Evaporative Cooling

Last post we talked about the psychrometric chart and the process that moist air follows on the chart when it is cooled until condensation begins.  You may be familiar with cooling systems that are variously termed “evaporative coolers”, “swamp coolers”, “desert coolers” and other names.  These systems work on something sort of like the inverse of the condensation process that we talked about last time.  Instead of cooling the moist air until liquid water condenses out, these systems evaporate liquid water into the air in order to cool it. 
On the psychrometric chart, the air follows a line of constant wet bulb temperature during an evaporative cooling process.  These lines go from lower right to upper left across the chart.

As the liquid water is evaporated into the air, the air increases in humidity ratio, and drops in temperature.  Obviously, no more water can be evaporated once the moist air reaches the line of 100% relative humidity.  So evaporative coolers work best in hot, dry climates (hence, the name “desert coolers”) because there is potentially a much larger temperature depression.  Immediately after leaving the cooler, the air will be at a much higher relative humidity than it started at, and may feel muggy—hence the name “swamp coolers”.

This figure shows the maximum possible temperature depression available as a function of the temperature and relative humidity of the incoming air.