This Month’s Puzzler
We operate a jet mill on superheated 300-psig steam to grind TiO2. The particles from the mill pass through a screen, a cyclone, a baghouse, a filter, and then a blower and silencer. The baghouse uses pulsed compressed air to clean the bags. Recycled solids from the cyclone go by screw conveyor back to the dryer upstream. Unfortunately, the system requires continual attention. The mill’s steam nozzles tend to corrode or erode. The baghouse elements frequently blind. The fins in the blower have bent or broken twice in the past year — no one remembers if this happened in previous years. Sometimes the cyclone bottoms become blocked. Our maintenance manager believes we need an additional cyclone and an oil heater before the first cyclone to keep condensate from forming in its inlet; he’s sure the cold air in the baghouse is condensing steam, causing the sludge we sometimes see in the bottom of the housing. We had been carefully maintaining the heat tracing and insulation but the maintenance budget was cut last year and the fall inspections were eliminated. The operating manager isn’t convinced we need to reinstate them. What do you think our problems are? Is the operating manager right?
Look First At The Baghouse
The essential problem is the blower failure, which probably is caused by the blinding of bags in the baghouse. This blinding may stem from condensation of steam wetting the bags or the pulsed-jet air system not working properly. Once wetted, the TiO2, which is a fine particle in the range of 2–0.5 microns in diameter when unclumped, will quickly blind the bags. As for the compressed air, things become more complex; common culprits worth investigating include: 1) moisture in the air; 2) cold air; 3) low pressure; 4) poor installation of the jets; 5) inadequate distribution of the air from the jets — air must hit all the bags, even the ones on the ends; and 6) deficient timing settings — air must rap the bags on a precise schedule allowing the manifolds to regain pressure before firing again.
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An ordinary pressure transmitter should suffice to confirm the pulse-jet settings, distribution and pressure. Use only one transmitter because there’s no way to calibrate two exactly the same. If the piping won’t recover fast enough after a pulse, increase the pipe diameters or create parallel pathways for air flow.
One of the key strategies for troubleshooting is to check that equipment is operating the way it was intended and to identify what changed and how performance has been affected. When possible, inspect the mill, screen, cyclone and baghouse — all the equipment upstream of the blower. Bring in the equipment vendors, if necessary. Because your maintenance department has been lax in record-keeping, perhaps you can get details about equipment purchases from the vendors.
Now, let’s consider the systemic problem: heat conservation. Keeping the ductwork and equipment hot is a challenge, especially with the saturation temperatures of 300-psig steam. The maintenance manager may have a point that a mineral oil may avoid condensation — but can’t ignore routine maintenance of heat tracing and insulation. Assuming the mill ran fine before the insulation degraded, investing in an oil heater may not be necessary. If cold spots are a problem, an alternative might be insertion heaters. Electric heaters could be expensive compared to oil heaters but are easier to install and maintain; they can be removed without all the expensive pipe and insulation.
The question of the additional cyclone is another matter. Again, assuming the mill ran well before the recent batch of troubles, then a cyclone may not be necessary. My advice would be to talk to the original designer. Did someone push for a second cyclone but get over-ruled by the accountants? If this is the case, you’ll have a challenge finding room for a second cyclone — not so much because of bulk but because ductwork is difficult to modify and because of the additional pressure drop. You may need to install a new blower with the cyclone.
The erosion or corrosion in the steam jets is something incidental to the actual problems with the heat conservation. However, the mill may work better if the nozzles are clad in ceramic.
As to who is correct? You’ll have to establish whether the heat tracing is working before deciding that the investments suggested by the maintenance manager are worth the price.
Dirk Willard, consultant
Wooster, Ohio
April’s Puzzler
My company sells hot melt glue, and I’ve been sent to a customer, ostensibly to participate in a hazard and operability (HAZOP) study about the processing of the glue — but in reality to settle a feud between the customer’s production manager and its corporate safety engineer.
In the customer’s process (Figure 1), hot melt first flows from a feed tank through a rotary screen. Then a high-pressure gear pump sends the fluid through an electric heat exchanger. After the exchanger, the melt goes to a duplex filter. Finally, it passes either to spray guns or gets recirculated to the tank. The hot melt should be between 350 and 370°F for the spray guns; its flash point is 400°F — production sometimes runs up to 395°F. The tank is maintained at 200°F by an electric heater when the line is down for a day or so.
