The concept of lowering the temperature of air by evaporating water in it is common knowledge. However, the process is adiabatic, meaning the total energy content of the air does not change. The evaporation of water requires energy, which it absorbs from the air. This cools the water, but the energy is still present now in the added vapour that increases the humidity in the room.

In figure 1, the air enters at 113F and is cooled to 90F while the moisture content goes up. The energy lost from the air is equal to the energy required for evaporation. So, the total energy content does not change. This is known as an ‘Adiabatic’ (No heat transfer) process.

Fig. 1 Psychrometric chart showing direct evaporation process

When an air cooler is operating in a poorly ventilated room, the increase in humidity reduces the body’s ability to remove its metabolic heat by perspiration. The problem is compounded by radiation from the walls and the ceiling. Therefore, an exhaust fan must also be provided to keep the humidity down. Also, direct exposure to any hot interior surface must also be avoided.

The feeling of comfort would be enhanced if some heat were to be removed before starting the water evaporation. This is called ‘Non-adiabatic’ (With heat transfer) process. By using a two-stage design, we could achieve the same temperature reduction with lower moisture content in the supplied air.

Figure 2 is a psychrometric chart showing the Two Stage process.

Fig.2 A Two-Stage Process

In the two-stage process, the upper line shows one air stream directly cooled by evaporation. This, in turn, partially cools the second stream, as shown in the horizontal line in the chart, through a heat exchanger. Notice that there is no increase in the moisture level up to this point. This is the non-adiabatic portion of the second air stream. The second stream then enters the direct, adiabatic stage, and is cooled to a supply temperature that is lower than the ambient wet bulb temperature. The upper stream is then discarded.

In this process, the horizontal line represents the “Indirect” stage and the slant line is the “Direct” stage. Hence it is called the “Indirect-Direct” process. Figure 3 is a drawing of the 2-stage cooler and figure 4 is a cross section of the heat exchanger.

Figure 3: Drawing of 2-stage cooler
Figure 4: Cross section of the heat exchanger

The Two-Stage Cooler

As the drawing in Figure 3 shows, the two-stage cooler comprises a single fan at one end that pushes the air for both the stages through the galvanized sheet metal heat exchanger in the middle. Here the two air streams pass through alternate channels. A pump (not shown), sprays water on the sides of one channel. The air and the sheet metal in that channel are directly cooled by the evaporating water. The metal then cools the indirect portion of the other stream by heat transfer. That air is then discarded. The cooled second air stream leaves the heat exchanger and enters the direct stage, which is a cellular cooling pad, Here, it is cooled by direct contact with water and enters the supply duct.


Figure 7 shows that this unit is working in the office of M/s Indigo Architects, Ahmedabad. It is installed at the roof and the air is ducted in, Both the unit and the duct are insulated in order to prevent any heat pickup from the exposed surfaces.

Fig. 5 Roof top installation
Fig. 6 Supply Duct

The Target

Figure 7 depicts the view of the cooled office space. The air enters from a grille near the top, next to the picture. It cools the entire double height space. The walls and the roof are designed such that they also resist the entry of heat through the structure. The cool surfaces also enhance the feeling of comfort provided by the cooling system that maintains conditions at about 32C and 60 per cent relative humidity at the peak of summer.


This product is not readily available in the market. Being hand-made, it requires skilled craftsmen, a rare commodity these days since they are being replaced by machines.