Coil Freeze-Up Protection; Inner Distribution Tube Steam Coils - Carrier Aero 39MN Series Installation, Start-Up And Service Instructions Manual

*
Refer to Fig. 91 when dripping steam supply main to condensate return.
NOTES:
1. Flange or union is located to facilitate coil removal.
2. When a bypass with control is required.
3. Flash trap can be used if pressure differential between supply and con-
densate return exceeds 5 psi.
4. Coils with different pressure drops require individual traps. This is often
caused by varying air velocities across the coil bank.
5. Dirt leg may be replaced with a strainer. If so, tee on drop can be
replaced by a reducing ell.
6. The petcock is not necessary with a bucket trap or any trap which has
provision for passing air. The great majority of high pressure return mains
terminate in hot wells or deaerators which vent the air.
Fig. 92 — Multiple Coil High Pressure Piping
* Refer to Fig. 91 when dripping supply to return.
NOTES:
1. Flange or union is located to facilitate coil removal.
2. When control valve is omitted on multiple coils in parallel air flow.
3. When a bypass with control is required.
4. Coils with different pressure drops require individual traps. This is often
caused by varying air velocities across the coil bank.
Fig. 93 — Multiple Coil Low Pressure
Piping Gravity Return
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NOTES:
1. Flange or union is located to facilitate coil removal.
2. To prevent water hammer, drain coil before admitting steam.
3. Do not exceed one foot of lift between trap discharge and
return main for each pound of pressure differential.
4. Do not use this arrangement for units handling outside air.
Fig. 94 — Condensate Lift to Overhead Return

Coil Freeze-Up Protection

WATER COILS — If a chilled water coil is applied with out-
side air, provisions must be made to prevent coil freeze-up. In-
stall a coil freeze-up thermostat to shut down the system if any
air temperature below 36°F is encountered entering the water
coil. Follow thermostat manufacturer's instructions.
When a water coil is applied downstream of a direct-expan-
sion (DX) coil, a freeze-up thermostat must be installed be-
tween the DX and water coil and electrically interlocked to turn
off the cooling to prevent freeze-up of the water coil.
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For outdoor-air application where intermittent chilled water
coil operation is possible, one of the following steps should be
taken:
• Install an auxiliary blower heater in cabinet to maintain
above-freezing temperature around coil while unit is
shut down.
• Drain coils and fill with an ethylene glycol solution suit-
able for the expected cold weather operation. Shut down
the system and drain coils. See Service section, Winter
Shutdown, page 187.
STEAM COILS — When used for preheating outdoor air in
pressure or vacuum systems, an immersion thermostat to con-
trol outdoor-air damper and fan motor is recommended. This
control is actuated when steam supply fails or condensate tem-
perature drops below an established level, such as 120 to
150°F. A vacuum breaker should also be used to equalize coil
pressure with the atmosphere when steam supply throttles
close. Steam should not be modulated when outdoor air is
below 40°F.
On low-pressure and vacuum steam-heating systems, the
thermostat may be replaced by a condensate drain with a ther-
mal element. This element opens and drains the coil when con-
densate temperature drops below 165°F. Note that condensate
drains are limited to 5 psig pressure.
INNER DISTRIBUTING TUBE STEAM COILS —
inner distributing tube (IDT) steam coil used in the Carrier
39M air-handling units has an inner tube pierced to facilitate
the distribution of the steam along the tube's length. The outer
tubes are expanded into plate fins. The completed assembly in-
cludes the supply and condensate header and side casings
125
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