The common thread is stickiness. If the dust or vapour phase of your stream is tacky at the temperature of the tube wall \u2014 and during heat-up it always is, because the wall starts cold \u2014 then the tube needs protection before that stream arrives. Full application details by fuel type are on our applications<\/a> page.<\/p>\n Lime powder and limestone powder are the default pre-coat materials, and for good reason: they are dry, cheap, alkaline, and available in the quantities a large vessel needs. Where the gas stream is acidic, the pre-coat does useful double duty, since the alkaline layer also starts neutralising acid components from the first minute of operation.<\/p>\n Avoid using process fly ash as a pre-coat. It is free, which is why it is tempting, but fly ash from the stream you are trying to protect against is usually the sticky material itself, and recycling it onto a clean tube defeats the purpose.<\/p>\n Do not let the differential pressure run away during pre-coating. If it climbs faster than expected, stop dosing and check for moisture in the powder before you add more.<\/p>\n<\/div>\n Filtration velocity decides whether the pre-coat lands evenly. At 2\u20135 cm\/s the powder follows the gas to every part of every tube and builds a layer of consistent thickness. Push the velocity above that band and the powder concentrates where the gas velocity is highest, so the tubes nearest the inlet get a thick layer while the far end of the compartment stays nearly bare.<\/p>\n Too low is also a problem. Below the band, the gas does not carry the powder through the full length of the compartment, and the layer builds unevenly from the bottom up. The window that works for pre-coating is the same window that works for operation, which is convenient: 2\u20135 cm\/s, or \u22481.2\u20133.0 m\u00b3\/min per m\u00b2 of filtration area.<\/p>\n Three failure signatures cover almost every case, and all three are readable from the pressure trend alone.<\/p>\n For reference, a clean tube starts at 300 Pa at 1 m\/min face velocity (clean), and system resistance in stable operation settles somewhere in the 900\u20131,500 Pa band. A pre-coat should move you part of the way from the first figure towards the second, not past it. Our published performance figures<\/a> give the full picture for both catalytic and non-catalytic grades.<\/p>\n Once normal duty begins, cleaning returns to its standard settings: 0.4\u20130.7 MPa, 0.1\u20130.15 s, triggered by differential pressure. Because the tube is rigid, the pulse releases the cake as a sheet without flexing the element, so there is no secondary dust and no pressure spike after each cycle. The pre-coat behaves exactly like an operating dust cake, and after the first few cleaning cycles it has effectively become one.<\/p>\n For the first campaign after a re-tube, set the cleaning trigger conservatively and let the cake run slightly thicker than you eventually will. A slightly thicker cake protects the tube, and you can tighten the setpoint once you have a week of pressure data to look at. The same approach applies to non-catalytic ceramic fibre filter tubes<\/a> running sticky duty without a DeNOx requirement.<\/p>\n It depends on filtration area rather than on vessel volume, so work from the area: dose until the differential pressure shows a clear, stable rise, and record that quantity for the next start-up. On the first pre-coat, weigh the powder in and log the pressure curve so the second one is repeatable instead of guessed.<\/p>\n No, and this is a common misunderstanding. A pre-coat protects the tube from sticky dust; it does not protect the vessel from condensation. Bring the vessel above the acid dew point first, then pre-coat. Powder laid onto a cold, damp wall will cake, and caked powder is worse than no powder.<\/p>\n No. Stop the pulse-jet system, or set it to a very long interval, until the layer is established. Cleaning during dosing removes the powder you have just applied and leaves the tube partly protected, which is the worst of the two outcomes because it looks like it worked.<\/p>\n At every cold start-up, and always after a shutdown long enough for the vessel to cool below dew point. On a plant that runs continuously, the operating dust cake effectively maintains itself and pre-coating is a start-up procedure rather than a routine one.<\/p>\n No. A lime or limestone pre-coat is chemically compatible with the ceramic and is removed by normal pulse-jet cleaning. What shortens life is the material the pre-coat is there to prevent: tar, oil mist and condensed acid working into the pore structure, which no cleaning cycle will remove.<\/p>\n Not sure whether your stream needs a pre-coat, or which powder suits it? Send us your fuel analysis, gas temperature profile and start-up sequence, and our engineers will specify the pre-coat material, dose rate and filtration velocity. Ask an engineer about pre-coating<\/a>.<\/p>\n<\/div>\nChoosing the pre-coat material<\/h2>\n
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How to pre-coat ceramic filter tubes, step by step<\/h2>\n
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![\"Fresh](\"https:\/\/anchor-filter.com\/wp-content\/uploads\/2026\/10\/ceramic-fiber-filter-tube-53.webp\")
<\/figure>\nHolding the right face velocity<\/h2>\n
Telling a good pre-coat from a bad one<\/h2>\n
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![\"Ceramic](\"https:\/\/anchor-filter.com\/wp-content\/uploads\/2026\/10\/ceramic-fiber-filter-tube-11.webp\")
<\/figure>\nAfter the pre-coat: cleaning settings and handover to duty<\/h2>\n
Frequently asked questions<\/h2>\n
How much pre-coat powder does a compartment need?<\/h3>\n
Can pre-coating replace proper start-up heating?<\/h3>\n
Should the pre-coat be applied with the cleaning system running?<\/h3>\n
How often should pre-coating be repeated?<\/h3>\n
Does a pre-coat shorten filter life if it is left in place?<\/h3>\n