Kruse (1991) analyzes methods of providing humidification.
Humidification can be provided in one of several forms. The need for clean air which is free from particles and ions usually eliminates humidifiers which use untreated water, treated water or steam from treated boiler systems. This leaves steam and evaporative styles which use deionized water or reverse-osmosis reject water. The analysis actually hinges around two issues; the cost of the water and the cost of the energy to convert it into a usable form. Regarding the cost of the water, if the building has a [Deionized Water] D.I. production system which uses a Reverse Osmosis [R.O.] unit, use the R.O. rejected water because it is essentially free. Otherwise, use the same source of D.I. water as the rest of the project, and pay the price. Regarding the energy required to use the water, the most expensive is usually electric energy. Each analysis must be site specific, of course. Packaged humidifiers can use electricity to convert D.I. water into steam. These may be your only choice for small projects, but they will consume lots of energy on large designs. Packaged humidifiers can also use steam as an energy source to convert D.I. water into steam. This is usually more cost effective since the plant steam will have usually been made by using a lower-cost fossil fuel instead of electricity. Steam generators can produce D.I. steam in a similar fashion, but on a larger scale. Evaporative humidifiers are the best economic choice... . This is especially true with tight humidity tolerances and in dry climates where an adiabatic "free cooling" effect can be obtained. Altogether, the best combination is evaporative humidification fed by R. O. rejected water.
Mumma (1997) provides the following advise regarding energy-efficient ultrasonic humidity control.
The ultrasonic humidifier is a more energy-efficient means of humidity control than steam humidification… Ultrasonic humidification requires a different approach when sizing the heating coil, which must not only be able to temper the mixed air but must also be able to provide sufficient heat to overcome the evaporative cooling effect of the humidifier (i.e., humidification energy). In steam humidification systems, the heating coil is not required to provide any of the humidification energy. Furthermore, the location of the SAT control sensor must be downstream of the ultrasonic humidifier. In addition, hunting is a situation that can frequently occur with ultrasonic humidification due to the multi-step nature of the device. The same hunting is not as likely with steam humidification if the steam control is fully modulating..
Brown (1993) identifies typical humidity ranges for laboratories.
For energy conservation reasons, the laboratory humidity ...[is typically] allowed to vary from a maximum of 50% RH when outdoor wet-bulb is greater than 54°F (12.2°C) to a minimum 40% RH when outdoor air wet-bulb is 50°F (10°C) or less. Supply air dew-point control ...[is] reset down accordingly. Face and bypass dampers ...[are] controlled to bypass air around the evaporative cooler to maintain supply air dew point.
Brown (1990) discusses humidification choices.
It is not possible to fully evaluate heat recovery potential and benefits without considering humidification. When humidity is added by steam, fuel-generated energy is always required. When humidity is provided with a direct evaporative cooler, all or a portion of the energy required to evaporate water can be supplied by recovered heat. While any supplemental energy required is fuel-generated, the source is not restricted to steam and the use of hot water is generally the preferred choice. As expected...the greatest requirement for humidification energy occurs in the colder/drier climates. Therefore, it is the author's opinion that a direct evaporative cooler be given highest consideration for humidification an energy conservation measure. … In some climates, direct evaporative cooling has an added benefit by providing simultaneous cooling and humidification.
