Engineering Calculation Tools
Generic baghouse calculators assume the flow figure you already have is the flow the filter actually sees. In EAF and ladle furnace dedusting the gas is hot, and the figure on hand is almost always in Nm³/h — using it directly can undersize the filter badly.
Air-to-Cloth Ratio
1.33 m/min
4.37 ft/min
EPA fabric filter design guidance places filtration velocity roughly in the 0.3-3.7 m/min (1-12 ft/min) band overall; pulse-jet systems tend to sit toward the high end and reverse-air/shaker systems toward the low end. The right value for your specific dust and cleaning method is an engineering question — talk to DNL's engineers.
The air-to-cloth ratio is the actual operating flow divided by the total cloth area:
ratio [m/min] = actualFlow [m³/h] / 60 / clothArea [m²]
actualFlow is the real volumetric flow of gas through the filter at operating temperature (m³/h) — not the normal-condition flow (Nm³/h); clothArea is the total filter area with every bag in service (m²).
Field engineers usually have their flow figure in Nm³/h (normal conditions, referenced to 0°C), but baghouse sizing needs the ACTUAL volumetric flow at operating temperature (m³/h). Hot gas occupies proportionally more volume than the same mass at normal conditions, so using the Nm³/h figure in place of m³/h undersizes the required cloth area.
Size an EAF dedusting system quoted at 400,000 Nm³/h, gas at 130°C, for a 1.0 m/min target velocity.
Using that 400,000 figure directly as if it were m³/h gives a required cloth area of 6,667 m².
But this is a normal-condition flow. Converted to the actual flow at 130°C operating temperature, per DIN 1343's 0°C reference, it comes to 590,372 m³/h.
Sizing on that actual flow, the cloth area actually required is 9,840 m².
That means the correctly calculated 9,840 m² is 47.6% larger than the 6,667 m² figure the Nm³/h shortcut gives — the gap between a filter that runs hot, blinds early and risks an emission breakthrough, and one that's sized right.
Assumptions: normal-condition reference of 0°C per DIN 1343, constant pressure, ideal gas behaviour.
This calculator gives the GROSS ratio, computed with every bag in service. When compartments are taken offline for cleaning, the same gas volume flows through less cloth, so the net filtration velocity at that moment is higher than the calculated gross ratio. Sizing on the gross ratio alone, without accounting for compartment count and cleaning cycle, is a known design error.
Off-gas must be deeply cooled before it reaches the bag filters, because the operating conditions of the bags set the limit. Where the bags are made of relatively low-cost materials, the off-gas temperature at the filter entrance must not exceed 120–140°C; in a shop reconstructed with higher furnace power, using bags of more expensive materials can allow this limit to rise to 200°C (Toulouevski & Zinurov, Innovation in Electric Arc Furnaces, 2nd ed., Chapter 14, p. 256).
This cooling is achieved by three methods: injecting finely atomised water into the hot off-gas flow, mixing it with cold secondary emissions at the duct junction, and drawing outside air through a dedicated valve.
This limit connects directly to what the tool needs: the actual volumetric flow to enter is the flow at the bag filter inlet, after cooling — not the flow leaving the furnace. The duct run between the furnace and the filter is therefore a cooling problem, not just a conveying one.
This is a completely different subject from the Nm³/h to m³/h conversion earlier on this page. That was one gas stream measured under different conditions — same mass, roughly 1.5 times more volume because it's hot. Here we're talking about different gas streams: primary furnace gas, secondary canopy-hood capture, and dilution air. The baghouse handles all of them; the furnace produces only the first.
According to Toulouevski & Zinurov, maximum gas evolution in the freeboard of a 120-ton furnace, including infiltrated air, is about 19,000 m³/h. By the time those gases reach the bag filters, dilution with air brings the amount up to 1.4–1.5 million m³/h — a factor of 70–80. About 10% of the entire electrical energy used for steelmaking goes to evacuating and purifying what is, in essence, an insignificant amount of primary process emissions (Toulouevski & Zinurov, Innovation in Electric Arc Furnaces, 2nd ed., Chapter 14.3.4, p. 268).
Toulouevski & Zinurov explain the cause in their own terms: most of the excess is caused not by the primary process emissions themselves, which are essentially insignificant, but by the need to capture secondary uncontrolled emissions through low-efficiency exhaust hoods above the furnace — because modern air-pollution standards require the complete absence of visible dust emissions above the mill.
Toulouevski & Zinurov also give the arithmetic, which can be checked by hand: to capture secondary emissions completely, the suction velocity of the air-gas mixture through the hood aperture, open from below, must not be less than 2 m/s. In a 120-ton furnace the hood aperture area is about 200 m², giving 200 × 2 × 3600 = 1,440,000 m³/h.
The practical consequence is plain: a baghouse sized from the furnace's primary gas volume alone would be undersized by a factor of 70–80, because the filter handles primary gas plus secondary capture plus dilution air.
One caveat follows directly from Toulouevski & Zinurov's own figures: the 70–80 factor is what a 120-ton furnace with a canopy-hood system produced, not a universal multiplier. A shop with a doghouse enclosure instead of hoods evacuates substantially less, as they state directly.
Values are calculated from actual flows (m³/h) at operating temperature. The 1.0 and 1.5 m/min velocities are illustrative, not recommendations — the right velocity depends on your dust and cleaning method.
| Actual Flow (m³/h) | Cloth Area @ 1.0 m/min (m²) | Cloth Area @ 1.5 m/min (m²) |
|---|---|---|
| 50,000 | 833 | 556 |
| 100,000 | 1,667 | 1,111 |
| 200,000 | 3,333 | 2,222 |
| 400,000 | 6,667 | 4,444 |
| 600,000 | 10,000 | 6,667 |
| 800,000 | 13,333 | 8,889 |
| 1,000,000 | 16,667 | 11,111 |
| 1,500,000 | 25,000 | 16,667 |
For an actual flow of 400,000 m³/h at a 1.0 m/min target velocity, you need approximately 6,667 m² of cloth area. See the reference table above for other flows.
Nm³/h expresses gas volume at normal (reference) conditions, such as 0°C; m³/h expresses the actual volume at operating temperature. Hot gas expands, so the same mass occupies far more volume. For example, a 400,000 Nm³/h EAF dedusting flow at 130°C, used directly as if it were m³/h, calculates to 6,667 m² of cloth area — but converted to the actual flow per DIN 1343 it is 590,372 m³/h, and the cloth area actually required is 9,840 m². The correct figure is 47.6% larger than the one calculated with the wrong unit.
No. According to Toulouevski & Zinurov, maximum gas evolution in the freeboard of a 120-ton furnace is about 19,000 m³/h — but by the time those gases reach the bag filters, secondary hood capture and dilution air bring the flow up to 1.4–1.5 million m³/h. A system sized only on the furnace's primary gas volume would come out undersized by a factor of 70–80, because the filter has to handle secondary capture and dilution air as well as the primary gas (Toulouevski & Zinurov, Innovation in Electric Arc Furnaces, 2nd ed., Chapter 14.3.4, p. 268).
It depends on the dust and the cleaning method. EPA fabric filter design guidance puts filtration velocity roughly in the 0.3-3.7 m/min (1-12 ft/min) band overall, with pulse-jet systems toward the high end and reverse-air and shaker systems toward the low end. We recommend working out the right figure for your project with DNL's engineers.
The gross ratio assumes every compartment in the baghouse is in service at the same time. In practice, one or more compartments are periodically taken offline for cleaning; the same gas flow then passes through less cloth, so the net filtration velocity at that moment is higher than the calculated gross ratio.
A filtration velocity higher than it should be increases pressure drop, causes bags to blind faster, shortens bag life, and raises the risk of an emission breakthrough. The exact impact depends on the dust and the equipment, so we don't attach numbers to it here.
