Coil Freeze-Up Protection; Water Coils; Steam Coils; Inner Distributing Tube Steam Coils - Carrier 39LA-25 Installation, Start-Up And Service Instructions Manual

39L UNIT SIZE
03-25
LEGEND
F — Full Circuit

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 50.

STEAM COILS

When used for preheating outdoor air in pressure or vacuum
systems, 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
39L 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 includes the
supply and condensate header and side casings which are built to
slant the fin/tube bundle back toward the condensate header. The
slanting of the assembly ensures that condensate will flow toward
the drains. This condensate must be removed through the return
piping to prevent premature failure of the coil. The fin/tube bundle
is slanted vertically for horizontal airflow coils, and horizontally
for vertical air-flow coils.

IDT STEAM COIL PIPING

The following piping guidelines will contribute to efficient coil
operation and long coil life:
1. Use full size coil outlets and return piping to the steam trap.
Do not bush return outlet to the coil. Run full size to the trap,
reduce at the trap.
2. Use float and thermostatic traps only for condensate removal.
Trap size selection should be based on the difference in pres-
sure between the steam supply main and the condensate
Table 12 — Steam Coil Connection Sizes
FACE AREA COIL TYPE
Small 1-Row
CIRCUITING CONNECTION
F
NOTE: Connection sizes are MPT - inches.
return main. It is good practice to select a trap with 3 times
the condensate rating of the coil to which it is connected.
3. Use thermostatic traps for venting only.
4. Use only 1/2-in., 15-degree swing check valves installed
horizontally, piped open to atmosphere, and located at least
12 in. above the condensate outlet. Do not use 45-degree, ver-
tical lift and ring check valves.
5. The supply valve must be sized for the maximum anticipated
steam load.
6. Do not drip steam mains into coil sections. Drip them on the
pressure side of the control valve and trap them into the return
main beyond the trap for the coil.
7. Do not use a single trap for two or more coils installed in
series. Where two or more coils are installed in a single bank,
in parallel, the use of a single trap is permissible, but only if
the load on each coil is equal. Where loads in the same coil
bank vary, best practice is to use a separate trap for each coil.
8. Variation in load on different coils in the same bank may be
caused by several factors. Two of the most common are
uneven airflow distribution across the coil and stratification
of inlet air across the coil.
9. Do not try to lift condensate above the coil return into an
overhead main, or drain into a main under pressure with a
modulating or on/off steam control valves. A pump and
receiver should be installed between the coil condensate traps
and overhead mains and return mains under pressure.
10. Use a strainer (3/32 in. mesh) on the steam supply side, as
shown in the piping diagrams, to avoid collection of scale or
other foreign matter in the inner tube distributing orifices.
NOTE: IDT coils must be installed with the tubes draining toward
the header end of the coil. Carrier's IDT steam coils are pitched
toward the header end as installed in the unit.
1. Ensure the AHU (air-handling unit) is installed level to main-
tain the inherent slope. Also ensure the unit is installed high
enough to allow the piping to be installed correctly, especially
the traps which require long drip legs.
2. Do not fail to provide all coils with the proper air vents to
eliminate non-condensible gases.
3. Do not support steam piping from the coil units. Both mains
and coil sections should be supported separately.
IDT Steam Coil Installation
Refer to drawings to position the coils properly with regard to the
location of the supply and return connections. Ensure that the IDT
coil is pitched with the tubes draining toward the header. Carrier's
AHUs provide proper coil pitch when the AHU is installed level.
Refer to schematic piping diagrams and piping connection notes
for the recommended piping methods.
36
CONNECTION SIZE
Inlet 2-1/2
Outlet 1-1/2