Feed Water Heaters: Functions, Types, Advantages And Operation

In this article, I will share the functions, types, advantages and operation of feed water heaters.

Feed water heaters are plant components used to preheat water before it enters the boiler.

Preheating boiler feed water is essential because it increases the thermal efficiency of the boiler.

There is more you will learn about feed water heaters in this article.

To give you a heads-up, I will discuss open-feed water heaters, closed-feed water heaters, low-pressure feed water heaters and high-pressure feed water heaters.

Now that you know what I’m going to discuss in this article, let me start with the function of feed water heaters.

The function of feed water heaters

1. Preheating boiler feed water.

The main function of the feed water heater is to preheat the water that goes into the boiler.

This water can be from the feed water tank or the condensate that is discharged from the condenser.

Preheating feed water is necessary for any steam plant to ensure that the thermal efficiency is improved.

With preheating the feed water, many advantages are experienced which I will talk about them further in the section below.

Types of feed water heaters

1. Open feed water heaters.

Another name for open feed water heater is a direct contact feed water heater.

In this type of feed water heater, the extracted steam from the turbine is used to heat the feed water.

The extracted steam is mixed with feed water to produce saturated water.

Saturated water contains energy and this makes it easier to change into steam with less heat.

Open-feed water heaters require two pumps to operate effectively since there is high pressure between the boiler and condenser.

These pumps are needed at the outlet and inlet to overcome the two pressures from the boiler and condenser.

Open-feed water heaters are classified into three types, which are namely;

Spray type
Tray type
Spray-tray type (combination of spray and try)

1. Spray-type feed water heater.

The spray-type feed water heater contains a nozzle that sprays feed water at a pressure of 20psi through steam which rises from the bottom of the tank.

The vessel that holds the feed water, is fed with extracted steam of low pressure.

When the spray feed water enters the tank under pressure, the steam also rises up and comes into contact with the feed water thus heating the water and eventually exiting the tank via a vent on top of the tank.

2. Tray-type feed water heater

The tray-type feed water heater also works under the same principle as the spray-type feed water heater.

The extracted low-pressure steam heats up the gas and leaves the tank through the vent at the top.

The only difference is that, instead of the water being sprayed, it falls on the trays.

When the feed water falls on the trays, its velocity is reduced and there is sufficient time for the steam to heat up the feed water before it falls off from the trays.

So, the feed water is heated at the trays and also when falls down from the trays.

This ensures that the feed water is sufficiently heated by steam.

3. Spray-tray type feed water heater

As the name suggests, this type of feed water heater consists of a spray and tray mechanism to heat up the feed water.

The combination of these two types of feed water heaters into a single system tends to improve the efficiency of the heating mechanism.

This is how spray-tray type feed water heater works; the feed water is sprayed through a nozzle under pressure just like in spray type.

The sprayed feed water falls on trays, and with the counter flow, the steam from the bottom heats up the feed water on the trays and also the one that falls inside the vessel.

The trays slow down the sprayed feed water to give it sufficient time to be heated by steam.

2. Closed feed water heaters.

Closed-feed water heaters have a simple design and do not require separates pump like the open-feed water heater.

This type of feed water heater is designed like the water tube fire boiler type.

You might like this: Boilers: Components, Types & Working Principle

It has tubes through which the feed water passes and the extracted steam heats up the water via the shell.

So, there is no direct contact between the feed water and the extracted steam.

The shell transfers the heat to the feed water via the shell and it’s condensed.

A pump is used to pump the feed water from the condenser to the boiler.

The closed-feed water heaters can be of two categories namely;

Closed type with drains pumped forward
Closed type with drains cascaded backwards.

The steam that is extracted between the stages of the steam turbine is used to heat the feed water.

Pressure-feed water heaters

They are categorized according to the type of extracted steam that they use to heat up the feed water and the location that is installed.

The pressure feed water heaters are divided into two three categories namely;

1. Low-pressure feed water heater.

The low-pressure feed water heater is used to heat the condensate discharge that is later pumped to the boiler.

From this, we can that this type of feed water heater is used in the condensate system between the condensate pump discharge and the boiler feed pump.

Between the condensate pump discharge and the boiler feed pump, there is low pressure.

Also, another point you should note is that it uses low-pressure steam extracted from the turbine for heating the condensate hence the name low-pressure feed water heater.

2. High-pressure feed water heater.

This type of feed water heater is located downstream of the boiler feed pump.

High-pressure extracted steam is used to heat the feed water

3. Intermediate pressure feed water heart

A feed water heater is located tube-side between the booster pump and the boiler feed pump.

The steam used for heating is usually extracted from an intermediate pressure turbine thus the name intermediate PFWH.

Working principle of feed water heaters

The general working principle of feed water heaters is that they use the extracted stream from the turbine to heat up water from different positions of the boiler (depending on the type of feed water heater) and direct the preheated water to the boiler for steam generation.

This is made possible by the different zones that are within the shell of feed water heaters.

Condensing zone (present in all feed water heaters) is where all the steam is condensed.

The non-condensable gasses are removed then the condensate enters the sub-cooling zone (optional) at its saturation temperature and is cooled by convective heat transfer from the incoming feed water.

The last and optional zone in the shell of the feed water heater is the desuperheating zone where the incoming steam enters the zone and transfers all its heat to the feedwater leaving the heater to the boiler.

Advantages of feed water heaters

1. Saves fuel.

Feed water heaters use heat that would be otherwise wasted to heat up the feed water.

The preheated feed water will enter the boiler at its saturation point, meaning less energy is going to be used to heat up the water to turn it into steam.

So, less fuel will be used and the heat in the boiler system is going to be used to the maximum.

This is a good thing since it reduces the operation cost of the boiler.

2. Increase boiler efficiency.

Preheating the feed water before it enters the boiler, provides it with increased thermal energy.

For quality steam to be generated, high thermal energy is needed inside the boiler.

The thermal energy from the boiler furnace and adding up the thermal energy from the feed water that is preheated will end up increasing the thermal capacity of the steam generated.

When the thermal capacity of the steam is increased this will end up increasing the overall efficiency of the boiler.

It should be noted that boiler efficiency is the percentage of the total absorbed heat value in the steam outlet and the total energy supplied.

We have two heat values that are provided at present, due to the preheated feed water and one energy value.

The output (steam generated) is greater than the input (energy provided for heating the steam).

3. Avoid thermal shock to the boiler.

The boiler is usually exposed to high temperatures at all times except when it’s not working.

If the feed water is not heated and it’s directly fed into the boiler with its high temperatures, this will result in thermal shock in the boiler; if the temperature differences are higher it will end up cracking the boiler tubes, and tube sheets and even cause a big explosion.

All these effects can be prevented if the feed water is preheated before it’s fed into the boiler.

4. Remove unwanted gasses from the feed water.

Heating feed water also helps in removal of the unwanted gasses like oxygen.

We all know that oxygen is a corrosive gas and it won’t do good to the boiler components.

By increasing the heat of feed water using steam, the level of dissolved gasses is reduced to the minimum.

5. Prolong the life of the boiler.

Preheating feed water increases the thermal efficiency of the boiler which means it won’t have to face a high energy value energy thus lengthening the life of the boiler pressure vessel.

Also, by preheating the feed water with the feed water heaters the corrosive gasses are removed from the feed water heater.

This automatically prevents corrosion hence the boiler life will be prolonged.

Final thoughts

Feed water heaters are essential because of the many advantages that they provide as you have read.

No boiler should be installed without feed water heaters.

Without forgetting something crucial, the feed water heaters can be oriented vertically or horizontally.

It’s always best to position the feed water heaters horizontally since it increases the stability of the water level, the downside is that it will occupy big space.

Also, it will be easier to maintain a feed water heater that is oriented horizontally.

The vertical orientation is great because it saves time but you will lack the benefits of horizontal orientation.

It will also be hard to control the volume of feed water and maintain it.

Thanks for reading the article, if you have any questions you should leave a comment.

See you in my next article.