reactor core. The steam produced drives the turbine. They are used in most nuclear power plants, but there are many types according to the reactor type.
The hot primary coolant (water 330°C; 626°F; 16MPa) is pumped into the steam generator through the primary inlet. High pressure of primary coolant is used to keep the water in the liquid state. Boiling of the primary coolant shall not occur. The liquid water flows through hundreds or thousands of tubes (usually 1.9 cm in diameter) inside the steam generator. The feedwater (secondary circuit) is heated from ~260°C 500°F to the boiling point of that fluid (280°C; 536°F; 6,5MPa). Heat is transferred through the walls of these tubes to the lower pressure secondary coolant located on the secondary side of the exchanger where the coolant evaporates to pressurized steam (saturated steam 280°C; 536°F; 6,5 MPa). The pressurized steam leaves the steam generator through a steam outlet and continues to the steam turbine. Heat transfer is accomplished without mixing the two fluids to prevent the secondary coolant from becoming radioactive. The primary coolant leaves (water 295°C; 563°F; 16MPa) the steam generator through the primary outlet and continues through a cold leg to a reactor coolant pump and then into the reactor.
Specific Enthalpy of Wet Steam
The specific enthalpy of saturated liquid water (x=0) and dry steam (x=1) can be picked from steam tables. In the case of wet steam, the actual enthalpy can be calculated with the vapor quality, x, and the specific enthalpies of saturated liquid water and dry steam:
hwet = hs x + (1 – x ) hl
where
hwet = enthalpy of wet steam (J/kg)
hs = enthalpy of “dry” steam (J/kg)
hl = enthalpy of saturated liquid water (J/kg)
As can be seen, wet steam will always have lower enthalpy than dry steam.