The application of . . .
Air filters used in ventilating systems
are called upon to remove an extremely wide variety of contaminants
from the air, ranging from soot and smoke and the common
dusts to mold spores, bacteria and pollen. Particle sizes
vary from less than a micron (.000039") to insects
and leaves. The concentration will vary hourly, daily and
seasonally.
These variables combined with filtering
requirements that range from the simple needs of a household
furnace to the absolute filtration required for a clean
room make air cleaning a complex science. Filter selection
should, therefore, be made by a qualified engineer or in
cooperation with the filter manufacturer.
The same variables that affect filter
selection also contribute to the problem of determining
when a filter has reached the tolerable problem of determining
when a filter has reached the tolerable limit of its effective
life span as a result of the accumulation of contaminants.
Because of wide variation in contaminant load in the air
being filtered, time is an uncertain measurement. Visual
inspection of the condition of the filter tells little or
nothing. The most widely used method of checking filter
condition is to measure pressure drop across the filter
by means of an air filter gage.
The Function of the Air
Filter Gage
A clarification of terminology is
essential to understanding the function of an air filter
gage. The air filter gage measures pressure drop across
the filter for the purpose of determining whether or not
the filter is operating within its design range of effective
utilization. It does not measure filter efficiency.
Filter efficiency as defined by the ASHRAE
GUIDE, "is synonymous with the term arrestance and
measures the ability of the air cleaner to remove particulate
matter from an air stream." Several methods of determining
filter efficiency exist, because of variations in filters
and application requirements.
One method is by weight, in which a carefully
compounded mixture of standardized dust is fed into the
air stream and the amount trapped by the filter related
to the amount released in the test determines the filter
efficiency in percentage. A second method, the dust spot
method originated by the National Bureau of Standards, compares
the opacity of the dust collected on filter paper from two
air samples of equal flow rate, one of which samples filtered
air. The efficiency of filter is thus evaluated on the basis
of the darkness of the dust spots. A third method measures
arrestance of DOP (Dioctyl-phthalate) smoke, a homogenous
aerosol of 0.3 micros size, to test a special class of high
efficiency filters used to control particulate matter of
high toxicity or where extreme cleanliness in required.
The "dust holding capacity"
of an air filter is defined by ASHRAE GUIDE as, "the
amount of a particular dust that an air filter can hold
and determines the operating life the air cleaner which
is fixed mainly by some figure of tolerable resistance."
Thus, an air filter gage, sensing the differential in static
pressure across n air filter, will permit the air filter
to be sued to its maximum dust holding capacity as determined
by he filter manufacturer and specified in terms of maximum
filter resistance. It should be noted that servicing a filter
when it has reached its point of specified maximum resistance
is the only method which will guarantee that the filters
are being used beyond the point where their increased resistance
could affect balance of the system, increase air leakage
and, for some types of filters, begin to pass contaminants.
Type of Filters and Recommended
Resistance for Servicing
All filter manufacturers supply
technical data which will permit full utilization of their
filters. This data will include initial resistance in inches
of water column for the filter at its rated air flow and
a recommended resistance at which point the filter should
be replaced or serviced. The following describes the broad
classifications or air filters:
Viscous Impingement Filters
have the filter element treated with an oil or adhesive
which holds dust particles coming in contact. The initial
resistance of a typical filter will usually run from .08
to .15" water, and servicing will be called for at
.50" water by means of a pressure actuated switch or
timer with overriding pressure switch operating an electrical
drive.
Dry Type Filters are
available in a multiplicity of materials, in varying thicknesses,
in batts, woven or bonded materials, natural or synthetic,
and in pleated form or in tubes or bags to obtain greater
filter area. Efficiencies are usually at least equivalent
to the viscous impingement type and may run to virtually
100%. Dust holding capacity is high and resistance values
vary so widely no rule of htumb figures are possible. The
manufacturer should always be consulted for proper initial
and final pressure drop figures for these filters.
Electronic Air Cleaners using
the electrostatic precipitation principle have no characteristic
increase in pressure drop as they accumulate dust and must
therefore be serviced on a pre-set schedule. The mechanical
filter normally used with such equipment should, however,
be serviced on the basis of increased pressure drop.
Significance of High or Low
Initial Resistance Readings Due
to the many variables inherent in an air distribution system,
the initial pressure drop in some systems may be found to
be below that specific by the filter manufacturer. If the
system has been carefully checked over and found to be in
order, this need not be a source of concern, as it simply
indicates that less than rated air volume is passing through
the filter. Generally, this will mean increased filter life
before the filter requires servicing. Conversely, if initial
pressure drop exceeds the filter manufacturer's rating,
it indicates that a greater volume of air is being handled
than the filter is rated for and filter life will be shortened.
Excessive variation from the filter manufacturer's published
initial resistance figure will probably indicate one or
more of the following problems:
Low Pressure Drop
1. Less than rated air volume being
handled due to over design of filter area, improper air
balancing of system, open bypasses, etc.
2. Incorrect filter installed.
3. Velocity influence.
4. Air leakage around filters, past frames, and possibly
through damaged filter.
IMPORTANT: An
initial pressure drop 30% to 40% or more below the rated
value for the filter means that approach velocities may
be sufficiently low to impair the efficiency of some types
of filters.
High Pressure Drop
1. Incorrect filter installed.
2. System handling more than rated air volume. Suggest thorough
check of system to be certain all controls are properly
set and operating. If unable to reduce pressure drop to
less than ten percent or fifteen percent above rated value
additional filter area should be provided or air volume
reduced.
IMPORTANT: An
initial pressure drop 10% to 15% or more above the rated
value for the filter, means that approach velocities may
be sufficiently high to impair the efficiency of some types
of filters.
Installation Check and Trouble
Shooting
Before putting your air filter gage
into service or in the event of initial pressure drop readings
that don't agree with the filter manufacturer's specified
pressure drop, make the following checks:
1. Check zero adjustment of the gage. Remove both tubing
leads or open vent valves to atmosphere.
2. Check all tubing connections for tightness from gage
to the static tip or fitting connection.
3. Check static pressure tips or fittings to be sure they
are into plugged.
4. Check installation of static tips or fittings. Angle
type static pressure tips must point directly into the air
stream. Flange type static pressure fittings should be mounted
on a duct wall in such a location that the opening is at
a right angle to the moving air stream. A velocity pressure
error can be created if the air blows directly into the
opening.
How to Install Dwyer®
Air Filter Gages and Switches
Dwyer® Air Filter Gages and Switches are offered in
a variety of types to meet the needs of your system. All
types are customarily mounted on the outside of the air
handling duct or plenum near the filter bank.
The type of static pressure tips used
and their location is of primary importance in securing
reliable readings. For maximum accuracy, it is essential
that the influence of the velocity of the air be eliminated
to permit sensing the true static pressure. Note that some
filter installations do not provide a straight duct approach
to the filter bank which may cause air to swirl and eddy.
Right angle static pressure tips give
the most accurate sensing and are furnished with 2000 Series
Magnehelic® gage and 250-AF Series Gages. Flush static
pressure fittings inserted at right angles to the flow are
lower in cost and less likely to plug up, but are more susceptible
to velocity and turbulence influence.
Tips should be located as recommended
by the specifying engineer or by the filter manufacturer.
In the absence of such recommendations, locate the tips
at least 12" upstream and downstream from the filters
in a zone of minimum turbulence.
Fig. 2-1. 2000 Series Magnehelic®
Gages are diaphragm actuated
pointer type gages. The gage is attached to the mounting
plate which is furnished and the mounting plate screwed
to the wall of the air duct. The static pressure tips are
installed through 7/16" holes in the filter wall with
tips pointed into air flow. The static tips are connected
to the plastic vent valves using the ¼" aluminum
tubing and compression fittings which are furnished. Zeroing
is accomplished by turning the vent valves to venting position
and then turning the zero adjustment scre3w in the face
of the gage with a screwdriver. For detailed instructions,
see Bulletins A-27 and A-28.
Fig. 3-1. 250-AF Series Solid
Plastic Gages are manometer
type gages which are customarily mounted directly on the
wall of the filter housing. The static pressure tips are
installed through 7/16" holes in the filter wall with
tips pointing into the air flow using the ¼"
aluminum tubing and compression fittings which are furnished.
Zeroing is accomplished by turning the vent valves (optional
at extra cost) to "vent" position - or by pulling
the connections from the gage body if vent valves are not
used - and sliding the adjustable scale until the zero mark
lies directly behind the indicating fluid meniscus.
Fig. 3-2. Shows
a typical hook-up of a Model 1823 compact, diaphragm actuated
differential pressure switch in parallel with a gage using
tees. The switch is se to energize an electric circuit when
the maximum allowable pressure drop is reached.
Fig 3-3. Shows
schematically how a Model 1638 switch with visual set point
adjustment can be hooked up to trigger the advancement of
an automatic roll filter when the segment in use becomes
too directly for efficient service.
For detailed instructions, see Bulletins
E-552 (Model No. 1638),E-53 (Model No. 1823) and E-55 (Air
filter switch service).
