By Controlled Environments
Created 2010-10-25 19:37
For most managers of critical cleaning operations, drying is synonymous with evaporation.
YOU HAVE TO HEAT THE PART, TOO
Consider this example of evaporative drying of water, which should be an easy task. Assume: a one quart stainless steel saucepan, half-full of water, on an electric stove. It is desired to evaporate all that water in five minutes.
The energy demand to evaporate this water is equivalent to one ton of refrigeration (12,000 BTU/hr). But since it is necessary to heat the stainless steel saucepan as well, to evaporate the contained water, the energy requirement is equivalent to the refrigeration requirement to cool a large home.
In other words, not only does water have a huge heat of vaporization vs. solvents (~1,000 BTU/lb vs. ~250 BTU/lb), but one can’t just evaporate the water from surfaces without wasting heat in heating the surface material too.
AIR AND WATER
Drying generally means evaporation of water. It takes a lot of energy, and a lot of time, to evaporate a little water. The rate of drying parts is limited by the rate at which heat can be transferred from hot air to the water, causing it to evaporate. Slow heat transfer from heated air to wet parts is normally the rate-limiting process step. Even worse, air doesn’t have a high capacity to carry heat or water. Consequently, huge volumes of hot air can be required.
Two factors affect that rate of removal:
THE DOMINANT FACTOR
The dominant factor is air temperature; and not air velocity.
Hotter supply air, at the same linear air velocity, evaporates the water films much more quickly, and therefore the cost of drying is lowered. This is a “double win,” as drying cycle time is shortened simultaneously with the power cost being decreased. Yes, the increase of air velocity does substantially shorten drying cycle times. But because all that extra air has to be heated, power costs increase somewhat.
THE CONCLUSION
It should be clear: dry parts with heated air at the highest temperature which does not cause part damage or affect material handling after drying. Dry parts with air flow at the value which produces the required quality of dryness at the required time.
In a subsequent column we will cover non-evaporative drying, including Marangoni drying systems.
John Durkee is the author of the book Management of Industrial Cleaning Technology and Processes, published by Elsevier (ISBN 0- 0804-48887). He is the author of the forthcoming book Solvent Cleaning for the 21st Century, also to be published by Elsevier, and is an independent consultant specializing in critical cleaning. You can contact him at PO Box 847, Hunt, TX 78024 or 122 Ridge Road West, Hunt, TX 78024; 830-238-7610; Fax 612-677-3170; or jdurkee@precisioncleaning.com.
YOU HAVE TO HEAT THE PART, TOO
Consider this example of evaporative drying of water, which should be an easy task. Assume: a one quart stainless steel saucepan, half-full of water, on an electric stove. It is desired to evaporate all that water in five minutes.
The energy demand to evaporate this water is equivalent to one ton of refrigeration (12,000 BTU/hr). But since it is necessary to heat the stainless steel saucepan as well, to evaporate the contained water, the energy requirement is equivalent to the refrigeration requirement to cool a large home.
In other words, not only does water have a huge heat of vaporization vs. solvents (~1,000 BTU/lb vs. ~250 BTU/lb), but one can’t just evaporate the water from surfaces without wasting heat in heating the surface material too.
AIR AND WATER
Drying generally means evaporation of water. It takes a lot of energy, and a lot of time, to evaporate a little water. The rate of drying parts is limited by the rate at which heat can be transferred from hot air to the water, causing it to evaporate. Slow heat transfer from heated air to wet parts is normally the rate-limiting process step. Even worse, air doesn’t have a high capacity to carry heat or water. Consequently, huge volumes of hot air can be required.
Two factors affect that rate of removal:
- The temperature of the hot air. Higher air temperatures produce higher rates of heat transfer because the rate of heat transfer is proportional to the difference between the temperature of the hot air and the temperature of the cold (less hot) water films covering the part surfaces.
- The velocity of the hot air as it moves across the part surface. Higher air velocities produce higher rates of heat transfer—chiefly by increasing the proportionality constant between temperature difference and heat transfer rate.
THE DOMINANT FACTOR
The dominant factor is air temperature; and not air velocity.
Hotter supply air, at the same linear air velocity, evaporates the water films much more quickly, and therefore the cost of drying is lowered. This is a “double win,” as drying cycle time is shortened simultaneously with the power cost being decreased. Yes, the increase of air velocity does substantially shorten drying cycle times. But because all that extra air has to be heated, power costs increase somewhat.
THE CONCLUSION
It should be clear: dry parts with heated air at the highest temperature which does not cause part damage or affect material handling after drying. Dry parts with air flow at the value which produces the required quality of dryness at the required time.
In a subsequent column we will cover non-evaporative drying, including Marangoni drying systems.
John Durkee is the author of the book Management of Industrial Cleaning Technology and Processes, published by Elsevier (ISBN 0- 0804-48887). He is the author of the forthcoming book Solvent Cleaning for the 21st Century, also to be published by Elsevier, and is an independent consultant specializing in critical cleaning. You can contact him at PO Box 847, Hunt, TX 78024 or 122 Ridge Road West, Hunt, TX 78024; 830-238-7610; Fax 612-677-3170; or jdurkee@precisioncleaning.com.
