Page 198 - Boiler Operator’s Handbook

Refrigeration & AC 183

New regulations and design standards for building ven- In the deep winter the moisture content of the con-
tilation that are primarily concerned with the distribu- ditioned air in an economy cooling system can get so low
tion of the ventilation air have led to the introduction of that the occupants encounter problems with static elec-
independent outside air supplies so don’t be surprised if tricity. They get everything from shock to clothing stick-
some of your equipment doesn’t have a mixing box with ing to them and hair looking … well, perhaps you’ve
connected outside air ductwork. seen it. Means of adding moisture to the conditioned
How much outside air is required for ventilation? air to maintain a comfortable level of humidity are de-
A general rule of thumb used to be 5 cfm (cubic feet per scribed later but some systems will have over-rides on
minute) for each person in the space. New standards the economy cooling to maintain desired humidity in
for ventilation produced by ASHRAE (the American the conditioned spaces and refrigeration or chilled wa-
Society of Heating, Refrigeration, and Air-conditioning ter cooling as needed for temperature control.
Engineers) now require as much as 20 cfm per person.
The design (essentially maximum) quantity for a given Apparatus Inlet
piece of air handling equipment is listed on the design If the inlet conditions don’t fall on that line drawn
drawings and you will find systems today that use a between the outside air and room conditions it can be
control interface with personnel access controls or the because the engineer has made allowances for heat
carbon dioxide content of the room air to determine gained between the room and the mixing box which can
actual values. As with night set-back temperature con- come from heated piping, lighting heat, or sun on the
trols the amount of outside air should be reduced when roof of the space, any one of which heats the room air
the building isn’t occupied or occupancy is very low. A that returns to the unit as return air flowing between the
round during night hours or weekends should include ceiling and the floor or roof above the space. It could also
verification that the ventilation air is off or at minimums be heat gained in ducts run outdoors to the equipment.
because it can cost a lot of money to heat or cool that air To represent that added heat gain in the return air erase
unnecessarily. the line from room air to outside air, plot the point for
the return air entering the equipment and draw a line
Economy Cooling from the outside air conditions through the inlet air con-
A feature of many air conditioning systems in the ditions down to the line for the same moisture level as
northern states is provision for economy cooling (not to the room air and draw a horizontal line that connects the
be confused with another concept called free cooling). room conditions with the end of the line you just drew.
When outside temperatures are low enough air handling Unless it’s an unusual air conditioning system, that
systems can be operated to take advantage of that colder should do it. When the air is simply heated its dry bulb
outside air to cool spaces in the buildings instead of cool- temperature is increased without changes in moisture so
ing the air with refrigeration equipment. Core spaces, in the line is always horizontal and the percent humidity
the middle of the building, will need cooling even in the decreases. You should be able to confirm the condition
winter because of the heat generated by people and lights by measuring the dry bulb temperature of the return air
alone. The savings in operating costs can be significant immediately before it enters the mixing box. If it doesn’t
and an operator should ensure that the economy cooling match check wet bulb conditions because the return air
is operating properly. When economy cooling is used the must have picked up some moisture somehow. Any in-
outside air ducts and dampers are larger so as much as crease in moisture content between room air and return
100% of the conditioned air flow can be drawn from out- air entering the mixing box is potentially problematic,
doors. Typically the systems are fitted with a return air possible caused by steam, water, or outside air leaks into
fan to draw the return air from the rooms and discharge it the return air stream.
outdoors or into the mixing box. Room temperature con-
trols are used to control the temperature of the supply Filters
air by mixing outside air and return air to produce the The equipment airflow, now a mix of outside and
required temperature of supply air. return air, passes through filters as it leaves the mixing
Economy cooling systems need to be watched to box. The filters are there to remove contaminants from
ensure the outdoor air is at minimum requirements the air like dust, pollen brought in with the outside air
during the summer. Only one of those large outdoor air (or produced by plants in the conditioned space) and
dampers that hangs up open can prevent adequate cool- any other solid airborne contaminants that might plug
ing and really pump up the electric bill. up the cooling and heating coils or bother occupants.