Deaerating steam systems efficiently

Elimination of non-condensable gases is ideally achieved by a combination of chemical and mechanical means. The Lalonde Systhermique team uses a four-step deaeration strategy:

Return as much condensate as possible

Since condensate is hot and already deaerated and chemically treated, returning more of it to the boiler will have a significant impact on costs. As more condensate is returned, less feedwater treatment is required and, more importantly, less fuel is required to power the boiler. In a Lalonde Systhermique Steam-Condensate Closed System (SCCS®), all the condensate is returned to the deaerator/boiler. On top of significant fuel savings, this means much lower water treatment costs.

Replace boiler feed tanks vented to the atmosphere by pressurized deaerators

Water exposed to air becomes saturated with oxygen, and the concentration varies with temperature: the higher the temperature, the lower the oxygen content. In conventional steam systems, the first step in deaeration often consists in heating the water in a boiler feed tank to drive off the oxygen. Typically the feed tank is operated at 85°C to 90°C and is vented to the atmosphere. This practice does not deaerate optimally and it generates energy losses since part of the steam used to heat the water escapes through the vent.

As much as possible, it is preferable to replace the feed tank with a pressurized deaerator in order to achieve a more complete deaeration and better energy efficiency.

Pressurized deaerator

Figure 1. Configuration of a pressurized deaerator.

If a liquid is at its saturation temperature, the solubility of a gas in it is zero, but the liquid must be strongly agitated or boiled to ensure it is completely deaerated. This is achieved in the head section of a deaerator by breaking the water into as many small drops as possible, and surrounding these drops with an atmosphere of steam. This generates a high surface area to mass ratio and allows rapid heat transfer from the steam to the water, which quickly attains steam saturation temperature. The dissolved gases are released and then carried with the excess steam to be vented (the addition of a Lalonde Systhermique thermo-eliminator allows the air to be vented while preventing the steam from escaping). The deaerated water falls to the storage section of the deaerator. A layer of steam is maintained above the water to ensure that air is not re-absorbed. 

Use chemical water treatment

In most plants, existing chemical treatments are appropriate. Historically, it’s at the mechanical level that conventional technical solutions aren’t optimal. There’s only so much that can be accomplished through chemical water-treatment alone.

Mechanical elimination of oxygen and carbon dioxide drastically reduces consumption of costly corrosion-control chemicals. In many of our projects, this decrease exceeds 50%.

It should be also noted that in many plants, water-treatment stations operate at full capacity, which emphasizes the importance of eliminating non-condensable gases mechanically as much as possible.

Use mechanical vents throughout the steam system

The use of a pressurized deaerator and chemical water treatment is not sufficient to completely deaerate a steam system. Unavoidably, some non-condensable gases will accumulate in heat exchangers if nothing is done about it. Removing non-condensable gases from heat exchangers improves heat transfer and helps provide a more uniform product temperature.

The Lalonde Systhermique thermo-eliminator can be installed on every strategic point of a steam system: on the deaerator, on piping and on any type of heat exchanger. The correct positioning of thermo-eliminators depends on a variety of factors and requires a good understanding of air movement and behavior in steam systems: the Lalonde Systhermique team has developped a solid experience over time, which allows us to insure an efficient use of thermo-eliminators.