Increasing Evaporation

[SCTrojan09]

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

 

[zdas04]

It sounds like you are using the steam coils to warm the water to try to increase the partial pressure of water vapor at the water surface. The process is controlled by the rate that you can supply the latent heat of vaporization. Blowing air through the water will affect the surface tension and reduce the amount of heat required slightly, but it is not going to be a huge difference. If you have compressed air to blow into the process, is there anyway to pull it off the compressor upstream of the after cooler? That could add a bunch of BTU's for free.

David

 

[SCTrojan09]

The air available to bubble through the liquid comes from a high volume blower as opposed to a compressor so there's not really much heat to use from that. The steam supply is currently boiling the water off for the most part rather than just heating it up and allowing it to evaporate.

 

[rmw]

Huh??

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

 

[MortenA]

If its already boiling then blowing air through the water wont help - or may actually have adverse effect as rmw write.

The evaporation is "energy controlled"

Best regards

Morten

 

[ione]

If we are talking about evaporation (a phenomenon involving only the free surface of water), there are some factors to promote it:

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.

 

[25362]

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.

 

[ione]

I consider heating as a mandatory requirement (no doubt on this), and just wanted to point out other factors which could promote evaporation rate: I think those listed in my previous post are undeniable.

 

[MortenA]

read the initial post!

"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

 

[ione]

It seemed to me it was obvious that saturated air above water surface must be removed (knowing what saturation means is enough to get this point): if air is already saturated no way to increase relative humidity (keeping constant pressure and temperature), that is no way to force other water to evaporate and migrate into air.

 

[vpl]

The way I read ione's post was to increase the air circulation above the tank (i.e. the surface air flow rate not to add air into the water/steam mixture as SCTrojan was thinking. Just from a theoretical viewpoint, having a higher circulation of surface air would decrease the chance for any of the evaporated/boiled water to recondense back into the tank.

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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[ione]

Patricia,

You’ve read my post the way I intended (so my English is not as bad as I thought). Thank for your contribution.

 

[danberry]

What governs the evaporation rate is the heat from the steam. While blowing air might lower the water temperature, it will not increase the evaporation rate significantly. you still need to provide the heat via the steam coil.

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.

 

[chicopee]

Instead of blowing air over that "soup" wouldn't it be better to create a partial vacuum under the lid with an exhaust fan?

 

[MortenA]

vlp/ione

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

 

[FredRosse]

To evaporate 1 pound of water, about 1000 BTU's of energy must be brought into the water. This energy balance applies to all options.

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.

 

[MortenA]

It is NOT a cooling tower - its an evaporator!

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