Page 203 - Boiler_Operators_Handbook,_Second_Edition

188 Boiler Operator’s Handbook

air would be supersaturated with moisture and droplets an area that contains CO because any one of them can
of water forming, producing what we refer to as fog. The call a utility or fire company that will come to the site
condition could occur as the return and outside air mix and, if they find CO in the building, shut off the fuel
resulting in wet spots in the mixing box or, worse yet, in supply and order the place evacuated. CO kills and it
the filters. One approach to eliminating the problem is a shouldn’t be treated lightly
mixing box design that introduces sufficient turbulence to
prevent droplets accumulating on the walls of the mixing Zone Dampers
box and ensuring good mixing before the combined air When required to serve several zones (an indepen-
streams reach the filters. The other approach is to heat the dent group of spaces with different heating and cooling
outside air before it’s mixed with the return air. requirements) the AHU can be a push through design
If a heating coil is used to boost the temperature with the fan mounted between the filter housing and the
of the outside air to prevent fogging at the apparatus coils and face and bypass dampers for each zone. Those
inlet it requires close attention when steam or hot wa- dampers installed after the coils mix the outputs of the
ter is used as the heating medium. It’s also true when two coils to provide a supply air temperature to the re-
large quantities of outside air are required for venti- spective zone controlled by that zone’s thermostat. The
lation (typical for movie theaters and similar venues) zone dampers are like face and bypass dampers but sim-
then the cooling coil (if using chilled water) and the ply direct air through either the heating coil, the cooling
heating coil all require attention. Why? So they are pro- coil, or a mix of the two to provide the desired supply air
tected from freezing. A poorly functioning steam trap temperature to a specific zone.
isn’t all that’s required for a steam coil to freeze up.
Sometimes the heating controls throttle a steam valve Fan Housing
so much that the pressure is inadequate to push the con- The normal arrangement of an air conditioning ap-
densate out of the coil and even so-called freeze proof paratus is to have the fan in line after the stuff already
coils will freeze; I had it happen. Hot water coils have covered making it what’s called a draw-thru apparatus.
also frozen when the flow to them is throttled enough If the fan or fans were located on the inlet it would be
that localized freezing in the coil is initiated and that referred to as a blow-thru apparatus. An advantage of
leads to blockage of part of the coil and freezing dam- the fan at the outlet is that it is in conditioned air. Blow
age. An idle cooling coil, unless drained for the winter, thru fans need regular attention to remove dust and dirt
can easily be frozen if the outdoor air heating coil fails. accumulating on the blades of the fan. That’s a typical
Today’s SCADA systems typically monitor apparatus problem with return air fans as well.
temperatures to warn of freezing conditions and you The fan can be powered by a motor contained with-
should conduct an immediate inspection of the unit to in the casing of the apparatus or a motor mounted out-
determine if failure can be averted. side the apparatus and connected by belt and pulley to
a shaft passing through the fan housing and supporting
Furnaces the fan wheels with separate bearings. A motor mounted
Gas fired and oil fired furnaces that heat the air inside the housing requires more space plus room to ac-
directly can contribute to the discomfort and under ex- cess it for maintenance and is usually restricted to RTUs
treme conditions injure the health of occupants. Even a (covered later) or very large AHUs. The inefficiency of
small crack in a furnace section can introduce enough the motor (3 to 7% of power consumed by the motor) is
CO (carbon monoxide) to poison occupants. If you have heat added to the air but the fan simply working on the
operating carbon monoxide detectors check their oper- air adds heat that is normally considered a load on the
ation regularly and don’t disable one if it alarms repeat- supply air in the engineer’s calculations.
edly because it’s probably telling you that you have a The most common service for large units today is
problem. Even a clean fire will produce CO under the variable air volume where the quantity of conditioned
right conditions. Unless shutdown introduces more se- air supplied to a particular zone is controlled by a
rious hazards, such as freezing occupants, shut the unit VAV box (described later) so the amount of air passing
down until the cause of the alarm is verified. Insist on through the apparatus is variable and, to limit duct pres-
having a portable analyzer if there are no fixed ones and sure and reduce operating costs the fan is fitted with a
use it to check all spaces supplied by air from a furnace variable speed controller that changes the fan speed to
and any complaint of uncomfortable conditions. Advise maintain a constant discharge pressure at the outlet of
the Owner that all personnel should be evacuated from the apparatus or, when equipped, the HEPA filters.