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Deaerators are mechanical devices that remove dissolved gases from boiler feed-water. Deaeration protects the steam system from the effects of corrosive gases. It accomplishes this by reducing the concentration of dissolved oxygen and carbon dioxide to a level where corrosion is minimised. A dissolved oxygen level of 5 parts per billion (ppb) or lower is needed to prevent corrosion in most high pressure ( >13.79 barg) boi lers. 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 deaerator.
The design of an effective deaeration system depends upon the amount of gases to be removed and the final gas (O₂) concentration desired. This in turn depends upon the ratio of boiler feed- water makeup to returned condensate and the operating pressure of the deaerator.
Deaerators use steam to heat the water to the full saturation temperature corresponding to the steam pressure in the deaerator and to scrub out and carry away dissolved gases. Steam flow may be parallel, cross, or counter to the water flow. The deaerator consists of a deaeration section, a storage tank, and a vent. In the deaeration section, steam bubbles through the water, both heating and agitating it. Steam i s cooled by incoming water and condensed at the vent condenser. Non-condensable gases and some steam are released through the vent. However, this should be optimised to provide satisfactory stripping, with minimised steam loss ( see Operational data, below).
Sudden increases in free or 'flash' steam can cause a spike in deaerator vessel pressure, resulting in re-oxygenation of the feed-water. A dedicated pressure regulating valve should be provided to maintain the deaerator at a constant pressure.

除氣器是一種用來除去鍋爐飼水中各種可溶解氣體，以保護蒸汽系統，避免氣體在其中產生腐蝕效果；其做法是降低水中含氧量及CO₂的量以達到蒸汽系統防蝕的目的。
在高壓鍋爐(>13.79 barg)系統中，為了防止腐蝕，最多必要保持5ppb以下的溶氧水準；而在低壓鍋爐系統中，溶氧濃度在43ppb以下還能容忍。限制溶氧濃度在5ppb以下，設備使用壽年可以延長或不必花太多維護成本。
基本上，溶解於鍋爐飼水中的CO₂經過除氣器後可以完全移除。
一套具有效率的除氣系統應視要除去的氣體量和需要保存的含氧濃度而設計；換句話說，要將鍋爐飼水補充至回流的凝結水量和除氣器的操作壓力列入設計考量。
除氣器的作用原理是以蒸汽加熱，使水達到當時除氣器中蒸汽壓力相對的飽和溫度，而把溶解其中的氣體連同蒸汽一併帶出，其流向可以與水的流向平行交叉或對沖。
整個除氣器可分為除氣器儲存槽及一套通氣管。在除氣器中部分蒸汽泡泡通過水翻攪的熱水中，此時蒸汽冷卻凝結，在通氣管的凝結器中，不會凝結的氣體及部分蒸汽則通過排放管排放。在此過程中必須有效脫氣，同時減少蒸汽損失。
蒸汽突然增加或閃蒸會造成除氣器槽壓突然升高，造成飼水再溶氧動作，所以要有一專用的調壓閥門維持除氣器的壓力值。