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Steam Quality  Controller

What is Steam Quality?   The “quality” of the steam refers to the proportion of water being in gaseous phase (steam) and in liquid phase in the steam actually flowing in the pipes. For example, a steam quality of 97% means that 97% of the mass of the steam is in gaseous state and 3% in liquid state. You might think that the steam traps installed along your steam network are taking care of removing that liquid but it is not the case. The liquid affecting the steam quality is in the form of very, very small droplets of water that are floating within the steam and being entrained with it. Bigger droplets (bigger than 1 micron) can be removed using steam separators, steam filters or steam coalescers but the smaller droplets will get through.

How to guarantee Steam Quality?  To improve the steam quality means to evaporate the liquid present in the steam to increase the gas/liquid mass ratio. You will need energy to do so since the change of phase from liquid to gas is significantly endothermic for water and requires about 970 BTU/lb at atmospheric pressure and about 905 BRU/lb at 60 psig. But how can we know that all the liquid phase present has actually been transformed into gas? The answer is: the temperature. The temperature of the liquid phase and the steam phase of saturated steam is the same at any given pressure. All the energy you are adding will evaporate the remaining liquid phase until no liquid remains and only gas is present. Assuming a constant steam pressure, this is the point where the temperature of the steam will increase and therefore becoming superheated steam. In others words, if you know you have a couple degrees of superheat to your steam, you also know that you have a 100% quality steam.

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