Introduction to Indirect Evaporative Cooling

What is indirect evaporative cooling?

Indirect Evaporative Cooling (IDEC) is a process where air is sensibly cooled by the effect of the evaporation of water across a heat exchanger. The advantage being that for most climactic conditions, there is a significant difference between the wet-bulb and the dry bulb temperatures at design conditions. This 'wet-bulb depression' allows the designer using indirect evaporative cooling to create supply air temperatures below the ambient dry-bulb temperature without using any refrigeration at all.

Take a Seattle design day of 85º/67º db/wb. If we bring in 100% OA (which is pretty common for IDEC systems) we will have, obviously, an OA condition of 85º/67º. If we have indoor air to exhaust and use as a heat sink in a traditional, dry air-to-air heat exchanger, we will have about 75º air to use to cool down the 85º OA. Assuming about a 70% efficiency for this type of heat exchanger, that means we can realistically drop the OA by about 70% of the difference from 85º to 75º or about 7 degrees. We should be able to get a resulting LAT from the HX of 78º. Note, however, that we will need some sort of refrigeration in our system to create the indoor environment of 75º from which we are taking conditioned air to cool the OA.

Now let's consider an IDEC system for the same service. This sort of system can take on many forms, including the exact same configuration as noted above, simply with the addition of a direct-evaporative media section in the exhaust air upstream of the air-to-air heat exchanger above. For this comparison, however, let's use a built-up Energy Labs IDEC system. This is essentially a closed-loop fluid cooler for air. An induced draft fan pulls OA upwards past water spray to encourage evaporation and the supply air is cooled across an internal heat exchanger without contacting the water.

Energy Labs IDEC Module

To make this realistic, let's give this service an actual CFM and pick a particular IDEC model. Let's say this is a 22K cfm service and let's pick the nominal I-220-48 IDEC unit. With 85º/67º OA conditions*, the effective temperature difference across the heat exchanger is not 10º (OA db of 85º-EA db of 75º) but actually 18º (OA db of 85º - OA WB of 67º). Note we did two things, we increased the overall temperature difference the heat exchanger sees, and we eliminated the need to have an available exhaust air stream exhausting pre-cooled air. Checking the performance of this particular IDEC unit, we see that it has an overall effectiveness of 69% at these conditions, and the LAT from this system is 72.5º/63.2. That's a 5.5 degree improvement in LAT, or, for this supply air quantity, nearly 11 additional tons of cooling. And we don't need to have any mechanical cooling anywhere in the building to achieve this leaving air condition.

Let's examine this cooling effect on a psychrometric chart:

(click for large image)

The first thing you should notice is that the cooling process is purely sensible--no humidification or dehumidification is performed. The other thing you should note is that the supply air wet bulb temperature is a few degrees cooler than the OA wet bulb temperature, 63º vs. 67º. This is of critical importance when applying direct evaporative cooling to these systems in an indirect/direct hybrid system.

In the end, however, you can see that about 25 tons of cooling was provided, at a mechanical cost of about 1" of static pressure drop and the operation of 3 3/4 HP of fan and pump energy for the IDEC unit.

This is very inexpensive and sustainable cooling. Of course, the delivery temperature is higher than typical for standard air-conditioning applications, but if viewed as a first stage of a multi-stage system, you can see that there is a compelling case to be made for using this sort of technology to at least partially offset cooling loads that would traditionally require compressorized cooling, and greatly expand the hours of available economizer function.

Resources you may find useful:
Energy Labs IDEC 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.