A dissolved oxygen level of 5 parts per billion (ppb) or lower is needed to prevent corrosion in most medium or high pressure (>200 pounds per square inch) boilers. While oxygen concentrations of up to 43 ppb may be tolerated in low-pressure boilers, equipment life is extended at little or no cost by limiting the oxygen concentration to 5 ppb. Dissolved carbon dioxide is essentially completely removed by the de-aerator.
Please understand that the main function of a de-aerator is to remove “Dissolved” gases not free air or free oxygen. The most important gases are oxygen & CO2.
How They Work
The design of an effective de-aeration system depends upon the amount of gases to be removed and the final oxygen gas concentration desired. This in turn depends upon the ratio of boiler feed water makeup to returned condensate and the operating pressure of the de-aerator.
Deaerators use steam to heat the water to the full saturation temperature corresponding to the steam pressure in the de-aerator and to scrub out and carry away dissolved gases. Steam flow may be parallel, cross, or counter to the water flow. The de-aerator consists of a de-aeration section, a storage tank, and a vent.
In the de-aeration section, steam bubbles through the water, both heating and agitating it. Steam is cooled by incoming water and condensed at the vent condenser. Non-condensable gases and some steam are released through the vent. Steam provided to the de-aerator provides physical stripping action and heats the mixture of returned condensate and boiler feed water makeup to saturation temperature. Most of the steam will condense, but a small fraction (usually 5% to 14%) must be vented to accommodate the stripping requirements.
Normal design practice is to calculate the steam required for heating and then make sure that the flow is sufficient for stripping as well. If the condensates return rate is high (>80%) and the condensate pressure is high in comparison to the de-aerator pressure, then very little steam is needed for heating and provisions may be made for condensing the surplus flash steam
De-aerator Steam Consumption
The de-aerator steam consumption is equal to the steam required to heat incoming water to its saturation temperature, plus the amount vented with the non-condensable gases, less any flashed steam from hot condensate or steam losses through failed traps.
The heat balance calculation is made with the incoming water at its lowest expected temperature. The vent rate is a function of de-aerator type, size (rated feed water capacity), and the amount of makeup water. The operating vent rate is at its maximum with the introduction of cold, oxygen-rich makeup water.
The de-aerator section and storage tank and all piping conveying hot water or steam should be adequately insulated to prevent the condensation of steam and loss of heat. This will reduce the steam consumption in de-aerator which is an additional cost to increase the life of equipment.
Sudden increases in free or “flash” steam can cause a spike in de-aerator vessel pressure, resulting in re-oxygenation of the feed water. A dedicated pressure-regulating valve should be provided to maintain the de-aerator at a constant pressure. This also helps in reducing steam consumption.
Deaerators provide the water storage capacity and the net positive suction head necessary at the boiler feed pump inlet. Returned condensate is mixed with makeup water within the de-aerator. Operating temperatures range from 215° to more than 350°F, which reduces the thermal shock on downstream preheating equipment and the boiler.