Sunday, February 11, 2018

Humidification Methods for Environmental Chambers

interior of biological environmental chamber with shelves and lighting
Environmental chambers for insect and plant research
are common applications requiring humidity control.\
Image courtesy Percival Scientific
Many environmental chambers utilized in research or production applications have a requirement to maintain a specified moisture level in the controlled space. Most often, the moisture level is specified as relative humidity. Some moisture levels may require the removal of moisture from the space, but the focus of this article is those instances where moisture must be added to the air in order to maintain the setpoint.

In designing a humidified environmental chamber, or specifying one for your own application, the challenge is to add moisture to the air in a fashion that does not result in detrimental effects on the contained materials or work. Additionally, the humidification system operation must work in concert with, not adversely to, other elements of the system that are tasked with maintaining other environmental conditions, such as temperature. With some awareness of the various methods available for humidification and their potential impact on your work and your budget, you will be better positioned to make an informed selection and achieve your desired outcome.

Several humidification methods are widely available for commercial use, all of which result in the same thing; adding water vapor to the air in the controlled space.

  • Steam introduction directly into the space: This direct addition of water vapor using low pressure steam should only be used when the sourced steam is positively known to be suitable for introduction into an occupied space. Any treatment chemicals used for efficient boiler and steam system operation could possibly travel with the steam into the controlled space. If the steam is clean, the main concerns are potential heating of the air by the sensible heat of the steam and the reliability of the steam source. An advantage of this method is that it is very rapid in response to a control signal. The hardware to regulate and control the steam injection can be costly, and present a challenge to some service techs.
  • Passive evaporating of water using the chamber air: Water will evaporate into air at differing rates under differing conditions. Directing a flow of air from the space over a water surface or falling stream of water will result in some degree of evaporation, with a corresponding increase in relative humidity. These systems come in many configurations designed to fulfill a range of requirements and available installation space, some with heating of the water to increase the evaporation rate. They are inexpensive and slow to respond. A major issue for some applications is the tendency for the water sitting in the evaporating pan to harbor biological contaminants. Frequent cleaning is a characteristic maintenance item for these systems. Cleaning reduces contamination levels and also removes the accumulated mineral deposits and other debris left behind by the evaporating water. This method of moisture addition will result in a small amount chamber air cooling.
  • Evaporating water using electric heat - This humidifier is similar to the passive evaporating method just described, but with the addition of a controlled heating system that can increase the evaporation rate. Consisting essentially of electric heaters immersed in water contained in a comparatively compact unit, these humidifiers can be selected to deliver very large amounts of vapor from a small package. The electric heat is the primary driver of evaporation. Heat applied to the water can be regulated in very fine increments to provide accurate humidity control, with little excursion from the setpoint. It is not uncommon for these units to boil the water contained in the unit, so one concern is the amount of sensible heat that is delivered with the water vapor and its potential impact on room temperature. Cost is moderate. Maintenance levels are low, but a treated water supply is recommended to minimize scale formation. Biological contamination is of little concern with these units due to the temperature at which they operate. Several configurations of this type of humidifier allow for a broad choice of installation options.
  • Evaporating water using steam - With essentially the same operation as described above for the electrically heated unit, a steam heated humidifier utilizes steam as the heat source. Precise control of a steam supply is generally more difficult and expensive than a comparable capacity electric heating system. These systems are generally available as larger capacity units.
  • Water dispersion devices - Several technologies are utilized to increase the surface area of a water supply and disperse it into the controlled space where it is evaporated by the air. Simple and inexpensive centrifugal atomizers create fog of tiny water droplets that evaporate as they contact the chamber air. Other devices employ compressed air and specialized spray nozzles to create very small water droplets that produce a similar result. Ultrasonic transducers aer also employed to atomize water into a fine mist. Centrifugal atomizers are probably the least expensive option, but may not provide the level of control accuracy needed, since they are not well suited to rapid on/off cycling. Ultrasonic transducers and spray nozzles can be sized or configured to provide better levels of output modulation, but come with a higher cost. Maintenance requirements for water dispersion devices should be considered. Another consideration for this type of humidification is water supply quality. Liquid water is dispersed in the air, then evaporated. Anything contained within the water droplet that is not water will be liberated in the controlled space. Over time, particulates and once dissolved minerals can accumulate on environmental chamber surfaces and require cleaning and removal.
There are many facets of humidification to consider for an environmental chamber or room. The less expensive first cost option may not be the best overall selection. Share your requirements with an environmental chamber specialist and leverage your own knowledge and experience with their application expertise to develop an effective solution.

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