Email this pageSteam Partial Pressure #1: Why the Temperature Doesn’t Rise
The necessity of removing air from equipment
Have you ever had the experience of the required temperature never being achieved even though the pressure gauge on the steam space of the heat exchanger indicates the designated pressure? In such cases, it is possible that the steam pressure in the steam chamber has dropped. This can be explained by the law of partial pressure. In this article, we will discuss this partial pressure.
Dalton’s law of partial pressure
Dalton’s law (of partial pressure) states that:
If different types of gas are mixed together, the total pressure of the gas mixture equals the sum of the partial pressures of each type of gas.
PTotal = P1 + P2 + ... + Pn
This law plays a role in air binding in equipment. In the equipment most of us use, particularly that used for batch operations, before steam enters the equipment the steam space of the heat exchanger contains air. Steam is then delivered into the steam space, where it is heated. If the air is not properly removed and displaced by the steam, the steam space becomes filled with a mixture of air and steam. If the pressure of the steam space under these conditions is measured, it will be the pressure of the mixture of air and steam; the pressure shown on the pressure gauge will not be the steam pressure. If we apply Dalton’s law, we find that:
PTotal = P1 (Air) + P2 (Steam)
So in this case the pressure of the steam is lower than the pressure of the mixture.
The reading on the pressure gauge for the mixture might lead one to expect the temperature will match the saturation pressure, but the actual steam pressure is lower so the temperature will never get as high as the expected value. If one fails to realize there is a mixture of gases at play, a curious phenomenon seems to occur - the pressure gauge shows the designated pressure but the temperature just won’t rise. We also know that air is used as a heat insulator, which is because air conducts heat very poorly. In this regard, it is no exaggeration to say that removing air from the steam space is the first step in the effective use of steam.
More Information:
http://en.wikipedia.org/wiki/Daltons_law
http://members.aol.com/profchm/dalton.html
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