User:Milton Beychok/Sandbox

A deaerator is a device that is widely used for the removal of air and other dissolved gases from the feedwater to steam generating boilers. In particular, the oxygen component of the dissolved air is quite corrosive and must be removed to very low levels.

Two basic types of deaerators exist today, the tray-type and the spray-type:

• The tray-type (also called the cascade-type) includes a vertical domed deaeration section mounted on top of a horizontal cylindrical vessel which serves as the deaerated boiler feedwater storage tank.

• The spray-type consist only of a horizontal (or vertical) cylindrical vessel which serves as both the deaeration section and the boiler feedwater storage tank.

Tray-type deaerator

(PD) Image: Milton Beychok
Figure 1: A schematic diagram of a typical tray-type deaerator.

The adjacent diagram depicts a typical conventional trayed deaerator. The actual construction details will vary from one manufacturer to another depending on their specific design requirements.

This is generally a horizontally mounted cylindrical steel vessel with dished ends, with internal and external fittings and in the tray-type style with a dome on the top. The size of the same depends on the unit capacity it is associated with. The storage section acts as a buffer for boiler feedwater supplying to the suction of the boiler feed pumps from a pipe connected to the bottom of the tank, generally in the mid portion.

During a cold start of the unit, it is possible that the water in the feed tank may be cold. At that time, the water has to be heated to bring it up to normal operating temperature so as to expel the dissolved gases. For this, a sparger pipe is provided inside the tank at the bottom level. Low pressure steam supplied to the sparger pipe provides the small amount of heat needed to increase the temperature of the feedwater.

Since the feed tank is always hot, sufficient external insulation (known also as lagging) is provided to minimize the heat loss.

At the top and in the mid portion of the feed tank an inverted domed vessel of sufficient size as dictated, is attached which is called the deaerator. This portion has internals something like perforated trays to breakdown the down flow of condensate water from the top into fine globules to separate dissolved gases. The heating steam, which is fed at the lower level of the dome, passes upwards to give good intermixing. A small vent pipe at the topmost point of this dome is provided for venting out the dissolved gases. Some designs may have a vent condenser to trap and recover any water particles escaping through this vent. The typical vent rate is in the range of 0.1% to 0.25% of total deaerator capacity.

Spray-type deaerator

(PD) Image: Milton Beychok
Figure 2: A schematic diagram of a typical spray-type deaerator.

The operation of a spray-type deaerator, in contrast to the tray-type principle, is based on pre-deaeration. Water is first sprayed into the steamed space of the tank, followed by final deaeration in the water volume of the tank, where steam is brought in close contact with the water to be deaerated.

The pre-deaeration is achieved by a condensate sprayer. Under all operating conditions, the sprayer assures heating up of the condensate to saturation temperature and ensures a very large area for mass transfer. As the solubility of oxygen in water at saturation conditions is practically zero, oxygen transfers from the water droplets to the surrounding steam. As the steam condensates on the water, the concentration of oxygen in the direct vicinity of the sprayer is increased making it possible to vent a small amount of steam with a relatively high oxygen concentration.

Final deaeration in the water tank is achieved by injection of steam deep under the water level. A proper designed steam rake, taking into account the hydro-dynamics in the water tank, will result in optimal contact between water and steam, allowing the oxygen to move from the water to the steam in accordance with Henry's law.

Deaeration steam source

The steam is generally tapped from an extraction point of the turbine to improve the cycle efficiency. The deaerator therefore is also termed as one of the feedwater heaters in the turbine cycle. It is also possible to use flash steam. Since the deaerator is always hot, sufficient insulation is provided to minimize the heat loss.

Achievable removal of oxygen

If operated properly, most deaerator vendors will guarantee that oxygen in the deaerated water will not exceed 7 ppb by weight (0.005 cm³/L).