Laminar flow hood – build a hepa filter flowhood fungifun hernia discal lumbar

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Once you decided on the size of the HEPA filter, you have to match a suitable blower to this particular filter. This is a very important step, so make sure to study the directions thoroughly.

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 with your filter) 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 particles like dust and hairs. It can be assumed that the static pressure for this prefilter at the working point is around 0.2"(50 pa)

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). Determining the correct blower for a filter consists of several steps:

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)

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 340m 3/h.( =200cfm)

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 400m 3/h.

In this example we have a blower that delivers 300m 3/h(according to our calculation it should deliver 340m 3/h though). This is a difference of mere 10%. The next stronger blower in this chart delivers 530m 3/h at 300pa, which is way too much.

In this case we go for the smaller(and cheaper) blower(nr.4) instead of the much bigger one(nr.1, which delivers 530m 3/h at 300pa), despite the fact that it’s generally recommended 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 may look a bit different, but similar.

The design in this pictorial uses an in-line centrifugal fan, another good option (actually the one most commonly used for flow hoods) is a squirrel cage blower (also called shaded pole blower).