The removal of dissolved gases from boiler feedwater is an essential process in a steam system. The presence of dissolved oxygen in feedwater causes rapid localized corrosion in boiler tubes. Carbon dioxide will dissolve in water, resulting in low pH levels and the production of corrosive carbonic acid. Low pH levels in feedwater causes severe acid attack throughout the boiler system. While dissolved gases and low pH levels in the feedwater can be controlled or removed by the addition of chemicals, it is more economical and thermally efficient to remove these gases mechanically. This mechanical process is known as deaeration and will increase the life of a steam system dramatically.

Deaeration is based on two scientific principles. The first principle can be described by Henry's Law. Henry's Law asserts that gas solubility in a solution decreases as the gas partial pressure above the solution decreases. The second scientific principle that governs deaeration is the relationship between gas solubility and temperature. Easily explained, gas solubility in a solution decreases as the temperature of the solution rises and approaches saturation temperature. A deaerator utilizes both of these natural processes to remove dissolved oxygen, carbon dioxide, and other non-condensable gases from boiler feedwater. The feedwater is sprayed in thin films into a steam atmosphere allowing it to become quickly heated to saturation. Spraying feedwater in thin films increases the surface area of the liquid in contact with the steam, which, in turn, provides more rapid oxygen removal and lower gas concentrations. This process reduces the solubility of all dissolved gases and removes it from the feedwater. The liberated gases are then vented from the deaerator.

There are three types of deaerator used in the industry

• Spray type
• Spray cum tray
• Vacuum

The incoming cold water first enters the spray valve compartment of the deaerator where it is sprayed in thin sheets through stainless steel spray valves. The water is sprayed into the steam atmosphere of the preheater section, where most of the non-condensable gases are removed.

This hot and partially deaerated water then passes from the preheater section to the deaerating steam scrubber section where final and complete deaeration is accomplished as the water is vigorously scrubbed with a large excess of oxygen-free steam. The deaerated water then spills over into the storage compartment.

Inlet steam is introduced through a steam distributor into a compartment called the deaerating scrubber. There, the steam collides at high velocity with the heated and partially deaerated water entering the scrubber from the preheater. As a result a vigorously scrubbing and mixing action occurs even at very low load conditions to mechanically scrub the remaining traces of non condensable gases out of the preheated water. Thus, the deaerated water discharging from the scrubber section is free of all measurable non-condensable gases. The steam separates from all deaerated water at the outlet of the scrubber section and passes into the preheater section for the initial heating of the water being sprayed through the spray valves. The vented steam is then condensed in the internal direct contact vent condenser with a very small wisp of steam being vented to atmosphere in order to carry away the non-condensable gases released from the water.

The internal direct contact vent condenser consists of the stainless steel plate in which the spray valves are mounted and the spray chamber where non-condensable gases are separated and concentrated before being discharged to atmosphere. The mixture of steam and non-condensable gases in the preheater section passes along the surface of the water spray cones in contact with the cold water so that most of the steam is condensed before entering the vent opening. A small amount of steam plus the non-condensable gases are then discharged through the vent pipe to atmosphere.

Steam enters the steam scrubber about two feet below the upper edge, against a slight water pressure. The large volume of steam passing upward through the scrubber acts as a steam lift to reduce the pressure against which the steam enters. In practice, the pressure loss through the scrubber is approximately 1/2 psi, which is of great advantage in obtaining the best deaeration. With 1/2psi pressure loss, the entering steam within the deaerator above the water surface. The difference in temperature diminishes as the water rises to the top of the scrubber, and as some of the water leaving the scrubber to be equal to the temperature of saturated steam.

Deaerator venting removes non-condensable gases along with some steam. In operation the rate is minimised, since steam venting represents heat loss which is not practically recoverable. However, insufficient venting defeats the purpose of the deaerator. Deaerator vent rates are a direct function of the amount of gases to be removed with higher rates being needed with the deaeration of high quantities of makeup water. Normally, it would not be necessary to vent at a rate higher than about 0.25% of operating capacity.