Evaporation Rates ```Name: Pauline Y. Status: educator Age: 30s Location: N/A Country: N/A Date: 3/5/2004 ``` Question: If I have two towels which are soaked with different amounts of water, will the rate of evaporation differ? Replies: Pauline, Please allow me to narrow the focus a bit. Let us assume that you spread two identical towels out under identical environmental conditions and then poured one cup of water into the middle of one towel and two cups of water into the middle of the second towel. Also let's assume that none of the water dripped away -- all was absorbed by the towels. The "wicking effect" of the towels' fibrils will determine how far the water spreads. The greater the exposed surface, the faster the evaporation. In general. the overall rate of evaporation will depend on the surface area exposed to the drying effects of the environmental conditions -- temperature, humidity, presence or absence of adjacent air movement. This sounds like a perfect opportunity for an experiment. Regards, ProfHoff 818 NEWTON BBS receives many inquiries about "evaporation rates" of water. While it seems to be so simple, the rate of evaporation is a very complex phenomenon. It depends upon so many parameters that it is virtually impossible to give an unqualified satisfactory answer. A partial list of variables that affect the evaporation rate is: temperature, relative humidity, surface area, air speed, air direction, presence of solutes in the water, cooling produced by the evaporation, ... And this is a short list. Vince Calder Pauline, Yes, MOST ABSOLUTELY they will be DIFFERENT. Let us imagine two pieces of fabric. Both are identical in size, shape, chemical makeup etc. THESE TOWELS ARE IDENTICAL IN EVERY WAY. Case A: This towel is drenched to the point to where there is NOTHING BUT WATER encompassing the towel. By this, I mean that surrounding air only "sees" the water on not even a square inch of towel. This CASE A is the fastest of the evaporation rates and is CONSTANT, with the assumption that ambient conditions (air around the towel / water "system") remain constant. This means that the RELATIVE HUMIDITY, moving air, TEMPERATURE etc... REMAIN CONSTANT ... By the way, this constant ambient condition assumption must be made for both CASE A & CASE B, RIGHT? Otherwise, we are just comparing apples to oranges. Why? When the air is trying to "drag off" or evaporate water from the towel, it can only see 100% water and will therefore dry off at a much quicker rate, IN THE BEGINNING. This leads us into CASE B: When sufficient water has been evaporated to where the air can "see" a little bit of the towel. Now, things have changed, right? Why? Well, the same amount of air has access to a decreasing amount of exposed water embedded in the towel. Case B: OK NOW. The towel is dampened just to the point to where there is ONLY ENOUGH WATER to cover the towel (barely). But the air is in direct contact with an increasing amount of area of the towel. Now, if you were to look at it this way: Now imagine your HAND IS THE AIR and there are now 1000 MARBLES ( marbles represent the water molecules ) laying on the ground. How many marbles per time would you be able to grab if these marbles were placed in the following arrangement a.) marbles are placed 1 inch apart? b.) marbles are placed 2 inches apart? . . . z.) marbles are placed 26 inches apart? You know just from common sense that in [ a.) ] you will be able to pick up the most since they are the closest together and your hands and shoulders will only allow you to pick up the marbles at a given rate. You also know that in [ z.) ] you will be able to pick up the least because they are so far apart. This, in a very crude manner, represents the decreasing availability of water ready to be evaporated. Do you see how this analogy compares to the statistics associated with how water molecules become decreasingly available for evaporation with every passing moment of time of evaporation? This point is the key. So this towel is soaking wet and was just taken out of the washing machine and placed in the laundry dryer. In the beginning, of course, the evaporation RATE will be constant and at its HIGHEST. However, in time, as the amount of "sites" that water molecules are residing at is DECREASING, so is the evaporation rate. At the risk of repeating myself, this explanation shows the following evaporation rate vs. time plot: http://members.cox.net/newton_aas_dw/evap_rate.jpg I hope this has helped some. Regards, Darin Wagner Click here to return to the General Topics Archives

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