Dew Point

The dew-point of flue gas is the temperature at which droplets of water will condense out of the vapour state. The dew point is dependent on a number of factors such as temperature, pressure, fuel type and sulphur content. There are two factors to consider with the dewpoint: In condensing boilers the flue gas temperature must fall to or below the dew point at the heat exchanger. This means that condensate will form inside the stack, which will require a liner to prevent corrosion or mould formation. In standard boilers the gases should exit the flue at or above the dew-point to avoid the formation of condensation in the flue. The presence of sulphur dioxide will increase the dew point very dramatically, which is just as well, since this avoids the formation of sulphurous acid in the stack. The dew point may be calculated after measurement of the sulphur dioxide content, or the moisture content of the flue gases may be directly assessed.