Air Handling Unit Components

It depends a lot on how well you want to filter the air - as in how small a particle you want to permit to get through the filter, and what sort of air you have to filter - as in kitchen hood grease fumes have a whole different set of requirements than say residential furnaces.

For sure, an industrial hygienist dislikes particles larger than 5 microns in the post-filtered air. Indeed, there usually are code requirements as well - the Minnesota Mechanical Code requires MERV 6. (MERV is rating standard I don't know much about, but frequently referenced).

How robust the filter media you choose, by the above criteria for degree of air cleanliness, effects the velocity of the air through the filter.

If your serious about this design ASHRAE Standard 52.1 would be a good place to start your research.

I hope the guest isn't designing systems for someone.

I included the source website since I copied this there:

Customer Service If you have questions about our products or services please refer to the information below or e-mail one of our friendly customer service representatives at support@aafilters.com. We can also be reached by phone at 1-888-844-5820 from 9am to 4pm Mon - Fri. Frequently Asked Questions • What is an efficiency or MERV rating? Most filters are labeled with a MERV (Minimum Efficiency Reporting Value) rating number, which measures a filter's ability to trap particles ranging in size from 3.0 microns to 10.0 microns. Residential filters commonly have MERV ratings of 1-11. The higher the MERV rating, the more efficient the filter is, and the more particles it can filter. • A MERV rating of 6 means the filter is 35% to 50% minimum efficient at capturing particles, sized 3.0-10.0 microns. • A MERV rating of 7 means the filter is 50% to 70% minimum efficient at capturing particles, sized 3.0-10.0 microns. • A MERV rating of 8 means the filter is 70% minimum efficient at capturing particles, sized 3.0-10.0 microns. • A MERV rating of 11 means the filter is 85% minimum efficient at capturing particles, sized 3.0-10.0 microns. MERV is an industry standard rating, so it can be used to compare filters made by different companies.

To answer you specific questions-

The filter goes at the entrance to the air handler, whether it is a "draw through" where air is drawn through the cooling and/or heating coils and then to the fan where it is discharged, or a "blow through" unit where air is sucked into a fan and then blown through the cooling and/or heating coils. In special conditions, like clean rooms, pharmaceutical manufacturing and surgical suites, there is a second set of "final" filters (HEPA- high efficiency particulate arrester) that go after the fan (usually outside the air handler).

Filter design is totally dependent on how dirty the air is, how often you want to change the filters and the level of pressure drop you choose to support. Very common filtration systems are pleated, 2 to 4 inches thick, standard dimension units- often with several filters used to cover the entire area. The filters can be installed with a flat surface or in a V-shaped housing for extended surface area.

you second question related to size of filter. Most filters are available in "standard" sizes ranging from 12" x 20" up through 20" x 25". The size of the filter "bed" should always have an air velocity of less than 500 feet per minute (FPM). Filter assemblies using V-boxes can often have actual filter velocities well below 200 FPM.

As discussed earlier- filters have different levels of capacities. IF you are handling a lot of air that has a relatively high amount of dirt or particulate in it, you might want to have a pre-filter assembly with low-to medium capacity filtration capacities followed by a primary filter rated for the effectiveness required. The pre-filters will be replaced twice as often as the primary filters.

Filter replacement is usually determined by a differential pressure gauge or switch / light assembly. For heavy duty filtration, it is very likely to have an initial pressure drop of about 0.25 to 0.35 inches water column (WC) with a final change out drop of 1.00 inches.