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The
Intelligent Use of Energy: Outside Air and Ventilation
Richard R. Vaillencourt, PE,
Canterbury Engineering Associates LLC
Editor’s Note: This is the second in a series of articles on
intelligent energy use in buildings by consulting engineer Richard
Vaillencourt. We hope you’ll find them helpful as you work to optimize
your building’s energy use.
Outside air is the most expensive air that building owners and managers
can use at their facilities. It is expensive because it is rarely the
temperature that you want it to be; therefore, you must apply heating or
cooling energy to make it the correct temperature. Of course when it is
at the right temperature, it is the least expensive air you can use
because you do not have to add energy to change its temperature.
Yet no matter the time of year, outside air is very desirable in terms
of indoor air quality, so it is important to optimize its use. In fact,
there are several regulations dictating the outside air volume that you
must bring in to your facility. The trick for using energy intelligently
here is to know when to maximize, or minimize, the volume of outside air
based on how close it is to the temperature that you need.
So what temperature do you need? The easiest answer: whatever the
discharge air temperature (DAT) is from your air-handling unit. For
example, if your DAT is 55 degrees F, then whenever the outside air is
55 F or less, you can use it for ‘free’ cooling – the classic
‘economizer cycle.’ Even if the outside air temperature is below 55 F,
it still can be used because in most cases you can blend very cold
outside air with the warm +/- 78 F return air to achieve the required
DAT.
But what about heating? It is very unlikely that outside air temperature
can ever provide free heating. So where else can you find free heat? In
your exhaust air. The air that is being exhausted from your building is
the temperature of your return air – typically the upper 70s F. If
outside air is colder than the exhaust, then you have a Delta T that
allows you to minimize the energy lost with the exhaust. With a heat
exchanger of various possible designs, the cold outside air can be
preheated by the energy in the exhaust. Remember, you already paid to
heat that air that you are throwing out. It may be cost-effective to
recycle some of it to lower your cost to heat the outside air needed to
replace the exhaust.
This is an important point: Every cubic foot of exhaust air will be
replaced with outside air. You can only exhaust the volume of air that
you return to the building. One way or another, the exhaust volume will
get back into your building and become a load on your heating or cooling
equipment. If you completely seal up a room and start an exhaust fan,
the fan will eventually stop exhausting air. The high negative pressure
on the room side of the fan will eventually be so large that the fan
will not be able to overcome the pressure difference, and air will stop
moving.
So pay attention to your exhausts. The only way that you can beat the
laws of thermodynamics is by refusing to play. Shut off your exhaust
fans when not needed. Reduce your exhaust volumes to the lowest possible
volume unless the outside air is the right temperature. Whenever
possible, recycle the energy that you paid for in the exhaust stream and
use it to precondition the makeup air.
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