How is it possible to cool the air below wet bulb temperature through evaporative cooling ?

Evaporative cooling is the oldest and most economical way of cooling at hot and dry regions. Humidity addition is the only major drawback of this technology. We all know that the maximum extent air can be cooled evaporatively is till the wet-bulb temperature of that area. But due to the presence of inherent design efficiencies of the system practically air can be cooled to a temperature of wet bulb + 2 to 3 Degree Celsius.

But now after the invention of two-stage indirect evaporative cooling, air temperatures below wet bulb temperature became practically possible. Indirect evaporative cooling is a sensible cooling process that uses evaporative cooling as the heat sink for removing sensible heat from the process air. These systems cool the primary air stream or process air stream with evaporatively cooled secondary air streams. Two streams are isolated from each other by means of a thin wall made of polymer or metal. Both air streams travel either opposite or at a right angle to each other on a continuous basis. Water is evaporated in the secondary stream, which cools the air and subsequently the wall material.

This saturated air is exhausted back to the ambient. On the other side of the wall, the process air or primary air stream losses the heat to the cooled wall and becomes cooler but remains as dry as it was to start with because no moisture was added to this stream. And in the second stage again this dry air is cooled down using traditional evaporative cooling and temperatures are further reduced to less than wet bulb temperatures.    

Plotting these processes on psychrometry will help us in understanding the concept of possibility to get air temperatures below wet bulb temperature through evaporative cooling. Initially ambient air passes through a heat exchanger which is sensibly cooled by spraying water on the same. Since there is no direct contact at this point between the air and water, humidity ratio remains constant or no humidity gets added to the air. Hence the process can be plotted as below.

In the above chart, we have taken initial ambient air at some point A and is cooled sensibly to point B. TA is the wet-bulb temperature of the air at A. TB is the wet-bulb temperature of the air at point B. 

Now in the second stage, this sensibly pre-cooled air comes in direct contact with water and gets evaporatively cooled. This process is drawn from B to C on the chart. Now TC is the wet-bulb temperature at point C.

Since the process from B to C is evaporative cooling, moisture addition takes place and process follows a constant wet-bulb line towards the saturation line. Now the TC will be wet bulb temperature at final point C which is equal to that of wet bulb temperature of the air at point B. Now it is clear than TC is less than TA. Here TA is the ambient wet-bulb temperature of inlet air and TC is the wet-bulb temperature of the air after two stages. Since the air stream is initially cooled in a sensible manner its wet-bulb temperature gets shifted to the left of the chart and is less than that of point A. Now if you drop vertical straight lines from the initial point and final point those are the dry bulb temperatures of air at those respective points and are denoted as DTA & DTC. It is clear that the dry bulb temperature of the air after two-stage cooling at point C ( DTC) is lesser than the wet-bulb temperature of ambient air or inlet air at point A (TA). Hence the value DTC is lower than that of TA. This proves that it is possible to cool the air below the wet-bulb temperature through evaporative cooling 

Suppose if the air is cooled directly with a simple single-stage evaporative cooling it would have theoretically achieved max TA or practically 3 or 4 degree higher than that due to the presence of inherent design inefficiencies.

An important point to note here is that this system works effectively at hot and dry climates only. A perfectly designed system can achieve best possible cooling else cooling to some extent nearer to that of wet bulb is possible at low kW/TR value and with less control on humidity.