Every filter has a different "resistance" when air blows through it at a certain speed, this resistance is called the "static pressure".
Press your hand against your mouth. Now try to blow through it. Dependant on how firm you press it against your mouth, you will have some difficulties blowing air out and you will feel some resistance, this is the static pressure.
Every filter has a different static pressure at the working point. The working point is where the amount of the air flowing through the filter is sufficient to meet the requirement of the laminar flow.
The static pressure is expressed in inch of water column in the english units, a typical value would be 1", the SI unit for pressure is Pa(Pascal).
1" water column is around 250 Pa. Each filter has a data sheet (consult the manufacturer if this is not the case) where the static pressure at the working point is entered. Before the air enters the blower it is usually pre-filtered by a furnace filter around 1"(2.5cm) thick placed in front of the blower to protect it and the HEPA filter from big paricles like dust and hair. It can be assumed that the static pressure for this prefilter at the working point is around 0.2"(50 Pa)
Matching a blower to the filter
According to Stamets (Paul Stamets and J.S.Chilton: The Mushroom Cultivator p. 347 ff) the air speed of the air flowing from the filter surface should be (at least) 100 feet per minute(fpm).(around 30 meter per minute or 0.5 meter per second).
1. Find out the area of your filter by multiplying the width and the hight in feet (for instance the smallest reasonably usable filter would be 2ft x 1ft)
2ft x 1ft = 2 ft2
2. Multiply the required air speed(the one Stamets specifies) with the area of your filter
100 ft/min x 2 ft2 = 200 ft3/min
So 200 ft3/min(cfm) is the amount of air your blower must deliver at the STATIC PRESSURE of the HEPA filter prefilter.
NOTE: 1 cfm= 1.7m3/h
So if you use the above filter with 1"(250Pa) static pressure and a furnace prefilter with a static pressure of 0.2"(50Pa) your blower must deliver 200 cfm(340m3/h) of air at a static pressure of 1.2"(300Pa).
Finding the correct blower
Every blower should have a data sheet with a characterisitc curve that shows the air output in dependance of the static pressure.
Here is such a set of curves for 4 blowers(numbered 1-4).
NOTE:Each model of a blower has his own characteristical curve. This chart shows the curvers for 4 different particular models of axial duct blowers.
What you can clearly see is that the bigger the static pressure the less air the blower delivers, up to the maximum static pressure where the air output is zero.
Sometimes these data are presented in form of a table.
By now you know how much your blower must deliver and at which static pressure. In our example it was 340m3/h at 300Pa.
So what we do now is to go in this chart and find the value of 300Pa(=1.2") at the vertical axis. Then you move to the right until you reach 340m3/h.(=200cfm)
Now, ideally there should be a curve of a blower going exactly trough this point, but this is not always the case.
So we have to choose a blower that best approximates our requirements.
In most cases we should choose a stronger blower, if it's not too strong.
How strong is too strong? I'd say the blower shouldn't deliver more that 20% more air than we calculated, so in our case the strongest acceptable blower would be around 400m3/h.
In this example we have a blower that delivers 300m3/h(according to our calculation it should deliver 340m3/h though). This is a difference of mere 10%. The next stronger blower in this chart delivers 530m3/h at 300Pa, which is way too much.
In this case I'd go for the smaller(and cheaper) blower(Nr.4) than the much bigger one(Nr.1, which delivers 530m3/h at 300Pa), despite the fact that it's generally recomended to take a stronger one.
NOTE: This chart is only an example for a set of 4 particular blowers. The curves of the blowers available to you will look a tad different, but similar.
Usually, if you search long enough, you will be able to find a blower that exactly matches your HEPA.