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To simplify the animation, a single internal float switch is illustrated on the surge chamber to represent the principle of float control. In reality, separate high and low, external float switches would be included, along with an external, low water cut off float.
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 the surge chamber (also referred to as an accumulation chamber) is vented to atmosphere and the deaerator chamber (DA chamber) is pressurized with internal spray tray baffles.
Both gravity and pumped condensate returns, along with makeup water are returned and blended together in the surge chamber of the tank. All the flows drop directly to the surface water.
The surge chamber serves as the holding tank for makeup water going to the DA chamber. Normally, all returning condensate (except small amount of returns above 212°F) are blended with the colder makeup water. This blending helps avoid sudden temperature changes in the DA chamber.
The blended condensate is transferred from the surge chamber to the pressurized DA chamber by the continuously running transfer pumps. A modulating level controller ensures a constant water level in the DA chamber by modulating the flow of blended condensate through the modulating transfer valve.
Next the condensate flows into the top of the DA chamber and through a spring-loaded, stainless steel spray nozzle that breaks down the water into a fine mist.
The droplets of mist then cascade down through a series of baffles in the spray tray. The cascading breaks down the droplets into a thin film. The thin film allows rapid heating of the water and release of non-condensable gases.
Steam constantly flows directly into the DA chamber through the submerged preheat tube and the preheat tube above the water line (below the tray). The injected steam:
Keeping the temperature of water 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).
As the condensate droplets fall from the spray tray, the rising steam scrubs (i.e., releases oxygen and other non-condensable gases) the falling droplets. As the droplets fall into the surface water of the DA chamber, the steam bubbling up from narrow slits in the preheat tube purges the last of the oxygen in the chamber water.
The released oxygen and other non-condensable gases rise up through the stray tray baffles and escape through the orifice vent valve at the top of the DA chamber. (Note: the vent should be piped to an area suitable for a small steam plume).
The size of the vent hole drilled in the orifice vent valve is based on the lbs/hr of the unit. The orifice vent must be kept closed during the operation of the unit.
This animation assumes a system designed for the boiler feed pumps to run continuously.
The transfer pumps of the surge chamber run continuously so that the modulating transfer valve always has water pressure available to it.
As the water level in the DA chamber falls, the modulating level controller detects the decrease and sends a signal to open the modulating transfer valve wider allowing an increased flow of blended condensate from the surge chamber into the DA chamber.
When the volume of blended water transferred to the DA chamber exceeds the volume of condensate returning to 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 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 shutting off flow of makeup water in the surge chamber.
Because the temperature of blended condensate is still cooler than the water stored in the DA chamber, even after cascading down the spray baffles, its presence causes the water pressure in the DA chamber to fall slightly.
The pressure sensors immediately detect the pressure decrease and sends signal to the steam regulators to increase the flow of steam into the submerged preheat tube and the preheat tube above the water line (below the tray).
The steam regulator includes a pressure reducing valve that ensures the steam pressure released into the preheat tube(s) does not exceed 5-7 psig.
The increased steam flow raises the pressure in the deaerator back to normal.
The modulating level controller senses water level restored and sends signal to modulating valve to reduce flow of blended condensate into the DA chamber. Cycle repeats.