Do we cool condensate in hotwell?

The temperature at which vaporization occurs at a given pressure is referred to as the saturation temperature. The saturation pressure for the given temperature is denoted as such. For instance, at 0.1 MPa, the saturation temperature for water is 99.6°C. A relationship exists between the saturation temperature and saturation pressure for a pure substance. If a substance is in a liquid state at the saturation temperature and pressure, it is termed a saturated liquid.

The extraction of as much work as possible from the steam generated in the steam generator is evidently advantageous. In simpler terms, the greater the amount of energy that can be extracted from the steam is directly proportional to the lower temperature and pressure at the outlet of the low-pressure turbine. In fact, if the turbine's exhaust is close to a perfect vacuum rather than atmospheric pressure, approximately 35% more energy can be extracted from the steam passing through the turbine.

Maintaining the low absolute pressure at the exhaust of the low-pressure turbine is facilitated by the condenser. The provision of this low pressure occurs through the condensation of steam into water. Typically, at the outlet of the low-pressure turbine, a kilogram of wet steam occupies 28 cubic meters. Upon condensation to water, one kilogram of water occupies 0.001 cubic meters. Thus, as one kilogram of steam condenses, nearly 38 cubic meters of previously steam-filled space become empty. It is this creation of empty space through steam condensation that establishes the vacuum in the condenser.

The cooling water, responsible for removing the latent heat of vaporization and inducing steam condensation, flows through the tubes. This condenser cooling water is drawn from a lake or river, pumped through the condenser tubes, and then returned to the lake or river. The inlet for the condenser cooling water is termed the inlet water box, while the outlet is referred to as the outlet water box.

On the condenser's steam side, the low-pressure turbine's exhaust passes through an exhaust trunk and enters the condenser shell, circulating around the tubes. The condensed water, or condensate, descends to the bottom of the condenser and accumulates in a hotwell.

1. Within the condenser shell, there is a substantial steam heating lane or steam excess lane that lacks cooling water tubes. This lane's purpose is to direct a portion of the exhaust steam to the bottom of the condenser. This achieves several objectives:
2. The hotwell is meticulously maintained to ensure that the water within it remains at its saturation temperature. This careful regulation is crucial, as any deviation from this state could lead to undesirable consequences for the overall efficiency of the steam/feedwater cycle.
3. In particular, allowing the water in the hotwell to become subcooled is deemed undesirable due to the potential for unnecessary heat loss from the condensate. Subcooling, in this context, refers to lowering the temperature of the water below its saturation temperature, a condition that could result in a less efficient energy utilization within the system.
4. The ramifications of permitting subcooling are significant. This unwanted loss of heat energy represents an inefficiency in the overall steam/feedwater cycle. To mitigate this issue, the lost heat would need to be reintroduced into the condensate, either through the feed heating system or the steam generator/boiler; therefore, more kilograms of fuel will be burnt in every second for the same power output thereby compromising the overall effectiveness and energy efficiency of the system. Normally, condensers are designed to provide condensate at the condenser saturation temperature (0°F subcooling).
5. The lower condenser tubes are compelled to expel an equivalent amount of steam as the upper tubes to utilize the entire tube surface for heat transfer. This process ensures optimal heat transfer efficiency.
6. Equalizing the thermal expansion of the tubes is crucial. Cooler lower tubes compared to the upper tubes could result in a bending force on the condenser shell and tubesheets.