Ask the Search Engineer

Blast air energy usage…Impact resistant steel…Rust removal…

Dear Search Engineer: With energy usage being on everyone's minds these days, we have a few large free-standing dust collectors that use shop air to blast the dust off of the bags/cartridge-type filters. They blast 24 hours every day if the blowers are on or off. Two of the big units have 20-inch inlets with 40-hp motors. Is it correct to think that during the run time the blast air isn't doing any good?—A.H., Albuquerque, NM

Dear A.H.: One reader writes in that the blowdown valve is useful while the system is in operation to blow the larger particles off of the filter media and to the bottom of the collector. Allowing the blowdown to cycle while the system is off removes more of the finer particles from the filters and allows them to do a better job when turned back on. The reader's system used a timer in the PLC to allow the blowdown to cycle for some time after the filter was shut off, but not continually. It may be an hour that it would continue the blowdown burst cycles. You could experiment with a low-cost timer to get to an optimum cleanliness versus usage ratio, and therefore cost savings.

Dear Search Engineer: I need to select a steel for impact resistance. The first part must be weldable as used. I would weld this to casing pipe. It is subject to impact as it is driving pipe into the ground. It may deform a little since it will not be reused. I prefer some type of tube profile if possible and would consider forging/casting. The second part is the driving tool. It is the intermediate part between the hammer and the above casing adapter, and it needs to resist impact and severe deformation. I machine a hex driver for rotation in the center and hammer on the outer circular edge. I am considering the high manganese steels for their work hardening properties. Any suggestions? —P.B., Newport, NH

Dear P.B.: Metal fabricators and ore and mineral processors have used Jalloy for such applications for many years now (sacrificial strike and wear plates, pry bars, etc.). It is strong, wear-resistant, and tough. Try your local metals supply house. They'll likely have a few "maintenance steels." For reference, check out http://www.400wearplate.com.

Dear Search Engineer: I need to remove rust from the machined surfaces of a large industrial machine. Is there a cleaning agent that can be applied and then washed or rubbed off that will remove all or most of this surface rust? —G.N., DN reader

Dear G.N.: Naval jelly (phosphoric acid) converts rust to iron phosphate and provides a good surface for painting. The general directions are to remove most of the rust first, then convert the invisible remainder for a good bond. Assuming you can dunk the whole machine, buy washing soda at a grocery store, add a couple of tablespoons per gallon of water, and apply 10-12V dc in the reverse direction of plating. A reader reports that this does remove the rust, but it leaves a scum that still takes some effort to remove. If at or slightly above the manufacturers' recommended speed, another option, wire wheels, can literally spark as they knock the rust off. Cleaning supply companies for the electroplating industry (e.g., Oakite, used to supply a phosphoric acid-based material, with cellulose as a "thickener" to make it more viscous, allowing it to be "painted" on a vertical surface and still cling to the rust. Evapo-Rust also will do an amazing labor-free job but the rusty surface must be submerged. It is reuseable, water-based, environmentally safe, and affordable even in 55-gallon quantities.

Dear Search Engineer: I am trying to design and manufacture a power generating system. Spec output is 100V ac, 10A. Do you know where I can obtain information on coupling the generator to the motor? And any information on a manufacturer of the generator and motor (5-10 hp)? —J.C., DN reader

Dear J.C.: One reader suggests you use a "standard" ac generator, 115V, 60 Hz (which may be purchased, coupled to any power source you may want, such as wind generators, gasoline engines, etc.) and connect it to a "standard" off-the-shelf ac power supply. If you select a "modern" switched power supply, you will end up with a low risk and simple piece of equipment to manufacture.

Dear Search Engineer: Can you tell me the formula for calculating the ideal gate resistor value (RG) for a P channel enhanced FET switching circuit driven from a CMOS microprocessor output. I understand that you could drive the FET directly from the CMOS output, but if you use a series resistor for the gate, you can reduce potential long-term damage to the CMOS output. Anyone know how to calculate this ideal value? —M.D., DN reader

Dear M.D.: Says one reader: There is no reason why you cannot drive a FET directly from a CMOS µP pin as long as the pin is current limited. However, you will get a sluggish response (the dv/dt will be proportional to current limit/capacitance), which will result in increased heat dissipation in the FET due to switching losses.

You can always put in a FET driver such as the MAX5078 as this will provide a high impedance to the µP, and give a good low impedance kick to the FET. To ensure good operation, make sure you have thick tracks going to the FET driver and have local decoupling (the output current will come directly from the rails).

A discrete solution will require a bit more design, but will provide adequate results.

Resistors are normally put in series with the gate primarily to stop ringing (due to the gate inductance and gate-source capacitance resonating). The current limiting is a secondary issue as you normally want the FET to turn on as fast as possible. 100Ù is normally good enough.

Dear Search Engineer: I would like to have suggestions for a tube handling fixture for a seven tank process. Presently, the tube diameter range is 14 to 88.9 mm and length varies from 3 to 12.5m. The weight of the tube bundle to be handled is in the range of 2,000 to 2,500 kg. Bundles are passed through various solutions along with rinsing. Bundles need to be tilted in order to allow escape air while dipping and to drain the liquid. One of the solutions is soap solution where the chances of the tube sliding down are more. Any suggestions? —A.M., DN reader

Dear A.M.: From the last sentence of your question, one reader deduces that you have no provision to stop slidings, so a perforated plate supporting the bundles at the down tipped end may do the trick.

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