95 Degree sweet spot

[kmdineen]

We know heating wet material will cause the water molecules to move faster and become water vapor. We also know if the vapor pressure of the wet material is greater than the vapor pressure of the air more water molecules will leave the wet surface than arrive and we will have evaporation. The greater the difference in vapor pressure between the wet surface and the air and a increased temperature at the boundary layer, the faster the evaporation takes place.
We also know air does not hold water. The idea that it is the air which determines the amount of water vapor which can be present through some sort of holding capacity is a eighteenth century idea which has shown to be false.
So once evaporation takes place the air temperature should be irrelevant as long as moisture is being purged from the affected area, but it is not. Net evaporation increases significantly with the ambient air temperature at 95 degrees instead of 70 degrees, why?
Evacuating moist air with the exhaust controller and air mover takes dehumidifier operating temperatures out of consideration. Does the 25 degree increase in temperature keep the water molecules moving fast enough so they don't fall back through the boundary layer? In theory is hotter better as long as the wet material is hotter than the air? Do we exhaust the room at 95 degrees do to worker and occupant safety? Do we exhaust the room at 95 degrees to keep the wet material temperatures from climbing to dangerous levels, off gassing voc's or voiding appliance warrantee's? Or is it just not necessary to increase the ambient air temperature past 95 degrees because there is enough energy at that temperature to keep all water molecules moving fast enough to stay suspended in the air?

 

[kmdineen]

Thanks Steve, but what I was looking for was if anyone knew why 95 degree ambient air worked best for drying. I asked this at a TES seminar and the instructor stated that "the air is really thirsty at 95 degrees", I was looking for something a little more scientific.
Here in CT the winters are very dry and the dehu's cause little grain depression after the first 24 hours of drying. I would like to use my E TES more often, exhaust or exchange moist air and eliminate the dehu on those jobs. However maintaining 95 degree ambient air temperature, using the E TES, is an issue when exchanging air.
So if air doesn't hold water and air doesn't get thirsty, why is 95 degrees the temperature you want in your drying chamber? Does anyone know?

PS the vapor pressure of the air is very close to the same at 70 or 95 degrees

 

[Desk Jockey]

I asked one of my supervisors who went to Jeremy's Reets drying school about the 95-temp.

He said that 95-degrees is what is taught in the class but that the 95-is a starting point.

He said you will need to do your calculations and depending on your evaporation you may need to increase or decrease the time when your exhaust comes on.

Do we exhaust the room at 95 degrees do to worker and occupant safety? Do we exhaust the room at 95 degrees to keep the wet material temperatures from climbing to dangerous levels, off gassing voc's or voiding appliance warrantee's?

I would think all the above!

 

[Craig Lindquist]

The reason why 95 is the sweet spot is primarily one of convection. If the ambient air was 70 and the affected surface was 110 evaporation would occur at a much faster rate. The problem that would be encountered is that because of the vast temperature difference, the affected surface would cool rapidily and would take a significant amount of time and energy to heat back up.

When the ambient air is held at 95 or around there, the temperature of the affected surface can be held with greater efficiency and evaporation can not only be rapid, but continuous.

I hope I worded that right so it can be understood. Basically, 95 is the best of both worlds.

 

[kmdineen]

Thanks for the replies. Using heat to dry requires a different mind set than the counting grains method. Using my E TES I realized that much of what I was taught about conventional drying involved how to make the dehumidifier work efficiently.
Craig's reply made a lot of sense. Not only about why 95 degrees is the sweet spot for heat drying, but also what to look for when purchasing heat drying systems. Will the system keep hot air blowing on wet material, at a constant temperature, while exhausting the moist air is something to look at when comparing systems.