There seems to be a lot of confusion when it comes to selecting a filter and then matching it to a blower. In this post I'll attempt to make a no-brainer fill in the blanks Aide-mémoire for people struggling with the math. Each line in this template will inform the next, that's why you need to solve the filter issue before tackling the blower. Choose the filter first then match the blower.

Not all High Efficiency Particulate Air filters are appropriate for laminar flow applications. HEPAs have a wide range of applications from furnace air filtration to Hospital and laboratory.

HEPA filters remove particulate such as micro-organisms, pollen, and mold spores from the air. HEPA filters used in clean benches should have a minimum filtration efficiency of 99.99% against airborne particles 0.3 microns in size. Filtration efficiency will be greater than 99.99% on particles that are larger and smaller than 0.3 microns.

My current filter is 99.999% down to .1 micron.

Not all kinds of flow are appropriate either, laminar flow is not just any ol wind coming out of any ol filter, as mentioned above. Laminar flow is orderly and parallel, turbulent flow however is chaotic and non linear. Laminar flow occurs when a fluid (like air, air behaves the same as a fluid) flows in parallel layers, with no disruption between the layers. ... Turbulent flow is a flow regime characterized by chaotic property changes. This includes rapid variation of pressure and flow velocity in space and time.

Don't expect laminar flow just because your fan has a HEPA filter on it, if you don't follow this template you can be assured that you will have turbulent flow.

Turbulence = contamination

This is a great example of what not to do:

**The Template****Step 1: The filter**After you've decided which HEPA filter you want you'll need to download the .pdf file for that filter or ask the retailer/manufacturer to supply the spec sheet which will have the velocity/resistance chart:

Here is a sample chart for a 2ft by 4ft HEPA which shows three different filters by thickness represented by the three different coloured lines. On the left you will notice that it's labelled "Resistance [W.g]" W.g stands for Water gauge and it's the unit of measure used to measure your static pressure (SP).

Before you can match any blower to your filter you need to find out how much resistance (Static pressure) your HEPA has at a

*velocity* of 100 feet/min +/- 10 FPM. 100ft/min is the minimum

*velocity* necessary to achieve laminar flow across the surface of your filter.

FPM refers to the "air speed" produced by the filter(at a specific static pressure), as measured in Feet Per Minute. CFM is a measure of the "air flow," stated in Cubic Feet per Minute. In other words, FPM is a measurement of how fast (velocity) the air moves and CFM measures how much air is moved (volume).

The red line show's exactly where the filter (green line) intersects with the W.g on the left and 100 feet/min on the bottom, it's now clear that this filter produces .5 inWG of static pressure at 100 feet per minute. Now it's time to figure out the

*volume* of air we need to move to achieve a

*velocity* of 100ft/min across the surface of the filter.

*1. HEPA measurements:*Length___ft x width___ft = ___ft²

*2. Flow rate:*100ft/minute x ____ft² =____ft³(CFM)

Maximum CFM for the filter is +20% of minimum CFM.

So let's use this template with the filter represented by that chart above.

*1. HEPA measurements:*L_4_ft x W_2_ft = __8_ft²

*2. Flow rate:*100ft/minute x __8__ft² =__800__ft³(CFM)

Maximum CFM for the filter is +20% of min CFM.

So this filter requires a blower that blows 800CFM at .5 inWG of static pressure (not including the prefilter).

Any filters that we use as prefilters will also produce static pressure, many standard 1" furnace filters usually have a resistance of around .4 inWG of static pressure, this needs to be added to the static pressure of your HEPA.

Let's add static pressure to the equation:

*1. HEPA measurements:*L_4_ft x W_2_ft = __8_ft²

*2. Flow rate:*100ft/minute x __8__ft² =__800__ft³(CFM)

Maximum CFM for the filter is +20% of CFM.

*3. Static Pressure:*HEPA SP _.5___+ prefilter SP_.4___= .9" W.g (SP)

Using these simple equations we can see that this filter requires a blower that is minimum 800CFM - max 960CFM @ .9 inWG (SP) to achieve 100ft/min.

**Step 2: The Blower**Now blowers have charts of their own, here's an example:

As you can see on the left hand side of this chart they've listed the static pressure, on the right in

**bold** they show the CFM at the corresponding SP.

We can see that this particular blower pushes 360CFM @ .8" SP....this blower is not strong enough, keep looking.

Here's an example of another type of blower chart:

This chart shows a blower (top line) that is 911CFM @ .9 inWG, this blower will match the above 2ft x 4ft filter

**Step 3: Application**Now do it for your own filter, just plug in your own numbers and you'll be gtg. The combined answers you get from line 2 and line 3 will give you the specs for your blower, as shown below.

**1. HEPA measurements:**

Length___ft x width___ft = ___ft²

**2. Flow rate:**

100ft/minute x ____ft² =____ft³(CFM)

Maximum CFM for the filter is +20% of minimum CFM.

**3. Static Pressure:**

HEPA SP ___+ prefilter SP____= ___ "W.g SP*Blower match* = 2. ____CFM@ 3.____"SP

**A note on "clogging the intake"**Blocking the air intake of your blower excessively will put unnecessary strain on the motor and will likely effect it's lifespan/efficiency. Chocking the intake can be done within reason however I suggest getting as close as possible to the proper output without choking the intake. No need for six prefilters to close the gaps of incomplete planning.

Here's an interesting HVAC video that clearly illustrates pressure drop in filters.

An average 1" pleated furnace filter will produce ~ .4 to .5 inWG in pressure drop.

Thanks to Sandman420 for this laminar flow smoke test video:

I hope that this write up clearly illustrates the relationship between filter, blower, and their respective charts.

Happy building.

The above filter and blower match up were used

in this build