The problem was that humidity and water was plugging the bags and accumulating in the collector & ducts. We depended on information that we got from the customer. We looked at the furnaces and they looked pretty standard. We asked them about the operation. What kind of salts they were using in the molten salt vats. We were told Barium Chloride. Most of the rest of the discussion then was about the existing dust collector. We suggested that the chief problem of corrosion was that moisture was condensing on the wall since the collector was located outdoors. They agreed that was probably correct. We discussed the possibility of locating the collector inside. We said we could do this because of the high ratio advanced technology style of dust collector used.
At no time was the subject of continuous seeding with limestone discussed. They did discuss that the collector had been operating for several years and were running at six inches pressure drop. That is normal for MAC collector designs. From our view, this was consistent with other furnaces that we knew had fabric collectors venting quench furnaces with molten salts of Barium Chloride or Sodium Chloride (standard salt like you have in a salt shaker).
They told us that they had been working with a representative from a well known cartridge dust collector supplier. They showed us sketches of some wooden hoods that they built to determine volume including temperatures in the hoods. These hoods were close coupled to the molten surfaces and they wanted us to bid that type of hood, which was unlike the ones that were already in place. They said they wanted us to bid cartridge collectors but we had never heard of a single installation on these quench operations. The temperatures that they measured were close to 200 degrees F and that was problematic, however the cloth bags of a baghouse could withstand 275 degrees. We would not bid a cartridge collector at those temperatures. There was no doubt that they were impressed with Donaldson and all the work they had done together on this project.
We did not specifically ask about Calcium chloride. There are various salts applied in these heat treating furnaces. Normally, when Calcium chloride is used they install water wash scrubbers. At American Air Filter, our colleague ran the scrubber department and they sold more than a hundred scrubbers that were venting either sodium chloride or calcium chloride heat treating operations. From the discussion, no “red flags" were raised and we judged this as a low risk application.
We assumed someone else went to the site to make measurements and lay out the ducts and fan since we never made any measurements. Next thing we heard that the cartridge dust collector was a lower price than our dust collector. We told the client that a cartridge collector could not work. We asked them to go back to the supplier and find a place where cartridge collectors had been applied on this type application. As it turned out they could not find a single place where they had applied a cartridge collector on salt bath quench operations. As a result they gave us the order.
After we installed the equipment there were complaints of water in the system. We were absolutely surprised since we were sure the wall temperatures were much higher than the dew point of the process air. We were sure that our design kept any condensation from the process gas stream. The operation could not produce moisture from feeding parts into the bath since wet parts would cause an explosion in the molten baths and might injure workers in the area.
We got samples of the salts. Six of the bottles were Barium Chloride and two bottles were labeled Heat Treat Mixture XX. It looked like the Calcium chloride salts that we used to melt ice since the salt was in the form of spheres 1/8 inch in diameter. The heat quench salt supplier told us that it was Calcium Chloride. They were reluctant to send more information so we ran the tests in our oven. We determined that the barium chloride was stable from 50 degrees up to 180 degrees F.
The calcium chloride would absorb water vapor and was a desiccant. How much and how fast the vapor was turned to liquid depended on the relative humidity and temperature. For instance, at 80 degrees and 80% humidity the liquid would not appear for more than twenty minutes. At 100 degrees and 50% humidity, it took several hours. Air flowing across the calcium chloride speeded the reaction but we had no equipment to measure the values. We found out the reaction would stop at 130 degrees F and higher. If the liquid formed it would take temperatures of over 160OF to dry out the calcium chloride.
We installed heaters in the ducts. Unfortunately, when the new bags were installed, the volume through the system went up and the heaters only increased the temperature to 110 from the previous reading of 105, We fed some baking soda into the collector to neutralize the calcium chloride but It was not enough to stop the formation of the water. It is obvious that the reactant must be fed continuously since it looks like the soda gets pulsed off before the end of the day and when the collector is shut down water is formed and plugs the bags.
This is an example of getting into trouble when we are not given all the information especially after the system was installed. We are competent but not magicians. The cartridge collector would not have lasted two weeks even with lime feed. We continuously shot ourselves in the leg on this project:
1) When we were installing the hoods, they insisted we move the hoods back at least six inches because they wanted access to a thermocouple opening. This meant that we would have more cold air blowing in and the exhaust temperature would be lower. Looking at it from hindsight this was the beginning of a lot of trouble. From the expected 200 degrees we were now at 110. At the time it seemed OK, since we were unaware of the calcium chloride. We should have insisted they move the location of the thermocouple probe instead, but we never suspected temperature was a problem. At the time we figured cooler is better.
2) Because the hood was moved back, this indicated we had to use the narrow high hoods to capture the fume instead of the squat ones close to the vats. The further away from the furnace, the higher the hood must be. These higher hoods drew in even more cooler air and the temperature dropped to near 100.
3) We put in heaters but they were sized wrong. Our sales rep had the same view that we originally had that the problem was with the water condensing on the walls and ducts. All he needed was to raise the temperature by 20 degrees and we would be home free. He figured that the higher volume after the bags were changed would further raise the temperature to the collector. The temperature coming from the duct venting the calcium chloride bath was 180 degrees, but most the reaction always had taken place on the bags. Between that and the insulated ducts he judged that the problem was solved. If engineering had selected the heaters, we would have designed them for 165 degree entrance temperature to the collector. When we visited after the heaters were installed. I was a bit surprised. Besides the client was complaining about the $2,400 they had to pay to wire the heaters. We measured the current in the heaters. It was about 25% of what I was expecting. The wrong heater was installed
Here we must not lose sight that the problem is related to the desiccant properties. We now needed to try to extricate ourselves from this mess that would allow us to look reasonably competent.
A) The most obvious approach is to put in a feed system to feed sodium carbonate to the system especially at the end of the shift. It looks like the pressure climbs overnight. It was not known if the collector runs all night. They probably do it more than one way. We believe they shut the fan off over the weekend and continue pulsing. That is probably the worst thing that they can do.
B) We might put a damper on the fan and cut the flow back when they are not operating so duct heaters heat the air to the collector to 160 degrees. This would dry out the calcium chloride and allow it to be removed from the collector.
C) We might put a big heater on the duct from the third hood that does not have a heater.
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