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Deaerators in industrial steam systems are mechanical devices used to remove air and other dissolved gases from boiler feedwater in order to protect the system from corrosion. Dissolved oxygen in boiler feedwater will attach to the walls of metal piping and other metallic equipment and will form oxides (rust). It also combines with any dissolved carbon dioxide to form carbonic acid that causes further corrosion. A dissolved oxygen level of 5 parts per billion (PPB) or lower is needed to prevent corrosion in most high pressure boilers, accomplished by reducing the concentration of dissolved oxygen and carbon dioxide to a level where corrosion is minimized.
The two major types of deaerators are the tray type and the spray type. In both cases, the major portion of gas removal is accomplished by spraying cold makeup water into a steam environment.
Tray-type deaerating heaters release dissolved gases in the incoming water by reducing it to a fine spray as it cascades over several rows of trays. The steam that makes intimate contact with the water droplets then scrubs the dissolved gases by its counter-current flow. The steam heats the water to within 3-5º F of the steam saturation temperature and it should remove all but the very last traces of oxygen. The deaerated water then falls to the storage space below, where a steam blanket protects it from recontamination.
Nozzles and trays should be inspected regularly to insure that they are free of deposits and are in their proper position.
Spray-type deaerating heaters work on the same general philosophy as the tray-type, but differ in their operation. Spring-loaded nozzles located in the top of the unit spray the water into a steam atmosphere that heats it. The steam heats the water, and at the elevated temperature the solubility of oxygen is extremely low and most of the dissolved gases are removed from the system by venting. The spray will reduce the dissolved oxygen content to 20-50 ppb, while the scrubber or trays further reduce the oxygen content to approximately 7 ppb or less.
During normal operation, the vent valve must be open to maintain a continuous plume of vented vapors and steam at least 18 inches long. If this valve is throttled too much, air and non-condensable gases will accumulate in the deaerator. This is known as air blanketing and can be remedied by increasing the vent rate.
For optimum oxygen removal, the water in the storage section must be heated to within 5º F of the temperature of the steam at saturation conditions. From inlet to outlet, the water is deaerated in less than 10 seconds. Call us today is you have additional questions about deaerators and how important they are to your entire system.