Humidification specifications and systems have often been found to be excessive and/or wasteful in datacenter facilities. A careful, site specific design approach to these energy-intensive systems is usually needed to avoid energy waste.
Objective 1: Design system to actual equipment requirements. Datacenter benchmarking has found a wide range of operation temperature and humidity setpoints in use at datacenters. The choice of room setpoints should be based upon actual equipment requirements and efficiency opportunities. Strategies: Use Widest Suitable Humidity Control Band. The tightest humidity control band recommended by ASHRAE is 40-55%, and larger humidity control bands are successfully used by datacenters. Over specifying the required humidity control significantly increases first cost and long term operating and maintenance costs.
> Specify Humidity Sensor Calibration Schedule. Humidity sensors tend to drift and require more frequent calibration than temperature sensors. An incorrect humidity sensor is less likely to be noticed than an erroneous temperature sensor, which could lead to extended out of specification datacenter humidity levels and excessive humidification energy costs. Regular sense calibration (or replacement) is required to maintain accurate humidity control.
> Provide Appropriate Sensor Redundancy. Humidity control is only as good as the humidity sensors used. To maintain very tight humidity control, multiple humidity sensors should be used. The use of a minimum of two humidity sensors increases the opportunity.
>Control Humidity with Dedicated Outdoor Air Unit. Typically, ventilation air represents the majority of humidity load in a datacenter. Controlling humidity with a single ventilation air handler (or a pair of redundant units with a common control loop) is a common method of eliminating simultaneous humidification and dehumidification.
Metric: kWh or therms per pound of humidification.
Objective 2: Eliminate over humidification and/or dehumidification. Humidification is very energy intensive, requiring the addition or removal of large amounts of heat and often degrading the efficiency of the entire cooling plant when in operation. Strategies: Ensure Proper Economizer Lockout. During periods of low absolute humidity (often measured as dewpoint), minimize the quantity of air brought into the Datacenter to avoid high humidification loading. Alternatively, humidify the air using heat recovered from the return air stream.
>Maintain Coil Temperature Above 55F. Coil temperatures below 55F can lead to localized areas of uncontrolled dehumidification on the coil, which can cause unnecessary energy use and decrease humidity control stability. A chilled water system with a chilled water setpoint of 50F or higher can greatly reducing uncontrolled dehumidification and the resultant humidification load.
>Centralize Humidity Control. Each datacenter space should have a single, centralized humidity control system. Multiple systems frequently end up 'fighting,' that is one unit will be humidifying a room while another unit is simultaneously dehumidifying the same room. A central humidity control system can completely eliminate this inefficient fighting and provide better control by eliminating or allowing rapid diagnosis of the causes of fighting (incorrect setpoints, failed sensors, etc.).
Objective 3: Use efficient humidification technology. When significant quantities of humidification are expected, investment in a high efficiency system is often justified by expected energy costs. Strategies: Use Waste Return Air Heat to Humidify. During periods of low outdoor air temperature, the warm return airstream can be humidified using an efficient adiabatic technology and mixed with cold, dry outdoor air to provide very low energy cost cool air at an appropriate humidity.
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Use Adiabatic Humidifiers for Humidity and Evaporative Cooling. Adiabatic humidifiers absorb heat from the air stream, providing both humidity and cooling with minimal energy input.
>Use Lower Power Humidification Technology. There are several options for lower power, non-isothermal humidification, including air or water pressure based 'fog' systems, air washers, and ultrasonic systems.
