The ability to decrease a power plant’s heat rate can have a profound effect on fuel costs. Optimizing the water level of your feedwater heaters is an effective way to achieve this objective. However, a basic familiarity with feedwater operation is critical to understanding the impact of this hardware – and how effective level control can impact your plant’s efficiency.
Feedwater Heater Level Control
There are normally six to seven stages of feedwater heating. At a capital cost of approximately US$1.2 million per feedwater heater, you need to achieve optimum heat transfer and improve terminal temperature difference to receive an acceptable long-term return on investment.
Feedwater heaters use the heat of condensation (energy available from the change from saturated steam to saturated liquid) to preheat water to the correct temperature for the boiler. This reduces the amount of fuel required to bring the water up to the temperature needed. During this process, shell and tube heat exchangers allow feedwater to pass through the tube side and extract steam from the turbine to the shell side. The primary benefit of this process is that the feedwater heater decreases the fuel costs by using recovered energy – rather than costly hot gas – to heat the water.
This image shows a standard high pressure feedwater heater (low pressure heaters are similar in design, but do not have a desuperheating zone). The three main zones of the feedwater heater are the:
1. Desuperheating zone
2. Condensing zone
3. Drain cooler or sub-cooling zone
Boiler feedwater enters the feedwater inlet while extraction steam flows into the steam inlet. The desuperheating zone cools the superheated steam to the point that the steam is saturated. The condensing zone extracts the energy from the steam/water mixture to preheat the boiler feedwater passing through the tube side. The drain cooler is incorporated to capture additional energy from the liquid.
Newer level control technologies, like Magnetrol’s 706 ECLIPSE® Guided Wave Radar, can optimize the condensing zone to transfer as much of the available energy as possible to the boiler feedwater. This enables you to augment the amount of available energy that you transfer, while also keeping your tubes cool enough to prevent premature hardware damage due to thermal overload.