129
Fig. 101 —
Multiple Coil Low Pressure
Piping Gravity Return
Fig. 102 —
Condensate Lift to Overhead Return
Coil Freeze-Up Protection
WATER COILS
If a chilled water coil is applied with outside air, provisions must
be made to prevent coil freeze-up. Install a coil freeze-up thermo
-
stat to shut down the system if any air temperature below 36°F is
encountered entering the water coil. Follow thermostat manufac
-
turer’s instructions.
When a water coil is applied downstream of a direct-expansion
(DX) coil, a freeze-up thermostat must be installed between the
DX and water coil and electrically interlocked to turn off the cool
-
ing to prevent freeze-up of the water coil.
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 208.
STEAM COILS
When used for preheating outdoor air in pressure or vacuum sys
-
tems, an immersion thermostat to control outdoor-air damper and
fan motor is recommended. This control is actuated when steam
supply fails or condensate temperature 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 thermo
-
stat may be replaced by a condensate drain with a thermal ele
-
ment. This element opens and drains the coil when condensate
temperature drops below 165°F. Note that condensate drains are
limited to 5 psig pressure.
INNER DISTRIBUTING TUBE STEAM COILS
The 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
*
Refer to Fig. 99 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. 100 —
Multiple Coil High Pressure Piping
a39-4311
* Refer to Fig. 99 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.
a39-4312
a39-2992
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.
