Breakdown of the Drying Process.

 

First we must understand what our problem is. Our problem is after extraction we must now remove the water that has soaked into the building materials. In order to do this we must change the liquid water to water vapor (gas). This is called evaporation.

 

Evaporation

 

Evaporation of water is directly related to the temperature of the water in relationship to the atmospheric pressure. The higher the water temperature, the closer the waters vapor pressure is to the atmospheric pressure, therefore the faster the rate of evaporation. The boiling point of water is a good example of this. At sea level the boiling point of water is 212F. As you go higher in altitude the boiling point decreases because it takes less energy to match the atmospheric pressure (which is lower the higher you go). In Denver Colorado the boiling point is around 202F. The top of Mount Everest the boiling point is around 156F.

 

Basically we have two choices. Increase the water temperature closer to the atmospheric pressure or lower the atmospheric pressure to the water vapor pressure.

 

A good example of lowering atmospheric pressure is freeze drying of documents. Documents are frozen to stop water damage and mold. These frozen documents are then placed in a vacuum that lowers the atmospheric pressure. The pressure in the ice becomes greater than the low atmospheric pressure created by the vacuum. This allows the ice to evaporate into water vapor (gas) without turning into liquid first, drying the documents without further damage.

 

Here is why you always need heat. The energy/heat required changing liquid water to water vapor leaves with the departing water vapor (gas); the remaining liquid water now has less energy or is cooler in temperature. Less energy or lower temperature water equals a slower rate of evaporation. We must now add back at least the amount of energy/heat that was lost through the evaporation process or evaporation slows dramatically.

 

We do get some heat from the dehumidifiers and air-movers; however, not in the quantity we get from heaters or in the quantity necessary to overcome the energy loss from the wet materials.

 

The above relates only to Evaporation. In order to dry something we must also take into account Condensation. Very important.

 

Condensation

 

Condensation is always occurring. Hot humid days produce sweating sticky humans. This is not because evaporation stopped; it is because there are lots of water molecules in the air in relationship to the potential of water molecules that can be in the air at that given temperature. Basically the Relative Humidity is high.

 

When the molecules of water in the air run into your sweaty body, some of these water vapor molecules lose energy and turn back into water. When these water vapor molecules are turning back into water, at a rate close to the rate of evaporation, you have a low “net evaporation” ratio. “Evaporation minus Condensation equals Net Evaporation”.

 

The lower the Relative Humidity of the air the less condensation is occurring, allowing for a higher net evaporation. This is our goal.

 

So how does heat make dryer air?

 

If I gave you 70F air with 100% Relative Humidity and told you to go dry a building. You would likely tell me I was out of my mind. If I take this same air and heat the air to 125F I now have air with only 18-19% Relative Humidity. If I gave you 125F air with 18-19% Relative Humidity and told you to go dry a building. You would likely thank me for the great air I provided you. Bottom line is air temperature manipulation takes lousy air conditions for drying and turns it into great air for drying.

 

Another method for creating lower humidity is dehumidifiers. Unfortunately dehumidifiers lose efficiency and have problems working in elevated temperatures over 90F degrees. This limits us on how close we can get the waters vapor pressure through temperature increase in relationship to the atmospheric pressure, limiting the rate of evaporation. Tenting, containment, focus (IR Lamps), etc… can change some of the dynamics allowing for easier use with dehumidifier’s.

 

Fastest method of water vapor removal is air exchange. Example: 2000cfm fan with lay flat ducting will exchange the air in a 2500sf building with 9’ ceilings once every 11 ¼ minutes.

 

Air exchange technique allows you to heat the water to a higher temperature for maximum evaporation while moving the evaporated moisture out of the building fast. This limits an increase in Relative Humidity that would potentially increase condensation rates and lower the net evaporation.

 

Net Evaporation is the goal

 

 

So first we create a high rate of sustainable evaporation. Then we limit condensation. Now we have high net evaporation.

 

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