TM 5-852-9/AFR 88-19, Vol. IX
coil is modulated and coils are subjected to outside air, they become vulnerable to freezing. The following
should therefore be considered.
(a) Antifreeze solutions. Circulating an antifreeze solution as the heat exchange medium can
avoid freezing and the system can operate down to very low temperatures. Some difficulties, however, can
be encountered. When coils carrying antifreeze solution pass below freezing air, the steam-antifreeze system
heat exchanger can be damaged if the automatic steam control valve or the steam trap serving the heat
exchanger fails. Manual bypass valves shall be installed around the control valves and trap assemblies to
prevent heat exchanger freeze-ups. It is also possible to freeze up the entire building system from outside air
blowing through the ventilating system when the circulating pump fails. This can also happen following a
power outage, when the fan automatically resumes operation but the antifreeze solution pump does not. It
is good practice to provide a low limit thermostat control in the air discharge from the coil to stop the fan
and close the outside air dampers if air temperatures drop to about 35EF.
(b) Steam with face and bypass damper control Coils using steam as the heating medium can
be used to heat outside air. However, some precautions must be taken to avoid freezing the coil. A bypass
should be incorporated around the heating coil to regulate downstream air temperatures. The steam supply
to the coil must not be modulated when air temperature to the coil is below 32EF. So-called nonfreeze or
steam distributing coils freeze easily under modulated steam supply. In addition, a vacuum breaker should
be used on the steam supply to the coil to permit complete coil drainage when steam is shut off. A low-limit
thermostat, set to actuate when any single foot of a 20-foot length of bulb is exposed to the set temperature,
should be installed on the downstream side of the coil. This thermostat should shut off the fan and close the
outside air damper when air leaving the coil is approaching freezing temperature (40EF.) Figure 4-1 shows
a control diagram for a heating and ventilating unit with face and bypass control. Note the many controls
required for satisfactory operation. Figure 4-2 shows one method in which steam could be used for preheating
to prevent frost closure of filters. Again note the complex system. Condensate will leave the coil more readily
if vertical nonfreeze coils are used. Steam traps with adequate capacity for the pressure differentials
encountered under operation must be used. Traps should be installed not less than 12 inches, and preferably
18 inches, below the bottom of the coils to provide sufficient static head for condensate drainage. Condensate
discharge from traps serving coils handling cold air is drained by gravity. The condensate piping from trap
to condensate receiver should be properly sized and sloped to reduce back pressure and permit drainage.
Much simpler operation can be obtained by using antifreeze instead of steam.
(c) Recirculating air. An effective ventilation system can be provided by reheating return air
and mixing it with outside air. This system should be considered if less outside air is needed during winter
than summer, and wherever coil freezing is to be avoided. A system that mixes return air with outside air
before it goes to the heating coil could also be used. Freezing can occur, however, when cold air and return
air do not mix properly before going through the heating coil. Air stratification can occur even after air passes
through a centrifugal circulation fan. To reduce stratification, warm air should be brought in at the bottom,
and cold air at the top, of the mixing box. Parallel blade mixing dampers should be set to maximize mixing
(d) Heat recovery system. Another effective ventilation system uses an air-to-air heat recovery
coil or a run-around coil heat recovery system. These systems should be considered where ventilation requires
use of 100 percent outside air and exhaust, for example, in maintenance shops or fuel storage areas. These
systems extract heat from the exhaust air stream and transfer it to the cold incoming outside air stream. This
transfer preheats the incoming outside air before it goes through the fan and heating coil assembly. These
types of systems must be evaluated on a life cycle cost basis.
(2) Outside air openings. Ventilation requires outside air openings for both intake and exhaust.
Improperly designed openings cause many problems in arctic areas, as high winds, blowing snow, and rain
come through these openings. Hoods, when used, should be protected from falling ice or constructed to
withstand damage from it.
(a) Exhaust openings. In low wind areas, exhaust openings of normal design will be adequate.
The openings can be designed with stormproof louvers. If hoods are used, they should extend a minimum
of 1 foot below the opening, and be sized to allow air passage without excessive flow loss. Even with fans
operating, high winds can prevent air exit. Winds blowing against the building horizontally can move upward
and into the hood. This can cause the exhaust fan to rotate backwards when not in operation, and the screens
can clog with frost or snow, preventing the system from functioning as intended. During design,