Deaerators: Purpose, Functions & Types
Industrial grade boilers are a major investment, making it extremely important to extend their lives and maintain their efficiency for as long as possible. Corrosion is a major factor in the failure of boilers, which is where the deaeration process comes in. Deaeration involves oxygen removal, as well as carbon dioxide removal, thus extending the life and efficiency of a boiler system – it is very pertinent that the makeup water entering a boiler is free of these dissolved gases. The major area of application for deaerators can be found within a boiler plant and boiler feed water systems.
As one of the leading worldwide deaerator manufacturers for over 30 years, Kansas City Deaerator Company (KCD) has worked with a long list of satisfied customers. Our extensive knowledge allows us to customize deaerator systems in virtually any industrial setting anywhere in the world.
We understand the investment you’ve made in your boiler system, and we’re prepared to help you keep it in great working order by installing a deaerator that will remove the dissolved gasses that cause corrosion and threaten the ability of the boiler to do its job. Our reputation for excellence is one you can count on, and your satisfaction is guaranteed. For more information about KCD and our products, call us today.
What is the Purpose of a Deaerator?
According to the U.S. Department of Energy, a deaerator’s job is to remove oxygen and other dissolved gases from boiler feedwater, creating clean, deaerated water. Deaerators are used in industrial applications such as power plants and chemical process industries to reduce corrosion and extend the life of a steam-generating boiler. A deaerator achieves this task by reducing the level of dissolved oxygen and carbon dioxide in the feedwater, which in turn reduces the amount of corrosive compounds within the steam system over time. When the deaerated water exits the deaerator, carbon dioxide concentrations will be zero and dissolved oxygen concentrations will be less than or equal to 7 ppb (parts per billion) (this can be tested through an extraction point). Deaerators also reduce the need for water treatment chemicals, such as an oxygen scavenger,
In addition to reducing dissolved gasses in the feedwater to reduce corrosion damage, a deaerator also raises the temperature of the feedwater before it enters the boiler, thus requiring less fuel for the boiler to heat the water hot enough to produce steam. As a result, the boiler system performs better, is more efficient overall, and helps maintain operating costs.
Why Remove Dissolved Gases from Feedwater?
The deaeration process is used to reduce corrosion and to remove dissolved gases in specific boiler feed water systems. The presence of dissolved gasses like oxygen and carbon dioxide causes accelerated corrosion that shortens the lifespan and reduces the efficiency of a boiler system. One of the more serious corrosion issues with dissolved oxygen is pitting. Although the metal loss with pitting is minimal and the corrosion level relatively low, pitting causes system failure just the same.
The degree to which the oxygen attacks the system depends on the amount of dissolved oxygen, the feedwater temperature, and the pH level of the water. Hot water does not cause corrosion by itself, but even the smallest amount of oxygen present in elevated water temperatures causes serious problems because the rise in temperature is just the accelerant the gas needs to cause corrosion.
Another destructive dissolved gas found in boiler feedwater is carbon dioxide. This gas creates a weak carbonic acid that attacks the metal inside boilers, feed systems, and condensate return/condensate systems.
Because these gasses have such a corrosive effect on these systems, they must be removed from all sources of water entering the system, which is where the deaeration process comes in.
The Function of a Deaerator
The Principle of Deaeration
Deaerators operate on Henry’s Law of Partial Pressure, a principle in physical chemistry that says that the quantity of dissolved gas in a liquid is directly proportional to the partial pressure above that liquid. When deaeration takes place, the partial pressure above the boiler feedwater is decreased as the water’s temperature rises. Simply stated, the solubility of the gases in the water is decreased as the temperature of the water increases.
The solubility of oxygen in water decreases as the water temperature increases. However, as the water temperature increases and approaches saturation temperature, so does the amount of water vapor in the atmosphere above the liquid. When the water reaches full saturation temperature (boiling point), theoretically there is zero oxygen left in the water. In conclusion, due to these physical characteristics, oxygen can be removed from water by raising the temperature and reducing the concentration of dissolved oxygen in the atmosphere above the water.
The Goals of a Deaerator
Measuring the technical efficiency of a deaerator depends on its operational effectiveness in the amount of oxygen content it removes from the boiler’s feed water. In addition, measurements are taken for the amount of gas that shows up at the inlet of the storage tank and what the amount is at the feedwater’s outlet.
To be considered at its optimal working condition, a deaerator needs to ensure that the cold feed water is heated enough to prevent thermal shock to the boiler. It also needs to remove the dissolved oxygen to reduce the need for water treatment chemicals and an oxygen scavenger, as well as eliminate all other dissolved gases to prevent corrosion and pitting. Lastly, it needs to do away with all other non-condensable gasses to bolster steam efficiency.
Types of Deaerators
For the deaerating process to occur, a few things must occur. 1) There must be a vessel in which the deaeration process can occur and protect the deaerated water from contamination. 2) More surface area needs to be created for the heating steam to contact the water. Creating surface area can be done with spray valves, where water is broken up into smaller droplets as it passes through each spray valve. 3) Retention time must be created for the steam atmosphere to remain in contact with the feed water. 4) An escape path for the oxygen and non-condensable gases must be available.
In general, there are two main types of deaerators used today: tray-type deaerators and spray-type deaerators. That said, reputable consulting engineers from all over the world recommend tray deaerators for most applications.
A tray-type deaerator typically has a deaeration section mounted above a horizontal feedwater storage compartment or storage section. Water enters the deaeration section into a stainless-steel enclosure through spray valves mounted to a horizontal header pipe. During the first stage of deaeration, the water is dispersed in a fine film or droplets above a section of trays. The water is deaerated in a second stage, as it cascades down through openings in the trays. Low-pressure steam enters the enclosure below the trays and flows upward counterflow to the water. The combined action of the 1st and 2nd stages of deaeration guarantees remarkably high performance, as it allows for longer contact time between steam and water. The steam strips the dissolved gases from the boiler feedwater and exits via the vent connection at the top of the vessel (or deaerator dome if it is a vertical deaerator). It is critical for proper operation that the vent line be opened sufficiently for steam venting, or the deaerator will not work properly, causing high oxygen content in the boiler feedwater system. The vent line usually includes a valve that allows just enough steam to escape with the vented gases to provide a small visible telltale plume of steam. The content of dissolved oxygen at this stage should be 7ppb or less. The deaerated feedwater flows down into the horizontal storage vessel from where it is pumped to the boiler.
There are two different types of tray deaerators: parallel flow and counterflow. KCD fabricates both types of pressure vessels; however, the vast majority of tray type deaerators that we fabricate are counterflow.
The first type of tray unit is a parallel downflow unit where steam travels in the same direction as the water. As the water enters through the water inlet on top of the tank, steam enters through the side of the deaerator and moves down through the trays with the water (hence, the name parallel downflow). The major disadvantage of a parallel downflow tray unit is that it does not comply with Heat Exchange Institute (HEI) standards because the steam is not contained within the stainless steel pressure retaining box. The shell of the pressure vessel is exposed to the high temperatures of the steam that has been used to remove the oxygen. Not being HEI compliant means that many customers, specifically those in the United States, cannot use a parallel downflow tray unit.
The other type of tray unit is a counterflow unit. Whereas the parallel flow unit had steam running the same direction as the water flow, counterflow units do the opposite. As water enters through the spray valves on the top side of the deaerator, the steam enters from below the tray enclosure box and is always contained within this box. This eliminates the opportunity for steam to encounter the shell of the chamber. This method is approved by the HEI and is the most used type of deaerator.
Counterflow Tray-Type Deaerator (per HEI):
KCD specializes in the design of counterflow tray-type deaerators, which prevent any carbon steel pressure retaining components from encountering corrosive undeaerated water or vent gases. The counterflow design is the only design that meets the requirements of the HEI and protects the vessel from all corrosive gases, not just concentrated gases.
Stages of Counterflow Deaerators:
Stage 1: Incoming water flows through KCD’s variable orifice spray valves and the steam-filled vent condenser chamber as a thin-walled, hollow cone spray pattern. Latent heat transfer is instantaneous because of the intimate water to steam exposure within the vent condenser section. As the water reaches the tray stack, at which point stage one is complete, its temperature is within 2ºF of the counterflowing saturated steam temperature, and most dissolved oxygen and free carbon dioxide has been removed. Nearly all of the steam has now condensed, permitting the non-condensable gases to be carried through by the remaining steam.
Stage 2: During stage two, the preheated water flows over the tray stack and is vigorously scrubbed by the counter flowing steam. The water zigzags its way through a counter-current of pure steam and leaves the tray stack virtually free of oxygen and carbon dioxide.
A spray-type deaerator is typically a single horizontal vessel which has a preheating section and a deaeration section. The boiler feedwater is sprayed into the preheating section through the inlet water connection, where it is preheated by the saturated environment of the deaerator. The feedwater is heated to its saturation temperature to facilitate the stripping of the dissolved gases and removal of oxygen content in the deaeration section. The preheated feedwater then flows into the deaeration section or spray scrubber section, where it meets the steam entering the system. The scrubber section holds the water in retention so it can fully mix with the incoming steam. In addition, the scrubber creates a path where the water must continuously mix with additional steam. The non-condensable gases are stripped out of the water and exit via the venting section/connection at the top of the vessel. The deaerated feedwater is pumped from the bottom of the vessel to the boiler.
While there are several distinct types of deaerators, tray deaerators are recommended by all reputable consulting engineers worldwide. Tray deaerators (counterflow types, not parallel downflow types) are the only deaerators approved by the Heat Exchange Institute (HEI) because they keep all gasses contained in stainless steel to protect the carbon steel vessel from corrosion. Tray counterflow deaerators are the choice where longevity and trouble-free operation is required.
The deaeration process in a custom tray type deaerator from Kansas City Deaerator Company takes place in an enclosure that eliminates any need for vessel lining or cladding, ensuring long life with little maintenance. The company serves customers throughout the United States with deaerators, accessories, pumps, and ancillary equipment.
Common Questions About Deaerators
Why are deaerators placed at height?
Deaerators are placed at height to provide sufficient NPSHa (Net Positive Suction Head available) to the boiler feedwater pumps – the boiler feed pumps are typically situated low in the ship for the same reason. Also, to allow air in the system to rise and enter the deaerator which, because of its name, will then discharge the air to either the atmosphere or the condenser. It’s important to deaerate the feed water at the point just prior to the feed pumps in order to avoid it passing to the boilers, where it will accelerate corrosion.
Is a deaerator a pressure vessel?
Not all deaerators are pressure vessels. Some deaerators called atmospheric deaerators have a temperature regulator rather than a pressure regulator to regulate steam. Temperature regulators are not as responsive as pressure regulators, which is why pressurized deaerators are more commonly used and come in a wide range of pressure ratings.
Visit Kansas City Deaerator Company at Deaerator.com to explore the availability and advantages of our tray deaerator. We have been in the deaerator industry since we incorporated in 1989. KCDC supplies deaerators and their accessories to the power and industrial sectors throughout the United States.