Air conditioning, heating, and fresh air intake during winter conditions and dry summer months cause electrostatic discharge (ESD) in cleanroom environments. ESD can result in costly damage to the equipment and products manufactured, as well as delays or downtime. The following looks at some of the forms of ESD control and their atributes.
De-correlation air atomizing: A de-correlation type system provides a humidification which generates equal sized micron droplets with equal spacing that do not attract to each other but oxidize into the air. It is completely non-wetting and forces the humidity into the air evenly and completely.
Droplet size: 4.2 microns
Powered by: Standard air compressor
Regulation required: Not required
Installation: In-room and in-duct
Air and water atomizing: Air atomizing humidifiers use compressed air to break down the water droplets in the nozzle. It provides uneven droplets between 7 to 20 microns.
Droplet size: 7 microns
Powered by: Regulated air compressor and requires water pressure
Regulation required: Compressed air and water pressure regulation required
Installation: In-room only
High pressure: High pressure water pumps push water through miniature holes which are sprayed into the air. The miniature holes can clog and require maintenance of the nozzle and water pump, and duct installation requires droplet filters and drainage.
Droplet size: 10 to 20 microns
Powered by: High pressure pump
Regulation required: Water pressure regulation is required
Installation: In-room and in-duct
Ultrasonic: Ultrasonic type humidifiers use a submerged vibrating disk to create a high-frequency oscillation, dispersing small water droplets that travel a short distance into the air.
Droplet size: 5 to 20 microns
Powered by: Vibrating disk
Regulation required: Not required
Installation: In-room and in-duct
Centrifugal: Centrifugal humidifiers deliver supply water to a fast-spinning disk that fractures water into large uneven droplets, which requires a fan, warm air, and high ceilings to distribute the humidity and evaporate the water. This type of humidifier is not recommended for cleanroom use.
Droplet size: 20 micron+
Powered by: Spinning disk
Regulation required: Not required
Installation: In-room only
Wet Media: Wet media humidifiers use a water absorbing mesh placed inside a duct or AHU airstream. It can be difficult to achieve a non-fluctuating relative humidity control. A wet media type humidifier is not recommended for cleanroom use.
Droplet size: No droplets, as the water evaporates
Powered by: Water pump
Regulation required: Not required
Installation: In-duct only
System selection
When selecting a humidification system for a cleanroom to remove ESD, it is recommended to take the following factors into consideration.
Installation in the room or in the duct?
Humidification systems can be installed in the duct as well as directly in the room. Some technologies create larger droplet size that may wet or create condensation on sensitive equipment.
Adiabatic humidifiers that can be installed inside the duct:
• De-correlationa atomizing can be installed in the duct and on the wall.
• Air atomizer, high pressure water, ultrasonic, and wet media all need special ducts with a drain and fan.
The humidification system must not introduce any mineral or particulates into the space.
When water evaporates any minerals present in the water can shoot out as “white residue” dust. To avoid this, install a water filter system. De-correlation air atomizing technology, air atomizing types, and high pressures system can use a filtered water system. For humidifiers using compressed air, there must be air filters installed inline to filter any particulates or oil from the compressed air going into the nozzle.
What is the annual maintenance cost involved?
If the cleanroom requires 24/7 ESD control, choose a humidifier that does not require a system shut-down to conduct maintenance. Sizing the system correctly and injecting the correct capacity of water into the air ensures correct relative humidity control.
The minimum required humidity for eliminating ESD is 30%. However, if you are already installing a humidification system, why not use it to make better conditions for the people who work in the space all day? Maintaining 50% relative humidity level can make the human body feel better and be more productive.
The key condition for the adiabatic humidifier to achieve 100% ESD elimination is the ability to maintain non-fluctuating humidity. The humidity level should not fluctuate more than plus/minus 1% throughout the controlled environment. Wetting and condensation causes fluctuation and uneven humidity levels and should be avoided. Maintaining the exact amount of water content in the air at all times will ground all the charges and maintain a static-free environment.
Ido Goldstein is Chief Engineer at Smart Fog Inc. He has over 20 years of experience in the humidification market. www.smartfog.com
