Potential for Evaporative Precoolers

In the last post we described a possible configuration for an evaporative precooler to lower total air conditioning operating costs at the expense of some additional capital costs (ducts, heat exchanger, evaporator) and perhaps a little additional maintenance costs on those items.   Of course, in order to make a decision on the payoff time, you have to know your climate conditions, your equipment costs, your operating costs, and the potential savings from the addition of the evaporative cooler.  In this post, we’ll provide a fuller description of the potential benefit you might be able to expect from an evaporative cooler for given inlet conditions.

Looking at this figure, it is easy to see that the total temperature depression (ΔT) achieved by the

evaporative cooler depends on how close you can drive the process toward saturated air (100% relative humidity).  It is not uncommon to assume that a relative humidity of 85% to 95% can be obtained with a typical evaporative cooling setup. 
Using a conservative value 85% as the maximum, we can form lines of constant temperature depression as shown here.

Each line corresponds to the starting point that will result in a given temperature depression (ΔT) assuming that the evaporative cooler can bring air to 85% relative humidity.
This function allows you to calculate the temperature depression from any arbitrary starting point under the assumption that the evaporative cooling process proceeds to 85% relative humidity.

where T is the temperature in deg F, and φ is the relative humidity in percentage.
Now, in real heat exchangers, not all of the temperature depression that is achieved with the evaporative cooler will be available for cooling the inlet air.  A common definition of heat exchanger effectiveness in terms of the temperatures used here might be:

where the numbers correspond to this setup from the last post:

As an example, suppose that we had outside air at 100 deg F, 25% relative humidity, and our heat exchanger effectiveness was 75%.  We could use the first equation above to determine that the temperature depression due to evaporative cooling would be approximately 25.3 deg F (1 to 2).  This would be the denominator in the effectiveness equation.  Then, using the effectiveness equation we determine that we could deliver precooled air to the chiller (4) at 81 deg F, rather than the original 100 deg F from using untreated outside air.