Data Sheet: Flash Condensing Deaerator Head

Cold make-up water and returned Condensate usually mix in the feedtank. Conventionally, both make-up water and condensate are fed to the feedtank above the water surface. However, this causes the following problems:

As the condensate heats the make-up water, the temperature of the make-up water rises. All water sources have a certain amount of dissolved gases mixed in them at ambient temperature. Cold water absorbs free oxygen and other gases that are liberated when heated. It is essential to remove the dissolved oxygen before it can be released in the boiler or the feedtank, to prevent corrosion of the tank, the boiler and the steam system. The removal of this oxygen can be done by three ways – thermal, mechanical and chemical. In chemical removal of oxygen, an oxygen scavenger like Sodium Sulphite is dosed to the feedtank, which absorbs the oxygen. However, this is detrimental because the addition of any chemical to the boiler water increases its TDS, again causing problems.
In mechanical de-aeration, water is stirred or sprayed, causing removal of oxygen from the feed water.

Thermal de-aeration uses the property of water shown in the graph . As is seen, the amount of dissolved oxygen in water is proportional to its temperature. So if we can heat the make-up water before it enters the feedtank, it will liberate the oxygen, thus preventing corrosion of the tank. Further, if the system used to preheat the make-up is made of Stainless Steel, corrosion will be negligible.
On larger boiler plants, pressurised de-aerators are installed and live steam is used to bring feed water temp above 100ºC to “drive off” the oxygen content. This action is normally enhanced by the steam “scrubbing” the feedwater. Freed oxygen and other gases are vented to the atmosphere. However, these are pressure vessels and are therefore expensive.

Accordingly, the De-aerator Head was developed – a compromise for fitting to any feedtank to drive off as much oxygen as possible at atmospheric pressure. The Steamline De-aerator Head uses a combination of thermal de-aeration and mechanical de-aeration. It has three restrictions to the flow – a nozzle in the make-up line, a baffle plate between the mixing head and the immersion tube, and a sparger in the immersion tube. Therefore it ensures that the oxygen in the make-up water is driven off by using the heat in the condensate which it is mixed with, and all the dissolved gases are released in the De-aerator Head before it enters the feed tank and the boiler. These are vented out by the Automatic Air Vent on top of the De-aerator Head.

In addition, sometimes Flash Steam is generated from high pressure Condensate. This flash steam will escape to the atmosphere and the heat will be lost. A third inlet is sometimes provided in a De-aerator Head to mix flash steam with make-up water, thus condensing the flash steam and saving its heat. This type of unit is called the Flash Condensing De-aerator Head, and this is the type of unit supplied to Weikfield.

Now it is seen that the quantity of flash steam generated is very small, since the condensate pressure was not 6 barg as originally assumed but only 0.5 barg. However, even water as 0.5 barg will flash to the extent of about 2 –3 % of the condensate by mass. Once the flash vessel has been removed, this flash steam will appear at the receiver of the pump, as well as the condensate inlet to the tank. This may seem negligible, but as the condensate is pumped to the feed tank under pressure (from the pump), the flash steam will be released more vigorously at the entry to the tank.
Even the loss of 1% flash steam can be quantified as under:

Qty of condensate = 3000 kg/hr
So flash steam = 0.01 X 3000 = 30 kg/hr
Cost of the heat in this steam = (350 X 24 X 30 X 540 X 10) / (10200 X 0.88 X 0.9)
= Rs. 1,68,450 per year

Further, if a thermo-mechanical deaerator head is not used, the oxygen in the feedwater will need to be controlled by dosing an oxygen scavenger, which could cost up to Rs. 50,000 per year.