철강, 코크스, 화학공업, 보일러 등의 배가스는 대부분 분무 또는 습식탈황을 거쳐 배출되며, 온도는 45~80℃까지 떨어진다. 이때, 배가스는 포화된 습윤 배가스로서, 배가스에는 다량의 수증기가 포함되어 있으며, 여기에는 절삭염, 삼산화황, 겔분진, 미세분진 등(헤이즈의 중요한 구성성분 모두)이 포함되어 있다.
Smoke whitening refers to the removal of some moisture from the smoke before it is discharged into the atmosphere, in order to prevent the chimney from emitting white smoke and reduce its impact on the environment. Normally, smoke whitening involves first cooling and condensing the smoke, followed by heating it. The main component of the air flue gas whitening unit is the BXB plate heat exchanger. In the plate heat exchanger, ambient air is used to cool the flue gas, thereby precipitating water from the flue gas. Afterwards, the flue gas is reheated to increase its temperature, so that there will be no "white smoke" when the flue gas is discharged into the atmospheric environment.