Handbook of air conditioning and refrigeration / Shan K

Ultrasonic Humidifiers
An ultrasonic humidifier uses a piezoelectric transducer to convert high-frequency mechanical electric
signals to a high-frequency ocscillation. The transducer is submerged into a water pan in which
water is well treated and demineralized. The negative oscillation produces a momentary vacuum
that causes the water to cavitate into vapor at low pressure. The positive oscillation creates a highcompression
wave which forces the water particles to leave the water surface and inject into the
airstream. Water films are then broken into a fine mist. Air containing this mist can be blown into
the space directly or through an air system by a fan.
If the water is well treated, it will be free of mineral deposits, and no filter is required downstream
from the humidifier. Ultrasonic humidifiers create less equipment noise than other atomizing
humidifiers. In the mid-1990s, ultrasonic humidification had been installed in telecommunication
projects. Ultrasonic atomizing humidifiers with small humidifying capacities have been used directly
in residences.
Case Study: White Plains Ultrasonic Humidification Project
Longo (1994) reported on a retrofit ultrasonic humidification project which is located in a 1950s
building in White Plains, New York. The humidification system in a totally new air system serves
space that houses solid-state complex multiswitching processors. Static charges created in a lowhumidity
environment can be detrimental to system components and cause failure.
The 100,000-cfm (47,200-L/s) air system consists of four supply fans (two standby), highefficiency
filters, and cooling coils with outside air economizer for free cooling. The ultrasonic
humidifiers were placed after the filters and upstream from the supply fan. There are altogether 43
ultrasonic units mounted on stainless-steel racks, each rated at 39.6 lb/h (18 kg/h) and 960 W. A
humidity sensor is installed in the return air duct. The controller turns the humidity system on one
stage at a time up to a total of six stages. A high-limit sensor set at 80 percent relative himidity
shuts down the system in case of overhumidification.
Water is treated by a series of prefilters, a reverse osmosis system, and deionization canisters.
Water quality is controlled by a resistivity probe. When water resistance reaches 0.02 M�, the
deionization canisters will be changed.
After completion, the actual running results in winter are excellent. As the equipment was
installed gradually, the supply air temperature was rescheduled, and the space relative humidity was
maintained precisely at 50 percent. The initial cost of the White Plains humidity system was
$350,000. Annual operating cost was $23,000, which was less than one-tenth of the annal operating
cost of an electric heating element humidity system.
Centrifugal Atomizing Humidifiers
These humidifiers use the centrifugal force produced by a rotating device such as a rotating cone,
blades of an axial fan, rotating disk, or rotating drum to break the liquid water film into fine mist or
to fling it into fine water droplets. When air is forced through an atomizing water spray, produced
by a pulverizing fan, water vapor is evaporated and added to air as a result of the mass concentration
difference between the saturated air film at the surface of the droplets and the ambient air. The
rotating device receives liquid water either from a pressurized water supply or by dipping into the
surface of a nonpressurized supply. The humidifying capacity of a pulverizing fan depends mainly
on the volume flow rate of the fan. Because of the oversaturation characteristics of the pulverizing
m˙ cap
AIR SYSTEMS: COMPONENTS—FANS, COILS, FILTERS, AND HUMIDIFIERS 15.77
fan, may vary from 50 to 150 lb/h (23 to 68 kg/h). Oversaturation is the excess amount of
water particles present in the moist air, and it is discussed in Chap. 20. Many different types of rotating
humidifiers with limiting humidifying capacities are used directly in the conditioning space
in residential and industrial applications. Most are portable.