An Evaporative Pre-cooler

We talked in an earlier post about evaporative cooling and how you can use it to cool off air by simply evaporating water.  Evaporative coolers work best in dry climates, and are much cheaper to operate than vapor compression systems since there is no electricity-hungry compressor.  Unfortunately, they result in a large increase in the relative humidity of the air which may render it unacceptable for some indoor air applications.  In this post we’ll talk about how evaporative coolers are used in combination with other air conditioning equipment to lower the overall cost of providing conditioned air.
This figure shows one way that such a system might be configured.  

Outside air (1) is cooled with an evaporative cooler (1-2) which lowers its temperature but significantly increases the relative humidity.  
The resulting cooler, humid air is passed through a heat exchanger (2-3) in order to cool the inlet air (1’-4) that is going to a regular chiller (4-SA). 
The various processes are shown here on a psychrometric chart.

The evaporative cooling process (1-2) follows a line of constant wet bulb temperature.  The evaporatively cooled air is then warmed in the heat exchanger (2-3) while the process air (1’-4)  is cooled off.  The air at (3) is then discarded back to the atmosphere. 
The benefit of this system can be seen by comparing the load on the chiller to bring air from (4) to (SA):

versus bringing air from (1’) to (SA) 

which is what we would need in the absence of the evaporative precooler.  In some climates, this can result in considerable savings in operating costs for the air conditioning system.  The price of this improvement is the capital and maintenance cost of additional ductwork, evaporative cooler, and heat exchanger.