Environmental Engineering Reference
In-Depth Information
Renewable-media filters were developed
to provide a filter surface that is slowly being moved to accommodate par-
ticle collection and HVAC system operating needs. They consist of a slowly
moving curtain/filter unit which advances in response to pressure drop or
a timer. As the medium becomes excessively soiled, it moves to a takeup roll
on the bottom of the filter system. When the roll has been completely soiled,
it must be removed and replaced. Need for replacement is usually signaled
by an alarm. Such filters have high arrestance (described later) efficiencies
(60 to 90%) and low dust spot efficiencies (20 to 30%). They are available as
both dry and viscous types. In viscous-media renewable filter systems, the
filter may pass through a reservoir of a viscous medium where it sheds its
dust load and is recoated.
d.
Renewable-media filters.
4.
Air flow resistance
Because of their nature, all air cleaning systems using the principle of filtra-
tion must be designed and operated with due consideration to the effects of
the filter on air flow. Filters with high packing densities have both increased
collection efficiencies and resistance to air flow. As a filter becomes soiled,
its resistance to air flow increases; as resistance increases, air flow decreases.
As indicated previously, extended-media filters are designed to provide
high collection efficiencies at acceptable air resistances described as pressure
drop (
P). Pressure drops associated with high-efficiency filters and filter
soiling must be considered in the design and operation of cleaner units and
HVAC systems. Of major importance is the selection and use of system
blowers that will develop sufficient static pressure to overcome resistance to
air flow and maintain desired volumetric air flows. As a consequence, blower
fans must be adequately sized in terms of volumetric flow rate and horse-
power rating.
For illustrative purposes, let us use two different fans for a small free-
standing residential air cleaner ( Table 12.1 ) . Though both fans have the
capacity to move 125 CFM of air at zero static pressure (no air resistance),
they differ in horsepower. Fan #1 has a higher horsepower rating than fan
#2. As resistance to air flow increases to 0.5” H
O, air flows associated with
2
both fans decrease. At 0.5” H
O
P, fan #2 can pull virtually no air. Fan
2
Table 12.1
Air Flow Rates Under Different
Static Pressures for Two Fans
Air flow rate (CFM)
Static pressure (“ H
O)
Fan #1
Fan #2
2
0 .0
125
125
0.1
120
115
0.2
115
105
0.3
110
98
0.4
105
85
0.5
100
 
 
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