Dissolved oxygen in feedwater causes serious corrosion damage in boilers. It attaches to the walls of metal piping and other equipment, it forms oxides (rust). Carbonic acid formed when dissolved carbon dioxide combines with water causes further corrosion.
Deaerators remove oxygen and other dissolved gases from water to prevent corrosion.
The Problem: Corrosion
Corrosion of iron or steel in boilers and feed water piping is caused by three fundamental factors:
- Feedwater temperature
- Feedwater pH value
- Feedwater oxygen content
Temperature and pH value influence the aggressiveness of corrosion. The higher the temperature, and the lower the pH value, the increased aggressiveness of the feedwater.
The dissolved oxygen content of the feedwater is a large factor in determining the amount of corrosion that will take place. The presence of oxygen and other non-condensable gases in feed water is a major cause of corrosion in feed water piping, boilers, and condensate handling equipment.
The Solution: Deaeration
Deaeration is the mechanical removal of dissolved gases from boiler feedwater. There are three principles that must be met in the design of any deaerator:
- The incoming feedwater must be heated to the full saturation temperature corresponding to the steam pressure maintained inside the deaerator. This will lower the solubility of the dissolved gases to zero.
- The heated feedwater must be mechanically agitated. This is accomplished in a tray deaerator by first spraying the water in a thin film into a steam atmosphere. Creating a thin film reduces the distance the gas bubble has to travel to be released from the water. Next, the water is cascaded over a bank of slotted trays, further reducing the surface tension of the water. This allows
for the removal of any gases not liberated by the initial spraying.
- Adequate steam supply must be passed through the water, in both the spray section and the tray section to sweep out the gases from the water.
Over the years, multiple styles of deaerators have been developed, such as the counter-flow tray type, atomizer type, packed tower type, and the parallel downflow type. Here at Altair, we offer two types of deaerators, packed tower, and parallel downflow, that meet the requirements of most installations.
There are many design and operational advantages of parallel downflow deaerators:
- Time proven design with thousands of installations worldwide
- Design is suitable for small to medium size plants
- Can meet outlet guarantees at varying plant conditions.
- High tray loading, resulting in higher outlet capacity for any given diameter.
- Large tray spilling edge resulting in high deaerating efficiency
- Low vent rate resulting in increased operating efficiency.
The main disadvantage of this style is that it is a more complicated design, which results in a slightly higher cost.
Because there are so many advantages to parallel downflow deaerators, they are our standard choice.
Parallel Downflow Operation
In our PD design, inlet water is sprayed into a steam atmosphere through variable orifice, spring loaded spray nozzle. This action heats the water to within 2 to 3 degrees of the steam temperature, while liberating 90% to 95% of the dissolved gases.
This preheated, partially departed water then flows down through a water seal for distribution over the tray bank. The water seal serves two functions – they prevent gases liberated in the initiate heating, from entering the tray bank, and they direct the steam to flow down through the trays before entering the upper heating section. The main function of the tray bank is to remove the remaining amounts of dissolved gases not liberated in the initial heating.
Since very little, or no heating takes place in the trays, the entire volume of steam is used to scrub out the remaining gases. The trays are slotted, and provide a great amount of spilling edge. This allows for a great amount of water surface area to be exposed to the steam.
Water and steam flow downward through the trays – this is where it gets its name.
The steam, after exiting the tray bank, then flows upward into the top of the deaerator where it is used to heat the incoming water being discharged by the spray nozzles. The steam is condensed by the colder inlet water. and a small amount is vented to atmosphere, along with the dissolved gases.
The deaerated water flows from the deaerator down into the storage tank. The stored water is covered by a steam blanket to maintain heat, pressure, and prevent re-contamination of the deaerated water.
Our proprietary packed tower design has been developed to meet the requirements for reliable deaeration. It includes multi-stage deaeration to deliver peak performance and is capable of producing completely deaerated water for small plants. Other advantages include:
- Low cost
- Low maintenance
- Ability to handle varying plant conditions
Two disadvantages of the packed tower design are it’s height requirement and it’s capacity limits – we usually reserve this style for small sized plants.
Packed Tower Operation
In our proprietary packed tower design, inlet water is sprayed into a steam atmosphere through a variable orifice, spring loaded spray nozzle. This action heats the water to within 2 to 3 degrees of the steam temperature, while liberating 90% to 95% of the dissolved gases.
The heated water flows down onto a distribution plate. This plate evenly distributes the water over the entire cross-sectional area of the tower packing.
As the water flows down through the distribution plate it enters a steam chest area, where the water is further heated by up-flowing steam, and more of the dissolved gases are liberated.
Any remaining dissolved gases are removed when the water flows down from the steam chest and then down through the packing tower. The packing tower function is to expose a greater surface area of the water, while up-flowing steam completes the deaeration process.
The water leaving the bottom of the packing tower is given a final scrubbing of steam. The steam entering the deaerator from below the packing tower is introduced through a fixed orifice steam distributor. This steam distributor directs high velocity steam through the down-flowing water leaving the bottom of the packing tower.
The departed water flows from the deaerator down into the storage tank. The stored water is covered by a steam blanket to maintain heat, pressure. and prevent re-contamination of the deaerated water.