TM-5-855-4
4-4. Coils and piping.
a. Tempering coils.
(1) Tempering coils are normally installed in makeup air units. The tempering coils is used
to heat the outside air in winter to prevent condensation on ducts and to prevent freeze-up of heating and
cooling coils in downstream air handling units.
(2) Steam tempering coils require careful design to prevent freeze up of the coil. Design
considerations will include large tubes of the steam distributing type, mounted vertically, with full steam
pressure on the coils at all times. Face and bypass dampers controlled by a downstream duct-mounted
thermostat, will be used for controlling the tempered air temperature. Controlling temperature with a
modulating steam valve is not acceptable because of the danger of freeze-up.
(3) Hot water tempering coils, utilizing a heat exchanger to maintain water temperature, a coil
pump to maintain flow, and thermostatically controlled face and bypass dampers can function
successfully in extreme weather with proper controls and alarms. A mixture of water and antifreeze will
be circulated through the coils to minimize the possibility of coil freeze-up should the controls or pump fail.
The coil and the heat exchanger will be sized including the derating due to the added antifreeze
component.
b. Air heating and cooling coils.
(1) Each hardened structure will be zoned and provided with at least one or more air-handling
units for cooling and dehumidification. For underground facilities under conversion, the primary
problem is one of dehumidification and reheat. During holding, the dehumidification load decreased, but
the cooling load increases. Both the cooling and the reheat coils will therefore be provided in two sections.
For cooling and dehumidification, one six-row section and one four-row section will be used, while for
reheat a four-row section and a two-row section will be selected. In this way the air-handling units can
meet the unusual load requirements for either dehumidification or cooling.
(2) Use of chilled water in unit-air-conditioners for individual rooms or zones has the advantage
of simplicity and flexibility of control. Chilled-water lines that pass through spaces with high dewpoints
or which are air-conditioned will be insulated to prevent condensation. Temperature control will be
accomplished by starting and stopping the fans in the units, by means of dampers to control the airflow
through the cooling coils, by regulating the flow or temperature of chilled water to the coils, or by a
combination of these methods. Heating coils will be installed in the air-conditioning units along with the
cooling coils, if desired.
(3) For central chilled water system and when. cooling is critical to the mission of the facility, a
loop-type system of chilled water distribution will be provided with necessary valving to isolate loop
segments in case of failure of a portion of the system.
c. Piping. The design of all piping systems and materials used in a hardened facility will conform
with nationally recognized codes, standards, manuals, and recommended practices.
Flexible
connectors, vibration eliminators, and expansion joints will be utilized to connect piping to HVAC
equipment which is subject to movement. Piping passing to and from RFI exclusion or containment areas
will be designed to preclude transmission of the RFI waves. Hydraulic transient pressures are covered in
TM 5-858-5. In-line-flow-through attenuators of the precharge-bladder-type are available in a wide range
of sizes and pressures. Figure 4-4 shows the construction features of a typical flow-through,
hydropneumatic, bladder-type attenuator.
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