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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 217.
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
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 airflow 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 (F & T) traps only for conden-
sate removal. Trap size selection should be based on the
difference in pressure between the steam supply main and
the condensate 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
/
-in., 15-degree swing check valves installed
1
2
horizontally, piped open to atmosphere, and located at
least 12 in. above the condensate outlet. Do not use 45-
degree, vertical lift and ring check valves.
5. The supply valve must be sized for the maximum antici-
pated 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.
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 stratifi-
cation of inlet air across the coil.
8. 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.
9. 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: The 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.
10. Ensure the AHU is installed level to maintain the inherent
slope. Also ensure the unit is installed high enough to al-
low the piping to be installed correctly, especially the
traps which require long drip legs.
11. Do not fail to provide all coils with the proper air vents to
eliminate noncondensable gases.
12. 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.
Refrigerant Piping, Direct-Expansion Coils
Direct-expansion (DX) coils are divided into 2 or 4 splits depend-
ing upon the unit size and coil circuiting. Each split requires its
own distributor nozzle, expansion valve, and suction piping. Suc-
tion connections are on the air entering side when the coil is prop-
erly installed. Matching distributor connections for each coil split
are on the air leaving side. See unit label or certified drawing to as-
sure connection to matching suction and liquid connections.
NOTE: Distributor nozzles are factory selected and installed for
adequate performance in many unit applications. For best perfor-
mance, use Carrier's AHUBuilder
for each unit and replace the factory-installed nozzles as required.
See the Distributor Nozzle Change-Out section on page 138 for
further details.
To prevent damage to the coil or coil headers: Do not use the
headers to lift the coil. Support the piping and coil connections
independently. Do not use the coil connections to support pip-
ing. When tightening coil connections, use a backup wrench
on the stub outs.
137
program to select nozzle sizes
®
CAUTION
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