Honeywell AUTOMATIC CONTROL, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
CONTROL FUNDAMENTALS
14
losses. The exhaust-air system may be incorporated into the
air conditioning unit, or it may be a separate remote exhaust.
Supply air is heated or cooled, humidified or dehumidified,
and discharged into the space.
DAMPER RETURN FAN
RETURN
AIR
EXHAUST
AIR
DAMPERS
OUTDOOR
AIR
MIXED
AIR
FILTER COIL SUPPLY FAN
SUPPLY
AIR
C2712
Fig. 15. Ventilation System Using Return Air.
Ventilation systems as shown in Figures 14 and 15 should
provide an acceptable indoor air quality, utilize outdoor air
for cooling (or to supplement cooling) when possible, and
maintain proper building pressurization.
For more information on ventilation, refer to the following
sections of this manual:
— Indoor Air Quality Fundamentals.
— Air Handling System Control Applications.
— Building Airflow System Control Applications.
FILTRATION
Air filtration is an important part of the central air handling
system and is usually considered part of the ventilation system.
Two basic types of filters are available: mechanical filters and
electrostatic precipitation filters (also called electronic air
cleaners). Mechanical filters are subdivided into standard and
high efficiency.
Filters are selected according to the degree of cleanliness
required, the amount and size of particles to be removed, and
acceptable maintenance requirements. High-efficiency
particulate air (HEPA) mechanical filters (Fig. 16) do not
release the collected particles and therefore can be used for
clean rooms and areas where toxic particles are released. HEPA
filters significantly increase system pressure drop, which must
be considered when selecting the fan. Figure 17 shows other
mechanical filters.
C2713
CELL
PLEATED PAPER
AIR FLO
W
Fig. 16. HEPA Filter.
PLEATED FILTER
BAG FILTER
Fig. 17. Mechanical Filters.
Other types of mechanical filters include strainers, viscous
coated filters, and diffusion filters. Straining removes particles
that are larger than the spaces in the mesh of a metal filter and
are often used as prefilters for electrostatic filters. In viscous
coated filters, the particles passing through the filter fibers
collide with the fibers and are held on the fiber surface.
Diffusion removes fine particles by using the turbulence present
in the air stream to drive particles to the fibers of the filter
surface.
An electrostatic filter (Fig. 18) provides a low pressure drop
but often requires a mechanical prefilter to collect large
particles and a mechanical after-filter to collect agglomerated
particles that may be blown off the electrostatic filter. An
electrostatic filter electrically charges particles passing through
an ionizing field and collects the charged particles on plates
with an opposite electrical charge. The plates may be coated
with an adhesive.