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On applications with a large volume (i.e., 20 percent or more of system load) of condensate returns, a two-tank design (i.e., surge tank and deaerator) is typically used. On a two-tank design, the modulating valve is a "transfer connection". On single-tank design (i.e., deaerator only), the modulating connection is a "make-up connection".
On the two-tank design, both the pumped returns and gravity returns are piped first into a surge tank rather than directly into the DA tank as illustrated on this animation, which is based on a single-tank design. The returns are then piped from the surge tank to the DA through a modulating "transfer" valve.
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.
Different designs of deaerators exist offering different levels of effectiveness. A .03 internal atmospheric deaerator, without a baffle tray, generally provides a lower priced option for applications using low-pressure boilers. However the level of deaeration is less effective (i.e., a higher concentration of oxygen remains) than .005 designs which use a spray tray of baffles.
The effectiveness of this deaerator is rated as .03 cc/liter -- meaning .03 cubic centimeters of oxygen per liter remain in the water within the tank of the deaerator. The highest rating in the steam industry is .005 cc/liter. To achieve the highest rating, an atmospheric deaerator with a tray of baffles or a pressurized deaerator should be selected.
The temperature design rating of feed water achieved and maintained in the tank on this deaerator design is 211°F. Other designs can provide a higher temperature rating, which leads to higher levels of deaeration.
The discharge pump sends water to boiler as needed. A water level control on boiler controls the discharge pumps (i.e., turning pumps on/off as needed depending on water level in boiler).
Gravity returns (condensate returns) flow into the tank hitting the inlet cascade baffle and then flow out either side of the baffle.
Both the pumped returns (condensate returns) and makeup water enters the tank through spring-loaded, stainless steel spray nozzles 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. The purpose of the spray box is to protect and to extend the life of the tank shell.
Steam is constantly pumped into submerged preheat tube that is below the surface water in the tank. The high temperature steam:
As the steam rises in the tank, it scrubs the condensate returns and makeup water falling to the surface water in the tank.
As the returning condensate and makeup water drop into the surface water, the steam bubbling up from narrow slits in the submerged preheat tube purges the last of the oxygen in the stored water.
The released oxygen and other non-condensable gases rise and escape through the vent at the top of the tank (Note: the vent should be piped to an area suitable for a small steam plume).
The deaerated water is stored until the boiler calls for water. It is then pumped from the bottom of the tank to the boiler.
If the volume of deaerated water pumped to boiler exceeds the volume of condensate returns, the water of level of the tank begins to fall.
The modulating level controller immediately detects any water level drop and sends signal to modulating valve to increase flow of makeup water.
The increased volume of cold makeup water causes the water temperature in the tank to decrease slightly.
The temperature sensor immediately detects the temperature decrease and sends a signal to the steam regulator to increase the flow of steam into the submerged preheat tube. (Note: the sensor must be installed below the water surface).
The increased steam flow raises water temperature back to normal.
When the modulating level controller senses that the water level is restored, it sends a signal to modulating valve to reduce flow of makeup water into tank.