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False Air Through Openings

Nobody ever fixes false air, because nobody can put a number on any single hole. Measure the draught once, walk the system with a tape, and this tells you what every opening is letting in — then sorts them worst first and works out what they cost you a year in fuel and fan power. Most plants find that three or four holes are most of the problem. Press YOUR OPENINGS to list them.

Why this matters now

Fan power, false air and unreliable gas measurements quietly increase fuel and electricity cost while weakening every heat and mass balance in the plant.

What this tool helps decide

Rank the openings costing heat, fan capacity and fuel so maintenance seals the highest-value leaks first, not the easiest gap to reach.

What subscription adds

Keep the gas-side audit together: save readings by fan, duct or machine, revisit the same equipment later and produce a report with findings, cost signals and priorities.

Connected to current industry priorities around energy efficiency, emissions measurement, waste heat and combustion optimisation reported in World Cement, CemNet and Global Cement.See process-tool plans

Every hole, worst first

The draught pulling the air in

How much suction at the leak The pressure inside the system compared with the room. Type it as a positive number — how far below atmospheric it sits.

That figure is in

Entry loss coefficient 0.5 for a plain sharp-edged hole, which is what almost every leak is. Lower for a rounded or bell-mouthed opening.

The air outside

Air temperatureDegrees C in the room.

Height above the seaMetres.

Which air density to use Your False Air.xls works the speed out at a fixed 15 °C reference. The air actually crossing the hole is at room temperature, which is the physical answer and reads about 2 % higher. Both are shown; pick which you want the numbers to follow.

The openings

Walk the system with a tape. For a round hole put the diameter in the width box and leave the height blank. “How many” lets you enter a row of identical bolt holes or a gap once.

Where it isWidth mmHeight mmRound dia mmHow many

What it costs

How hot the gas is where the air leaks in Degrees C. The cold air has to be heated from room temperature up to this.

Fuel calorific valuekcal per kg.

Fuel pricePer tonne, your money.

Fan pressurePa, total across the fan.

Fan efficiencyPer cent.

Electricity pricePer kWh.

Hours a yearRunning hours.

Process gas flow, if you know it Nm³/h. Only used to show the leak as a percentage. Leave 0 if you do not know it.

HOW MUCH IS GETTING IN

WHAT IT COSTS YOU

THE HOLES THEMSELVES

False air getting in

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By weight

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Share of your process gas

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At room conditions

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Speed through the holes

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Heat wasted warming it

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Fuel it burns

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Fan power to move it

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Costing you a year

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Worst three are

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Openings counted

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Total open area

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Worst single opening

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and it is

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Air density used

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Every opening, sorted worst first

What to do about it

Check the leak against a measured flow

If you have traversed the duct, compare the flow before and after the leaking section. The difference should be close to what this page says.

Gas flow by traverse

How the numbers are worked out

The leak itself is your own False Air.xls, term for term. Air is pushed through the hole by the pressure difference and loses some of it entering: v = √(2Δp / (ρ(1+K))), with K = 0.5 for a sharp-edged opening. Then Q = v × A × 3600 and m = Q × ρ. Checked against your worked example to six figures: a 152.4 × 152.4 mm hole at −0.1 inH₂O gives 5.20696 m/s, 435.368 m³/h and 533.312 kg/h — all three exactly.

One thing your sheet and the physics disagree about. Your spreadsheet works the velocity out with air density at a fixed 15 °C reference. But the air crossing the hole is the air in the room, at room temperature, so strictly the density should be the ambient one. It reads about 2 % higher. Neither is wrong — they answer slightly different questions — so the panel lets you choose, and it opens on your sheet's convention so your own numbers reproduce.

Why the pressure matters more than the hole. Velocity goes with the square root of the draught. Double the suction and the leak only rises by 41 %. But area is linear — double the hole and you double the leak. That is why closing one big opening beats chasing the draught, and why a plant that fixes its three worst holes usually gets most of the benefit.

What it costs. The heat is m × 0.24 × (process temp − room temp) in kcal/h — the same published specific heat of air used in the mill heat balance. Divided by your fuel's calorific value that is kilogrammes of fuel an hour. The fan term is Q × Δp / (3600 × 1000 × efficiency), and it assumes the extra air has to be dragged through the whole system resistance, not just through the hole. That is the right assumption for a leak upstream of the fan and it is the usual case, but if your leak is on the discharge side the fan cost does not apply.

What this cannot tell you. It cannot find the holes. It measures the ones you find and tells you which are worth the scaffolding. The draught also varies along a system — if your openings are spread over a long duct or a whole preheater, run them in groups at the draught each group actually sees, rather than one figure for everything.