Particle sizes vary from less than 1 micron, up to the size of insects and leaves. The concentration varies 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 cleanroom, make air cleaning a complex science.
These same variables contribute to the problem of determining when a filter has reached the limit of its effective lifespan; in other words, determining exactly when it has accumulated as many contaminants as it can hold and still do a tolerable job of filtering the air.
Because of the wide variations in contaminant loading, time is an uncertain measurement; even visual inspection can tell little or nothing.
For technicians who need to keep close tabs on their clients’ filter conditions, the most widely used method is to measure pressure drop across the filter by means of an air filter gage.
It does not measure filter efficiency.
An air filter gage, sensing the differential in static pressure across an air filter, permits the air filter to be used to its maximum dust-holding capacity as determined by the filter manufacturer and specified in terms of maximum filter resistance.
There’s more than IAQ at stake. Filters must be serviced when they reach their point of specified maximum resistance. The results of not servicing a loaded filter can include:
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.
However, 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:
Important: An initial pressure drop that’s 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:
Important: An initial pressure drop of 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.
1. Check the zero adjustment of the gage, including both tubing leads, or open vent valves to the 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 plugged in and correctly connected.
4. Check installation of static tips or fittings.
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. 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 in. upstream and downstream from the filters in a zone of minimum turbulence.
The following describes the broad classifications or air filters:
The initial resistance of a typical filter will usually run from 0.08 to 0.15 in. wc; servicing will be called for at 0.50 in. wc by means of a pressure-actuated switch, or a timer with an overriding pressure switch operating an electrical drive.
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 that no rule-of-thumb figures are possible. The manufacturer should always be consulted for proper initial and final pressure drop figures for these filters.
The mechanical filter normally used with such equipment should, however, be serviced on the basis of increased pressure drop.