A Sticky Subject, Part 2
In Part 1, I began looking at how to use reverse-pulsing dust collectors to control solid particulate emission and recover final product. As explained, these devices use a filtration media that permits a cake composed of the dust particles to form on it; the cake performs the filtration. Reverse pulsing controls the thickness of this cake on the media. Each pulse will remove a layer of cake and control the pressure drop and filtration efficiency.
Implicitly, exhaust streams in dryers contain moisture. If this moisture is permitted to condense, it will create a "sticky situation" and lead to blinding, buildup and bridging. So, how do you deal with these evils?
You first have to know that they are occurring. The easiest method to determine whether they are is to monitor the differential pressure across the baghouse. (A manometer is used to measure this differential pressure, or pressure drop; see the "Tools of the Trade" series; links at bottom of the page.) Differential pressure is a measure of how much resistance there is to the air passing through the media. For fabric dust collectors, the pressure drop usually is in the range of 5 to 7 in. w.c., but each baghouse is different and, as such, will have different pressure drops. When your system is clean, you should note the clean operating differential pressure. After it has run for a while and is operating in a steady state, you should again note the differential pressure. If the unit is operating at a pressure outside of these two points for any extended time, there is a problem. The causes of a higher pressure reading can be numerous; blinding of the bags due to moisture is one of them. Taking a sample of the cake will show whether the problem is moisture related.
Another method of determining if the bags have blinded is to monitor the exhaust fan (if it is an induced system). The fan's performance will move away from its design point on the fan curve if the fan is not able to overcome this pressure. This will result in the fan becoming starved of air, and the system will simply not operate properly. If the system uses only a forced-draft fan, the baghouse pressure will increase and the system will start dusting from every possible aperture.
Unfortunately, with moisture problems, one occurrence can cause a major problem. By that, I mean if you have run for years without any problem and, for some reason, condensation forms inside the baghouse, it can take the plant down.
All condensation problems are caused by the gas temperature dropping below the dewpoint temperature of the gas. By now, I am sure you all know what the dewpoint temperature means. To avoid condensation problems, the answer is to maintain the exhaust stream above the dewpoint temperature.
To achieve this, the exhaust setpoint must be set above the dewpoint. Be careful, however, that it is not elevated above the allowable temperature for the fabric. If it needs to be, replace the fabric with a higher temperature product.
But this is not all. The dust collector and ducting insulation must be sufficient to prevent cold spots that would permit localized condensation. Additionally, heat sinks need to be insulated appropriately to maintain the temperature. Sources of cold air ingress must be eliminated.
In some critical applications, heat tracing the baghouse or providing a method of boosting the exhaust gas temperature can be used to prevent condensation. In my opinion, these measures should be employed only as a last option as they require capital expense and use valuable energy to achieve the desired objective.
In Part 3, I'll look at potential trouble spots.