Increasing Evaporation
Increasing Evaporation
(OP)
Right now we are evaporating waste water in a tank with steam powered heating coils running through it. The tank has a lid over it with a fan blowing air into it then out a pipe through the roof. This doesn't seem to be very efficient and I was thinking that running an air line into the tank to bubble up through the water would significantly increase the evaporation rate thus decreasing the time and heating energy required. We already have similar set ups in adjacent tanks for air agitation so it should be an easy addition. Will this help much or is it not significant enough to worry about.
Thanks in advance for your input
Thanks in advance for your input





RE: Increasing Evaporation
David
RE: Increasing Evaporation
RE: Increasing Evaporation
Boiling is evaporation but evaporation is not necessarily boiling. If you are trying to get rid of the water what do you care if it evaporates or if it boils?
Unless you are using preheated air, and air above the boiling point of the water, any air you put into the tank, below the surface or above the surface is removing heat that could be evaporating (boiling) water.
rmw
RE: Increasing Evaporation
The evaporation is "energy controlled"
Best regards
Morten
RE: Increasing Evaporation
1. Relative humidity of the surrounding air: the dryer is the air in contact with the water surface the higher is the evaporation rate.
2. The temperature of the surrounding air: the higher is the air temperature the higher the evaporation rate (as the relative humidity decreases with temperature)
3. Increase water surface.
4. Remove faster the air over the water surface.
So keeping in mind point 4, blowing air over the water surface should increase the evaporation rate.
RE: Increasing Evaporation
To ione,
Let's not forget that vaporization of water involves heat of evaporation with eventual cooling of the remaining mass, therefore heating is needed, in line with the inputs by zdas04 and MortenA.
RE: Increasing Evaporation
RE: Increasing Evaporation
"Right now we are evaporating waste water in a tank with steam powered heating coils running through it" - its forced evaporation! So unless the air that leaves is fully water saturated then IMHO it wont change adding additional air flow - and if the air is colder than the water then it will have adverse effect.
If the air is saturated then it must be assumed that the water is hotter than required - hence higher heat loss to ambient through e.g. vessel wall. then increasing air flow might help.
Best regards
Morten
RE: Increasing Evaporation
RE: Increasing Evaporation
If the "SC" in "SCTrojan" stands for "South Carolina," the air there can be pretty close to 100% humidity at times without adding additional moisture to it.
Patricia Lougheed
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RE: Increasing Evaporation
You've read my post the way I intended (so my English is not as bad as I thought). Thank for your contribution.
RE: Increasing Evaporation
If the incoming water is very hot, blowing air will get the water down to wet bulb temperature quickly but to evaporate the rest will require heat input.
RE: Increasing Evaporation
RE: Increasing Evaporation
Think about it again - the evaporation is caused by adding heat. Adding air thats colder than the liquid will slow evaporation!
Saturated air or not - where would the heat go - if not for evaporation - and where will the steam go?
OK - as i said - if the air is close to sat. then some energy will go to raise the heat of the water/air - and there will thus be more heat required for this and there will be additional energy loss to ambient.
Best regards
Morten
RE: Increasing Evaporation
Forced mixing of water and cooling air will allow evaporation to take place at a lower temperature, depending on the mixing air temperature and humidity. Sufficient air flow circulation is required to take away the humidified air and bring in freash cooling air.
Mixing can be accomplished by forcing the cooling air thru the water reservoir, generally efficient from a heat transfer standpoint, but high in blower energy consumption.
Mixing can usually be more efficiently accomplished by spraying the heated water into the air stream, (or onto large surface areas upon which the water impinges) which gives good surface area for efficient evaopration. This is similar to conventional evaporative cooling tower practice, and is a well established technology.
RE: Increasing Evaporation
The heat is added - and the vessel (i assume) is hotter than ambient!
Dosnt matter how you mix it - adding it will require energy to heat the air - if you eenrgy input is the limiting factor - then your evaporation will go down.
Best regards Morten