5 Micron Filter Cloth: What Micron Ratings Mean in Felt
Blog · 10 November 2026 · 9 min read
TL;DR: A "5 micron filter cloth" rating names a particle size the cloth is said to capture. It is neither a fibre size nor an emission level. Fibre fineness is stated in decitex, which an Elsevier textbook defines as the weight in grams of 10,000 metres of fibre, so a lower dtex means a finer fibre. Stack emissions are a third measure, a mass of dust per normal cubic metre of gas (mg/Nm³), and no micron rating converts into one. A better purchase order for filter felt specifies the dust profile, the gas conditions and the emission target, and lets the felt be engineered to them.
What does a 5 micron filter cloth rating actually specify?
A micron rating on filter cloth is the particle size a supplier says the medium is designed to capture, given in micrometres (µm), each one-millionth of a metre. The popular reading treats it as a sharp cut-off: particles of 5 µm and above stop, and smaller ones pass. That picture comes from woven mesh, which screens at a single plane, and it does not describe needle felt.
Filtration literature qualifies the cut-off model. A 2018 review of filter cloth in the International Journal of Clothing Science notes that some screening fabrics are designed only to capture particles above a set size, while other systems tolerate some solids passing until cake filtration takes over. A baghouse bag is the second kind, because the dust cake on the felt does much of the fine capture.
Efficiency also changes with particle size, which one micron figure hides. FabCo Industrial Services, which sells baghouse filters and services, illustrates this with a hypothetical filter that may rate 95% efficient at 5 microns but 99% at 10 microns. A micron number printed without an efficiency percentage and a test method is a product description, not a performance guarantee.
Filter cloth micron size chart: particle size, fibre diameter and emissions
Five different numbers can end up on one line of a filter-bag enquiry. Particle size and fibre diameter are both lengths in microns, one describing the dust and the other the felt, while decitex describes the fibre by mass and mg/Nm³ describes what leaves the stack.
| Number | What it measures | Unit | Where it comes from | What it cannot tell you |
|---|---|---|---|---|
| Micron rating | Particle size the cloth is said to capture | µm, a length | Supplier catalogue or listing | Efficiency at that size, or the test behind it |
| Dust particle size | Size of the particles in the gas | µm, a distribution | Laboratory analysis of a dust sample | How the dust behaves once it forms a cake |
| Fibre fineness | Mass per length of one fibre | dtex, grams per 10,000 m | Fibre or felt datasheet | Diameter, unless the polymer density is known |
| Fibre diameter | Physical width of one fibre | µm, a length | Microscopy, or calculation from dtex and density | Pore size alone, which also depends on felt weight and needling |
| Stack emission | Mass of dust per volume of cleaned gas | mg/Nm³, a concentration | Stack test or emissions guarantee | The size of the particles that escaped |
For scale, research from Foshan University published in MDPI Buildings in 2024 puts primary silica fume particles at 0.1 to 1 µm, well below a 5 µm rating. That gap is one reason a dust's own size distribution matters more than the label on the cloth.
Is a 5 micron filter bag the same as 5 mg/Nm³ emissions?
No. A 5 micron rating describes particle size, while 5 mg/Nm³ describes the mass of dust left in each normal cubic metre of cleaned gas, a volume corrected to reference temperature and pressure. Indian suppliers quote outlet performance in the second unit: TECHFLOW, which sells bag filters, states that bag filters can achieve emission control less than 5mg/Nm3, without naming the dust, test data or conditions.
No published relationship converts a cloth's micron rating into an outlet concentration. Masturlal's own guidance on reducing emissions names finer fibres on the dust side, correct bag fit, sealed tube sheets, powder leak testing, and cleaning gentle enough to leave the dust cake in place. Only the first of those is a property of the cloth, and none of them is a micron figure.
Decitex sets fibre fineness, and fineness shapes pores in felt
- Decitex (dtex)
- the mass in grams of 10,000 metres of a fibre, so a lower dtex value means a finer fibre.
Decitex is a direct count, meaning a higher number signals a coarser fibre. Denier uses a 9,000 metre reference length and tex uses 1,000 metres, and an Elsevier text on cut-protective textiles gives the conversions: multiply denier by 1.11 to get decitex, and divide dtex by 10 to get tex.
Textile engineering states fineness by mass rather than width because diameter varies along a fibre and cross-sections are not always circular. For a round, solid fibre, the Elsevier textbook Textile Calculation gives decitex ≈ 7.85 × 10⁻³ × density (g/cm³) × diameter² (µm), so at a fixed density, diameter scales with the square root of dtex. Its worked example puts a 40 µm cotton fibre at about 19.09 dtex; cotton is not a filtration polymer, but the method carries over.
The same textbook supplies the geometry behind fineness: for a round fibre, surface area per unit volume equals 4 divided by diameter, so halving the diameter doubles the surface. Masturlal states the consequence for felt as lower dtex giving finer fibre, more surface and finer pores at the same felt weight. The step from surface to pore size is the felt maker's engineering position, and a sample swatch tested on the plant's own dust is where a buyer checks it.
How do you convert dtex to fibre diameter for polyester and aramid?
Converting dtex to microns needs the polymer's density, which the textbook formula leaves to the user. VNPOLYFIBER, which sells polyester staple fibre, gives polyester's density as 1.38 g/cm³ and a round-fibre formula of diameter (µm) ≈ 11.89 × √(denier ÷ density), consistent with the textbook's denier version. Its own published diameter table sits below that formula by a factor of about √2, so neither should drive a purchasing decision without independent checking.
Aramid needs a density from the fibre producer's datasheet, as does any blend. The dtex of a microfibre surface layer is also a datasheet value, so a buyer comparing two "5 micron" quotes learns more by asking each supplier for the polymer, the face-layer dtex and the test basis.
Mesh-size ratings do not describe how needle felt captures dust
Needle felt is the dominant baghouse medium, and it does not capture dust through a mesh opening. The 2018 International Journal of Clothing Science review describes needle-felt nonwovens as the most commonly used filtration fabrics and notes that multilayer media combine layers varying in porosity or fibre diameter, such as a fine-fibre face over a coarser support.
Depth filtration is the capture of dust inside the fibre matrix of a filter medium. Surface filtration is the capture of dust on the face of the medium, where it forms a cake that cleaning pulses can release. Masturlal's account of pressure drop (ΔP) follows from the difference: ΔP rises with over-dense media, dust embedded deep in the felt or a cake that will not release, and surface capture with clean release keeps it low and stable.
Tighter structures carry a cost. Roxia, which sells filter cloths, notes that non-woven media can retain fine particles an equivalent woven cloth would pass, but blind more readily under high solids loading; it writes about liquid filtration, but the trade-off carries over to dust. Two felts with the same micron label can therefore behave very differently, one holding fine dust on its face and the other letting it work into the fibres.
Masturlal uses ultra-microfibre surface layers to catch fine particles at the felt surface, and offers them among the media for its pulse-jet filter bags. A same-size replacement bag cannot change a collector's air-to-cloth ratio, so where more cloth area is needed, Masturlal points to a different geometry such as its pleated filter elements, which also carry ultra-microfibre surface layers.
What micron rating filter cloth do I need for cement dust?
No single micron rating fits cement dust, because one plant runs several dust streams (kilns, raw mills, clinker coolers, coal mills and packers), each with its own temperature, chemistry and abrasion. The right felt follows from the stream, not from a number on a listing.
How fine are cement, coal and silica fume dust?
Published particle size figures exist mainly for silica fume, and they show how far a dust can drift from its nominal size. Primary silica fume particles run 0.1 to 1 µm in the Foshan University research cited above, but densified material agglomerates heavily.
In a 2024 laboratory study by Foshan University researchers in MDPI Buildings, densified silica fume .
That sample was a wet suspension prepared for concrete, not baghouse inlet dust. Cement and coal dust have no single representative published particle size distribution; the profile shifts with the mill, kiln, fuel and collection point, so it has to be measured from a sample taken at the collector. Masturlal treats ferro-silicon furnace off-gas, with its very fine silica fume at high volume, as a case for ultra-microfibre surface filtration.
What do Indian cement-plant listings specify?
The "5 µm" label does appear in Indian procurement. Baroda Industrial Fabrics, which sells filter bags from Vadodara, lists its cement-plant bag as a 600 gsm aramid felt with a "5 µm" micron rating and a 250°C maximum, with no test method or efficiency stated for the rating.
Masturlal specifies cement felts by stream instead: PPS and meta-aramid media, microfibre surface layers where emission limits are tight, and anti-static construction for coal mills, where fine coal dust moving through polyester builds charge and creates a fire and explosion risk. Its reverse-air filter bags add fibreglass media with PTFE, graphite or silicone finishes for continuous, low-stress baghouses.
Specify the dust profile on the purchase order, not a micron number
A purchase order that names the dust and the duty gives a felt maker what a micron rating cannot. Macrokun, which sells mesh filters, lists particle size distribution alongside flow rate, chemical compatibility, temperature and cleaning needs as selection factors. For needle felt, Masturlal asks for the following before recommending a spec.
Collector and bag geometry. Give the collector type, the bag diameter and length, and the quantity. Pulse-jet, reverse-air and shaker collectors stress felt differently, so the format comes first.
Gas temperature and moisture. State the normal and peak gas temperature and the moisture content. These decide which fibre survives, since acid dew point and hydrolysis attack the wrong fibre.
Dust chemistry and particle size. Send a dust analysis and a measured particle size distribution from a sample taken at the collector. Abrasion, stickiness and combustibility belong here too, because they set the finish.
Current bag specification. Describe the bag in service: fibre, felt weight, finish and service life. That record gives the application engineer a baseline to improve on.
Emission target and pressure drop. State the outlet limit in mg/Nm³ the plant must meet and the ΔP the system runs at today. These are the outcomes the felt is engineered for, and the micron rating is not one of them.
From that dust profile, Masturlal's process moves to a recommended spec and a sample swatch, then bags built to the collector drawing, then field support.
Sources
Fibre Fineness - an overview | ScienceDirect Topics (Elsevier (Textile Calculation, Madan Lal Regar et al.), 2023-01-01)
A Review Article on Applications of Filter Cloth (International Journal of Clothing Science (Scientific & Academic Publishing); authors Zerin & Datta, Bangladesh, 2018-01-01)
What Are Baghouse Filter Ratings? (FabCo Industrial Services (Fabco), 2024-10-17)
Research on Mechanical Properties of Silica Fume Cementitious Materials Excited by Wet Grinding Methods (Zhao, Li, Li & Li, Foshan University, published in MDPI Buildings, 2024-11-25)
ESP vs Bag Filters in Sugar Mills | TECHFLOW (TECHFLOW)
Linear Density - an overview | ScienceDirect Topics (Elsevier (R. Chattopadhyay et al., Textile Calculation), 2023-01-01)
What is a Filter Cloth(Filter Fabric) and what is it used for - Roxia (Roxia, 2026-01-01)
Dust Collector Bags - Filter Bag For Cement Plant Manufacturer from Vadodara (Baroda Industrial Fabrics (Vadodara), via IndiaMART listing)
Frequently asked questions
- What does 5 micron filter cloth mean?
- 5 micron filter cloth is cloth a supplier says is designed to capture particles around 5 micrometres in size. In needle felt the figure is not a sharp cut-off, because the dust cake also captures finer particles. Without an efficiency percentage and a test method, a 5 micron rating describes the product rather than guaranteeing its performance.
- Is a 5 micron filter bag the same as 5 mg/Nm³ emissions?
- A 5 micron filter bag is not the same as 5 mg/Nm³ emissions. Micron measures particle size, a length, while mg/Nm³ measures the mass of dust per normal cubic metre of cleaned gas. No published relationship converts one into the other, and emissions also depend on bag fit, tube-sheet seals, leaks and cleaning.
- What micron rating filter cloth do I need for cement dust?
- Cement dust has no single correct micron rating, because kilns, raw mills, clinker coolers, coal mills and packers produce different dust at different temperatures. Felt for cement duty is better specified by gas temperature, dust chemistry, measured particle size and emission target, which then determine the fibre, decitex, finish and any microfibre surface layer.
- What is decitex, and how does fibre fineness relate to a micron rating?
- Decitex is the weight in grams of 10,000 metres of fibre, so a lower dtex means a finer fibre. Decitex describes the felt's fibres by mass, while a micron rating names a particle size the cloth is said to capture. Lower dtex gives more fibre surface at the same felt weight, but no formula converts dtex into a micron rating.
- Can I convert dtex to fibre diameter for polyester or aramid?
- Converting dtex to fibre diameter requires the fibre's density as well as the dtex value. For a round, solid fibre, the Elsevier textbook Textile Calculation gives decitex as 7.85 × 10⁻³ times density times diameter squared. Polyester and aramid diameters therefore need density figures taken from the fibre producer's datasheet.
- Is a finer filter cloth always better for dust collection?
- A finer filter cloth is not always better for dust collection. Tighter structures capture finer particles but raise resistance to airflow and blind more readily under heavy dust loading, which pushes pressure drop and fan power up. The aim is a felt face fine enough to hold dust at the surface and open enough to release it cleanly.
- What should I send a supplier to get a filter bag quote?
- A filter bag quote needs the collector type, bag diameter and length, quantity, gas temperature, moisture, dust chemistry, measured particle size, the current bag specification and the emission target. With that dust profile, a felt maker such as Masturlal can recommend the fibre, decitex and finish, then confirm the choice with a sample swatch.
Get a felt specified for your dust profile
Before the next purchase order says "5 micron", gather the dust profile and emission target for the collector in question, so the felt can be specified to them rather than to a label. You can see how Masturlal specifies felt for your industry and start from the application closest to your plant.