Page 208 - Boiler_Operators_Handbook,_Second_Edition

Refrigeration & AC 193

its performance. I don’t think that it’s the maintenance the differential before the cooling or heating gets started
crew’s carelessness, is just cheap construction. I’ve seen again. It’s not the best (remember I mentioned I wanted
them ready to fall through the roof and so corroded that ground source?) but my electric bills dropped to 38% of
I wonder what’s preventing them collapsing. They sel- what they were.
dom use more efficient chilled water sources. Normal- Instead of controlling the unit with a thermostat in
ly they contain a complete refrigeration system with the space or sensing temperature in the return air duct
air cooled condensers and a DX coil. DX standing for the outlet temperature of air conditioning apparatus can
direct expansion which is an acronym that separates be controlled by an independent system controller ac-
them from separated systems that supply chilled water cording to the lowest air temperature required at any
and means the air is cooled directly by the refrigerant. outlet in the air distribution system. All other outlets in
Their operating cost is easily twice that of a unit using the system control the temperature of supply air to the
chilled water. They frequently are direct fired for heat- space by a thermostat in that space through use of a re-
ing which would be more efficient than steam or hot heat coil or a variable air volume box.
water from a boiler if the burners and controls weren’t New regulations and design standards for build-
so cheap that they are less efficient than a steam or hot ing ventilation that are primarily concerned with the
water operation. distribution of the ventilation air have led to the intro-
I have spent an evening at a meeting in a local duction of independent outside air supplies so don’t
school during the winter listening to the RTU and con- be surprised if some of your equipment doesn’t have a
necting ductwork creak, snap, and emit other sounds mixing box with connected outside air ductwork. Sep-
that challenge a box of Rice Krispies (but at several arate air handling units, frequently called ventilation
times the volume) while wondering if that night would air units, provide outside air distributed to the building
be when the ductwork or the RTU comes crashing down through its own independent system of ducts and reg-
because it has worked loose. Called for problems with a isters. Those units can be operated to deliver air at the
potential leak from an RTU I found it shifted on its curb room design conditions so the ventilation air does not
(a curb, by the way, is a raised portion of the roof that have any effect on the operation of the heating and cool-
supports one) until one corner of the RTU had slid over ing equipment.
enough to expose the indoors to the outdoors. They’re One of the major considerations for load of an air
where I find most of the access panels lacking half their conditioning system is sun light. A well-designed sys-
screws so unfiltered outside air leaks into them as well. tem will account for variations in solar heat gain at dif-
If you’re stuck with operating and maintaining ferent exposures (North, South, East, and West) through
RTUs, may God bless you. All I can say about them is the use of separate AHUs, face and bypass dampers,
do your best and pay attention to those creaks, moans and/or reheat coils for each exposure. Of course the
and snaps. If you can isolate them you may be able to bigwigs in any office building insist on having corner
eliminate them. offices with double exposures and unless their space has
independent temperature controls there will be periods
Air Conditioning Systems Control when other occupants of the zone are uncomfortable.
The normal air conditioning apparatus control con-
sists of a thermostat that starts and stops the fan, com- The TAB Report
pressor and associated electrical equipment. That would In any large installation the construction project
be typical of our good old window units and the system usually calls for air balancing. Technicians will take
in your home. While it’s inexpensive at first cost the op- readings and adjust air flows and temperatures, record
erating costs are considerably higher than equipment them, and show the actual measured air flow at each ap-
that can operate continuously and modulate to control paratus, main distribution duct, VAV boxes, and regis-
space temperatures. When I had my heat pump replaced ters. That document should also be kept handy so you
I had the new one fitted with a two speed scroll com- can compare the setup conditions with actual.
pressor. It means the fan runs more but the compressor
shifts back and forth between half and full speed (hi and Convectors
lo on the thermostat) to keep it running longer, some- Convectors are usually floor mounted and usually
times constantly at high heating and cooling loads. Ev- below windows. They contain their own fan or fans to
ery time a compressor shuts down the refrigerant pres- circulate room air over refrigerant or water coils cooling
sures balance out and the compressor has to reinstate and/or heating the room air to adjust the room air for