What is direct evaporative cooling?

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.

Stay Cool with Direct Evaporative Cooling
Stay Cool with Direct Evaporative Cooling

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

Misting System
Misting System

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.

Facebook Prineville DC Cooling Diagram
Facebook’s Prineville DC Cooling Diagram


Facebook's Prineville DC Misting System
Facebook’s Prineville DC Misting Nozzles

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.

Evaporative Cooling Module form Premier Industries, Inc.
Evaporative Cooling Module form Premier Industries, Inc.

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


5 thoughts on “What is direct evaporative cooling?

  1. Dear Dan Mascola:
    Thank you for the useful paper: “What is direct evaporative cooling?”, July 21, 2014.
    I would like to inform you how to reduce data center energy consumption for cooling about 95% using the unique evaporative cooling process
    through the Maisotsenko Cycle (M-Cycle). There is the proven at the market technology from Coolerado Corporation (USA), which significantly reduces the total cost of ownership (TCO) for a data center.
    I hope this information will be helpful for your renewable energy efficiency initiatives for data centers.
    For example, the National Snow and Ice Data Center (NSIDC), at the University of Colorado at Boulder has a 95% reduction in energy for cooling through the M-Cycle, using Coolerado Air Conditioners. It has become the most energy-efficient data center in the United States and has won the 2011 CO-Labs Green Data Center of the Year Award (see attachments 1 and 2).
    Conclusions from this report are:
    ∙ Cooling energy is reduced as much 97% (about what is takes to run your car’s A/C)
    ∙ This is a prototype project of what can be done on a small-scale operating data center
    ∙ Overall center carbon and energy use will be reduced by 70% (with solar & virtualization)
    ∙ Payback on the cooling system is about 3 years (difference of CRAC replacement vs. Coolerado solution and energy savings): http://www.facilitiesnet.com/site/pressreleases/Coolerado-Data-Center-Customers-Reducing-Cooling-Costs-Up-To-90-Percent–28152

    The video links below illustrate how the NSIDC is saving significant energy by the use of Coolerado systems through the M-Cycle:


    The NSIDC has released the official One Sheet on their winning of the 2012 Green Enterprise IT Award – via the Uptime Institute (a very prestigious authority institute for the Data Center community).

    What is the real difference between the Coolerado technology and the best existing cooling technologies for data centers?
    Please find attachment 3 and direct your attention to slide 3 on Coolerado’s value proposition in data centers. The Maisotsenko Cycle’s superior thermodynamics allow us to deliver much colder air than conventional indirect evaporative coolers (IEC). This enables us to satisfy set points with no, or greatly reduced, backup mechanical cooling, which helps get our installed cost under $600k/MW. Furthermore, we deliver industry leading Power Usage Effectiveness’s (PUE) without requiring owners to accept fresh air or introduce high water/humidity flows into their data centers.
    I’m actually most excited about how our design numbers are coming out against a pumped refrigerant economizer in the northeastern US, a region which should favor economizing as it is cool most of the year and humid in the summer. We are first-cost competitive while cutting annual energy usage and connect load in HALF against a claimed 1.15PUE baseline. We don’t need any computer room air conditioner (CRAC) units, water, or refrigerant runs inside the DC, which the client loves. Our outdoor footprint is about 1,200sqft/MW. My analyses predict we can deliver this level of performance in any climate.

    For more information about the M-Cycle, please visit our web sites at http://www.coolerado.com and http://www.idalex.com

    If you have any questions, please let me know.

    Best regards,

    Dr., Prof. Valeriy Maisotsenko

    Chief Scientist and Founder
    Coolerado Corp. and Idalex Inc.
    Phone: 303-375-0878, ext.106
    Direct phone: 720-974-9616
    Fax: 303-375-1693
    Cell: 720-220-9347

  2. It’s quite convenient for a certain place especially home to have a reliable cooling system. Both aircon and evaporative cooling are the top choices but when justify it evaporative cooling has it’s advantages. I have also realized that evaporative cooling is cost effective and eco friendly as state on this site http://brivis.com.au/blog/what-is-evaporative-cooling-and-how-does-it-work/ I have also noticed that most establishments use evaporative cooling during summer time like coffee shops and restaurants. They place the misting system on top in order to totally spread the effect.

  3. With extensive usage of data, thanks to the growth in technology, heating up of data centers is obvious. However, applying the cooling techniques that you’ve mentioned will help in keeping the temperature under control.

  4. I appreciate you helping me learn more about evaporative cooling. I am only particular with air-conditioning units and this information is quite new to me. Not only that, you have also taught me about its other types.

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