Drying – Enter Evaporative Cooling . . .

In reading the last couple of blogs it would be easy for the reader to come to the conclusion that the addition of heat is all that is needed to dry things.  It would be nice and easy if that was the only consideration.  The fact is that evaporative cooling is working against us in our effort to increase temperature.  It’s time that we take a look at evapoative cooling and explore how we might possibly reduce its effect on the drying process.

Note – The therodynamic rigor for the following discussion is extremely complex. For those wishing to “crunch the numbers” there are a plenty of sources that will provide the tools to do that.  For our purpose here, it is the concept that is important – I’ll save you the detail for now.

Any time matter changes from one phase to another (solid to liquid, liquid to gas) there is heat either released or absorbed.  By heat, we mean BTUs, not temperature.  A BTU (British Thermal Unit) is defined as the amount of heat that is required to raise the temperature of one pound of liquid water one degree Fahrenheit at standard atmospheric pressure.  BTUs are what change temperature.  Add BTUs of heat and temperature rises.  Take BTUs away and temperature lowers.

Chart showing the change in temperature of water resulting from the introduction of BTUs of heat.
When BTUs of heat are added to water, the temperature increases. At two temperatures, when the water melts and when it turns to steam, the temperature stands still although BTUs are still being added. At these two temperatures, the water is said to be “changing phase.”

Although we commonly only think of phase changes at the melting and boiling points, the same rule applies whenever there is a phase change.  When water evaporates, it changes phase from a liquid to a gas.  As a consequence, BTUs are absorbed.  We are all familiar with evaporative cooling.  Our bodies employ evaporative cooling to maintain a constant body temperature.  Moisture is produced and evaporates from the surface of the skin to lower our body temperature on a hot day.  On and extremely hot and HUMID day, the body produces more and more moisture in attempt to produce the required cooling effect but, because of the high relative humidity, not all of the moisture is able to evaporate.  The excessive moisture is what we commonly refer to as “sweat.”

Although we commonly think of water changing from liquid to vapor only at the boiling point, phase change by evaporation occurs at any temperature, not just at the boiling temperature.

Now it gets interesting (and maddening) – – In earlier blogs we said that temperature (we now know that this means more BTUs) is good for drying because it decreases relative humidity of the air surrounding the parts and increases the vapor pressure of the water being dried from parts.  But as always happens in nature, there is a balance.  As it turns out, decreased relative humidity and increased vapor pressure both accelerate the rate of evaporation.  When the rate of evaporation is increased, the expenditure of BTUs is increased thereby creating a cooling effect.  So now we’re in a position where we must not only provide BTUs to decrease relative humidity and increase vapor pressure but also enough additional BTUs to overcome the effect of evaporative cooling resulting from the otherwise beneficial factors in drying process.

In the drying process, there are many factors in addition to relative humidity and vapor pressure that impact the amount of heat lost to evaporation.  In an upcoming blog, I’ll discuss some of these and how the effects of evaporative cooling might be minimized in the drying process to make it faster and more efficient.

–  FJF  –

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