Introduction to Direct Evaporative Cooling

Now that we have discussed Indirect evaporative cooling, let's move on to the next question: What is direct evaporative cooling?

Direct Evaporative Cooling is a process where air is sensibly cooled by the effect of the evaporation of water directly into the delivered air stream. This is typically accomplished by use of a wetted absorbent media in the air stream, most commonly Munters CelDek (pdf) or GlasDek (pdf).

Evaporative media in an Energy Labs Unit

This process has the advantage over IDEC systems in being much more efficient (with efficiencies in the range of 90% easily attainable), but with one major difference: Direct Evaporative cooling is an adiabatic process. This means that there is no energy added to or removed from the airstream. The enthalpy of the air is unchanged, even as the sensible temperature is cooled.

How is this possible? Well, essentially you trade sensible heat for latent heat. As you reduce the dry-bulb temperature of the air, you concurrently increase the humidity ratio of the air. What you lose in sensible heat, you make up in the heat embodied in the evaporative phase-change of the water.

What does this look like on a psychrometric chart? Take that Seattle design day* of 85º/67º db/wb. Let's bring in 22,000 CFM of 100% OA. If we select a direct evaporative system with an 89% efficiency, the leaving air temperature will be about 69º/67º. Note that the wet bulb is essentially unchanged. The chart of this process is below:

(click for larger image)

Essentially, for this service, you get about 32 tons of sensible cooling but and zero tons of total cooling. If you were to allow this supply air to warm sensibly to a room temperature of 75º, you would find that the space RH would be close to 70%, which would probably not be acceptable for standard comfort cooling applications. However, in high-sensible cooling applications, like, say, data centers, this method of cooling has great application.

Additionally, since the resultant indoor conditions depend greatly on the outdoor air conditions, Direct evaporative cooling can provide acceptable air conditions for much of the year in a cool, dry climate like Seattle. In fact, any time the ambient wet bulb temperature is 53º or less, the direct evaporative cooling can provide supply air almost identical to that off of a 55º cooling coil, with pressure drops at the media on the order of half that of a standard cooling coil! Direct evaporative cooling can used to essentially greatly extend the hours of economizer performance available on almost any cooling system.

But the benefits do not end there--because pre-cooling with direct evap systems upstream of a cooling coil can significantly decrease energy costs for sufficiently dry ambient conditions:



Resources you may find useful:
Energy Labs direct evaporative performance calculator
Energy Labs Direct/Indirect Evaporative Systems Engineering Guide (booklet format)


*Note: When applying evaporative systems, often it is necessary to consider the performance of the system at the ASHRAE evaporative design day conditions, in addition to the sensible design day conditions that we commonly use. And, additionally, it can use what would normally be unwanted space heat in the return air to provide beneficial humidification in times of low humidity.