What is a Deaerator? A Practical Guide for Facility Engineers
A deaerator is one of the most important — and most underappreciated — components in a steam boiler system. Its job is straightforward: remove dissolved gases from the boiler feedwater before they reach the boiler. Done well, this single function protects the boiler, condensate return piping, and associated equipment from some of the most destructive corrosion mechanisms in steam system operation. Done poorly — or with an improperly sized or maintained unit — the consequences show up as pitted tubes, corroded condensate lines, and shortened equipment life. This article covers how deaeration works, the main design types and when each applies, sizing considerations, and what to watch for when a deaerator isn’t performing as it should.
1. Why Feedwater Quality Matters
Two dissolved gases are the primary targets of deaeration: oxygen (O₂) and carbon dioxide (CO₂). Both enter the system in makeup water and, to a lesser degree, in returned condensate exposed to atmosphere.
Dissolved oxygen is the more immediately damaging of the two. It causes pitting corrosion — highly localized attack that forms deep pits in boiler tubes, headers, and feedwater piping. Even at low concentrations, oxygen pitting can perforate a boiler tube relatively quickly, and the damage is concentrated rather than uniform, making it difficult to detect until a failure occurs. The threshold that most boiler operators target is dissolved oxygen below 7 parts per billion (ppb) in the deaerated feedwater — a level achievable with a properly designed and operating deaerator.
Carbon dioxide is less immediately destructive but causes significant long-term damage to condensate return systems. CO₂ dissolves in condensate to form carbonic acid (H₂CO₃), which lowers the pH of the condensate and attacks carbon steel condensate piping, fittings, and traps. The result is grooving corrosion — a thinning and eventual perforation of the pipe wall that is characteristic of carbonic acid attack. Removing CO₂ at the deaerator protects the entire condensate return system downstream.
2. How Deaeration Works
The operating principle behind thermal deaeration is Henry’s Law: the solubility of a gas in a liquid decreases as the temperature of the liquid increases and as the partial pressure of that gas above the liquid decreases. A deaerator exploits both effects simultaneously.
Incoming feedwater is heated to its saturation temperature by direct contact with steam. As the water approaches saturation temperature, the solubility of dissolved gases drops sharply — oxygen and CO₂ are liberated from solution and driven into the vapor space above the water. A vent at the top of the deaeration section allows these non-condensable gases to escape to atmosphere (or to a vent condenser, which recovers some of the steam that carries the gases out). The steam used for heating condenses into the feedwater, becoming part of the treated water supply.
The effectiveness of this process depends on achieving thorough contact between the incoming water and the steam, and on allowing adequate time for the liberated gases to escape before the water is collected in the storage section. This is where design differences between deaerator types become significant.
3. Deaerator Designs: Tray, Spray, and Constant Recycle
Three deaerator designs are commonly used in industrial and commercial steam systems, each with different operating characteristics and best-fit applications. The WC Rouse diagrams below illustrate each design.
Tray Type Deaerator
In a tray-type deaerator, incoming feedwater is distributed over a stack of perforated trays mounted in a vertical domed deaeration column that sits on top of the storage vessel. As water cascades down through the trays, it breaks into thin films and droplets that maximize surface area contact with steam rising upward through the column. This counterflow arrangement — water falling, steam rising — is highly effective at liberating dissolved gases, and the tray design is generally considered the most reliable for achieving low dissolved oxygen levels consistently.
Tray-type deaerators are well-suited for systems with up to approximately 50% pumped condensate returns. Above that threshold, the high flow rates associated with condensate return pumps — which are typically sized to pump 2–3x the actual condensate flow rate — can exceed the deaeration column’s capacity to heat and treat the incoming water fast enough, potentially compromising deaeration performance. For systems with higher return rates, a surge tank upstream of the deaerator should be considered to buffer the flow.
Spray Type Deaerator
A spray-type deaerator distributes incoming feedwater through spray nozzles in the deaeration section, creating a fine mist that contacts steam directly. The large surface area created by the spray pattern promotes rapid heat transfer and gas liberation. A scrubber section below the spray provides additional contact time before the treated water falls into the storage vessel.
Spray-type deaerators are generally recommended for systems with up to approximately 30% pumped condensate returns — a lower threshold than tray-type designs, reflecting the greater sensitivity of the spray mechanism to flow surges. The spray nozzles are also a maintenance consideration: they can clog or wear over time, degrading deaeration performance without obvious external symptoms. Periodic inspection of the nozzle condition is an important part of deaerator maintenance on spray-type units.
Constant Recycle Deaerator
The constant recycle deaerator uses a recycle pump to continuously circulate a portion of the deaerated water from the storage vessel back through the deaeration section. This recirculation maintains a steady flow through the deaeration zone regardless of variations in the incoming feedwater flow, which makes the design inherently more tolerant of flow swings than either the tray or spray design.
This characteristic makes the constant recycle design well-suited for systems with high or variable condensate return rates, or where the incoming flow is intermittent rather than steady. The tradeoff is the added complexity and operating cost of the recycle pump, which runs continuously. Where condensate return rates are high or the load profile is highly variable, the deaeration performance stability offered by the constant recycle design frequently justifies this tradeoff.
4. Atmospheric vs. Pressurized Deaerators
Deaerators are also classified by their operating pressure: atmospheric (also called open) and pressurized (pressure-type).
Atmospheric deaerators operate at essentially zero gauge pressure, with the steam supply at or near atmospheric pressure. The saturation temperature at atmospheric pressure is 212°F (100°C), which sets the upper limit for feedwater temperature leaving the deaerator. Atmospheric units are simpler and less expensive, and are suited to lower-pressure steam systems and smaller applications where the feedwater temperature entering the boiler does not need to be above 212°F.
Pressurized deaerators operate at elevated pressure — typically 5 to 15 PSIG in most industrial steam applications, though higher-pressure units exist for large power boiler applications. The elevated pressure raises the saturation temperature proportionally: at 5 PSIG, saturation temperature is approximately 227°F; at 15 PSIG, approximately 250°F. The higher feedwater temperature reduces the thermal shock to the boiler when cold feedwater enters, reduces the energy required to bring the feedwater to boiling in the boiler, and improves dissolved gas removal efficiency. For these reasons, pressurized deaerators are the standard choice for most industrial high-pressure steam systems. The operating pressure of the deaerator must be coordinated with the boiler feed pump NPSH requirements and the system design to ensure reliable pump operation.
5. Deaerator Sizing
Proper sizing of a deaerator involves two related but distinct calculations: the capacity of the deaeration section and the capacity of the storage vessel.
The deaeration section must be sized to handle the maximum expected feedwater flow rate — the combined total of makeup water and condensate returns at peak boiler demand. This is not simply the boiler’s rated evaporation rate; it must account for flash losses, blowdown, and any process steam that does not return as condensate. Undersizing the deaeration section means the incoming water cannot be heated to saturation temperature fast enough, and dissolved oxygen removal will be incomplete even if the equipment is otherwise functioning correctly.
The storage vessel is typically sized to provide 10 to 20 minutes of feedwater supply at the boiler’s full rated demand. This buffer ensures that short-term imbalances between feedwater supply and boiler demand — caused by load swings, condensate pump cycling, or makeup water supply interruptions — do not starve the boiler feed pumps. A storage vessel sized too small will cause boiler feed pump low-level trips during demand peaks; too large adds unnecessary capital cost and increases the risk of oxygen ingress through the vent.
When condensate return rates are high — generally above 50% for tray-type systems or 30% for spray-type — a surge tank upstream of the deaerator should be evaluated. Condensate return pumps are typically sized to pump 2 to 3 times the actual condensate flow rate, meaning the deaerator can receive surges of water well in excess of average flow. A surge tank absorbs these surges and feeds the deaerator at a steady, controlled rate via constant-speed transfer pumps, protecting deaeration performance. For a detailed discussion of surge tank application, see our article on deaerator surge tanks.
6. Signs of Trouble and What They Mean
Pressure fluctuations. Unexpected pressure swings in a pressurized deaerator typically point to a steam supply control issue — the steam valve may be hunting, or the pressure control setpoint and differential may need adjustment. Pressure drops can also be caused by surges of cold condensate entering the vessel faster than the steam supply can compensate, in which case the underlying issue is in the condensate return system or the deaerator’s inlet flow control. Persistent pressure instability warrants investigation before it causes boiler feed pump operational problems.
Unusual noise or water hammer. Knocking, banging, or vibration in or near the deaerator is often caused by subcooled condensate — condensate that has not fully flashed to steam — entering the deaeration vessel and flashing suddenly on contact with the steam space. This creates pressure pulses that can be heard and felt. It can also occur when steam contacts pooled cool water in the inlet piping. The cause is typically in the condensate return system: a failed steam trap allowing liquid to back up, or condensate return at temperatures below the deaerator operating pressure saturation temperature.
Rising dissolved oxygen readings. Increasing dissolved oxygen in the deaerated feedwater is a direct indication of degraded deaeration performance. Common causes include: spray nozzles that are clogged or worn (spray-type units), damaged or fouled trays (tray-type units), inadequate steam supply to maintain saturation temperature, a malfunctioning or undersized vent that is not allowing liberated gases to escape, or an undersized deaeration section being overwhelmed by peak flow. Dissolved oxygen should be measured regularly as part of the water treatment program; a target of less than 7 ppb is standard for most boiler applications.
Visible corrosion. External corrosion on the deaerator vessel, associated piping, or connections warrants inspection. Deaerators are pressure vessels subject to periodic inspection requirements under state boiler codes — the same inspection intervals that apply to the boiler itself apply to deaerators in most jurisdictions. Internal corrosion found during inspection may indicate that the deaerator has been operating outside its design parameters, or that the chemical treatment program is not adequately protecting the vessel internals.
Working With W.C. Rouse on Deaerator Systems
Deaerator selection, sizing, and integration into the broader feedwater system requires coordinating a number of variables — boiler load profile, condensate return rates, operating pressure, available steam supply, feed pump NPSH requirements, and water treatment program. Getting these right from the start produces a system that protects equipment and operates reliably for decades. W.C. Rouse represents deaerator manufacturers and has the application experience to help you select, size, and specify the right system for your facility — and to diagnose and correct problems in existing installations.
We work with facility managers and plant engineers across North Carolina, South Carolina, Tennessee, Georgia, and Florida. Call us at 336-299-3035, email us at sales@wcrouse.com, or visit wcrouse.com.
Frequently Asked Questions
What is a deaerator and what does it do?
A deaerator removes dissolved oxygen and carbon dioxide from boiler feedwater before it enters the boiler. Dissolved oxygen causes pitting corrosion in the boiler; CO₂ forms carbonic acid that attacks condensate return piping. Removing these gases at the deaerator protects the entire steam system from corrosion-driven failures.
What is the difference between a tray-type and spray-type deaerator?
In a tray-type deaerator, water cascades over perforated trays in a counterflow arrangement with rising steam, maximizing contact and gas liberation. Tray-type units handle up to approximately 50% pumped condensate returns reliably. Spray-type deaerators distribute water through spray nozzles to create a fine mist for direct steam contact. They are effective but more sensitive to flow surges — generally recommended for systems with up to 30% pumped returns — and require periodic inspection of the spray nozzles to maintain performance.
What is a constant recycle deaerator, and when is it used?
A constant recycle deaerator uses a recycle pump to continuously circulate a portion of deaerated water back through the deaeration section, maintaining stable flow through the deaeration zone regardless of incoming flow variations. This makes it well-suited for systems with high or variable condensate return rates, or where intermittent loads create significant flow swings that would challenge a tray or spray design.
How do I know if my deaerator is working properly?
The most direct indicator is dissolved oxygen measurement in the deaerated feedwater — a properly operating deaerator should consistently achieve dissolved oxygen below 7 ppb. Warning signs that performance may be degraded include rising dissolved oxygen readings, pressure fluctuations in the vessel, unusual noise or water hammer, and visible corrosion on the vessel or associated piping. Any of these warrants investigation by a qualified boiler professional before a manageable issue becomes a system-wide problem.
When does a deaerator need a surge tank?
A surge tank should be evaluated when pumped condensate returns exceed approximately 50% of boiler load for tray-type systems, or 30% for spray-type systems. Condensate return pumps are typically sized to pump 2 to 3 times the actual condensate flow rate, which can send surges of water to the deaerator well above its design flow. A surge tank absorbs these surges and feeds the deaerator at a steady, controlled rate, protecting deaeration performance and preventing the deaerator from being overwhelmed during peak return periods.
