How Evaporative Cooling Works

In order to fully understand why the modern form of evaporative cooling is so viable even in the somewhat moist climatic conditions of the coastal regions of the Arabian Gulf and Red Sea, we must first explain Humidity (Absolute and Relative) and the difference between wet-bulb and dry-bulb temperatures.

Humidity: absolute and relative

Humidity is a complex and widely misunderstood topic. Many times the radio weather report mentions a high temperature and a maximum humidity, giving the false impression that relative humidity will be 90% or more when the temperature is above 40°C. As we will see, this is not the case, even in the most humid conditions here in the Gulf. In fact it is typically the opposite, when the temperature is at its maximum the relative humidity is at its lowest - this is why we only see mist or fog in the early morning or late evening and never in the middle of the day!

All atmospheric air contains water in gaseous form. The amount of water it contains is expressed as humidity. The most commonly used measure is relative humidity. When air is holding all the water it can, it is said to be saturated, and the relative humidity is 100%. If there is less than the maximum possible amount of water in the air, the relative humidity is expressed as a percentage of the amount it could contain. Thus, at 50% relative humidity, the air is holding half as much water as it would if it were saturated.

One very important fact to remember is that the amount of water air can hold depends on the temperature. If air at 50% relative humidity is cooled, its ability to hold water is reduced, and if it is cooled far enough, it can no longer hold all the water it contained at the higher temperature. This cooling is what causes condensation, dew, and even rainfall: the air can no longer hold all the water it could when it was warmer, and the excess “falls out.” In other words, any time air is cooled, the relative humidity increases, simply because the cool air can hold less water than it could when it was warmer. The amount of water in the air has not changed, but the water-holding capacity of the air is reduced. Conversely, when cool air is heated, the relative humidity decreases, even though the actual amount of water in the air is unchanged.

The actual amount of water contained in air is known as absolute humidity, expressed as mass of water per unit mass of air (kg H2O/kg Air.) On a given day, the absolute humidity varies only slightly; it is temperature change which is responsible for the wide variations in relative humidity, as the following example shows: Anyone who lives in the Gulf region is familiar with humid summer days. The early morning temperature can be as high as 30 degrees Celsius, and the relative humidity as much as 90%. Taking this rather extreme case, it is interesting to note that as the temperature rises to 40°C, the relative humidity has fallen below 50%, and at 44°C, to just 40%. Also, at these conditions, modern evaporative cooling can produce temperatures of just 31°C.

Temperature: wet-bulb and dry-bulb

The numbers in this example are derived using psychrometry, the science of measurement of humidity in atmospheric air. The conditions prevailing at any given time and location can be found by using an instrument known, appropriately, as a psychrometer. In its simplest form a psychrometer consists of nothing more than two ordinary thermometers, one of which has a wet sock wrapped around its mercury bulb. These two thermometers are therefore known as “wet-bulb” and “dry-bulb” thermometers. The dry bulb, of course, simply gives the normal temperature reading that is quoted in weather reports. The wet bulb, on the other hand, will show a lower temperature, due to the evaporation of water from the sock. The lower the humidity, the lower the wet-bulb temperature. Given these two temperatures, the Psychrometric Chart can then be consulted to give relative and absolute humidity, the dew point (the temperature which equates to 100% humidity) and several other more obscure quantities useful to the engineer. For the purposes of this discussion, however, the most important factors are the wet-bulb and dry-bulb temperatures.

The wet-bulb temperature, by its very nature, represents the limit of performance for evaporative cooling. After all, the wet bulb temperature is achieved by evaporation of water, and is the lowest possible temperature an evaporative cooler can produce. Actual performance of evaporative cooling depends on the effectiveness of the particular unit, expressed as a percentage of the possible temperature drop, which is the difference between the dry-bulb and wet-bulb readings.

The old-fashioned type of desert cooler mentioned earlier, with its grass mat or hemp pads, generally has an effectiveness of less than 20%. For this reason, such coolers are only useful in extremely dry conditions. In the example above, at 44°C with 40% RH, this type of cooler would only produce a temperature of about 41°C. We can readily see why the traditional form of evaporative cooling is often considered unsuitable for Gulf Region applications.

Modern technology, of course, has developed evaporative cooling to the point that any desired degree of effectiveness can be achieved, right up to 100%. Of course 100% effectiveness would also produce 100% relative humidity, which is not desirable in most situations. Modern evaporative coolers are therefore designed for effectiveness ratings of 60-80%, depending on the application, typically giving ambient relative humidity levels of 50-60%, the optimum range for people and equipment.

One caveat: conditions in the Gulf Region do make it vital that any evaporative cooling installation be carefully engineered - a poorly-designed system can prove to be ineffective.

Don't believe the radio!
Many times the radio weather report mentions a high temperature and a maximum humidity, giving the false impression that relative humidity will be 90% or more when the temperature is above 40°C. As we will see, this is not the case, even in the most humid conditions here in the Gulf. In fact it is typically the opposite, when the temperature is at its maximum the relative humidity is at its lowest - this is why we only see mist or fog in the early morning or late evening and never in the middle of the day!