Feed Water Tank Sizing

The primary goal of a feed water tank is to collect condensate from the steam distribution system, there are basically two types of feed water tanks:

1. Atmospheric tanks: very much used on small steam systems, constructed with carbon steel (shorter life) or stainless steel (longer life).
2. Fully pressurized: pressurized ASME vessels, known as deaerators can come also in two types (1) Spray type, and (2) Tray-type.

Those guys play a big role in energy efficiency and maintaining steam quality. In a typical steam system, steam is generated in the boiler, distributed through the plant, and (optional) condensate is returned to the boiler.

When condensate has returned the chances are high of it bringing non-condensate contamination, as much as the make-up water from the water system.

The presence of non-condensate can provoke serious corrosion in the piping and equipment, and due to their low specific heat, they negatively impact the heat transfers.

So here's why we usually come for a feed water tank to complement the steam system.

To design a preliminary feed water tank it is needed four pieces of information:

1. Boiler's power (hp): to know how much steam the system is capable of generating.
2. Residence time of water: usually 10 minutes to deaerate if a surge condensate tank exists or 15~20 minutes if don't.
3. Average converting factor: a boiler on average can generate 15,6 kg of steam from each hp (or 34,5 lb/hp).
4. Water density at operating temperature: the recommended temperature to operate is -2°C from saturation temperature.

Then, voilà:

It's important to make sure that the feed water tank has a capacity that can hold at least the amount of boiler water needed for 10 minutes of consumption at maximum output, this volume is known as Minimum Working Volume. For systems that use a condensate setup without a surge tank, it's recommended to size the feedwater tank to accommodate 15-20 minutes' worth of boiler mass flow. This will ensure that the system is prepared for changes in condensate returns.

Source: Carey Merritt - Process Steam Systems