Handbook of air conditioning and refrigeration / Shan K

15.84 CHAPTER FIFTEEN
TABLE 15.4 Comparison of Various Types of Humidifiers
Heating Pneumatic
Description Steam element Air washer Ultrasonic atomizing
Humidifying capacity Small to large Small Large Small to large Medium and large
Oversaturation No No Yes Possible Possible
Quality of
humidification
Good Good Good Good Good
Dripping of water in
conditioned space
No No No No No
Energy consumption Need more energy if Same as steam Saving energy if Saving energy if Saving energy if
and operation cost cold air supply is cold air supply is cold air supply is cold air supply is
needed, so operating
cost is higher
needed in winter needed in winter needed in winter
Initial cost High High Medium Medium High
Maintenance work
and maintenance
cost
Low Medium High Medium Medium
Capacity control Modulation On/off or
modulation
Modulation Multiple stages Modulation
Response to control Fast Slow Medium Fast Fast
Space occupied Small Medium Large Small Medium
Equipment noise Medium Low Medium Low High
Various types of humidifiers are compared in Table 15.4. For commercial buildings that need a
warm air supply during winter and for which a steam supply is available, a steam humidifier is
often the most suitable choice. For a small packaged unit serving a health care facility, a heating
element humidifier may be selected. For a telecommunications center with an appreciable amount
of space heat gain, an ultrasonic humidifier is often the right choice. Ultrasonic humidifiers are also
often used in residences for their comparatively low equipment noise. For a factory in which space
air contains lint and other dirt that should be cleaned, or for which a cold air supply is needed to
offset the cooling load in winter, an air washer may be the suitable choice.
When air washers or atomizing humidifiers are selected as the humidifiers, oversaturation may
result in the supply air discharged from the air washer or atomizing humidifier. As oversaturation
causes wetted surfaces and dampness along the air distribution passage, the following remedies are
recommended to prevent IAQ problems:
● All-metal (fiber-free) silencer or active silencer should be used, if required.
● Interior surfaces of ducts that contact supply air must be galvanized metal sheets.
● Duct heat gain may be beneficial to eliminate oversaturation.
● Periodic monitoring of the moisture and dirt conditions of the ducts and silencers should be
provided.
ASHRAE/IESNA Standard 90.1-1999 mandates that when a zone is served by a system or systems
with both humidification and dehumidification, means (such as limit switches, mechanical stops,
software program for DDC control) shall be provided to prevent simultaneous operation of humidification
and dehumidification equipment. Exceptions are zones served by desiccant dehumidification
systems, which are used with direct evaporative cooling in series, or zones where specific
humidity levels are required such as computer rooms, museums, and hospitals. Standard 90.1-1999
also mandates that humidifiers with preheating jackets mounted in the airstream shall have an automatic
valve to shut off preheat operation when humidification is not in use.