Also, some facilities, such as health care facilities, have specific requirements governing the rate at which outside air must be brought into the facility. So most facilities must install humidification systems to increase building relative humidity during heating seasons. They require no additional energy to heat the water for evaporation. Rather, they inject water in micron-sized droplets small enough that they readily evaporate in the airstream. Various system types are available that use atomizing nozzles, rotating discs and ultrasonics to inject the moisture into the airstream.
Adiabatic systems offer the advantage of low energy cost.
Isothermal systems use an external heat source to generate steam, which is then injected into the airstream. Units are available as packaged electric or steam-driven systems. Since the steam injected into the airstream is generated within the unit itself, there is no problem with boiler feedwater chemicals typically found in most steam systems. Isothermal humidification systems offer the advantages of low first costs and low maintenance costs, and they can increase relative humidity to almost any value. Their primary disadvantage is the energy costs associated with the steam generation.