In an HRSG (Heat Recovery Steam Generator), the evaporator section is responsible for converting the heat from hot exhaust gases into steam. During this process, the temperature curves of the gas and water/steam can exhibit different behaviors.
The primary reason for the difference in temperature behavior is the heat transfer mechanism occurring within the evaporator. As the hot exhaust gases pass through the evaporator tubes, heat is transferred from the gas to the water/steam flowing inside these tubes.
Here's a breakdown of the temperature behavior in the evaporator section:
Gas Temperature Drop: The hot exhaust gases enter the evaporator at a high temperature. As they come into contact with the cooler evaporator tubes, heat is transferred from the gas to the water/steam. This heat transfer causes a drop in the temperature of the gas, as it gives up its thermal energy to convert water into steam.
Water/Steam Temperature Remains Constant: The water/steam flowing inside the evaporator tubes undergoes a phase change from liquid to vapor as it absorbs the heat transferred from the gas. During this phase change process, the temperature of the water/steam remains relatively constant, even though it absorbs a significant amount of heat energy. This is due to the energy being used to convert the water into steam rather than increasing the temperature.
By maintaining a relatively constant temperature, the evaporator ensures that the water/steam reaches the desired saturation point without overheating. This saturation point corresponds to the temperature at which the water has absorbed enough heat energy to transition into saturated steam at a given pressure.
In summary, the temperature behavior in the evaporator section of an HRSG is a result of heat transfer from the gas to the water/steam. While the gas temperature drops as it transfers heat, the water/steam temperature remains constant as it undergoes a phase change from liquid to saturated steam.