TM-5-855-4
is first warmed, the rock surface is below the air dewpoint. Moisture from the air condenses on the rock,
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adds to the existing seepage, and reduces the room latent load. The latent heat of condensation released
adds to the sensible heat flux penetrating the rock. Upon continued heating, the rock surface temperature
rises. When the surface temperature is above the air dewpoint, part of the air sensible heat is converted
and used to evaporate some of the seepage at the rock surface, thus reducing the heat flux penetrating the
rock. The moisture added to the air increases the room latent load.
f. Vapor barriers or thermal insulating materials in direct contact with rock surrounding
underground spaces are not generally recommended.
(1) Hydrostatic pressures generated because of the depth of an underground chamber are greater
than can be restrained by ordinary vapor barrier materials or even by moderately heavy concrete liners.
Hydrostatic heads up to 43 psi could develop 100 feet below the water table depending on the over-burden
permeability.
(2) Insulating material applied directly to rock walls or to concrete in contact with such walls is
likely to become wet, either by condensation or from ground water or both, with possible damage to the
insulating materials or to the fastenings.
(3) A concrete liner may be installed in an underground space to improve the appearance or to
reduce the chances of spalling, but should not be considered effective either as thermal insulation or as a
vapor barrier. The dehumidification load in such a space is the same as that for a bare chamber.
g. If the walls, ceiling, and floor of an internal structure are vapor proof, the water vapor to be
removed is equal to that liberated by the equipment and personnel within the structure. Conditions in the
annular space do not directly affect conditions within the structure. If the walls, ceiling, and floor of the
internal structure are pervious, the water vapor to be removed is the sum of the water vapor liberated by
personnel and equipment and that entering the internal structure through the walls, ceiling, and floor by
permeation or by convection from the annular space.
2-9. Air distribution and fire protection.
a. General configuratwn.
(1) A central AC! system has the advantage of lower chilled and hot water piping first-cost and
lower noise generation when the unit is remote from the conditioned spaces. Disadvantages are large,
long ducts, inflexibility under moderate load, and the inherent unreliability of a single system when
compared to installation of multiple units.
(2) On the other hand, using a large number of self-contained air-conditioners , one for each room
or zone, simplifies the zoning and control problems, improves the overall reliability, and avoids the use of
large, long insulated ducts. Noise may be a problem if such equipment is used because human occupants
may be situated near the source of the noise. Self-contained air-conditioners include condensing units in
pre-assembles cases. For underground use, these condensers will be water cooled.
(3) Fresh air will be ducted to the return side of the air-conditioner in proportion to the population
in the room or zone being conditioned. This allows the air to be tempered or bypassed through a
conditioning coil before entering the occupied space..
(4) Self-contained air-conditioners will be furnished with hot water or steam coils when heating is
required or arranged to serve as heat pumps and, thus, warm as well as cool and dehumidify spaces when
required. Most of the heat for warming a space with a heat pump arrangement is taken from the water
used at other times to cool the condenser.
b. Distribution. Prior to design of the air distribution system, the designer of HVAC systems for
multi-room structures will analyze the requirements for each room. Areas containing odors, toxic
vapors, dust, and other contaminants will be designated as contaminant areas. All other areas will be
designated as non-contaminant areas.
(1) Contaminant areas will be maintained at a lower pressure relative to adjacent rooms to ensure
that contaminants generated within the area will not escape to other areas. To obtain the maximum
utilization of ventilating air, exhausts from toilets, and kitchens (properly degreased) will be discharged
into unoccupied equipment rooms.
(2) Filtered air will be distributed in a manner to give the most effective results in providing
uniform air quality for occupants. Filtration requirements will be specified as a function of the location
of the facility and the air quality required to accomplish its mission. A duct system will be used wherever
feasible, except between areas or rooms where pressure differences are to be maintained.
(3) In structures not provided with central AC or air-handling equipment, circulation or
recirculation of air can be obtained by the proper placement of floor or wall fans. During seal-up periods
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