Well, perhaps a bit more clarification. The evaporation of non-atomized water is not, strictly speaking, the only mechanism for moist air enthalpy change. The enthalpy (or energy content) of moist air at pressure Pb, temperature t (°C) and mixing ratio r (g H20 / kg dry air) is defined by: h [kJ / kg moist] = 1.00464 t + 0.001846 r x t + 2.5 r Note: by convention, the enthalpy of dry air ( r = 0 ) at 0°C is equal to zero. Negative values of enthalpy are actually possible and indicate that the energy content of the air / vapor mixture is less than the energy content of dry air at 0°C. From this it can clearly be seen that as the water content (truly gaseous water) increases, the enthalpy of the resulting mixture increases. That is, it contains (or can contain) a greater amount of heat. So, *for any given pressure*, the higher the humidity, the higher the heat transfer capacity will be. However, as has been said, the higher the humidity, the less dense the air. So, there are a couple of competing things happening here; 1. As the humidity increases, the enthalpy, and heat transfer efficiency, increases for any given pressure. 2. As the humidity increases, the density (and hence pressure) decreases. The enthalpy changes can be readily obtained from the equation above, or using a standard psychrometric chart. The pressure change is not as easy to calculate, since unfortunately PV=nRT applies to ideal gases only. And, while saturated steam can *almost* be considered ideal, water vapor cannot. Primarily due to the issues Frank raised about non-atomized clusters, and combinability issues. Keith Hughes '86 Westy Tiico "Marvin"
> Date: Fri, 28 Jun 2002 23:32:29 +0100 > From: Clive Smith <clive.harman-smith@NTLWORLD.COM> > Subject: Re: Humid Air - Some Comments > > Absolutely wonderful > > And talking of wonderment, I wonder if its the last word? |
Please note - During the past 17 years of operation, several gigabytes of
Vanagon mail messages have been archived. Searching the entire collection
will take up to five minutes to complete. Please be patient!
Return to the archives @ gerry.vanagon.com
The vanagon mailing list archives are copyright (c) 1994-2011, and may not be reproduced without the express written permission of the list administrators. Posting messages to this mailing list grants a license to the mailing list administrators to reproduce the message in a compilation, either printed or electronic. All compilations will be not-for-profit, with any excess proceeds going to the Vanagon mailing list.
Any profits from list compilations go exclusively towards the management and operation of the Vanagon mailing list and vanagon mailing list web site.