Using Filter Aids

The Quality Air Management Institute

Lesson 13; Using Filter Aids

Review of Filter Cake Formation

In filtration of dust, especially with pulse jet collectors, there are times when the dust has characteristics such that
it will not form a stable filter cake or the filter cake is partially or completely removed during the cleaning cycle.
The filter cake with a felted media, spun bond media and with cellulose media should normally penetrate from 20
to 45% of the depth of the media. In a well functioning cake, the dust below the surface is wedged and
interlocked. This interlocked layer is further held in place by the threads or fibers, which wrap around the dust
holding the residual cake in place. The initial sub surface cake is usually permanent or semi-permanent. With
certain porous and light weight dusts, there can be an additional filter cake on the media surface. In these cases
the pressure drop can be very low. The penetration of the dust through the filter and associated cake is a function
of the frequency of pulsing, For this reason the cake, in these cases, should be maintained as thick as possible.

As a case in point there was a collector installed to vent some woodworking machines. The collector required
cleaning every fifty minutes. The pressure drop was maintained at less than two inches and the outlet from the
dust collector was below ambient levels of particulate. The only limitation came that they needed to reduce
cleaning frequency to 30 minutes, because when a row of bags cleaned so much dust fell into the hopper that the
rotary feeder in the collection hopper bridged. Of course another approach would have been to increase the size
of the feeder.



Pleated filter elements

However with pleated filter elements, the cake must not be allowed to get no more than 25% of the width of the
pleats. The cake can bridge across the pleats and render the media below the dust bridge ineffective.



Defective Filter Cake Formation

If the cake is removed or partially removed during the cleaning cycles serious bleeding or leakage will occur. Also
the pressure drop may increase and the filter elements will become blinded. The dust may become imbedded in
the filter media and form an impervious coating.

A very common condition is when the contaminated gas stream contained liquid along with solid particulate.
Several possibilities can occur depending on the dust characteristics and loading in the gas stream:

The dust may not be wetted by the liquid. In that case the dust is coated and lubricated so that the dust
penetrates through the media and presumed cake. In a pulsed collector, running at less than 3 inch pressure
drop, even dust particles larger than 50 microns penetrated through a sixteen ounce felted medium. This can
occur if the liquid load is much larger than the particulate load.

When the dust is wetted by the liquid, the cake can form a paste or a paint. The pressure drop will
gradually increase until changing the filter elements is indicated. Often in welding operations, there is a residual
coating of oil on the surface to be welded and the elements can have short life. Fortunately the load of particulate
with their associated hydrocarbons is often in the 1 to 10 grains per thousand cubic feet of airflow.



FILTER AIDS

The process of remedying problems of defective cake formation is usually accomplished by the application of filter
aids in operating dust collectors and dust collector systems. This process is sometimes referred to a seeding or
conditioning of filter media. It consists of introducing powder into the system that will form an effective filter cake
either on a permanent or continuous basis. The filter aids should have the following characteristics.

High porosity

Low density

Will not react physically or chemically with the compounds that are collected

Inert and non-flammable

Granular in shape and able to interlock below the media surface

Produce a low pressure drop cake

Disposable

Easily applied through manual or automatic feeders

Not Hygroscopic

Inexpensive

Common Selections and limitations

Lime or limestone. This is a commonly recommended as an additive but has limitations in many cases.
It can react with chlorides and sulfates to form harmful cake. Calcium sulfate is plaster-of-paris, which is
hygroscopic and can form an impervious coating. Calcium chloride is highly hygroscopic to the point of being a
desiccant at ambient conditions.

Flour can mix with oils and form a dough which can plug the bags especially when the cleaning jet gets
below the dew point. It absorbs water easily.

Baking Soda or baking powder can cause corrosion to the collector if condensation occurs.

Powdered sugar can form a frosting when subjected to moisture.

Sand can be an option but some sand is too coarse to form a good cake

Aluminum Salts often have excellent characteristics

Commercial Formulations available through various sources are readily available.



Determining the quantity of additive necessary

First we must estimate load for a continuous process.

20,000 CFM for welding smoke, Load of particulate 3 grains per 1000 CFM, 65 lb /cu.ft. and oil
load 50%. By experimentation we determine that if particulate level is 90% solids and 10% Oil, a stable cake and
a low pressure drop operation is possible.

20,000/1000 x 3gr per 1000. = 60 grains per minute or 360 grains per hour. That is a total load
of 2880 grains per 8 hour day. If 10% is oil we have 288 grains per day of oil.

We will assume the additive will blot up the oil and we determined, from experience, that the additive should be
nine times the load of oil. Then 288 grains x 9 will equal 2592 grains or 2592 grains / 7000 grains per lb = 0.37
pounds of additive per shift.

Next we should determine pre-coat necessary for initial conditioning

20,000 CFM at 14:1 ratio. With fabric collector or 40 cartridges with pleated elements at 250 sq. ft per cartridge.

We will use commonly available aluminum salts. Prefer not to use limestone, lime or diatomaceous earth.

Fabric Collector

20,000 / 14 = 1428 sq. ft

25 lb per cubic foot additive at 1/32 inch thick

1/32 x 1428 sq,ft, x144 sq in,/sq. ft = 6426/1728 = 3.7 cu cft

3.7 x 25 lb = 92 lb



Cartridge Collector

20,000 CFM / (250 x 40) =10,000 sq. ft

25 lb per cu ft at 1/64 th thickness because of narrow pleats

1/64 x10,000 sq ft x144 sq. inch= 22500 cu in /1728 =13 cu ft

13 x25 = 325 lb



Feeding Pre-coat

Feed pre-coat into the operating system of collector with fan energized, slowly at no more than 25 grains per cu ft
with cleaning system not operating until pressure drop increases by two inches

Shut down fan and run cleaning cycle for four complete cycles (this off-line cleaning). Restart collector and
continue process again with fan running.

Remove collected seeding dust from hopper and repeat procedure .



Other considerations

Sometimes a membrane laminated bags can operate when other forms of seeding will not solve the problem of
cake formation. Occasionally a dust will be blown apart by the pulse cleaning system. In that case often only a
shaker collector will function satisfactorily.