TM-5-855-4
CHAPTER 2
DESIGN
CONSIDERATIONS
2-1.
General.
a. To fulfill the basic objectives of the prime mission material/personnel (PMMP), the HVAC system
must prevent CB contamination and ensure survival and operation of occupants and equipment in a
degree consistent with the other elements of the facility as required by the PMMP or specified by facility
designers or both. The methodology for integrating the HVAC design in the total design of an effective
facilities is covered in TM 5-858-1 in the context of availability, survivability, endurance, performance,
technical and cost effectiveness, trade offs and optimization, and functional and interface compatibility.
HVAC includes both nonprotective and protective elements, such as hardened structures, reservoirs,
tunnels, and penetrations.
b. In particular, the hardened air-entrainment subsystem (AES), which ensures the continuous or
periodic transfer of air between the atmosphere and the facility, is covered in TM 5-858-5. The AES design
includes ports, ducts and chambers, blast valves, dust removal devices, and booster blowers. The TM 5-
858-5 also covers the design of fastener shock-resistant attachment/isolation of equipment/structures,
penetration protection (access/egress or umbilical), hydraulic surge protection (circuits, reservoirs), and
EMP protection.
c. TM 5-858-7 presents design guidelines for the facility support systems: power supply, waste-heat
rejection, air quality control, utilities, and services. Each one of these has a direct impact on HVAC
design which cannot be considered in total isolation but must be integrated in the total system-
engineering approach.
(1) As an example, consider the total parasitic load which includes power for the coolant
circulation pump, the refrigeration systems (largely compressors), and the air circulation fans. The
parasitic load will often be dominated by fan power demand. As a result, at least a conceptual design of
the HVAC is required to size the newer supply.
(2) The size of, the power supply in turn determines the power cycle heat rejection and the
combustion air requirements. Both of these elements are part of the HVAC design, which must now be
reevaluated to include these loads.
d. The air-conditioning (AC) of hardened `structures aboveground is essentially designed like
conventional AC to hold the interior temperature, relative humidity (RH), and air supply at levels and
volumes suitable for the intended use of the space.
(1) Underground this holding phase is preceded by a so called conversion phase, due to the much
longer time interval required to warm up or cool down the initial temperature and RH of the underground
space to the desired levels. The process of conversion must include the simultaneous control of
temperature and humidity. Neither the addition or extraction of heat alone, nor the use of ventilating air
alone, will ordinarily be sufficient for conversion purposes within acceptable time frames.
(2) The latent dehumidification load is usually greater than the sensible load during conversion;
however, the sensible heat rejected by the dehumidifiers will be reused to heat the space, except for
refrigerated storage cool-down below initial temperature levels. During conversion the structure will not
be used for either production or storage, except in cases of emergency.
e. In hardened structures, ventilation alone will not suffice since dissipating the heat with outside
air quickly becomes impractical. Therefore, a minimum quantity of outside air will be introduced with
provision for complete recirculation and some degree of AC to provide for a greater latitude in occupancy
and operational loads. During the seal-up period, the recirculation and cooling of interior air will permit
continued operation and occupancy that may otherwise be prohibited. AC systems will be kept simple and
designed for minimum maintenance.
2-2. Makeup air.
a. The proper quantities of outside air required for personnel are determined by pressurization, air
lock scavenging, occupant metabolism, and other special requirements, such as for smoke purge systems.
2-1
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