Filter effects on Fan Speed...with performance ratings.

[Bluefront]

So I made a new filter housing for my wood case project. And I determined to test the effects of a few filters I have used. The housing just attaches to a 120mm fan by the foam-lined wood frame. The filter opening is 5"x7".....and it mounts the fan blades aprox 1 1/2" from the filter. This leaves an air gap that promotes better airflow at a lower noise level.



For the test I used a medium speed Yate Loon fan and a Fanmate2 speed controller. I found very similar effects on fan speed going from 11v to 5v, when using the different filters. So for this test the numbers are from the 11v test. Using a different fan would no doubt give different results, so any numbers you might get with this same test, would be different. But I'm interested in the restriction results of the different filters, not in the exact fan speed. Here's what the test setup looks like.....





Here's a photo of the different filters I used. I also tested a wire grill, and a case cut-out from an Antec Aria (a low restriction design).....from the upper left you see:

A Hamilton Beach pleated paper filter
A cutout of a Nissan Altima Cabin filter also pleated material
A piece of a plastic electrostatic filter from a furnace filter
A piece of a plastic extrusion (gutter guard) to hold the thin filters, keeping them from getting sucked into the fan.
A pieced together piece of flat vent filter material from Home Depot
A piece of a foam window AC filter
The Yate Loon fan
The wire grill
The cut-out from the Aria case.



The Yate loon will do 1600rpms @11v in free air sitting vertical, ambient of 28C, about 50% relative humidity. In the wood housing the rpms remain the same.....with no filters. Here are the drops in rpm using the different filters/restrictions.....

A wire grill.....rpm drop 0rpm.

The Antec Aria cut-out.....rpm drop -10rpms

The Electrostatic filter.......rpm drop -50rpms.

The extruded plastic piece.....rpm drop -10rpms.

The extruded plastic plus the ac foam filter.....rpm drop -80 rpms

The extruded plastic plus the vent filter.....rpm drop -90rpms.

The Nissan Altima cabin filter......-170rpms

The Hamilton beach filter......-190 rpms.

The numbers speak for themselves, but do not tell how good at filtering dust each filter is. From my experience, the two pleated filters are by far the best at filtering dust. Plus those two can go for a much longer time without cleaning or replacement. IMHO.....if you want the best filter, you will get the most restriction, and the higher rpm drop. This will also increase the noise level slightly. So like almost everything, you will have to make a decision about what you want.

[cAPSLOCK]

Interesting comparison, but when you say increase the noise level, do you mean that the noise will increase because the fans have to be run at a higher voltage to achieve the same airflow, or turbulence noise?

[Bluefront]

No...the noise increase is strictly from the restriction. There's probably a term for this effect, but it escapes me. I don't think the term is turbulance. You can easily demonstrate this effect by moving a card or something toward the intake side of the running fan. You'll hear a change in the sound. This is the same effect as installing a very restrictive filter.

In this test, the voltage was fixed at 11V. But the sound differs with diferent filters......usually more restrictive equals louder. I am unable to determine if a particular filter is louder because of the type of material used, only that more restriction makes it louder.

What was interesting was the wire grill and the Aria cut-out. They had almost no effect when on the intake side of the fan. But when on the exhaust side, the rpm drop was much greater.....due to back-pressure. Something to consider when dealing with grills.

[Bluefront]

So just how effective at dust filtration, are these various filters? Unfortunately most of these filters I have used, have no way to measure.....no figures are published that I can find. But......If you browse the furnace filter section of a large hardware store, you can find out. The specs are on some packages.

I wanted to try the best available.....a 3M Filtrete filter, called "ultra allergen". It has a performance rating of 1250. That was the best of the bunch at Home Depot. You'll have to cut out a section of the filter....not too hard, but took me about one hour. This filter has a small aluminum frame on each side of the filter material.....keeps the filter from being sucked in. This frame is glued to the filter, which makes the cutting-out procedure very easy.







This filter has a large number of closely-spaced pleats. And this is important both to lessen restriction, and to extend the life of the filter.....before cleaning is necessary. I calculated the total surface area of the filter as you see it as 280 square inches. As a comparison, the surface area of a flat filter placed over a 120 mm fan is about 20 square inches or less.....depending on the area of the hub and fan motor braces.

Here's a list of various filters with their performance rating....off the 3M package.

Filtrete Ultra Allergen......................1250
Filtrete Micro allergen......................1000
Non-electrostatic 3-month filter.........160-430
Washable filter.................................20-200
Fiberglass filter.................................50-60

So how does this filter affect my computer? Ambient today of 28.5C. Computer idling at 1229rpms @35C. When I place the filter assy on the fan, after a few minutes the RPM rises slightly to 1300 @36C. Yeah you pay a price for using a good filter....but it's worth it if you have to deal with dust, from pets or whatever. This filter cost me $16......I could cut maybe 6 computer filters from it if I was really careful. Not bad...

[BrianE]

So how does this filter affect my computer? Ambient today of 28.5C. Computer idling at 1229rpms @35C. When I place the filter assy on the fan, after a few minutes the RPM rises slightly to 1300 @36C.

Hi Bluefront, thanks for doing these tests - they are quite informative. One thing I noticed though, was that in your previous tests there was generally an RMP drop as filters became more restrictive. Why does the RPM increase in this case? To me that implies that the 3M is less restrictive than the "before" setup.

Also, I like how nice the wooden frames you made for the filters look. How do you attach the filter into it or make sure that it seals well? I've thought of some holders for filtering material before (none of which were built ) but sealing it (while making the holder re-usable) always seemed tricky.

[Bluefront]

I guess I wasn't clear enough....In the first series of tests on different filters, the fan voltage was fixed at 11V. When I changed filters the rpm dropped depending on how restrictive each filter was.

In this last filter test, the fan speed was controlled by a sensor. So when I installed the filter, the temp started to rise because of less airflow......and as the temp rose, the sensor increased the fan-speed to compensate. I ended up with about the same temp at a slightly higher rpm. That's how this fan controller is supposed to work (Noisemagic Thermalcontrol NMT-3)

Here's a photo of the rear of the filter housing.....it's a square section made of four pieces of wood glued together. I wrapped some heavy black duct tape around it to strengthen it. The inside of this square is lined with 3mm thick foam.....it was made to be a snug fit around the externally mounted 120mm case fan. Just pushes on...



Here's a view of the front of the filter housing....there is a small foam covered ledge running around the inside of the housing. The various filters rest against this sealing ledge. If you're careful cutting out these filters, they seal well against the sides and the ledge.



If I were willing, I'm sure this whole filter design could get a patent. I've never seen anything like it for sale anywhere. The filter is about 1.5" from the fan blades, with a step that gives the 120mm fan the ability to draw from the entire area of the filter. The whole thing installes/removes in seconds with no tools. Neat device.....

[dchrsf]

^^Links/Images broken

[jaganath]

All the images are displaying correctly in my browser (standard IE).

[Bluefront]

Humm...these images are being hosted by Photobucket, which is fairly reliable. So if an image won't load, try back later.

Here's one more view of this setup.....there is a small insert used in this picture that holds a flat filter. the insert is held tightly by some window screen clips. I'll probably never use a filter like this low-restriction type, but you could.....

[Longwalker]

All else being equal, air flow through a filter is inversely proportional to the size of particles the filter will trap. A MERV 11 filter like the Filtrete 1250 ('1250' is just a 3M model number) will capture at least 60% of particles larger than one micron. The price for this, however, is that pulling air through a MERV 11 filter requires close to twice the fan power as pulling air through a MERV 8 filter that blocks 70% of particles larger than 3 microns. As a result, if you want to keep your fans quiet, you're better off using a Filtrete 600 filter rated MERV 8 than a Filtrete 1250 rated MERV 11. The best filter available is not necessarily the best for every use.

[Bluefront]

Without getting too technical here.....the "1250" number on this particular filter is not the model number, but rather an MRP (microparticle performance rating) assigned by 3M to compare the efficiency of their various filter models with each other and with different types/brands. This number is comparable to the "MERV" number you mentioned.

The other thing to be aware of when comparing pleated filters, is the number of pleats per the width. More pleats equals more surface area......which translates to better airflow and longer life.

In the real-world conditions, each of us has a different air quality to deal width.....in my home my pet parrot is the major problem, the major creator of dust (feather dust). This dust ranges in size from .5-10.0 microns, which is well within the ability of this filter to capture most of it. Using a less effective filter would catch dirt, but would let the feather dust right through.

If you don't have pets, particularly birds, and have a clean house without carpets, never open your windows, have a HVAC system with good filters....you probably don't need a filter like this 3M model. Not all of us are so lucky......

[Anvar]

In the real-world conditions, each of us has a different air quality to deal width.....in my home my pet parrot is the major problem, the major creator of dust (feather dust).

Easily solved, just pluck the parrot, done.

[Bluefront]

Oh yeah....I'd get one feather. After that I wouldn't have any fingers left....

[monkiman]

I'm doomed - 2 cockatiels+ 1 hairy dog = big honking gobs of dust