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this rule is violated, even at very low face velocities. The same
result is often experienced if after-market fin coatings are
applied.
If a NuFin hydronic coil is installed with correct airflow, but
opposite piping hand, and counterflow is maintained, steps
must be taken to ensure that the coil is continuously vented,
and that the water velocity is maintained to prevent the coil
from air-binding.
Hot or cold areas of the coil face (or otherwise broad tem-
perature differences and stratification) are usually indications
that one or more circuits are air-locked internally. This can
result in coil freeze-up (a condition NOT covered by warranty).
Refrigerant coils may be rotated for opposite hand applica-
tions, maintaining the proper airflow direction.
Do not reposition the distributor(s), they will perform equal-
ly well in upflow or downflow positions. When soldering
expansion valves to up-feed distributors, use the minimum
satisfactory amount of solder to prevent damaging the valve or
plugging passages.
DIRECT EXPANSION COILS — Rotate the coil in vertical
plane and reinstall. Distributor must be on downstream side of
coil. (Refer to Fig. 51).
CHILLED WATER AND HOT WATER COILS — These
coils can be rotated. If coil is rotated in vertical plane and rein-
stalled with counterflow maintained, supply will be at the top of
the coil and return will be at the bottom. Ensure coil is continu-
ously vented and water velocity is maintained to prevent air
binding.
CAUTION
Chilled and hot water coils must not be rotated horizon-
tally. If coils are rotated horizontally, severe water blow-off
will result.
STEAM INNER DISTRIBUTING TUBE COILS — Rotate
in horizontal plane and reinstall. See Fig. 51.
PIPING — Direct expansion, chilled water, and hot water
coils should always be piped for counterflow. (Fluid should
enter the coil at the leaving-air side.) Steam coils must have the
condensate connection at bottom of coil.
To determine intervals for cleaning coils in contaminated air
operations, pressure taps should be installed across the coils
and checked periodically. Abnormal air pressure drop will indi-
cate a need for cleaning the coils.
Annual maintenance should include:
1. Clean the line strainers.
2. Blow down the dirt leg.
3. Clean and check operation of steam traps.
4. Check operation of control valves.
5. Check the operation of check valves to prevent conden-
sate flowback.
6. Check operation of thermostatic air vents, if used. A float
and thermostatic trap will contain a thermostatic air vent.
When the bellows is ruptured, it will fail closed.
7. Check operation of vacuum breakers.
8. Check operation of the thermal protection devices used
for freeze-up protection.
9. Steam or condensate should not be allowed to remain in
the coil during the off season.This will prevent the forma-
tion and build up of acids.
There are additional precautions and control strategies, as
found in various catalogues and in the ASHRAE Fundamentals
Handbook and in the Carrier System Design Guide — Piping
Section, when the entering-air temperature to the coil falls be-
low 35 F. These conditions occur when IDT coils are used for
pre-heat and/or face and bypass applications.
Freeze up protection:
1. Use a strainer in the supply line and the dirt leg ahead of
the trap.
DX AND ALL
WATER COILS
STEAM COILS
ONLY
Fig. 51 — Coil Rotation
2. Use a vacuum breaker in the return.
3. Do not use overhead returns from the coil. A floodback
can occur.
4. An immersion thermostat to control outdoor-air dampers
and the fan motor is recommended. This control is acti-
vated when the steam supply fails or the condensate
temperature drops below a predetermined temperature,
usually 120 F.
5. On low pressure and vacuum systems, the immersion
thermostat may be replaced by a condensate drain with a
thermal element. This element opens and drains the coil
when the condensate temperature drops below 165 F.
Note the thermal condensate drain is limited to 5 psig
pressure. At greater coil pressures they will not open.
In spite of the precautions listed above, a coil may still
freeze up. An oversize capacity coil, at partial load, with a
modulating steam control valve will occasionally freeze.
Freezing occurs in the 20 F to 35 F range of entering-air
temperatures. A better installation would be an undersize coil,
with an on/off control valve with thermostatic control in the
outside air, set at 35 F air temperature, installed downstream of
the first coil; or setting the minimum steam pressure at 5 psig.
Filters
FILTER SECTIONS — See Table 14 for filter data. Filters
are field supplied.
Flat filter section can use 2-in. or 4-in. thick filters. The flat
filter section as shipped accepts 2-in. filters. Remove spacer in
each filter track to provide the 4-in. space required to accom-
modate 4-in. filters.
On all filter sections, filters are pushed into the track until
they touch the opposite side of the unit. Any remaining space is
taken up by the adjustable 2-piece sheet metal spacer. See Fig.
52 for filter arrangements.
42
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