Limestone is a chemically stable powder, that is injected into the baghouse to establish
uniform porous dust cake on the filter bags for maximum air flow with enhanced efficiency
and protection from moisture, oily or sticky particulates, bleed-through and especially
to delay blinding to increase bag life.
Procedure Steps for Filter Bag Precoating
The amount of precoat utilized should be a minimum of .10 lbs
per square ft. of baghouse filter cloth area.
Prior to injection of precoat:
a. Insure that all previous dust has been evacuated from baghouse hoppers,
then turn off hopper dust removal equipment.
b. Deactivate (lock-out) the bag cleaning cycle to prevent filter bag cleaning
during the precoat period.
c. Operate the system at approximately 50 percent of design air flow to the baghouse
for resulting air flow velocity of approximately 2000 ft./min. through the inlet duct,
but not less 2000 ft./min. (typical minimum dust carrying air velocity).
Inject precoat at the feed rate of .3-lbs per minute per 1000 ACFM of reduced gas
flow to the baghouse.
(Ex. 50,000 ACFM to baghouse - 1,000 ACFM x .3-lbs = Feed rate of 15 lbs per minute.)
For Baghouses operating under suction the precoat can be injected through ports of inspection
doors in ducts or hoppers. For positive pressure baghouses, the precoat should be injected
into duct at the suction side of the positive pressure system fan.
Isolate each compartment and inspect filter bags for adequate precoat cake (approx. 1/16"
thick) and check inside of hopper to be sure there is not a large amount of precaot dropout.
Once the bag precoating is complete, increase the air volume for normal
design flow levels to the baghouse and turn on the hopper dust removal equipment.
However, do not reactivate the filter bag cleaning cycle until a minimum differential
pressure of 3" - 4" s.p.w.g. is measured across the bags: this is to ensure that a seasoned
cake of precoat has been achieved on the filter bags prior to activation of the initial