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Formula for Evaporation loss of water

12/25/2007 11:37 AM

Does anyone have a formula for the evaporation of water given 1) surface area, 2) barometric pressure, 3) ambient temperature above the surface of the water, 4) temperature of the water, 5) humidity of the air above the water? For the temps, either f or C will do. For area, either feet or meters (square) will do. For barometric pressure, I know Inches of Mercury, rather than millibars, but if you have the conversion formula, its time I commit that one to memory also.

I'm not looking for 12 decimal place accuracy, just a good value for calculation of water volume loss over time in a fairly large (for a Jacuzzi, anyway) pool of water that seems to require replenishment too often to account for by evaporation alone. But since it is heated 24/7 to 90-92 degrees f, and the building it is in cycles from ambient outdoor to 70 degrees f every 24 hours, with humidity staying in the 20-30% range, I can't assume away evaporation as the cause. I can plug in all the variables, including the changes, using averages over a 24 period, and averaging for the year, so those inaccuracies can be fairly well accounted for.

Thanks

Micah

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#1

Re: Formula for Evaporation loss of water

12/25/2007 5:16 PM

I don't have any formulas for you, but a good place to check is: http://www.awwa.org/

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#3
In reply to #1

Re: Formula for Evaporation loss of water

12/25/2007 8:22 PM

Kilowatt0:

Thanks. I'll check it out.

Micah

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#2

Re: Formula for Evaporation loss of water

12/25/2007 6:03 PM

micahd02:

1-- go to this website www.air-dispersion.com/source.html
2-- select whether you want SI metric units or customary USA units
3-- select Evaporation From A Non-Boiling Pool and you will find three different methods for finding the evaporation rate from a liquid pool of any liquid

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#6
In reply to #2

Re: Formula for Evaporation loss of water

12/26/2007 10:13 AM

Nice!

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#4

Re: Formula for Evaporation loss of water

12/25/2007 8:44 PM

I don't have an equation but back in my school days we weighed a specific amount and waited a few hours and went back and re-weighed the container of water and then determined that it was x grams/min. or something like that.

Everything was tabulated:

Size and weight of container, amount of water, Water Temp., ambient Temp. , etc...

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#5

Re: Formula for Evaporation loss of water

12/25/2007 10:58 PM

Power plant cooling towers have water cascading over them much like a hot tub with the water jets running,

the make-up water is expected to be about 2 percent of the system volume per day as I recall.

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#7

Re: Formula for Evaporation loss of water

12/26/2007 11:08 AM

My Dad is 76 yrs old; in a conversation with him over the phone he said that he recalled seeng that in a Refrig/AC. Tech Book! Check them out!

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#8

Re: Formula for Evaporation loss of water

12/26/2007 12:39 PM

From Wiki pedia...

Factors influencing the rate of evaporation

Concentration of the substance evaporating in the air: If the air already has a high concentration of the substance evaporating, then the given substance will evaporate more slowly.

Concentration of other substances in the air: If the air is already saturated with other substances, it can have a lower capacity for the substance evaporating.

Flow rate of air: This is in part related to the concentration points above. If fresh air is moving over the substance all the time, then the concentration of the substance in the air is less likely to go up with time, thus encouraging faster evaporation. This is result of the boundary layer at the evaporation surface decreasing with flow velocity, decreasing the diffusion distance in the stagnant layer.

Concentration of other substances in the liquid: If the liquid contains other substances, it will have a lower capacity for evaporation.

Temperature of the substance: If the substance is hotter, then evaporation will be faster.

Inter-molecular forces: The stronger the forces keeping the molecules together in the liquid state the more energy that must be input in order to evaporate them.

Surface Area: A substance which has a larger surface area will evaporate faster as there are more surface molecules which are able to escape.

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#9

Re: Formula for Evaporation loss of water

12/26/2007 1:15 PM

This is a hemical engineering calculation problem. Would suggest you start with looking at Bennett and Meyers " Momentum, Heat and Mass Transfer" by McGraw-Hill Book Company. [Another factor that you will need is the velocity of the air past the surface, since the instant you evaporate water into the air above the surface the humidity above the surface increases.]

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#10

Re: Formula for Evaporation loss of water

01/02/2008 4:48 AM

It looks as though the effect of turbulence in the water and in the air above it, and also splashing caused by humans entering and leaving it, will have a significant influence as well! It also looks as though any formula will need to be determined by local experimentation.

Or is this the sort of thing that would support a research graduate doing a PhD thesis? What's the value of the information and would such an approach be worthwhile? It's possible to imagine a queue of applicants for this one, especially if cocktails were thrown in.......

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#11

Re: Formula for Evaporation loss of water

01/02/2008 9:45 AM

PWSlack makes a very good point about the water lost as bathers enter and exit the water. Think about all the water a swim suit picks up.

Don't be overly concerned unless the water level is rising -- ie: Welcome to my ool. Notice there is no "P" in it. Please keep it that way.

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