Page 201 - Boiler Operator’s Handbook

186 Boiler Operator’s Handbook

screws that are just long enough to do the job so you prevent bypassing, structural problems with the con-
don’t waste time removing and replacing long ones. struction of the air handling unit or simply that the coil
Hinged access doors need to have those hinges is dirty. Note that a dirty coil is detected by bypass fac-
lubricated. Hear a squeak when you’re opening one? tor, not by higher than normal outlet temperatures. A
That’s the AHU telling you it needs lubrication. You can higher than design air pressure drop across the coil is
also note that the hinges are worn down (cheap ones do also an indication. A dirty coil, like a dirty filter, changes
that in a few years). You may have to replace them to the air flow which normally results in cooler unit outlet
get the door to fit properly but the first thing I always temperatures because the coil has less air to cool. These
try is nylon washers. Remove the hinge pin, raise the points are shown on the chart for cooling and reheat of
door, slip the nylon washer in where the hinge has worn Figure 5-44.
down, and replace the pin. I would prefer Teflon wash-
ers but they’re hard to find. Ultraviolet Lights
Keep the access openings in condition so they seal When you encounter an apparatus with ultravio-
out infiltration (air leaking in unfiltered) and you’ll have let lights inside they’re there to kill bacteria and other
less trouble with your AHU. growths that occur on the cooling coil because its cool,
Back to the psychrometric chart, locate the point on moist condition is an excellent breeding ground for all
the chart where the dry bulb and wet bulb temperatures those nasty things. They’re high intensity lights that can
at the outlet of the cooling coil intersect then mark and injure your eyes which is why you should find signs
circle it. Then draw a line from the point of inlet condi- on the access doors warning of exposure to them. You
tions through the point of outlet conditions extended to should be furnished with a set of UV filtering eyeglasses
the curve at the left side of the graph. The line between to wear when inspecting the lights because you have to
the inlet and outlet points indicates the changes in the open the access door and look in at them to see if they’re
air as it is cooled. The dry bulb temperature decreases working.
and the moisture content decreases. Extending that line When inspecting the cooling and heating coils turn
to a point where the line intersects the curve on the left off the lights using the switch that should be nearby and,
marks the ADP (apparatus dew point) which is the tem- when ready to close the unit up, turn the lights back on
perature of the cooling coil, either the refrigerant in it and make sure they’re all on before replacing the cov-
or close to the chilled water entering temperature. The ers or closing the access doors. Check the upstream side
difference between the ADP and the air temperature of the bulbs while you’re looking at the coils to detect
leaving the cooling coil is due to the fact that some of any dust or debris that has impinged on them. Wipe that
the air passes through the coil without contacting the side with a white glove or cloth for best results. If you
coil, actually bypassing it. Dividing the difference be- end up with a dirty glove or cloth you should look for
tween the coil outlet dry bulb temperature and the ADP, problems with the filters.
then dividing the result by the difference between the
coil inlet dry bulb temperature
and the ADP produces a value
called a bypass factor. Multiplied
by 100 it’s the percent of the air
that didn’t come in contact with
the coil. A low bypass factor would
be unusual and could be attribut-
ed to a replacement coil or change
during construction where a coil
with a lower bypass factor was in-
stalled. A high bypass factor, one
higher than indicated by the origi-
nal design drawings or more than
20%, can indicate leakage around
the coil due to changes in the air
handler’s structure, corrosion and
wasting away of metal baffles that
Figure 5-44. Psychrometric chart, reheat application