Air Conditioning With A "Swamp Cooler": the Psychrometric Chart

 As the climate heats up and energy resources become more and more stressed, interest in alternative approaches to compressor-based A/C has increased.  A lot.  The basis of many alternatives circulates around evaporative cooling technology, most commonly referred to as the swamp cooler.

As a kid I remember swamp coolers in two different situations.  For some time we lived in the Four Corners area, where Colorado, Utah, Arizona and New Mexico meet at one point.  The area is high desert, hot and dry in the summer; and that's perfect for the old-style swamp cooler.  The version we had in our house took hot and dry air from the outside and blew it through a water-soaked membrane.  Evaporation occurred, which cooled the air and also added some welcome humidity to the air, which usually had a very low relative humidity.  The cooled and wetter air was blown into the house using the same heater ducts that were used to heat the house in the wintertime.

The other situation was during summertime visits to relatives in Oklahoma.  At that time compressor-type A/C was expensive, so they couldn't afford to get that.  So they used swamp coolers there, too.  But in that case, while the air temperature was comparable to what we got in the Four Corners area, the humidity was much higher.  In that case, the swamp cooler was less effective for two reasons.  First, the high humidity reduced the amount of evaporation that could occur in the swamp cooler.  The second has to do with our perception of comfort.  When in a high-humidity environment WE also are less able to cool ourselves, because our sweat is less able to evaporate.  The end result was that the swamp cooler in Oklahoma really didn't make me feel any cooler than just staying outside in the shade, hoping for a bit of wind to come by.

This is where the Psychrometric Chart comes in, to help us understand what's happening.  I'm working on an upcoming post where I (hopefully) explain how a relatively new evaporative cooling technology based on something called the "Maisotsenko Cycle" works, and how a version of it can be relatively-easily added to your basic evaporative cooler, to significantly improve its performance; and the explanation heavily depends on use of the Psychrometric Chart.

Anyway, back to our simple swamp cooler.  Today I measured the exterior peak temperature and ambient relative humidity and got 32.2C (just shy of 90F) and about 37% relative humidity.  I plotted that point on a copy of my Psychrometric Chart and it looks like this:

The vertical axis is the temperature, but the relative humidity curves are the upward-trending ones as you look from left to right.  The dark point shows the conditions at our house.  Another set of curves are straight lines that go down from left to right, not quite at a 45 degree slope.  Those are lines of constant wet-bulb temperature, and give the temperature of the water in the swamp cooler membrane.  In this case we get about 70 degrees Fahrenheit.  That sounds pretty good, dropping the exterior air temperature down to 70F:  but that's the temperature that the WATER gets to.  My previous experiments with a home-brew swamp cooler show that the exit air temperature can be ten degrees higher than that, perhaps more if the air flow is excessive.  This means that the air temperature coming of out my swamp cooler might be about 80F.  Better than 90, but that wet-bulb temperature sure sounds better.

Some may wonder why the air and water temperatures aren't the same.  I think that's because the air carries off the heat extracted from the evaporating water.  Also based on my experiments, excess air flow also can be a factor.

The difference between the wet-bulb temperature of 70F and the exit air temperature makes the use of a slightly-more complicated system attractive.  That's an indirect evaporative cooler, where the chilled water is piped into the interior space and passed through an air-water heat exchanger.  In this case, the water also is continuously circulated through the evaporative cooler because we want to use the chilled water to cool the house.  A level sensor detects when the water level in the chiller falls to the point where it needs to be topped-up.

In either case, once the exterior relative humidity rises above about 50% they become pretty ineffective as an A/C system.  But there is a way to get the water in the chiller colder, approaching the dew point temperature (rather than the higher wet-bulb temperature).  In the case of my example, the difference is about 5 degrees C,  which would produce water at about 60F, ten degrees colder yet.  More on that in another post, which includes a more in-depth exploration of the Psychometric Chart.

Some may wonder why the wet-bulb temperature is higher than the dew point.  I think that is because there are two effects that are in equilibrium at the wet bulb temperature.  The first is the heat extracted from water by evaporation.  The second is the heat contained in the air being transferred to the wet bulb.