Question

How do open feed water heaters differ from closed feed water heaters?

Short answer

Answer: The main differences between open and closed feed water heaters are that open feed water heaters use direct contact between steam and feed water to transfer heat, while closed feed water heaters use a heat exchanger for indirect heat transfer. In terms of efficiency, open feed water heaters generally have lower efficiency compared to closed feed water heaters due to heat loss from evaporation. Closed feed water heaters offer better control over the temperature of the feed water and minimize heat loss, but are more complex and expensive to design, build, and maintain.

Step-by-step solution

Introduction

Open and closed feed water heaters are used in power plants to heat up feed water before it enters the boiler. This is done to increase the efficiency of the Rankine cycle. The key difference between the two types of feed water heaters lies in the way they transfer heat to the feed water.

Open Feed Water Heaters

An open feed water heater, also known as a direct-contact or mixing heater, is a type of feed water heater where feed water directly comes in contact with the steam extracted from the turbine. This direct mixing results in the feed water being heated up and the steam condensing back into water. The mixture of feed water and condensed steam is then supplied to the boiler.

Closed Feed Water Heaters

Closed feed water heaters, also known as indirect-contact or shell-and-tube heaters, operate by transferring heat through a heat exchanger. In this case, the extracted steam from the turbine and the feed water do not come into direct contact. The steam flows on one side of the heat exchanger tubes while the feed water flows on the other side. The heat from the steam is transferred to the feed water via the tubes, heating up the feed water and condensing the steam back into water. The condensed steam is then returned to the system through a condensate pump.

Efficiency

The efficiency of open feed water heaters is generally lower than that of closed feed water heaters. This is because open feed water heaters rely on direct mixing of steam with feed water, which results in some heat loss due to evaporation. In closed feed water heaters, the heat transfer occurs through the heat exchanger tubes, minimizing heat loss and allowing for a higher level of control over the temperature of the feed water. However, closed feed water heaters are generally more complex and expensive to design, build, and maintain compared to open feed water heaters.

Comparison

In summary, the main differences between open and closed feed water heaters are: 1. Open feed water heaters use direct contact between steam and feed water to transfer heat, whereas closed feed water heaters use a heat exchanger for indirect heat transfer. 2. Open feed water heaters generally have lower efficiency compared to closed feed water heaters due to heat loss from evaporation. 3. Closed feed water heaters offer better control over the temperature of the feed water and minimize heat loss, but are more complex and expensive to design, build, and maintain. Overall, choosing between an open or closed feed water heater depends on the specific requirements, budget, and efficiency goals of the power plant.