AbstractThe experience in the operation of atmospheric deaerators demonstrates that decreasing the bicarbonate alkalinity of deaerated water reduces the efficiency of deaerator operation. The available publications on the deaeration do not provide information on the causes of this reduction at a bicarbonate alkalinity of less than 0.2 mg-equiv/dm3. There is a problem of carbonic acid removal from the coolant at thermal power plants where steam condensate is used as feed water. AO Sibtechenergo’s specialists faced this problem during commissioning of atmospheric deaerators operating at a low bicarbonate alkalinity of the deaerated water. During commissioning of the deaerators, the content of carbonic acid in the deaerated water often changed with no apparent causes. On completion of the commissioning, carbonic acid had a lower content in the deaerated water and was even absent under certain operating conditions. An experimental deaerator where distillate with low bicarbonate alkalinity was used as deaerated water was tested. Carbonic acid was not removed from the deaerated water if its content did not exceed 3.0 mg/dm3 and the bicarbonate hardness was below 0.06 mg-equiv/dm3. Adding tap water to the investigated distillate in an amount of 0.22% decreased the content of carbonic acid in the deaerated water. With an increase in the amount of added tap water to 0.44%, carbonic acid was completely removed from the deaerated water. At thermal power plants with a closed steam-water path (circuit), carbonic acid can be removed from the closed circuit by increasing the steam vent from district water heaters and deaerators above the standard value.