Heat exchange during evaporation
Heat exchange during evaporation
(OP)
Hi guys this is my first posting.
We are having a debate over on a window cleaning forum about water evaporation. Someone suggests that using hot water on a hot pane of glass creates less evaporation due to the fact that the heat exchange is much lower. And with cold water because of the heat exchange being more rapid more of it would evaporate. This came from a reputable company, and after doing some extensive searching on the Internet I could find no confirmation of this.
Any information or help on this would be appreciated.
Thanks,
Peter
We are having a debate over on a window cleaning forum about water evaporation. Someone suggests that using hot water on a hot pane of glass creates less evaporation due to the fact that the heat exchange is much lower. And with cold water because of the heat exchange being more rapid more of it would evaporate. This came from a reputable company, and after doing some extensive searching on the Internet I could find no confirmation of this.
Any information or help on this would be appreciated.
Thanks,
Peter





RE: Heat exchange during evaporation
Rates of evaporation are conditioned by many factors, among them: vapor pressure (as function of temperature), vapor diffusivity in air, air speed, air humidity, air temperature, exposed surface geometry and temperature, etc.
This link may prove to be useful:
http://van.physics.uiuc.edu/qa/listing.php?id=1440
RE: Heat exchange during evaporation
RE: Heat exchange during evaporation
RE: Heat exchange during evaporation
I had a look at the web link you gave me but I am afraid it is above me.
Given that these 4 points are the same...
1) The temperature of the water at the air-water surface
2) The humidity of the air
3) The area of the air-water surface
4) The temperature of the air
I read that the actual evaporation process can change some of the 4 points above, and accept that. But could say 1 Litre of cold water possible evaporate quicker than 1 Litre of hot water. BTW these panes of glass is only heated by the sun.
Thanks again,
Peter
RE: Heat exchange during evaporation
One thing I would like to add the water is purified. Would that make any difference? I shouldn't think so.
Peter
RE: Heat exchange during evaporation
1. the only heat source for evaporation comes from the warm glass pane kept at a temperature T, and heat is transferred by conduction
2. the starting water film thickness x is equal for the warmer and colder water at Tw, and the final thickness is zero
This is substantiated by the general transient heat transfer equation for time, t
When dividing the estimated times for cold and for hot water we obtain:
Where H=latent heat of evaporation; ρ= water density; k=water's thermal conductivity; c=cold; h=hot.
It is seen that although colder water has a greater H.ρ/k, its larger T-Tw may overcompensate and make t smaller.
RE: Heat exchange during evaporation
I2I
RE: Heat exchange during evaporation
It is assumed that natural air convection currents and undersaturation would remove the vapors as they form. It is also assumed that the latent heat of evaporation is supplied by the window pane (heated by the sun). If we assume also heat gain by solar radiation then the effects may be even enhanced.
RE: Heat exchange during evaporation
The answer would amaze anyone who studied heat transfer and thermo. and the reason had nothing to do with water properties.
RE: Heat exchange during evaporation
Best regards
Morten
RE: Heat exchange during evaporation
Peter
RE: Heat exchange during evaporation
I've read thread1088-156114 on the Mpmemba effect and couldn't find a satisfying explanation for its actually taking place.
From unsteady state heat transfer principles applied to freezing machines, the time t is estimated
where λ=latent heat of freezing; ρ=density of ice; k=thermal conductivity of ice, ΔT=temperature difference between unfrozen water and the chilling surface.
Since ΔT is greater for the warmer water (thus taking longer to freeze), the only other sufficiently important factor to affect time and offset the effect of ΔT is the size of the crystals, measured by x.
It appears that the better distribution of warmer water in the icecream mixture leads to smaller and more homogeneous ice crystals. This results in reduced chilling times.
I wonder whether I'm on the right track. Any comments ?
RE: Heat exchange during evaporation
A correcting comment: I remember the logic of the postings that stated the impossibility of the Mpmemba effect due to the fact that warm water must reach first the temperature of cold water, thus warm water would take longer to freeze.
It appears, however, that the time for heat transfer through ice predominates over that through the water phase.
The correct equation for the freezing time would be:
and although a larger ?T through warmer water prolongs the time for heat transfer, the influence of x2/ΔTice through ice is much stronger.
Still looking forward to receiving comments.
RE: Heat exchange during evaporation