TM-5-855-4
when fresh filtered air cannot be supplied, this continued recirculation of air will extend the time of
occupancy.
c. Air motion. Air motion in comfort air-conditioned spaces should be at a design rate of 50 fpm. In
areas where people will be seated, such as in offices, control rooms, and personnel support areas, room air
motion should be ,between 25 and 35 fpm.
d. Fire protection. The designer of HVAC systems for hardened structures will take special
precautions to prevent the spread of fires through ducts and shafts.
(1) Fire dampers and smoke detectors will be installed in all duct systems in accordance with
National Fire Protection Association (NFPA) publication 90A. In the event of a fire, flow of smoke from
the fire zone will be inhibited from spreading to required interior ways of exit access, interior enclosed
stirs and ramps, passageways, and designated refuge areas.
(2) Smoke control and purge systems will be included as an integral part of the HVAC systems.
Such systems will involve HVAC systems alone or in combination with other systems such as emergency
venting, pressurizing systems, and fire suppression systems and are covered in the ASHRAE Design of
Smoke Control Systems for Buildings.
e. Radio frequency interference (RFI) protection.
(1) Supply and return air ducts serving rooms with sensitive electronic equipment will be
equipped with RFI filters to prevent transmission of RFI into the electronic equipment room. This RFI
protection requirement is usually included in the design criteria.
(2) To maintain the attenuation of the RFI shielded room at the prescribed level, a waveguide filter
will be installed in the duct which will result in attenuation equal to the center area attenuation of the
room. The air-duct wave guide filter will be specified in terms of the attenuation over a specified range of
RFI frequencies and the allowable air pressure drop across the filter in accordance with TM 5-855-5 and
TM 5-858-5.
f. Internal structures.
(1) To conserve space an internal structure will be cooled by utilizing the annular space between
the structure and surrounding rock as a cold-air plenum held at or near the initial rock temperature. The
cool air in the annular space plenum is distributed into the internal structure to maintain the desired
interior conditions.
(2) Air from the internal structure, except for that exhausted, will be discharged directly into the
annular space through a cooling coil that lowers the air temperature to that of the surrounding rock.
Leakage of air between the plenum and access passageways will be prevented to avoid discharge of cold
air into warm passageways.
2-10. Chemical, biological, and radiological (CBR) protection.
a. General. Protection against CB agents and radiological fallout will be provided if the facility is to
continue to function regardless of attack. There are not varying degrees of CBR protection, and it will be
continuous if the effects of covert attack are to be reduced.
(1) Air filtration, pressurization, and personnel decontamination are the three basic principles of
CBR protection. Radiation shielding and CBR detection are not part of this manual. Entrances and
decontamination facilities (covered in chapter 6) will permit egress and ingress without endangering the
occupants of the facility.
(2) CBR agents may occur as gases, liquids, or solids and can be airborne, waterborne, or surface
contaminants. Airborne agents are chemical toxic gases and CB aerosols. Surface contaminants are too
heavy to remain suspended in the air. They can be either liquids or solid. Liquids may be chemical or
biological agents. Solid contaminants may be biological or radiological agents.
b. Pressurization.
(1) Dependable exclusion of airborne agents is ensured by sealing possible leakage sources,
providing pressurization, and filtering the makeup air. The internal overpressure P in inches of water
(in. wg) needed to prevent infiltration from winds of velocity v in miles per hour (mph) is 110 percent of
the velocity head equivalent of the wind or
P' =
(2) Air input needed to achieve the desired overpressure is determined by the exhaust and leakage
rate of the installation and is independent of the size of the installation. Contaminants are effectively
removed from air by passing them through a standard filter unit developed by AMCCOM-CRDC. After
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