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Water that leaves a direct evaporative cooler at the bottom of the unit has a temperature close to that of the wet bulb temperature of the outgoing airstream. In the two-stage evaporative cooling setup, that chilled water is then led through an air-to-water heat exchanger to pre-cool the hot air before it is cooled down further by the direct
In terms of energy balance, nothing has changed compared to the standard direct evaporative cooling principle; overall, it is still an adiabatic cooling process. However, whereas the direct evaporative cooler has a uniformly distributed temperature profile at the side of the outlet air, the two-stage evaporative cooler shows a vertical temperature gradient: further down the air temperature gradually decreases.
Typically, the lower 60% of the outgoing air has a specific enthalpy lower than that of the inlet air. Whereas the temperature of the outlet air from a direct evaporative cooler is restricted to the wet bulb temperature, in this case, when taking only the lower 50% of the outlet air from the two-stage evaporative cooler the temperature can even be lower than the initial wet bulb temperature. The corresponding wet bulb efficiency will then be higher than 100%.
The figure below shows the cooling process of a standard direct evaporative cooler.
Figure 1 – Cooling process of a standard direct evaporative cooler
As described above, there is a uniform vertical temperature profile. Air is cooled adiabatically to 1.7 K above the initial wet bulb temperature, corresponding to a wet bulb efficiency of 90%, close to the physical limit of 100%.
The figure below shows the cooling process of the two-stage evaporative cooler in which the exact same amount of air is flowing through the system as in the configuration above.
Figure 2 – Cooling process of the two-stage evaporative cooler
The first stage (the air-to-water heat exchanger) pre-cools the hot outdoor air to a uniform temperature of 23.8 °C, without changing its moisture content. Due to the fact that there is a large temperature difference between water supplied to and extracted from the direct evaporative cooler, contrary to the previous configuration, air leaving the second stage (the direct evaporative cooler) now does show a vertical temperature gradient, as described above.
The lower 50% of the supply air has an average temperature of 18.3 °C, 2.7 K below the initial wet bulb temperature, corresponding to a wet bulb efficiency of 116%.
Furthermore, although achieving a lower temperature compared to the previous configuration, less moisture has been added; the resulting absolute humidity is 11.4 g/kg instead of 15.0 g/kg.
The upper 50% of the supply air has an average temperature of 25.4 °C, which is higher than that of the supply air in the previous configuration, but may still be low enough to serve other purposes.
More information and specific details are available in the documents listed in our DOWNLOAD CENTER.