Environmental Engineering Reference
In-Depth Information
horsepower required to draw air through a filter at a constant or near-
constant rate increases with increased resistance to air flow. Fans with higher
horsepower ratings are more expensive to purchase and operate. However,
an adequately sized fan is essential for proper system performance.
If resistance becomes excessive, performance will decline significantly
even when a properly sized fan is used. This occurs if filters are not replaced
when they reach their design resistance values. Maximum acceptable resis-
tance values for products are provided by manufacturers, who also provide
resistance values for clean filters at their rated air flow.
In many HVAC system applications, a differential pressure gauge mon-
itors pressure differences (
P) upstream and downstream of the filter. The
pressure drop increases as the filter becomes soiled. In some systems, an
alarm sounds when the pressure drop exceeds a predetermined value. The
alarm signals maintenance personnel that filters need to be changed to main-
tain desired system air flows. In other cases, maintenance personnel period-
ically check pressure gauges to determine filter change requirements.
Sensors or pressure drop indicators are not present in most residential
and many HVAC applications. Because of the variability of dust loading on
filters, particularly in HVAC systems, it is difficult to know exactly when
filters need to be replaced. In such cases, filter replacement is a matter of
judgment by service personnel and homeowners. Filters are often replaced
on a routine schedule irrespective of their condition. Since excessively soiled
filters cause decreased air flows and increased operating costs in HVAC and
home heating and cooling systems, it is important that operators implement
an appropriate service plan.
∆
B.
Electrostatic air cleaners
Electrostatic air cleaners remove airborne particles by electrostatic forces.
Three basic designs are used: ionizing plates, and the charged-media ionizing
and nonionizing types.
1.
Ionizing-plate cleaners
Ionizing-plate electrostatic air cleaners are widely used to collect airborne
particles in HVAC systems and residential applications. Their operation is
based on the principle that airborne particles can be given a positive or
negative charge and then collected on metal plates with the opposite charge.
In industrial applications, particles are negatively charged; in indoor appli-
cations, they are positively charged.
Both single- and two-stage electronic cleaners are available for use in
indoor applications. A two-stage cleaner is illustrated in
Figure 12.9
.
In the
first stage, a high electric potential (12,000 volts) is applied to thin, vertical,
tungsten wires. Electrons are accelerated toward the positively charged ion-
izing wires. The accelerated electrons strike air molecules, stripping them of
