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Dissolved oxygen is a major cause of corrosion in boilers. A deaerator is designed to remove the dissolved oxygen and carbon dioxide in boiler feed water to extend the life of a boiler. The removal of dissolved gases is known as deaeration. Two processes are available - mechanical or chemical.
Mechanical deaeration is the preferred method of removing the dissolved gases in boiler feed water. While chemicals can be used to treat the dissolved oxygen, most boiler feed water also contains other compounds such as dissolved carbon dioxide, some carbonates and bicarbonates which are not removed by oxygen inhibitors.
Deaerators are rated based on the cubic centimeters of oxygen per liter that remain in the water of the tank. Different designs of deaerators offer different levels of effectiveness. The highest rating in the steam industry is .005 cc/liter.
Applications with a large volume of condensate returns (i.e., 20 percent or more of system load) typically use either a two-tank design or a two-chamber design. In the two-tank design both the surge tank and deaerator are free standing. In contrast, a two-chamber design has one tank with two chambers -- one-chamber functions as a surge tank and the other as a deaerator.
This animation illustrates a two-chamber system where both the surge chamber (also referred to as an accumulation chamber) and the deaerator chamber (DA chamber) are vented to atmosphere. This style has no spray tray baffles in the DA chamber.
Both gravity and pumped condensate returns, along with the colder makeup water are returned and blended together in the surge chamber, which serves as a holding tank for the makeup water going to the DA chamber. This blending helps increase the temperature of water and avoids sudden temperature changes in the water of the DA chamber.
As the gravity condensate returns and city makeup water flow into the surge chamber, the water first hits the inlet cascade baffle and then flows out either side of the baffle and falling to the water surface.
The continuously running transfer pump[s] simultaneously:
The re-circulated water, along with the pumped condensate pass through spring-loaded, stainless steel spray nozzles in the surge chamber that break down the water into a fine mist that allows for rapid heating of the water. The mist hits the sides of the spray box, and then drips down through the rising steam that is escaping from the DA chamber through the opening in the divider plate separating the two chambers. The purpose of the spray box is to protect and to extend the life of the tank shell.
Blended condensate is transferred from the surge chamber to the DA chamber by the continuously running transfer pumps that maintain a constant flow of water through the modulating transfer valve. A modulating level controller on DA chamber ensures a constant water level in the DA chamber by modulating the flow of blended condensate through the modulating transfer valve.
The blended condensate next flows through an external heater assembly where steam is directly injected that pressurizes the water to about 3 psig or 220°F. The pressurized water then enters the DA chamber through a spring-loaded, stainless steel spray nozzle where it flashes back to atmospheric conditions (i.e., 212°F).
The droplets of water mist (now at 212°F) fall into the surface water of the DA chamber. The flashing steam:
The released oxygen and other non-condensable gases rise over the top of the divider plate between the two chambers into the surge chamber where the gases are vented to atmosphere. (Note: the vent should be piped to an area suitable for a small steam plume).
Keeping the temperature at the saturation point (i.e., boiling point) is critical for effective removal of oxygen and non-condensable gases. Therefore, preventing wide temperature fluctuations within the DA chamber deaerator is critical to achieving and maintaining its rated performance level (e.g., .005 cubic centimeters of oxygen remaining per liter of water).
Water level controls on the boiler control the boiler feed pumps on the DA chamber. This animation assumes a system designed for the boiler feed pumps to run continuously.
As the boiler feed pumps run continuously, the water level in the DA chamber drops slightly.
The modulating level controller immediately detects any water level drop and sends a signal to the modulating transfer valve to increase flow of blended water through the modulating transfer valve.
The increased water flow leads to a temperature drop in the water flowing through the external heater assembly into the DA chamber.
The temperature sensor, located in the piping between the external heater assembly and the spray nozzle in the DA chamber, immediately detects any temperature decrease and sends a signal to the steam regulator to increase the flow of steam into the external heater assembly.
The steam regulator includes a pressure reducing valve that ensures the steam pressure released into the external heater assembly does not exceed 5-7 psig.
The increased flow of blended water through the modulating transfer valve causes the water level in the DA chamber to return to normal.
The increased steam flow raises the water temperature so that the water entering the DA chamber is maintained at 220°F.
When the volume of blended condensate transferred to the DA chamber exceeds the volume of condensate returning into the surge chamber, the water level in the surge chamber begins to fall.
The flow of makeup water into the surge chamber is controlled by a solenoid valve that in turn is controlled by float switches in the surge chamber. While actual DA's typically are built using separate (one at high and one at low water levels), float switches external to the tank, this animation uses a single float switch that is internal to the surge chamber for illustrating principle.
At the low water level, a float switch sends a signal to open the solenoid valve letting makeup water flow into the surge chamber.
As the makeup water flows into the surge chamber, the water level begins to rise.
At high water level, a float switch sends signal to solenoid valve shutting off flow of makeup water in the surge chamber.
When the modulating level controller senses water level in the DA chamber is restored, it sends a signal to modulating valve to reduce flow of blended water into the DA chamber.
When the flow is reduced, temperature of transfer water in the external heater assembly will begin to increase above 220°F. The temperature sensor immediately detects any temperature increase and sends a signal to the steam regulator to decrease the flow of steam into the external heater assembly.
The decreased steam flow lowers the temperature of the water entering the DA chamber to maintain it at 220°F.
Cycle repeats.