211
heating coils, the actual drop may not be measurable, thus of
insignificant consequence.
It is important that the airflow direction of the NuFin coil be ad
-
hered to when latent cooling is possible. Significant moisture car
-
ryover from the face of the dehumidifying coil will result if 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 oppo
-
site piping hand, and counterflow is maintained, steps must be tak
-
en 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 temperature
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 applications,
maintaining the proper airflow direction.
Do not reposition the distributor(s); they will perform equally 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. 197.)
CHILLED WATER AND HOT WATER COILS
These coils can be rotated. If coil is rotated in vertical plane and
reinstalled with counterflow maintained, supply will be at the top
of the coil and return will be at the bottom. Ensure coil is
continuously vented and water velocity is maintained to prevent
air binding.
STEAM INNER DISTRIBUTING TUBE COILS
Rotate in horizontal plane and reinstall. See Fig. 197.
PIPING
Direct expansion, chilled water, and hot water coils should always
be piped for counterflow. (Fluid should enter the coil at the leav
-
ing-air side.) Steam coils must have the condensate connection at
bottom of coil.
To determine intervals for cleaning coils in contaminated air oper
-
ations, pressure taps should be installed across the coils and
checked periodically. Abnormal air pressure drop will indicate 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 for
-
mation and build-up of acids.
There are additional precautions and control strategies, as found in
various catalogs and in the ASHRAE Fundamentals Handbook
and in the Carrier System Design Guide — Piping Section, when
the entering-air temperature to the coil falls below 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.
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 tem
-
perature drops below a predetermined temperature, usual
-
ly 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 ther
-
mostatic control in the outside air, set at 35°F air temperature, in
-
stalled downstream of the first coil; or setting the minimum steam
pressure at 5 psig.
Minihelic Gage
The Minihelic gage is a pressure gage that is a selectable option in
AHU
Builder
®
. It gives job site personnel the ability to view the
pressure drop across flat and angled filters, which gives an indica
-
tion of filter integrity. See Fig. 198 for a picture of a typical Mini
-
helic gage.
CAUTION
Chilled and hot water coils must not be rotated horizontally. If
coils are rotated horizontally, severe water blow-off will result.
DX AND ALL
WATER COIL
S
S
TEAM COIL
S
ONLY
Fig. 197 —
Coil Rotation
a39-2388
