[AlmostThere]

Hi guys, I'm pretty green at mass transfer and have a pretty simple question that I don't know the relation for.

I need to calculate the minimum air volumetric flowrate for a cooling tower scenario, which boils down to some basic mass transfer.

The minimum air volumetric flowrate occurs at 100% saturation of the air with water vapour. 

If I have air at the conditions of dry bulb temp 22.8 degC, wet bulb temp 13.3 degC and humidity 38% flowing countercurrently with a water stream flowing at 75 kg/h at 31.3 degC, what is the minimum volumetric air flowrate?

I hope I'm not missing any important information...

[typhoon2028]

Please start with a flow diagram.  Label your flows and properties of each flow.

[AlmostThere]

Here's one I made.  I forgot to say that the exit temperature of the air is the same as the inlet water.

The numbers are not as important as the equation/method to solve this.

The mass flowrate of water is actually not constant... because water transfers into the air.

The loss by evaporation can be calculated as E = 0.00085 * Constant Water Mass Flowrate * ΔT of the water in/out (10degC in this case).
This loss by evaporation is made up by another supplemental water stream that's part of the cooling tower.
This is why the water flowrate doesn't change in and out of the tower although water has evaporated into the air.

[typhoon2028]

Your diagram actually depicts a cross flow, but can we work with it.

You left out the 38% humidity in the incoming air (into the cooling tower).

To start, you should assume a steady-state process.

The mass flowrate of water is actually not constant... because water transfers into the air.

I do not believe this to be true.  At 100% humidity, you should be able to calculate the percentage of water in the air exit stream.  This amount minus the original water content in the 38% humidity incoming air is the total water removed from the cooling tower.

Remember to balance your flows in and your flows out.