Quality Air Management

Baghouse Dust Collector

Saturday, January 22, 2011

Condensation in Dust Collectors

London, Ontario Installation; Cartridge Dust collector retrofit on Plasma cutting stations
The client complained of having to service the cartridges every 1-2 days because they would plug up. The pressure drop across the collector would rise to 8-10 inches water column. The cleaning system was totally redesigned, and six 36” high ratio style cartridge filters replaced twenty-four 26” tandem cartridges. 80/20 paper blend cartridges were installed temporarily until special anti-pinch style polyester filters could be supplied. Within two days the paper cartridges blew apart, mostly at the closed end-cap. They did run the pulsed cleaning system at 100psi instead of 85psi, with no regulator on the line. However, that was not enough to rip the media apart, so, something else was the cause. Upon inspection of the cartridges, we observed that the media was dry but had the look of being wetted. Also there were watermark stains on the clean side of the media. There was an accumulator tank on the compressed air line leading to the collector. The maintenance people told us the accumulator was installed because the valve manifold wasn’t large enough to hold enough residual pressure during a pulse. The manifold was just fine. What was happening is that the air line was very long (over 200 feet) from the compressor to the collector. Moisture would condense in the line then drop out at two elbows, which was the low point just before going up to the manifold. This choked the line which made the manifold appear like it was too small, and then suddenly a slug of water would blow through to the valves and into the cartridges. We recommended taking out the accumulator tank and, just before the connection to the valve manifold, installing an air line coalescing filter, top quality dryer, and a regulator set for 85psi. We also recommended an automatic drain valve system on the manifold tank. The collector now runs continuously at 3-3.5inch pressure drop. The client says they’ve never been able to control the contaminants at the plasma stations so well since they installed the system 1.5 years prior to the retrofit.

Maine Installation; Energy Recovery from Trash and garbage.
This collector installation was venting a large room where garbage was dumped. Front-end loaders took this garbage and carried it to the hoppers that fed an incinerator. Steam was produced that fed a boiler. The pulse jet collector vented 65,000 ACFM at ambient conditions. It was running at a 15:1 filter ratio and at 2 “ water column from January to June. In June the pressure drop started to creep upwards about 1/8 of inch per week. This collector was well instrumented with continuous recording of wet bulb and dry bulb temperatures as well as pressure drop. We compared the pressure drop increases with the weather reports in the local newspapers. The increases occurred early in the morning on days when the wet bulb and dry bulb temperature were closer than 5 degrees F. There were several kinds of trash being handled in the facility. We recommended that they not load the wet trash into the incinerator until after 10:00 a.m. This stopped the rising pressure drop problem. Since they shut the system down on weekends we recommended that they clean the collector for two hours on Saturday afternoon with the outlet fan damper 90% closed. This was almost as effective as off line cleaning and the dust was not blown back into the loading room during the procedure. The dust collector ran for at least two years, at less than three inches water gauge pressure drop after implementing these recommendations.
General Comments: The cleaning system was running at 85 psig. Under typical conditions the compressed air expands to critical pressure which is 37psig. beyond this pressure, the pressure to velocity conversion stops and from 37 psig to atmospheric pressure, 0 psig, the energy is turned to heat from turbulence. This nullifies the refrigeration cycle as the compressed air expands to critical pressure. This collector used converging diverging nozzles which had a complete conversion of pressure to energy so that the refrigeration cycle was reducing the temperature in the jet by approximately 5-8 degrees F.Actually the turbulence below 37 psig causes some heat regain but the jet is still 5-8 degrees cooler despite this. Without the regain it would be about 9-12 degrees cooler. With a converging diverging nozzles the amount of cooling from expanded compressed air in the jet is a bit colder but the amount of induced air from the plenum is almost twice as much as with an ordinary orifice so the jet temperature is about 6-8 degrees cooler but not enough to make a difference. In Maine, the problem was mainly in the summer when the trash was wet from people dumping beer and other associated liquids. They did not have the problem in the winter when the trash was dry.
There were two other approaches that could have been used to counter the rising pressure drop:
1) Larger pulse valves and eliminating the nozzles. However, this would increase air consumption by over 35%.
2) Manifold heaters could be installed that would raise the temperature of the cleaning jet above ambient even to the point where wet garbage could be processed in high humidity conditions.
In either case, the collector could not handle vent volume where the gas entering the collector has condensed water droplets.
Low pressure compressed air, in the range of 7 to 22 psig is often employed for pulse jet cleaning systems. These have the same effect as the cleaning system with converging diverging nozzles since no turbulence occurs as complete expansion occurs in the orifice or nozzle. The best remedies are as follows:
1. Locate the low pressure compressor near the pulse valves and insulate the manifold leading to the pulse valves.
2. Use a manifold heater in the compressed air header, same as described above.
Other Comments and observations. There are many other installations in energy recovery plants that use high ratio reverse air fan collectors, The temperature regain on the reverse air fan is higher than ambient and eliminates condensation considerations described above.

East Tennessee Installation; Powder coating
This plant in the upper elevations in the mountains used a pulse jet collector to vent a powder coating operation that coated the internals of residential wash machines. This pulse jet collector started in July and ran until the middle of the winter when it developed a creeping rise in pressure drop characteristics. The wet and dry bulb spread was usually over 15 degrees F except early in the morning when it was about ten degrees. Investigation of the operation was conducted and we measured wet and dry bulb temperatures with a sling psychrometer mounted through a hole in the main vent duct. What we discovered is the booths were manually washed with a hot water hose every morning. At times the gas would go through the dew point for several minutes and then immediately go back to operation at a wide dew point spread. We recommended mounting a heater with a damper on a branch line to the vent system. The heater was triggered by pressure switches on the hot water hoses in each booth. This eliminated the creeping pressure drop problem permanently.

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