Deaeration: Overview of Deaerators and Their Role in Boilers
Deaerating has proven to be incredibly beneficial for lowering boiler maintenance needs, operating costs, and system downtime. Deaeration extends the equipment’s lifespan, increases operating efficiency, and more. Below is a brief guide to how deaerators work, why they are important, and different kinds of deaerators.
Deaerator Operating Principle
Deaeration is the process of removing air from a substance, wherein the air has become mixed in with the substance. In the case of boilers, this substance is the makeup water that the boiler needs to function. Deaeration works because the solubility of gasses in water decreases as the water temperature rises – this is known as Charles’ Law. As water is heated to the saturation temperature at the given pressure, gasses begin to come out of the solution and can be separated and removed. This “purifies” the water so it is free from component gasses.
Why Deaeration of Water is Done
Prior to deaeration, boiler makeup water contains oxygen, CO2, or other gasses. These gasses cause serious harm to boiler parts and function. If CO2 is not removed, it leaves the boiler with steam. As the steam condenses, CO2 combines with H2O to create Carbonic Acid (H2CO3). The Carbonic Acid causes corrosion damage to piping, heat exchangers, etc. Oxygen itself is up to ten times more corrosive than CO2, especially when exposed to the higher temperatures at which boilers operate. In addition, oxygen corrosion frequently occurs in localized areas, allowing it to dig through a section of metal and cause failure quickly.
When oxygen and CO2 are present together, they can be up to 40% more corrosive than either of them individually. If makeup water is untreated, 16 gpm of makeup can dissolve up to 50 lbs of iron in one week of operation.
Boiler Deaerator Design and Function
Boiler deaerator systems are designed to efficiently remove oxygen and other gasses. They deaerate water mechanically through Temperature, Time, Turbulence, and Thin-Film (reduces surface area and agitates the water).
At the most basic level, makeup water enters the deaerator vessel and interacts with the steam inside the tank. The water temperature rises, freeing dissolved gasses from the fluid (Charles’ Law). The freed gas escapes through a vent at the top. Deaerator venting is essential so unwanted gasses can exit the vessel.
The deaeration process requires sufficient time for heating the water and the release of gas. The target temperature is dependent on the deaerator’s pressure. For example, water will need to be 227°F for a DA operating at 5psig, or 212°F for an atmospheric DA, and so on.
As a part of the process, the deaerator has ways to mechanically reduce the water’s surface tension. By breaking the surface tension, the gases can escape more easily. The two most commonly encountered methods for breaking surface tension are spray-type and tray-type. Spray-type deaerators spray the makeup water to break the surface tension, for example. These, and some other methods, create a thin film. Beyond reducing surface tension, the thinner flow of water reduces the time needed to reach the appropriate temperature.
Spray-Type Deaerator
As mentioned above, spray-type deaerators use a spray nozzle to reduce surface tension. There are a few different versions, but we’ll discuss the two-stage spray type here. In general, the nozzle sprays makeup water through the vessel’s steam atmosphere into a stainless steel receiver. At this point, the water will be within a few degrees of the saturation temperature and most of the corrosive non-condensable gases are removed.
From the receiver, the water flows into a secondary “scrubbing” section where incoming steam heats it for the final stage of deaeration. After this, the deaerated water falls into the vessel’s storage section.
The deaerated water is ready for use by the boiler. Once treated, the water is now considered feedwater. A feedwater pump takes the deaerated water from the bottom of the vessel to the boiler(s). This design below serves as both the boiler feedwater storage tank and the deaerator.
Spray-type deaerators operate best at stable load conditions, with a temperature rise of at least 50°F over the incoming water temperature, above 25% of design load (some designs), and with condensate returns of 30% or less. When operated outside these parameters, performance will suffer.
At low loads (less than 25%) there is not enough steam flow to complete the deaeration process. When higher levels of condensate are returned, these designs can experience pressure decay, which is an uncontrolled drop in the deaerator’s pressure when incoming water flow, temporarily exceeds the current volume of steam being introduced into the deaerator. It takes a moment for the steam control valve to sense the pressure drop and admit more steam.
The pressure decay causes two issues: an increased chance of cavitation going into feedwater pumps, and deaeration performance suffers until the steam control valve is able to increase steam flow.
Spray-type deaerators are often paired with Surge Tanks to avoid the conditions that lead to pressure decay. The Surge Tank acts as a buffer between unsteady flow conditions, feeding the Deaerator a more stable volume of condensate returns. This allows the steam valve to respond to the gradual changes in temperature which controls the pressure.
Tray Type Deaerator
In the Tray Type design, the deaerator internals are stacked vertically, above the water storage tank. This different approach is often called the ‘dual effect’. Boiler makeup water enters from the top and filters down through perforated trays.
This water is usually introduced by means of a spray nozzle or tube. Some designs may call for the piping of makeup and condensate into the top; others will indicate makeup and pumped returns only through the top of the tray section and trap the returns directly into the storage vessel.
As makeup water flows through the trays, low-pressure steam flows up through the perforations, rubbing against the makeup water. The contact causes unwanted gasses to separate from the incoming water and release through the deaerator vent valve at the top. The trays assist with the functions of Thin Film and Agitation (and Contact Time).
Now deaerated, the feedwater continues down into the storage tank, where it sits until it is pumped into the steam boiler system.
The tray-type design:
- Is more forgiving of higher levels of condensate returns versus a spray-type
- performs well with higher inlet water temperatures
- works well throughout low load conditions
The Dual Effect Tray Type deaerator (counter flow and co-current flow designs) will give preference to flash steam for deaeration and handle greater blended feedwater approach temperatures (within 10°F of operating temperature). The design will provide substantially improved turndown performance below ≤20% of the load range. As operating loads decrease under the design maximum, the ratio of surface to throughput water increases, and thinner films for the release of gases are provided. This ensures effective deaeration under all inlet water temperatures and flow conditions (a tray-type deaerator can give preference to any flash steam for deaeration whereas a 2-stage spray or spray-scrubber type cannot).
There are other variations of ‘tray-like’ deaerators. Packed column types share the same principles as the tray type, using pawl ring packing in lieu of trays, but are limited in size range due to the necessary height and smaller diameter of the deaeration column section.
Generally, tray-type deaerators (without a surge tank) are best used in facilities where condensate returns are less than 50%. Tray-type deaerators are physically taller than spray-types, making it important to take the boiler room’s design into account when estimating shipping and installation costs. Some tray-type designs can be separated – with a removable upper tray section – for easier shipping, but require reassembly in the field.
Constant Recycle Deaerators
A different approach to deaerator design is the Constant Recycle-type. With these, makeup water is sprayed into the deaerator through a stainless steel spring-loaded nozzle into a stainless steel internal vent condenser which is located in the mixing section. The incoming water is heated instantly by direct contact with steam. Returned condensate enters the deaerator below the water level, eliminating pressure decay caused by surging returns.
The deaerated water is then pumped into the deaerating section where it is blasted through stainless steel wide-angle, full-cone unrestricted nozzles. The last traces of oxygen are shaken out at the source of the purest steam. The pumped transfer rate is approximately 125% of the deaerator capacity, which enables the deaerator to furnish the boiler with deaerated water from start-up. Deaeration is accomplished from 0% to 100% load, and thermal stratification is eliminated.
Excess deaerated water, which is not required by the boiler, is recycled into the deaerating section through the compartment overflow. This deaerated water is blended with makeup water and is constantly re-scrubbed. Non-condensable vapors are expelled from the top of the deaerator through the internal vent condenser.
The Dual Effect Constant Recycle Dual Compartment deaerator will give preference to flash steam for deaeration and handle greater blended feedwater approach temperatures (within 10°F of operating temperature). The design will provide a complete full range turndown from 0% to 100% of the rated load range, regardless of plant load modulation conditions. This design is well suited for handling steam-powered condensate pump returns and is resistant to pressure decay issues.
The dual compartment design lends itself to an expansion of the mixing section to act as system surge capacity (in lieu of a separate surge tank station). This design can be provided with extended pressure vessel warranties of up to [10] years. Where height restrictions may be a concern, these systems present a low profile. The cost of the system is greater than the 2-Stage Spray/ Scrubber units and equal to or a bit more than the Tray-Type on a unit-for-unit basis.
Pressurized or Atmospheric Deaeration?
One option to consider is the use of atmospheric or pressurized deaerator designs. Both can be used effectively in many systems, but require the previously mentioned temperature limitations to be observed.
One advantage of atmosphere designs, they do not require annual, internal inspections – since the atmospheric tank is not a pressure vessel subject to such inspections. This means the deaerator is online and available year-round. With pressurized designs, consideration must be given to this annual inspection.
There are options for handling the annual inspection. If your facility is a manufacturing plant with a scheduled whole-plant shutdown, this is a non-issue. Whereas, a healthcare facility will need to stagger boiler shutdowns to keep critical steam going to the facility. The online boilers will need feedwater. Rental feedwater (or deaerator) systems or provisions that bypass the vessel can continue to supply the boiler. For the latter approach, your deaerator system will need a surge tank, with valving and piping (included at the time of installation) that the boiler’s feedwater pumps can draw water from during the deaerator’s inspection.
Chemical Deaeration
Chemical deaeration can remove gases from a boiler system using chemicals called oxygen scavengers, such as Sodium Sulfite or Hydrazine. They’re best used in conjunction with a deaerator system. Oxygen scavengers can remove any gases left over by the deaerator. If used alone, the process is often incomplete or expensive.
When shutting down a boiler for a few days or weeks, a boiler technician will perform chemical deaeration during a wet layup to protect the boiler during downtime.
Necessary Conditions for Boiler Deaerator Function
In order for deaeration to occur, water must reach saturation temperature. The deaerator operating pressure determines the relative saturation temperature. At 5 psig, a typical pressurized deaerator’s operating temperature, water’s saturation temperature is 227°F—for sea level conditions. For a more comprehensive guide to the water and steam flows required for deaeration at different conditions, check out the US Department of Energy’s Deaerator Calculator.
Reaching the saturation temperature does not mean immediate deaeration; the more time water has in the turbulent system, the more gasses are released. For example, in the Tray System, the friction of downward flowing water and upward drifting steam creates turbulence that frees gasses. Additionally, a consistent flow of feedwater and slow temperature changes help the system regulate steam demands and perform efficiently.
Other Benefits of Deaeration
Deaerating protects your boiler in another way: it provides the boiler with hot feedwater, which reduces the chance of thermal shock. Thermal shock can occur in a boiler due to the pressure vessel’s materials expanding and contracting quickly.
Deaerators can increase boiler efficiency, as well. Making low-pressure systems trapless and pumping the condensate directly into the deaerator can save up to 6 percent in fuel. Exhaust steam and flash steam – formerly lost to the atmosphere – can be used by the deaerator to preheat makeup water.
In high-pressure systems, the use of continuous blowdown heat recovery (CBHR) can be used in conjunction with the deaerator. The flash steam from the CBHR can be used by the deaerator, instead of being wasted to the atmosphere. Up to 3 percent fuel savings are possible with a payback often realized in a matter of months.
Conclusion
From protecting your boiler from damage to increasing operating efficiency, a deaerator is a great investment to make. It will give your boiler more years of productivity with fewer maintenance interruptions. Contact the W.C. Rouse team for all your boiler operation or repair needs!
