Thread: DIY Room Air Filter

Its quite possible to do. I used one 105 cfm fan in a solar heater and it does blow a fair bit of air.
I would do some calculations as to changes of air in the room per hour to make sure that the fans will do what you would expect. A google search will help you out with the calcs. I think they normally work on 6 + changes of air an hour for filtration. Also remember to derate your fans by 50% due to sucking through the filters and blockage of filter material.
Let us know how it goes.

Shane.

My girl suffers bad hayfever and i'm building a positive displacement filtered air system to pump filtered air from the ceiling cavity into the bedroom so at least she can sleep well. I bought from Systemair in Perth last week a fan kit (see below), a 10 micron filter (600 x 600 x 50mm), and a fabric prefilter to protect the main filter. I've build a light plywood box to house the filters and had the wiring done (fan comes with a normal power plug attached). All going well it will be running tomorrow. Following is an extract of an email i sent to another friend over east who also suffers hayfever and the essence of it explains what i did.

i just bought a little fan, some duct work, an inlet filter cartridge to filter out the pollen and a variable speed fan controller and am in the process of building a little filter housing from plywood to mount in the roof cavity. The idea is that we will very slightly pressurise the bedroom with filtered air and this should in theory at least displace any pollen laiden air from inside the bedroom with fresh filtered pollen-free air. The fan is very well made and i expect it will last a long time.

I bought a kit called an ILK150HP that included fan, duct, inlet and outlet grills. The fan is a Fantech Plus TD 500/150 with a 2 speed motor and i have also bought a variable speed controller called a Fantech VA2.8 (fan draws 50 watts on high speed and i guess about 20 watts on low speed). This enables the fan to run from flat out down to about 20% of its total capacity and will very definitely minimise the amount of power you consume. These fans are designed for domestic/residential use and are also designed for moving hot air. Fantech make a range of fans in this style that would suit your application including a TD250/100 (nominal 250m3/hr flow rate and 100mm diameter duct), a TD350/125 (nominal 350m3/hr and 125mm dia duct). Some of these fans are two speed. The best way would be to put a variable speed controller on one then you have got good control over the amount of air that is moving and the amount of noise (very important). My fan on low speed (without a variable speed controller) would annoy me if it were running while i was watching TV and it was close by.

You can also get a VA2.0 speed controller for about $20 less than the VA2.8 but they do not provide adequate motor protection. The VA2.8 has a red light in the switch so you can see if the fan is running from the other side of the room and it has a minimum speed setting which protects the motor from running too slow (and overheating and burning out) you would need Mike to set the minimum speed setting when the fan was installed. The Fantech technical guy i spoke to today said don't touch the VA2.0 even though it is plenty big enough to run my fan.

The TD350/125 is $250 + GST
The TD500/150 is $280 + GST (my one)
The kit with duct etc that i bought ILK150HP was $370 + GST
The VA2.8 was $70 + GST

Following three links should give you necessary info.

http://www.fantechplus.com
http://www.fantechplus.com/pages/fan...oductgroupid=6
http://www.fantechplus.com/images/pd...itBrochure.pdf

There are a few things you need to consider. If you're trying to deal with hayfever/pollen then an internal recirculation system as you propose wont provide any slightly positive pressure anywhere in the house and therefore you may well still get pollen laiden air blowing in one side of your house and out the other. Ensuring good window seals should help - consider tape. Consider where you put the air inlet if you go for a positive pressure type system with regards operation in summer - it may blow hot air in. With a recirc system as you propose you may want to use insulated ductwork if you run the duct through the ceiling for the same reason. Room changes per hour are important - calculate volume of room in m3 (L x W x H) and then divide that number into the hourly flow rate of the fan. Say the room is 45m3 and the fan does 350m3/hr then you will in theory get nearly 8 changes per hour. Remember that the fan rating is usually given without any ductwork or other resistance hanging off the end of it. As soon as you add duct work, bends, suction and discharge grills etc the actual flow rate will drop (as a guess it could drop by between about 10 and 30% but that is only a guess)

Hope that long disertation helps

I have been thinking about doing this for some time and I once did a search and I found this

It is based on a box fan which should not be to hard to get but I am not sure how effective they would be. eg; cfm and changes of air an hour for filtration

Bathroom and computer fans are not designed or able to provide air flow against any sort of back pressure that proper (HEPA) room filters generate. These types of fans will simply not move enough air thru the filter and they will just end up cavitating (trashing the same air in the same place) and heating the air.

To move air through filters media you need a proper impeller or squirrel cage type fan.

You can build your own room air filter - it's not that hard I have built several - but make sure you start with decent fan and filter.

Originally Posted by Jigsaw

Has anyone made their own Room Air Filter.
The one's that recycle the air within the room.

Yes, I have built one for use in my finishing shed. I converted an old evaporative cooler that I picked up from one of the hard rubbish collection days. Here are the details of how I did this.

Works very well as the motor is designed to blow/suck through soggy and wet filtering material. It has 3 speeds but because of the size of the original unit (and I didn't want to cut up the original metalwork) is a bit bulky, but in my case that didn't matter.


Peter.

I am in the middle of putting together a small/portable spray booth for some reno's i am about to tackle and I picked up one of these fans on ebay for $60 and it works a treat.

http://cgi.ebay.com.au/NEW-HYDROPONI...QQcmdZViewItem

I am definatley going to rig this up in the workshop when i am done, i just bought a 6m (length of ducting) but i am not sure if i should use it to suck the dirty air out, or blow the clean air in...

joez

Thanks for all of the replies.
I will endevour to respond to all of the issues raised.
The computer fans I have are 110mm sq, not the ones installed in PC's.
When I did the calcs it appeared it would do 6 changes per hour. I had thought about the extra load provided by the filter but had no way of knowing how much to allow for.
On the weekend I measured the current drawn by all of the fans and also listening to them whiz around. I completely covered the fans with a sheet of wood to see if the current changed or if the fans started complaining if they had a total blockout and they didn't.
I did not think of the evapourative cooler fan, however we got rid of one on Friday from work into the skip. I had thought of using it the workshop to keep me cool but thought the moisture it pushes out would rust things in the shed.

I thought they used the squirrel motors on the purchased units so that the dust was not going through the motor, but I am not sure of this. I don't know why they would be more efficent than axial fans.
I also thought of a 6 computer fan assembly at both end of the box, one drawing dusty air in and the other pulling the clean air out.

Another thought also was to just connect the standard 4inch dust collector to the box to draw air through. It pulls about 650cfm and is designed to pull under load. It just means that instead of dusty air going into the collector, it would get clean air from the finer filters.

Thanks for all of the input, I am not sure what my next step is yet. I will see if I can get back into the shed in the next few weeks.

Originally Posted by Jigsaw

On the weekend I measured the current drawn by all of the fans and also listening to them whiz around. I completely covered the fans with a sheet of wood to see if the current changed or if the fans started complaining if they had a total blockout and they didn't.

That actually proves my point, even completely blocking them means they are not taking any significant load (they are just turning in the air in which they sit) which is why their current doesn't change. For example, a Table Saw will triple or quadruple its current draw when placed under load. Impellers and squirrel cages compress their air and don't allow it to escape so easily. Computer fans allow too much air to escape around the sides and in between the blades and cannot generate any decent sort of back pressure.

If you want to test how well a fan can push air through a filter (particularly one it gets partially blocked), build a box open at both ends that encloses the fan put the fan at one of the openings and the filter media at the other, or for testing purposes you can seal the box with a sheet of timber. Now make a manometer from a piece of plastic tubing fashioned in the shape of a U and some water at the bottom of the U. Place one end inside the section of the box between the fan and the sealed section of the box and the other side in atmosphere and measure the pressure the fan generates. A reasonable value would be something like 75- 150 mm of water.

Thanks BobL it is all starting to make sense now. Even when I looked at the specs on the compter fans they mentioned leakage etc at the blade tips.
I have never looked inside a purchased Room Air Filter to see what type of motor or fan type was inside.

I also got some filter material recently that is used for air conditioning and I tried putting this in front of the fans to see if the air would come through and I could not feel it. It was a 30micron filter material. I was not sure if I was supposed to feel it though.

On another note I purchased a 1hp Dust Collector recently and I read somewhere that the horespower rating was determined as the power consumed just before the unit fails. I measured the current drawn by the Dust Collector and it was 2amps which is less than 3/4 of a horsepower.

I took home the meter we purchased at work to give it a try out.
It is a clamp style meter that is capable of measuring the current through a 3 core flex. Meters normally require you to clamp the active line in the switchboard which I am not qualified to do, but this tool is great.

Originally Posted by Jigsaw

Thanks BobL it is all starting to make sense now. Even when I looked at the specs on the compter fans they mentioned leakage etc at the blade tips.
I have never looked inside a purchased Room Air Filter to see what type of motor or fan type was inside.

As most room air conditioners have very poor or limited filtering I would not use these are a model for a room filter either. However, a big evaporative aircon unit squirrel cage would be a fair start

I also got some filter material recently that is used for air conditioning and I tried putting this in front of the fans to see if the air would come through and I could not feel it. It was a 30micron filter material. I was not sure if I was supposed to feel it though.

30 microns is not much of an air filter - you need to get under 1 (and preferably under 0.5) micron to get health effective filtering. 30 Microns is just something to stop chunks blocking the gaps in the heating/cooling coil fins of an air conditioner. Sub 1 micron filters generate a significant back pressure unless they are arranged as very large surface area - see final comment.

On another note I purchased a 1hp Dust Collector recently and I read somewhere that the horespower rating was determined as the power consumed just before the unit fails. I measured the current drawn by the Dust Collector and it was 2amps which is less than 3/4 of a horsepower.

HP ratings on electrical motors more of a guide than anything that makes real sense as what one would like to know is how much work it can do rather than the power it consumes.

I took home the meter we purchased at work to give it a try out.
It is a clamp style meter that is capable of measuring the current through a 3 core flex. Meters normally require you to clamp the active line in the switchboard which I am not qualified to do, but this tool is great.

Yeah they are handy gismos. Mine is one that has to clamp on the Active only so I made up a short extension that has the active separated out from the N and E wire.

1 HP dust collectors have a nominal flow rate of 600 cfm at 6" of water back pressure. For all practical purposes their flow rate drops to 2/3 that when connected to a 30 micron filter or bag, and to around half when that filter starts to clog it can drop even more. One way to ease back pressure is to make the surface area of the filter larger. This is why Pleated filters work much better both in terms of micron rating and minimized back pressure but very little will beat a proper paper folded HEPA filter. These have around 150 mm thick folds and take a long time to clog up if they are shielded with 30 micron filter media.