Direct evaporative cooling is technique used to remove heat simply by evaporating water within an airstream. Our bodies use the same process when we sweat. More and more new data centers are using direct evaporative cooling as their primary cooling system for energy efficiency purposes. Direct evaporative cooling systems differ from mechanical cooling systems (such as DX CRAC units or chilled water CRAH units) in that they do not require electricity to cool the air. This article describes how directive evaporative cooling works.
The (simple) Physics behind direct evaporative cooling
If you’ve ever been to Las Vegas in the summer, I am sure you’ve seen one of these misting systems outside restaurants or by the poolside
They’re used for the same purpose as data centers, to cool the air. When a mist of water is blown into hot air, it evaporates. The mist delivery is important because misting, as opposed to haphazardly spraying like an open garden hose, increases the air surface contact for the tiny water bubbles. Misting allows the water to evaporate more effectively.
In order to cause water to evaporate, you need to add heat to the water – just like you need to add heat if you want to boil a cold pot of water. In our Las Vegas poolside case, the heat comes from the air itself. Heat is transferred from the air to the water causing it to evaporate. As heat leaves the air, the air gets colder…voila! You can now enjoy your mojito comfortably.
The same principal occurs during sweating. Our bodies create little droplets of water on the skin. This water evaporates leaving the skin, and the body, cooler.
Know that direct evaporative cooling is more effective in dry places than wet places, so the data center location matters. For the non-technical, this is because the evaporation process is more efficient in dry air and less efficient in wet/humid air. For the more-technical, the web-bulb temperature is the key driver that will determine the effectiveness of a direct evaporative cooling system.
What it looks like in a data centers
The system design for a data center is straight forward. The misting system just needs to be placed in the supply airstream that will be fed into the data hall. It basically replaces the ‘cooling coil’. The other common cooling components (supply fans, return fans, economizers, dampers, and louvers) all remain the same. Cooling occurs where the mist is delivered. The photos below, from Facebook’s Open Compute Project, show the mechanical diagram of the entire cooling system and the misting nozzles in action.
Direct evaporate cooling systems require some unique considerations that differ from mechanical cooling systems. These might include, but are not limited to:
- Water storage tanks
- Water purification/filtering process to prevent piping and nozzle clogs
- Mist pumping system
- Mist control system to regulate water pressure and flow
- Airflow control system to regulate temperature and humidity within the data hall
Other types of evaporative cooling
Misting is not the only way to introduce water into the airstream. Forcing air through a wet material (think of a wet piece of corrugated cardboard) is another evaporative cooling delivery mechanism. This is sometimes referred to as “Air Washing” and is also used in data center environments. The concept is the same.
Indirect evaporative cooling can also be used to cool an airstream. The benefit of this design is that the water does not come in direct contact with the conditioned air. The drawback is that you need a separate heat exchanger to keep the water out of the airstream, increasing the initial system cost. There can be many types of designs for indirect evaporative cooling systems. But if you’ve ever hosed down your condenser coils on a really hot day, you’re performing a poor-mans indirect evaporative cooling system. Environments that have tight humidity thresholds would benefit from indirect evaporative cooling. The latest ASHRAE thermal guidelines specify a rather wide humidity window (allowable of 20% to 80% RH), making direct evaporative cooling feasible for data centers environments.
More Interesting Information
- 2011 ASHRAE Thermal Guidelines
- Do you know the difference between CRAC vs CRAH?
- Facebook’s Open Compute Project publicizes their data center designs and best practices. It is a recommended read.