As technology use becomes increasingly integral to our daily lives, we need the growth of information technology (IT) infrastructure to keep pace with demand. Operating and maintaining sensitive IT equipment efficiently underlines the need for accurate and reliable control of space temperature and humidity. The humidity levels within data centers are particularly important because excessive humidity creates a risk of condensation, and inadequate humidity can promote electrostatic discharge at electronic equipment. Proper humidifier selection can also impact building cooling loads and total energy consumption within a data center.

Just as there are many ways to cool a data center, there are also numerous technologies available for humidification. Understanding the benefits and drawbacks of each solution lends to more robust system design decisions, and ultimately to a better operating environment. This article will outline and discuss a few humidification options so that engineers can evaluate their data center and choose the best humidification system based on their needs.


Why Humidify Data Centers?

Temperature control is likely the first topic that comes to an engineer’s mind when talking about data center environmental control. However, humidity is an equally important component for effective operation. Recommended operating humidity levels within a data center are constrained by an upper and lower boundary as described in the 2011 ASHRAE Thermal Guidelines for Data Processing Environments. The chart and table in Figure 1 show ranges of temperature and humidity criteria for various data processing environments sorted by class. Each class of data center has specific recommendations for low-end relative humidity control.

Beyond the humidity level recommendations provided by ASHRAE, many data center owners further define the space conditions through an end-user specific service level agreement (SLA). The SLA is typically applied in a colocation facility where an owner is leasing white space to various tenants, and it guarantees that certain space conditions are provided, one of which is thermal environment. SLAs must be considered early on in the design of a project, as they can have impacts on the sizing of the mechanical equipment, especially if a tighter-than-standard thermal environment window is prescribed.

Low-end humidity monitoring and control within data centers are recommended as one method to help mitigate the effects of static charge buildup and potential for electrostatic discharge (ESD) for sensitive electronic components. The main threat of ESD comes from occupants moving throughout the space, accumulating charge, and then touching equipment without being properly grounded, though this is a rare occurrence. A static discharge event at a piece of electrical equipment can lead to equipment failure and downtime.

Over the last 15 years, electronic equipment manufacturers conducted trials and studies to further understand the relationship between humidity levels and how they affect the buildup of static electricity. One study concluded that the rate of static electricity buildup on a person walking within the space decreased significantly as the relative humidity levels increased (Evans, 2004-2008, p. 4). Other industry research leans toward the conclusion that equal charging occurs independent of the humidity within the space. However, as humidity increases, the potential for electrostatic discharge decreases due to the boundary layer of water molecules (Evans, 2004-2008, p. 4). Nonetheless, the general consensus in the field of research into ESD vs. relative humidity levels is that increased relative humidity within the space decreases the potential risk of unwanted electrostatic discharge.


How to Humidify

Humidification can be accomplished with numerous commercially-marketed technologies. Each project will have varying humidification needs and project constraints, so the engineer must complete a thorough evaluation in each instance; this guarantees that each project implements the ideal humidification method. When selecting a system, consideration should be given to design choices, such as: whether to humidify at a system level or local to the server spaces; utility routing and serviceability; introducing noise and water sources within or near sensitive equipment; and the number of humidifiers necessary to meet capacity requirements.


Adiabatic and Isothermal Humidification

All of the humidification techniques discussed here are categorized into one of the following processes: adiabatic or isothermal. These two processes vary significantly and must be fully understood so that a humidification method can be selected. The adiabatic process utilizes energy from the air stream to absorb water at a rate of approximately 1,000 btu/lb of water. This, in turn, reduces the temperature of the humidified airstream.

Conversely, with an isothermal process, the energy source required for the humidification is external to the airstream and can result in slightly warmer leaving air temperatures in some applications. If designed and operated properly, adiabatic humidification can result in a significant decrease in compressor driven cooling energy. In certain climate regions, adiabatic cooling and humidification can be used exclusively to meet the cooling needs of a data center.

The are a variety of marketed technologies that fit into one of these processes and vary only in their specific method of introducing water droplets to an airstream. Some of the humidification methods available for use include:

  • Heated pan humidifier (isothermal)
  • Direct steam injection humidifier (isothermal)
  • Self-contained steam humidifier (isothermal)
    • Electrode humidifier
    • Resistance humidifier
      • Infrared humidifier
      • Gas fired humidifier
  • Atomizing humidifiers (adiabatic)
    • Ultrasonic humidifier
    • High-pressure humidifier
  • Compressed air nozzle humidifier (adiabatic)
  • Wetted media (adiabatic)


Isothermal Humidifiers

In HVAC applications, isothermal humidifiers are the traditional option for humidification. On a fundamental level, these humidifiers generate steam by boiling water. Any number of heat sources can be used, such as electric, steam to steam, fuel-fired boilers, infrared light, etc. Once generated, the steam is then injected into an airstream for absorption.

Isothermal humidifiers are frequently chosen due to their ease of install, maintenance, and reliability, and because they can be easily retrofitted into a system. Steam nozzles and tubes do not generally require replacement or cleaning as frequently due to the fact that precipitates and minerals are removed from the water in the boiling vessel. Another benefit of isothermal steam generation is that the steam helps to sanitize the airstream. As stated previously, the term “isothermal” is a misnomer in that injecting steam into the system can raise the temperature of the air. This temperature rise should be considered when selecting these systems, and these are less frequently selected for data centers because they can have a negative impact on the power utilization effectiveness (PUE) rating.


Adiabatic Humidifiers

Atomizing humidifiers humidify air through an adiabatic process. The energy required for vaporization is supplied from the airstream; therefore, the velocity of air traveling through the atomizing humidification section of an air handler needs to be within the tolerances specified by the equipment manufacturer. If the velocity increases beyond the recommended maximum, the absorption efficiency will significantly decrease, which results in the need for additional capacity.

Another variable in the efficiency of these systems is the absorption distance within the humidification section of the duct or air handler. Absorption distance is most often measured from the humidification assembly to the next component, if mounted within an air handler, which is typically a mist eliminator. The average droplet size also affects the absorption efficiency of the system; it is highly dependent on the technology used and the manufacturer’s design.


High-Pressure Humidifier

High-pressure humidification systems are typically mounted as arrays within an air handling unit. These humidification arrays consist of numerous nozzles that are capable of supplying a mist of water to the airstream, with average droplet sizes around 10 microns. In order to create such a fine mist, water with pressures ranging from 800 psi to 1,200 psi is supplied to the humidification assembly and sent through impeller or impingement style nozzles, referenced in Figures 4 and 5. High-pressure water is typically supplied by high pressure pumps, such as positive displacement pumps, and is controlled to maintain supply pressure to the humidification system.

Impeller style nozzles consist of a small orifice at the end of the nozzle that atomizes high-pressure water as it exits the nozzle. Impingement style nozzles direct high-pressure water towards an impingement pin. Upon contact with the pin, the water atomizes into a fine mist. An array can be split into multiple stages depending on the level of humidity control required within a space. Often, fast acting solenoid valves are utilized to control the staging of these humidifiers. Since high-pressure humidification systems use nozzles with such small orifices, it is typically recommended by the manufacturer to use water that has been filtered and purified through a reverse-osmosis process; using purified water reduces the risk of clogged nozzles due to particulates within the water.


Compressed Air Nozzle Humidifier

Compressed air nozzle humidification, as seen in Figure 6, is similar to high-pressure humidification except that compressed air is used in place of a high-pressure pumping skid. Water can then be fed at a lower pressure compared to the high-pressure system. Water droplet sizes using this technology average 7.5 microns in size. Water is typically purified before being supplied to these systems, but specific requirements should be verified with the manufacturer during system design.

This type of humidification system eliminates the need for a high-pressure pump skid but will require the installation of a compressed air system. If the building already has a compressed air system installed, such as those required for large valve actuation, this humidification system may be a good candidate. Compressed air nozzle humidifiers are typically mounted within an air handling unit for data center applications.


Ultrasonic Humidifiers

Ultrasonic humidifiers utilize a series of piezoelectric transducers to create cavitation within a basin of water. This cavitation then creates a fine mist of water vapor with an average droplet size of one micron. Ultrasonic humidifiers are typically mounted within an air handling unit for data center cooling applications or in downstream supply ductwork. The makeup water that is supplied to the basin often needs to be purified through both reverse-osmosis filtration and a deionization skid. This allows for the ultrasonic process to work more effectively and produce mineral-free humidification.

Ultrasonic humidifiers typically require supply water pressures ranging from 30 psi to 80 psi depending on the manufacturer; this eliminates the requirement for a high-pressure pumping skid when compared to the high-pressure humidification system above. Piezoelectric motors have low power requirements. Therefore, they create an additional benefit in that ultrasonic humidifiers can use nearly 90% less power, depending on the humidifier output capacity, when compared to an equivalently sized electrode steam humidifier (Stulz, 2015, p. 7). Multiple ultrasonic humidifiers can be stacked on racks within an air handling unit to achieve a larger humidification output if required. Racked units can be controlled to operate as one stage or can be configured for multiple stages.


Wetted Media

Another choice for humidification is wetted media. Wetted media assemblies, often referred to as direct evaporative coolers/humidifiers, offer a mix of benefits and drawbacks similar to the other humidification approaches. Wetted media works by spraying or dripping purified water over layered polyester fabric, as shown in Figure 7. The water wicks down the fabric media, saturating it along the way. Excess water is recirculated to the top of the media via a pumping system, where the cycle repeats.

Benefits to utilizing wetted media for humidification include lower power consumption, low-pressure water distribution, and significant additional compressor-free cooling. Pump packages and controllers still require power, but less when compared to steam generators or high-pressure atomization systems. If distribution or generator power is at a premium, these savings can have a sizeable impact on an electrical system design. Next, reverse osmosis/deionized (RO/DI) or softened water can be delivered to the humidifier at low pressures. These lower pressures save money on pump power, but also minimize the danger presented by leaks, both to sensitive data center equipment and to personnel. Lower pressure piping can be joined through more conventional means and with less specialized equipment.

Another benefit of evaporative humidifiers is that the heat of vaporization required to introduce water into the airstream also cools the air. Different equipment manufacturers list additional cooling benefits upwards of 20°F; however, the actual benefit depends greatly on the desired design conditions and the manner in which an airside system is operated.

When deploying wetted media as a source of humidification, performance and efficiency need to be taken into consideration. Unlike many of the aforementioned humidification solutions, wetted media efficiency is not measured directly by how much water is absorbed versus supplied, but rather as a percentage difference between dry bulb and wet bulb temperatures at the discharge of the humidifier. Theoretically, a 100% efficient direct-evaporative humidifier would have a discharge condition that is on the saturation line of the psychrometric chart. This efficiency is important to consider when sizing a supply/makeup water source to wetted media humidifiers.

Another point of concern with wetted media humidifiers is air velocity. Manufacturers rate and recommend humidifiers based on a certain range of air volume and velocity. In many instances, this requires the addition of a bypass section in an air tunnel in order to maintain the total design air volume, while still maintaining design volume through the humidifier. An improperly sized bypass can result in water carryover, sputtering, or incomplete absorption.

Another point of consideration is media degradation from mineral deposits or soiling on the media pad. Depending on incoming water quality, mineral deposits may begin to accumulate on the media itself, as well as debris or airborne pollutants that were missed by upstream filtration. Also, high concentrations of dissolved solids within the humidifier basin will require periodic blowdown. The useful life of the wetted media varies based on use, but the media will need to be replaced at some interval during operation.


Humidification Makeup Water

When designing a humidification system, proper provisions must be made to ensure that an adequately sized and designed makeup water source is available. Makeup water sources should be sized to provide the required makeup water volume to the humidifiers while also taking into account any losses from the humidifiers due to absorption inefficiencies and other factors. To ensure proper operation, makeup water should also be provided at appropriate pressures as required for the particular humidification system being implemented.

Required makeup water quality highly depends on the type of system that is utilized and must be carefully coordinated with the manufacturer during design. Makeup water to humidification systems can vary from domestic water that has been filtered through particulate filters to water that has passed through a reverse osmosis filtration skid and deionization unit. Proper design of the water filtration assembly is crucial to the long-term operation of the humidification system.

In conclusion, as data center installations continue to grow throughout the world, every climate zone will require an individual analysis to determine site humidification requirements. Properly designed isothermal and adiabatic humidifiers both mitigate the effects of electrostatic discharge on electrical equipment. Where isothermal humidifiers add heat to an airstream, thereby increasing cooling load, adiabatic humidifiers are able to humidify while reducing compressor based cooling load. As energy codes tighten and data center owners strive towards a PUE of 1.0, the added cooling load reduction from adiabatic humidification will help engineers meet these requirements. Though selecting an appropriate humidification system varies by project, understanding the methods of humidification and all of the options available is an important first step in design. ES



Evans, T. (2004-2008). Humidification Strategies for Data Centers and Network Rooms. American Power Conversion.

Stulz (2015). Stulz Ultrasonic Humidification Systems.