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Old 11-25-2022, 09:28 AM
sfl sfl is offline
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Default DIY - Air Purifier

Hi,

I am considering building a Air Purifier, I need it because I am working in a garage and it can become very dusty over the time.

I can build a wooden box embedding a fan and putting a G4/HEPA filter in front of it or behind it.

Questions:

1. is it better to place the filter in front of the fan or behind ? is it make any difference ? I'd prefer to place it behind where the air is sucked in so that my fan is "never" getting dirty.

2. Which kind of filter ? G4/HEPA ok, but can I built it using a filter tissue or is it better to buy an already framed filter, which is the different ?

I'd prefer to built my own filter using the G4 tissue and frame it in wood, this will fit my box measures.

What do you think ? any suggestions ?


thank you
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Old 11-25-2022, 02:51 PM
printer2 printer2 is offline
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Front or behind, the fan will not care. As long as you have the edges of the material sealed (or clamped) to the frame so no air sneaks around then you are good to go. It is always best to collect the offending particles by collecting them at the source rather than trying to filter it out of the air. If just this filter, place it closer to where the particles are coming from. No idea of your situation but it may be something to consider. Other than noise being an issue not a lot to figure out.
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Old 11-25-2022, 06:30 PM
Rudy4 Rudy4 is offline
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I have a box fan mounted to a simple wood frame that holds two 20" by 20" furnace filters. It's just visible at the top of the photo here. It has a standard furnace filter that sits in front of a filter that's maybe 20 microns. Both filters are on the intake side of the fan. It clears out nuisance dust pretty well, but if there's fine stuff in the air I wear a fine particulate mask.

If you're anticipating using a HEPA filter you'll need a carefully designed fan that can pull an adequate amount of air through the filter.

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Old 11-26-2022, 04:58 PM
LFL Steve LFL Steve is online now
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Quote:
Originally Posted by sfl View Post
Hi,

I am considering building a Air Purifier, I need it because I am working in a garage and it can become very dusty over the time.

I can build a wooden box embedding a fan and putting a G4/HEPA filter in front of it or behind it.

Questions:

1. is it better to place the filter in front of the fan or behind ? is it make any difference ? I'd prefer to place it behind where the air is sucked in so that my fan is "never" getting dirty.

2. Which kind of filter ? G4/HEPA ok, but can I built it using a filter tissue or is it better to buy an already framed filter, which is the different ?

I'd prefer to built my own filter using the G4 tissue and frame it in wood, this will fit my box measures.

What do you think ? any suggestions ?

thank you

1. In HVAC design it is preferable to have filters, coils, etc upstream of the fan, on the low pressure "suction" side. There are a variety of aerodynamic reasons for this, and there is also the practical advantage that air pressure will tend to draw the filter tight to the frame so you'll have less trouble with leaking, in addition to the advantage you mentioned that the fan, motor, and bearings will have less dust.

2. G4 and HEPA are toward opposite ends of the spectrum for particle sizes they arrest: G4 is for quite coarse, HEPA is for very fine. It's the very tiny particles that are bad for your lungs, PM10s and smaller. Larger particles get trapped on your mucous membranes in your ingeniously designed airway and don't make it deep into the lungs. But as a practical matter, G4 is actually considered appropriate for wood dust, and you'll certainly get much more airflow through a G4 than through a HEPA given the same fan and filter surface area. Filter selection is a tradeoff between pore size and airflow (I'm leaving out pressure differential and other factors not relevant to a simple residential system).

Total area of the filter surface matters. Think of the filter as a collection of tiny pores, and each individual pore allows only a tiny amount of airflow. The more pores, the more airflow. This is one reason for using pleated filters. The more and deeper the pleats, the more surface area, so the more airflow.

printer2 is correct that it's better to catch the particles where they are generated, before they diffuse into the general air circulation. Check out Rockler's website for some systems that do this. In a shop setting, you'd typically use a 4" hose with its inlet right at the dust source, and the hose leading to a dust collector system. Some shops have a hose inlet at each tool, and operable gates ("blast" gates) so that only the one in current use is open, maximizing flow at that point. There is an endless selection of hoses, connectors, Y fittings, blast gates, etc; very elaborate systems can be built.

Any filtration is better than no filtration, so by all means, build your gizmo! But even with filtration, shop air is usually dirtier than outside air. When weather allows, open the garage door. If possible, roll your tools outside and work out on the driveway.
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