Adam K Dean

Notes on software, systems, engineering, and things

2.2 The particle filter

Cat W/C Air Filtration System ·

The second research topic is the particle filter: what dust a litter room puts in the air, which filter grade is worth having, and how much airflow it costs. I gave ChatGPT a brief with the plan and the findings from 2.1, on GPT-6 at the Pro setting. It worked for about 27 minutes.

The short version: H13 HEPA isn't worth chasing here. The stack becomes a washable hair mesh, a pleated ePM1 filter before the carbon, and the best filter, an E11 or E12, after the carbon, where it also catches dust the carbon sheds. Face area matters more than grade. IKEA's £5 FÖRNUFTIG filter, the thing that started this project, is the first one to test.

What a litter room puts in the air

There's no good size data for litter dust. It depends on the brand, how the granules are made, and how hard the cats dig. Some litter makers count everything below 250 µm as dust and aim to keep it under 1% by volume. That's far bigger than PM10, so "99% dust-free" on a bag says little about the fine particles that stay in the air.

Cat allergen is better studied. A study of nine homes found about three quarters of the airborne Fel d 1 on particles of 5 µm and up. On average a quarter was on particles of 2.5 µm and below, but that ranged from 10 to 62% between homes. A hair screen on its own would miss a good share of it.

Hair is a maintenance problem more than a filtration one: it mats over filter faces. And if the litter is a bentonite clay, its safety data sheet is the place to check for crystalline silica. Silica gel litter is a different thing, amorphous silica. A 2025 conference abstract, partly based on industry dust data, estimated that normal use of bentonite litter isn't a significant breathing risk, but said the estimate may be low for homes with several pets.

So the particle stage has two jobs. It keeps hair and coarse dust off the carbon, and it catches fine particles somewhere in the air's path.

The grades

The EN 1822 classes are rated at the particle size that gets through most easily, so they're a worst case:

Class Minimum efficiency Gets through
E10 85% 15%
E11 95% 5%
E12 99.5% 0.5%
H13 99.95% 0.05%
H14 99.995% 0.005%

E is EPA and H is HEPA. The general ventilation grades are different tests again. An ISO 16890 ePM1 60% filter takes 60% of the PM1 fraction as a whole, not 60% at every size below 1 µm. The older EN 779 G4 is measured on a coarse test dust, so it doesn't mean 90% of PM2.5. And MERV is the American system, with its own size bands. None of them converts exactly into another.

Why not H13

The room gets clean air at the airflow times the filter's efficiency, as in 2.1:

Qclean=Q×ηQ_{\text{clean}} = Q \times \eta

At 36 m³/h:

Grade η\eta QcleanQ_{\text{clean}}
E11 0.95 34.2 m³/h
E12 0.995 35.8 m³/h
H13 0.9995 36.0 m³/h

An E11 filter needs only 5% more airflow to clean the room as fast as an H13, and an E12 needs less than half a percent more. So for a room unit the grade above E12 barely matters, and the airflow does. A good H13 panel that doesn't cost any airflow would be fine. The grade doesn't set the resistance on its own: the construction does.

Face area and pleats

Air speed through a filter face is the airflow over the face area:

v=Q3600×Av = \frac{Q}{3600 \times A}

with QQ in m³/h and AA in m². For a 200 × 200 mm face at 36 m³/h that's 0.25 m/s. For FÖRNUFTIG's 390 × 250 mm face it's 0.10 m/s.

ChatGPT found published pressure drops for real filters and scaled them to a 0.1 m² face, assuming the drop rises in step with air speed, which is close enough for clean fibrous media:

Filter Published At 36 m³/h At 60 m³/h
G4 synthetic mat, 20 mm 30 Pa at 1.0 m/s 3 Pa 5 Pa
Pleated ePM1 55%, 48 mm 110 Pa at 2.3 m/s 5 Pa 8 Pa
E11 mini-pleat, 50 mm pleats 90 Pa at 0.45 m/s 20 Pa 34 Pa
E11 mini-pleat, 125 mm pleats 45 Pa at 0.45 m/s 10 Pa 17 Pa
H13 mini-pleat, 66 mm 115 Pa at 0.45 m/s 25 Pa 42 Pa
Flat ePM1 65% sheet, 4.5 mm 120 Pa at 0.17 m/s 72 Pa 120 Pa

Two lessons. A flat sheet of fine media is a trap: 120 Pa at a speed this unit will actually run at. And deeper pleats help a lot: the same E11 media in 125 mm pleats has half the drop of 50 mm pleats. Pleating multiplies the media area by about twice the pleat depth over the pitch, so 40 mm pleats at a 4 mm pitch give 20 times the face area.

The target is about 0.1 m² per fine filter face. That's bigger than the 220 × 220 mm I'd been picturing, and bigger than the A1's bed in one direction, but the bed limits the printed parts, not the filter. The frame can be joined around a larger panel.

The stack

Room air → hair mesh → pleated ePM1 → plain carbon → carbon for bases → E11 or E12 → fan → room.

The best filter goes last because the carbon granules shed dust. Particles from the room meet both filters, and two in a row combine as

η=1−(1−η1)(1−η2)\eta = 1 - (1 - \eta_1)(1 - \eta_2)

but dust made inside the carbon cartridge only meets the one after it. The ePM1 filter in front isn't a perfect barrier, but it keeps most of the dust off the carbon without paying for two EPA filters.

The estimated pressure budget, clean:

Stage At 36 m³/h At 60 m³/h
Hair mesh, 0.3 to 0.8 mm openings 1 to 3 Pa 2 to 5 Pa
Pleated ePM1 50 to 60%, about 45 mm deep 5 to 12 Pa 8 to 20 Pa
E11 or E12, pleated 20 to 30 Pa 35 to 50 Pa
Particle stages 26 to 45 Pa 45 to 75 Pa

That leaves out the carbon beds, the grilles and the turns inside the case, which is the next topic. These are estimates from published data, not measurements, and a thin consumer filter won't necessarily match a 50 mm industrial one.

Sealing

If a share bb of the air goes round the filter instead of through it:

ηeff=(1−b)×η\eta_{\text{eff}} = (1 - b) \times \eta

With 1% bypass, an H13 filter's 99.95% becomes 98.95%. What gets through goes from 0.05% to 1.05%, 21 times as much. But the room still gets 99% of the clean air. A leak ruins a HEPA claim long before it ruins the room cleaning. The catch is that a gap has far less resistance than the media, so a small gap can carry far more than its share of the air.

The housing gets a continuous seal, not a push fit: a flat seating land 8 to 12 mm wide, a 2 to 3 mm closed-cell gasket (EPDM, silicone or neoprene), and a clamping frame with stops so the screws can't crush the filter. The joins between printed parts need sealing too. I won't assume a 3D-printed wall is airtight.

Filters to design around

Filter Size Grade Price
IKEA FÖRNUFTIG particle filter 390 × 250 × 20 mm E12 £5
IKEA STARKVIND particle filter 370 × 290 × 40 mm E12 £12
IKEA UPPÅTVIND particle filter about 200 × 250 × 25 mm E12 £3
ADD Filtration G4 pads, pack of ten 290 × 290 × 20 mm G4 £5.50 + VAT

IKEA's filter pages don't give a class. The grade comes from the purifier pages, which say EPA 12. FÖRNUFTIG's filter has a 0.0975 m² face for £5, so it's the obvious first test. IKEA doesn't publish its pressure drop, though, and a filter in my case doesn't inherit IKEA's testing. I'll measure it before I design the housing around it. For the ePM1 stage, a pleated panel from a filter supplier, with its test report and pressure curve, beats cutting media to size.

Charged media and humidity

Many synthetic filters use an electrostatic charge to catch fine particles with less resistance. The charge can fade, and the efficiency with it, without any change in pressure drop. A 2025 lab study loaded charged filter media with a salt aerosol at 80% humidity: efficiency fell by 43.2% while the pressure drop rose by just 1 Pa. That's one test, not a prediction for IKEA's filter, but it shows that the pressure sensor won't see this kind of failure. When I buy, I'll ask for the minimum efficiency with the charge removed (ePM1,min in ISO 16890), not just the headline figure.

When to change them

Stage Check Change when
Hair mesh Weekly at first Hair starts covering it. Wash and dry fully
ePM1 Monthly It eats too much of the pressure budget, or the airflow can't be held
E11 or E12 After the first carbon fill, then monthly Its pressure rises, or carbon dust loads it fast

As starting triggers: a stage reading 10 to 20 Pa above its clean value, or the unit losing about 20% of its airflow at the same setting. IKEA's six months is a backstop, not a lifespan. Fine filters can't be washed, only replaced. And a rising pressure shows loading, not a hole or a leak, so the seals get a look at every change.

What changes in the design

What I checked

I had Claude check sixteen of the claims against the sources, reading the standards summaries, data sheets, papers and shop pages themselves. I worked through the arithmetic myself, including ChatGPT's scaling of the pressure drops to a 0.1 m² face.

These held up: the EN 1822, EN 779 and MERV figures, the cat allergen particle sizes, every published pressure drop in the table, the IKEA filters' sizes and prices, the six-month replacement advice, and the electret humidity result.

These needed correcting, and the post above uses the corrected versions:

The rest, including the pressure budget for the hair mesh and the replacement triggers, comes from ChatGPT's answer as engineering estimates, and wasn't checked claim by claim.

Still open

Sources

Papers and standards:

Manufacturers and shops: