Air affects steam quality too! What I am not sure about is how this affects the heat of vapourisation. If the amount of air mixed in with the steam is not known, could this lead to an over estimate or an under estimate of the heat of vapourisation?
Anyway, don't you think a humidity probe would be an appropriate means determining how much air is in the steam? Harry http://www.spiraxsarco.com/resources/steam-engineering-tutorials/steam-engineering-principles-and-heat-transfer/steam-quality.asp from this page on Steam Quality Steam should be available at the point of use: * In the correct quantity * At the correct temperature and pressure * Free from air and incondensable gases * Clean * Dry Air can also enter the system in solution with the boiler feedwater. Make-up water and condensate, exposed to the atmosphere, will readily absorb nitrogen, oxygen and carbon dioxide: the main components of atmospheric air. When the water is heated in the boiler, these gases are released with the steam and carried into the distribution system. Atmospheric air consists of 78% nitrogen, 21% oxygen and 0.03% carbon dioxide, by volume analysis. However, the solubility of oxygen is roughly twice that of nitrogen, whilst carbon dioxide has a solubility roughly 30 times greater than oxygen! This means that 'air' dissolved in the boiler feedwater will contain much larger proportions of carbon dioxide and oxygen: both of which cause corrosion in the boiler and the pipework. The temperature of the feedtank is maintained at a temperature typically no less than 80°C so that oxygen and carbon dioxide can be liberated back to the atmosphere, as the solubility of these dissolved gases decreases with increasing temperature. The concentration of dissolved carbon dioxide is also kept to a minimum by demineralising and degassing the make-up water at the external water treatment stage. The concentration of dissolved gas in the water can be determined using Henry's Law. This states that the mass of gas that can be dissolved by a given volume of liquid is directly proportional to the partial pressure of the gas. This is only true however if the temperature is constant, and there is no chemical reaction between the liquid and the gas.
