TM-5-855-4
2-11.
Economic factors.
a. Space utilization.
(1) The selection and operation of equipment within a hardened facility are governed primarily
by requirements other than economy, especially underground. The economics of equipment selection and
operation will be compromised where dictated by facility mission requirements. The HVAC designer
will review the equipment configuration and space allocation to provide a familiarity with maximum
..
utilization of excavated space, minimum consumption of energy, and optimum hardness design to ensure
successful completion of the mission.
(2) Trade-offs must be made in efficiency and noise when using smaller ducts with higher
velocities and small high capacity equipment such as fans, coils, and boilers. Noise will be kept within
limits set by Occupational Safety and Health Administration (OSHA); however, for each space, and where
necessary, such design consideration as grouping and isolation of equipment and noise attenuation will
be provided for maximum utilization of space.
b. Economy of operation.
(1) Facilities designed for uninterrupted power have continuous operation prime movers. Waste
heat from jacket water and engine exhaust will be recovered to heat the facility and domestic hot water.
Lube oil heat recovery may also be practical. In a gas turbine cycle, the thermal efficiency is
approximately 12 to 60 percent with the remainder of the fuel energy discharged in the exhaust or through
radiation. A diesel engine rejects approximately 30 percent of the input fuel energy to the jacket water and
30 percent to the exhaust gases.
(2) Practically all the heat transferred to the engine jacket water can be utilized but exhaust heat
recovery is limited to 300 F leaving gas temperature to prevent condensation of water vapor and acids in
the exhaust piping. Depending on the initial gas temperature, approximately 50 to 60 percent of the
available exhaust heat can be recovered. Heat recovery methods are covered in detail in the ASHRAE
Handbook, Systems.
(3) In frigid and temperate climatic zones, air-to-air heat exchangers or heat pipes will be
installed in outside air and return air ducts for sensible heat reclamation. Where conditions permit,
duct-mounted rotary air desiccant wheels will be installed in air-conditioning exhaust and outside air
ducts for latent and sensible heat recovery.
(4) Where facility hardness requirements and interior humidity design conditions permit,
outside air will be used to cool the facility when ambient dry-bulb temperature is 640 F or lower.
(5) Where high-radiant, heat-producing equipment, such as ovens, furnaces, and infrared
devices are to be installed, consideration will be given to isolating such equipment by the use of metal
panels through which water at normal temperature is circulated to carry off this high heat, thus reducing
the load on the air-conditioning system. Similarly, the selection of liquid coolant-type power units,
having water jackets through which either water at normal temperatures or condenser water can be
circulated, will reduce the load on the "air-conditioning system.
2-12. Survivability and reliability.
a. General. Survivability and reliability of hardened structures are discussed in TM 5-858-1. By
way of illustration, some of the HVAC applications of these considerations are included in this manual.
Heating and Air-conditioning equipment installed in hardened facilities will be of such design or
otherwise protected to withstand the shock (ground motion) and overpressure effects of weapons.
Experience with the ballistic missile programs has proven that standard air-conditioning equipment can
be utilized in hardened facilities if properly designed and protected.
b. Redundancy of equipment.
(1) Systems requiring a high degree of reliability will include redundant units which will
automatically start and maintain the load should the operating unit fail. The required degree of
reliability is based on the function of the facility, allowable downtime for critical systems, type of facility
operation (continuous or standby), type of system operation (remote or local), and degree of maintenance.
(2) Fans and pumps in critical HVAC systems will be installed in multiples of two or three. The
degree of reliability will determine whether units will be installed in multiples of two with each unit
designed to carry 100 percent of the load or in multiples of three with each unit designed to carry 50 percent
of the load.
(3) Controls will be arranged to keep one of the units in near new condition, operating it only as
required for maintenance. In some cases, bypasses for control valves will be required for AC reliability _
where single AC units are used. Remotely operated valves in critical fluid systems will require two
valves in a series to ensure reliability of facility isolation during the button-up phase. Computer cooling
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