Adding humidity to data centers is one of many habits that has been carried over from mainframe computer rooms to the cooling of modern data centers. There is a growing opinion that humidification is unnecessary, but if you decide to humidify, there are much greener and less costly ways to do so than the primitive infrared and steam-generating systems that have been used for years.
Computer room air conditioning (CRAC) units have traditionally humidified the space using infrared humidifiers. These devices use light bulbs similar to what you would see at a fast food restaurant to heat and then boil water. The boiled water vapor (steam) is absorbed into the airstream and increases the space humidity. This is extremely inefficient from an energy standpoint for three reasons:
- It takes 970 BTU (approx .3KW) to boil a pound of water. Add in some inefficiency as well as the energy required to heat the water from room temperature to the boiling point and you are using approximately 40 kW to add 100 lbs. of moisture to the space.
- The heat that is used to boil the water raises the temperature of the air. This heat has to be removed and adds load to the air conditioning system. The 40 kW that was used to boil water is approximately 12 tons that now has to be removed.
- Each unit typically has its own control system, and these standalone control systems often fight each other. Sensors need periodic calibration and a large deadband to insure adjacent units are acting together and not fighting each other. A huge amount of energy is wasted if some units are humidifying a space and others are dehumidifying.
There is a much greener way to humidify your space through the use of adiabatic humidification. This process uses an airstream’s tendency to absorb water vapor when it is put in contact with a wet surface or small particles of water. The most common types of adiabatic humidification are:
- Evaporative type: The air stream is put in contact with wet media. Similar to “swamp coolers” used in desert climates. This technology has been used successfully in the clean room industry for years.
- Ultrasonic type: This device uses high-frequency vibrations to break water into very fine droplets that are easily absorbed by the air stream.
- Atomizing type: This device uses compressed air or a high-pressure water pump to atomize the water into very small particles that are easily absorbed by the air stream.
These three styles all have different application issues, space requirements, maintenance requirements and first cost, but all three will consume around one kW to provide 100 lbs. of moisture. They also have the additional benefit of providing “free cooling” as a result of the evaporation process.
The benefit and energy savings available by using adiabatic humidification are so large that pending changes to California’s Title 24 energy standards will require that any humidification in a data center be done using adiabatic-type systems. This recognition of the amount of energy that can be saved compared with infrared or steam-generating humidification means that this concept will quickly spread to other areas of the country. In the meantime, you can save large amounts of energy in your existing data center by implementing adiabatic technology.
The first step is to determine how much humidity your space actually needs. The industry standard has been approximately 20 lbs/hr of moisture capacity per CRAC unit. This is probably more than is needed in a properly constructed building without an air-side economizer, but any engineer or qualified equipment supplier should be able to help with the calculation.
The next step is to decide where to put the adiabatic humidifiers. There are at least two companies (Humidifirst and Stultz) that make ultrasonic humidifiers, which are designed to be direct replacements for CRAC unit infrared humidifiers. Custom Mechanical Systems makes a cabinet-type evaporative humidifier that can go directly in the space or in a mechanical room. There are a number of manufacturers of atomizing-type humidifiers that can be mounted directly in the space or in an adjacent mechanical room. As stated above, each type of humidifier has its own application issues, space requirements, maintenance requirements and first cost, but with a little investigation, you can quickly determine which will work best for you.
The third step is to decide on a controls strategy. If you are retrofitting into an existing unit, you can likely reuse that unit’s standalone controls. However, a more effective strategy may be to monitor humidity at several locations in the space and control the humidifier based on these average readings. This will eliminate the phenomenon of having units fighting each other. Any of the humidifier manufacturers should be able to provide you with a global control system.
The last step is to disconnect the existing infrared systems and install the new adiabatic system. In most cases, this can be done without any downtime or without any impact to the data center operations. The amount of energy saved with these systems is so large that the payback will be very short. Typical paybacks are less than one year. Some of the named manufacturers will finance the purchase and installation cost so that you can pay for the equipment out of the energy savings. This is a painless way to reduce energy consumption, reduce maintenance and possibly create new capacity for IT load.
There are a lot of ideas about how to save energy in a data center. Very few of these ideas combine the obvious reduction in energy, the ease of implementation, the low risk and low first cost that can be realized by replacing your existing humidification system with an adiabatic-type system.
ABOUT THE AUTHOR: Dan Hyman, co-founder and Principal at Custom Mechanical Systems, has 25 years of experience designing custom HVAC systems for mission-critical facilities, such as clean rooms, hospitals, research labs and data centers.
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