A Sticky Subject, Part 1
Many drying systems employ reverse-pulsing (also referred to as pulse-jet or pulsing) dust collectors to control the solid particulate emission and recover the final product. The physical action of reverse pulsing releases a volume of compressed air at a substantially higher pressure than the operating or atmospheric pressures. On expanding, the air “shocks” the bag, effectively shaking the media and, together with certain inducing devices, forces air in the opposite direction, the reverse of the exhaust airflow (hence, reverse pulsing).
Reverse-pulsing dust collectors rely on media filtration to clean the air. Media filtration systems separate the process air (dirty side) from the filtered air (clean side) by means of the filter elements. Dust-laden or “dirty” air is introduced from the dryer’s exhaust stream and clean air is exhausted from the filter.
In using media filtration technology, a number of factors will influence the separation efficiency. Obviously, proper media selection and adequate surface area are crucial. So is taking into account the solids loading of the incoming air and the particle size and bulk density of the solids.
Many people are misinformed, believing that the filter media actually performs the filtration. It doesn’t! The media is an aid that permits a cake composed of the dust particles to form on it. This cake is actually the medium that performs the filtration. Reverse pulsing controls the thickness of this cake on the media. Each pulse will remove a layer of the cake and control the pressure drop and filtration efficiency.
This is all very good and well, but what if the cake becomes solid and starts sticking to the media and to other layers of cake? This is a common problem in baghouses on dryers.
Implicitly, exhaust streams in dryers contain moisture. If this moisture is permitted to condense, it will create a “sticky situation” -- literally. The condensed water can lead to layers of wet dust building up on the bags’ fabric, causing a gooey cake. As the cake thickens, hardens and shrinks, it prevents the air from flowing through the fabric. When the air can no longer pass through the bags, it is called blinding of the bags. Blinding causes the pressure drop across the bags to rise.
A buildup of sludge also can form inside the hopper of the dust collector, preventing the dust from flowing out and exiting the collector.
There are other potentially sticky and nasty products in dryer exhaust streams. Carryover of uncombusted fuel can cause similar problems. Because heavy fuel oils (HFOs), light fuel oils (LFOs) and gases are oil-based, a sticky oily residue can be deposited on the media and filter housing surface. This can also cause blinding, buildup and bridging. Other than stating that for efficient operation, you need to tune the heat source to provide stoichiometric combustion, this aspect will not be further discussed.
In my next column, I’ll look at how you deal with these evils.