CHAPTER 4
HVAC EQUIPMENT
a. Conventional, commercially available HVAC equipment is general adaptable to use in hardened
,. facilities whether aboveground or underground. The equipment reviewed in this chapter is well known to
the experienced designers. The comments included are therefore mostly limited to potential areas of
application with suggested solutions to the design considerations of chapter 2.
b. In a typical system, fresh air is drawn via a tunnel or shaft through an air filter of the
conventional type and a CB filter; then it is drawn through a tempering coil. If close control of humidity is
required in temperate and aired areas, the air will be passed through an air washer or sprayed coil during
the summer months. In most cases, such high maintenance devices are not necessary since cooling coils
are sufficient for summer dehumidification and duct-type steam humidifiers for winter humidification.
c. The conditioned outside air is ducted to various rooms or zones where zone air handling units or
built-up units containing heating and cooling coils, filter, and fans will ingest, mix and condition the
fresh and return air prior to distribution to the conditioned spaces. Special purpose areas with low
humidity and wide temperature range requirements, such as storage areas for machinery, may be
conditioned with chemical or mechanical dehumidifiers.
4-2. Air cleaners.
a. Criteria. The criteria for air cleaners in a hardened facility will address the removal of airborne
contaminants from outside air brought in for air breathing equipment, environmental air within the
structure, and outside air brought in for human consumption. Each requirement is unique in that the
contaminant characteristics and the required filter efficiencies vary for each application.
b. Air washers. Air washers are used primarily in industrial air-conditioning. A central station
air washer is bulky and requires more space than conventional coils and chiller. For this reason, an air
washer will not be cost effective if installed in hardened facilities which require excavation of rock. Air
washer design is covered in ASHRAE Handbook, Equipment.
c. Dust separators. Dust separators will be utilized to remove normal airborne contaminants during
standby operation. Usage under other circumstances is not recommended due to decontamination and
disposal problems.
d. Air filters. ,
(1) Industrial ventilation will generally require only removal of the coarser air dust particles.
Administrative areas and areas, containing equipment sensitive to dust buildup will require removal of
the smaller components of atmospheric dust. Cost, space, pressure drop, and effectiveness determine filter
and filter media selection. Electronic air cleaners or high efficiency dry filters will be used for small
particle removal. For critical installation, such as deep buried facilities, redundant high efficiency filter
banks will be provided with provisions to keep one filter bank on stream while the other is being serviced.
(2) To increase the useful life of the high efficiency filters, prefilters will be installed
particularly in areas of high dust concentration. Prefilters will also be installed upstream of the
tempering coils to eliminate dust buildup downstream. At rated flow, the pressure drop will not exceed 0.3
in. wg when the unit is clean. Special electrostatic or other types of self-cleaning filters are not
recommended; conventional disposable units are preferred. Prefilter installation will permit easy
removal and replacement of the prefilters without undue leakage. Filters having a 90 percent efficiency
for the removal of 50-micron particles will suffice if fallout protection is the only consideration.
(3) Automatic filters of the moving-curtain or replenishable-media-type will be used for remote
installations because of the small amount of attention they require. This type of filter will also be used as
a prefilter or medium efficiency filter to save space, or for large airflows. For medium and high
efficiency requirements replaceable cells of the dry media type will otherwise be used. Typical
performances are summarized in table 4-1. Air filters are also covered in the ASHRAE Handbook,
Equipment.
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