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I read that it "is a positive pressure against the environment". But I am not too sure what that means.

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Saturation pressure is vapor pressure $p_{vap}$. The statement is saying to have vapor pressure greater than external (atmospheric) pressure $p_{vap} > p$. In a hypothetical system where a liquid is evaporating with no boundaries, we could evaporate with $p_{vap} = p$. In an actual evaporator with physical boundaries (i.e. a container), the evaporating liquid must "push out" existing vapor. To do so, the existing vapor must "flow out" of the container. Flow requires a pressure difference. Hence $p_{vap} > p$ for effective operation in real evaporators.

One point of confusion remains in the original statement. The phrase "atmospheric pressure" should be replaced by "applied pressure". Consider the applied pressure to first order as the pressure felt in the tank before evaporation starts.

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  • $\begingroup$ I've replaced the words "external pressure" by "applied pressure" and clarified a bit further. $\endgroup$ – Jeffrey J Weimer Aug 20 '18 at 21:25
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This isn't always true. Industrial low pressure ammonia vapor refrigeration was usually operated below external pressure so you didn't poison people or contaminate stuff in the event of a leak. It was relatively easy to remove atmospheric leakage into the systems, and it didn't pose any great threat to the system. With modern equipment, you don't want anything getting into the system. The systems are designed to be hermetically sealed and they don't suffer contamination well. Nobody wants to purge leaked atmosphere out of there refer system once a day like was common practice in the past. These old systems basically used to require an operator to run them, like old boilers and everything else back then. Old icemakers used to be a problem because many operated near atmospheric pressure on the low side, and if there was a high side leak, the low side would suck air and you lost your cool. This is why they all had little bubble windows in them.

See table 2.9 in the following FAO report on seawater ice making for fisheries. http://www.fao.org/docrep/006/Y5013E/y5013e05.htm

And as a practical matter, detecting a leak is easier if the low side is below atmospheric, but locating the leak is easier with positive pressure, and except for residential comfort air, you won't often be shutting a system down to go on a leak hunt.

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  • $\begingroup$ Even in this case, $p_{vap} > p_{applied}$ still applies for useful operation. It is now the case that $p_{applied} < p_{external}$, where $p_{external}$ is the pressure outside the container (presumably atmospheric pressure). $\endgroup$ – Jeffrey J Weimer Aug 20 '18 at 21:28

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