Deaerators: Introduction
If you are working with boilers or steam systems, then you already know that fuel quality, burner efficiency, and control systems are important.
But there is one area that is often ignored until problems start showing up, and that is boiler feedwater quality.
Water may look clean, but it always contains dissolved gases, mainly oxygen and carbon dioxide. These gases do not create problems at room temperature, but once water enters hot boiler systems, they become highly corrosive.
Over time, this corrosion damages boiler tubes, feedwater piping, steam drums, and even condensate return lines.
This is where deaerators play a very important role. A deaerator is not just another tank in the boiler house. It is a carefully designed mechanical device whose only job is to remove these harmful gases before the water enters the boiler.
Let us now understand how deaerators work and why they are so critical for reliable boiler operation.
Why deaeration is critical in boiler systems
Boiler systems operate under high temperature and pressure, which makes chemical reactions happen much faster. When oxygen is present in hot water, it reacts with iron and steel surfaces and starts corrosion almost immediately.
1. Oxygen as the main corrosion driver
Oxygen is the most aggressive gas present in feedwater. Even a few parts per million can cause deep pitting corrosion. This type of corrosion is dangerous because it does not thin the metal evenly.
Instead, it creates small deep holes that can suddenly rupture under pressure, leading to tube failures and unplanned shutdowns.
2. Carbon dioxide and condensate line damage
Carbon dioxide mainly causes problems in condensate systems. When CO₂ mixes with water, it forms carbonic acid, which attacks piping and steam traps from inside.
Many plants face repeated condensate pipe leakages without realizing that the real cause is carbon dioxide that was never removed from feedwater.
3. Other non-condensable gases affecting heat transfer
Apart from oxygen and carbon dioxide, small amounts of air and other gases can also remain in the system. These gases collect on heat transfer surfaces and form thin insulating layers.
Even a very small gas film can significantly reduce heat transfer, forcing the boiler to burn more fuel to achieve the same output.
Because of all these reasons, removing gases before water enters the boiler becomes far more effective than trying to control corrosion after damage has already started.
What exactly is a deaerator
A deaerator is a mechanical device that removes dissolved gases from boiler feedwater by using heat and direct contact between steam and water. It does not use filters or membranes. Instead, it uses physical conditions that naturally force gases to leave the water.
In most industrial applications, deaerators also serve as feedwater storage tanks. This means they not only remove gases but also provide a hot reserve of treated water that can be supplied continuously to the boiler.
So, in simple words, a deaerator performs two important jobs. First, it removes harmful gases. Second, it stores hot, ready-to-use feedwater for stable boiler operation.
Deaerator working principle
The deaerator working principle is based on the fact that hot water cannot hold dissolved gases easily. When water temperature increases and surrounding gas pressure decreases, gases naturally come out of solution.
Inside a deaerator, feedwater enters through spray nozzles or flows over trays as thin layers. At the same time, low-pressure steam enters from below and comes in direct contact with the water.
1. Heating of feedwater
The incoming steam quickly heats the water close to its saturation temperature. As temperature increases, the solubility of gases in water drops sharply.
2. Maximum contact between steam and water
By spraying water into fine droplets or spreading it in thin sheets, the surface area between steam and water becomes very large. This allows gases to escape more easily and quickly.
3. Reduction of gas pressure around water
As steam fills the vessel, the partial pressure of oxygen and carbon dioxide around the water decreases. This further forces the gases to leave the water.
4. Continuous venting of released gases
The released gases are continuously removed through a small vent at the top of the deaerator. This prevents gas accumulation inside the vessel and maintains effective deaeration.
5. Storage of deaerated hot water
After gas removal, the hot water flows into the storage section where it is maintained near saturation temperature. From here, it is pumped to the boiler as feedwater.
So the deaerator simply creates conditions where gases cannot remain dissolved in water and are naturally driven out.
Types of Deaerator Used in Boiler Systems
In industrial boiler plants, deaerators are designed in different mechanical forms, but the basic purpose remains the same, which is to remove dissolved gases from boiler feedwater before it enters the boiler.
Based on construction and operating method, deaerators are commonly classified into three main types, and each type is selected depending on plant size, pressure conditions, and efficiency requirements.
The three commonly used types of deaerator are spray type, tray type, and vacuum type. Let us understand each one in simple terms.
1) Spray Type
The spray type deaerator is one of the simplest designs used for removing dissolved gases from feedwater. In this type, the main method of gas removal is achieved by increasing the water temperature and reducing gas solubility through direct steam contact.
1. How feedwater enters the vessel
In a spray type deaerator, boiler feedwater enters the vessel through specially designed spray nozzles. These nozzles break the incoming water into fine droplets, which increases the surface area of water exposed to steam inside the vessel.
2. Role of steam in gas removal
Low-pressure steam is supplied into the deaerator vessel and comes into direct contact with the sprayed water droplets. This steam heats the water rapidly and also reduces the partial pressure of dissolved gases around the water. Because of this combined effect, oxygen and carbon dioxide are forced out of the water.
3. Preheating and deaeration zones
Inside the vessel, the water first passes through a preheating section where its temperature is raised close to saturation temperature. At this stage, most of the dissolved gases start leaving the water. After this, the water enters the deaeration section where steam and water droplets are separated and gases are continuously vented out from the top of the vessel.
4. Collection of deaerated water
After gas removal, the hot deaerated water flows down into the storage section of the deaerator vessel. From here, it is pumped to the boiler as feedwater. Maintaining high temperature in this storage section also helps prevent re-absorption of gases.
Spray type deaerators are generally used in smaller boiler systems where moderate deaeration efficiency is acceptable and plant layout is simple.
2) Tray Type
Tray type deaerators are widely used in large industrial and power plant boiler systems because they provide more efficient gas removal compared to spray-only designs. In this type, both spray nozzles and trays are used to improve contact between steam and water.
1. Combined action of spray and trays
In a tray type deaerator, feedwater is first sprayed through perforated nozzles and then flows downward over a series of trays arranged inside the deaeration section. This creates multiple thin water layers, which increases the time and area of contact between steam and water.
2. Better heat transfer and gas stripping
As water flows over each tray, it mixes continuously with rising steam. This repeated contact ensures uniform heating and more effective stripping of dissolved gases. Compared to spray-only systems, tray systems allow more complete removal of oxygen and carbon dioxide.
3. Steam distribution inside the vessel
Low-pressure steam enters through a steam distribution pipe located below the trays. This ensures that steam flows upward evenly across all trays, maintaining uniform temperature throughout the deaeration section.
4. Storage of treated feedwater
After passing through all trays, the deaerated hot water collects in the storage section at the bottom of the vessel. From here, boiler feed pumps deliver this water to the boiler. Because the water is stored at near saturation temperature, gas re-entry is minimized.
Tray type deaerators are preferred in applications where high reliability and maximum corrosion protection are required, especially in continuous-duty boiler systems.
3) Vacuum Type
Vacuum type deaerators operate on the same basic principle as tray type deaerators, but with one additional feature that improves gas removal under certain conditions.
1. Use of vacuum to assist gas removal
In vacuum type deaerators, a vacuum pump is connected to the vent line at the top of the vessel. This pump reduces the pressure inside the deaeration section, which further lowers the partial pressure of dissolved gases and makes it easier for gases to leave the water.
2. Lower vent losses compared to open vent systems
Because gases are actively removed using a vacuum pump, less steam escapes through the vent. This reduces steam losses and improves overall thermal efficiency of the system.
3. Suitable for low-temperature feedwater systems
Vacuum deaerators are useful where feedwater temperatures are relatively low and heating to full saturation temperature is difficult. By lowering pressure, gas solubility is reduced even at lower temperatures, allowing effective deaeration.
4. Similar internal construction to tray type
Internally, vacuum type deaerators often use trays and spray nozzles similar to tray type designs. The main difference lies in how gases are removed from the vessel rather than how water and steam contact occurs.
Vacuum type deaerators are commonly used in special applications where steam availability is limited or where minimizing vent losses is important for energy savings.
Why physical laws support deaerator operation
Deaerators work reliably because they are based on well-known physical principles rather than complicated chemical reactions.
1. Role of Henry’s Law
Henry’s Law states that the amount of gas dissolved in a liquid is proportional to the partial pressure of that gas above the liquid at constant temperature. In a deaerator, steam replaces air, which reduces the partial pressure of oxygen and carbon dioxide. This encourages the gases to escape from the water.
2. Effect of temperature on gas solubility
As water temperature increases, gas solubility decreases rapidly. At room temperature, water can hold significant oxygen. But near boiling point, oxygen content becomes almost zero. By heating feedwater inside the deaerator, most of the dissolved gases are forced out naturally.
This is why mechanical deaeration is so effective and reliable in industrial boiler systems.
How deaerators reduce chemical usage and operating cost
If feedwater enters the boiler with high oxygen content, chemical oxygen scavengers must be added in larger quantities. This increases chemical cost and also increases total dissolved solids in boiler water.
1. Reduction in chemical dosing
When mechanical deaeration removes most of the oxygen, only small amounts of chemical scavengers are needed. This directly reduces chemical expenses.
2. Lower blowdown requirement
High dissolved solids force frequent blowdowns to maintain water quality. Reduced chemical usage means lower dissolved solids, which reduces blowdown frequency.
3. Less heat and water loss
Every blowdown removes hot water from the boiler. Less blowdown means less fuel is required to heat makeup water, improving overall efficiency.
So deaerators not only protect equipment but also help in saving fuel, water, and chemicals.
Impact of deaeration on overall plant reliability
Good deaeration has a direct effect on how smoothly the entire steam system operates.
1. Longer life of boiler tubes and piping
With less corrosion, metal surfaces remain intact for longer periods. This reduces tube replacements and welding jobs.
2. Better performance of steam traps and valves
Corrosion products often block or damage steam traps and control valves. Clean feedwater reduces these failures.
3. Stable boiler operation
When heat transfer surfaces remain clean, boilers respond better to load changes and operate more efficiently.
4. Reduced unplanned shutdowns
Most sudden boiler failures are related to corrosion or deposits. Good deaeration reduces the root causes of such failures.
In simple terms, deaerators improve not just boiler health but the reliability of the entire plant.
Why deaerators are a smart long-term investment
From a financial and operational point of view, deaerators provide excellent return on investment.
1. Reduced maintenance cost
Fewer tube failures, fewer pipe leaks, and less valve damage reduce maintenance expenses.
2. Improved energy efficiency
Clean heat transfer surfaces allow boilers to operate closer to design efficiency.
3. Higher plant availability
Fewer breakdowns mean higher production uptime.
4. Lower chemical and water cost
Reduced treatment and blowdown directly reduce operating expenses.
Even though deaerators may not look as important as boilers or turbines, they quietly protect the most expensive components in the steam system.
What we learn today?
If I have to summarize the importance of deaerators in one simple line, it would be this: a deaerator protects your boiler long before problems start appearing.
By removing oxygen, carbon dioxide, and other non-condensable gases, deaerators prevent corrosion, improve efficiency, reduce chemical usage, and increase system reliability.
They work quietly in the background, but their impact is seen across the entire steam and condensate network.
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