Protection against watery and acidic condensations on the filter bags during start up

• Not for leakage test, but to protect the bags physically and chemically before start-up, precoating should be done.
• Without precoating, the virgin filter bags still have a high permeability. Due to the low flow resistance, the a/c ratio will be high during start-up, especially when the fan runs at full draft. Fine free flowing dust can cause immediate bleed-through and blinding of the bags.
• A precoating with good agglomerating dust reduces air permeability and initial a/c ratio and builds up a coherent dust cake, which protects the filter medium from dust penetration.
• When a combustion process is started up, vapour can condense in the still cold dust collector and form sulphuric acid. When hydrated lime Ca(OH)2 is used as a precoating, it will react and neutralize the acid, thus protecting the bags from chemical attack. But also unburned hydrocarbons will be separated on the dust cake and cannot penetrate the filter medium. Before precoating, the cleaning system should be turned off, to keep the dust cake on the bags until the regular operating conditions are reached.
• 200 g of hydrated lime Ca(OH)2 per 1 m² of filter area are sufficient for precoating. As a result of precoating, the differential pressure must increase at least by 3 – 4 mbar, otherwise the lime dust did not reach the filter bags but fell into the hopper.