Air Density Considerations

Foundries in Mexico City
It was a nice trip for our service engineer in Mexico City with a stay at the Camino Real, freely translated as the royal road. It was one of many sites visited that week all with the same complaint. The pressure drops of the systems were much lower than expected. The result was that power was wasted and in the case of an arc furnace the electrodes burned up at an alarming rate.
The problem boiled down to the fact that using the Industrial Ventilation manual produced higher pressure drops than those calculated in their procedures. The pressure drop across the collectors was lower than for historical numbers. After a couple of days in the office we came to a startling conclusion. The pressure drop was related to the Reynolds number.
We came up with a procedure that was quite simple. To figure pressure drops all you had to do was convert air flows to SCFM instead of ACFM. It worked like a charm. We were then able to modify fan speeds to suit each application. The fan speeds were selected based on actual density at the proper temperature and altitude. This procedure can be used to figure pressure drop at any density due to altitude or gas temperature.

Venturi Scrubber Modifications for fan exhausters.
The normal procedure is to use two factors one for the pressure at 30-49 inches of water for a typical venturi scrubber system and the other for the temperature. If the temperature is calculated to be 160oF a common value exhausting a melt furnace the density will be too high and the required pressure will not be developed. The pressure developed by the fan would be more than 40% less than using the multipliers found in the fan catalogs. The fan belt drive must be modified to compensate for these radical differences in density.

Vacuum Conveying System for GM Brake Shoe Plant in Ohio
The collector was designed to run eight inches of vacuum. Inspection revealed that the pressure drop across the bags was 5.5 inches water gauge of pressure drop. It was a 40 bag collector cylindrical in shape with a high inlet. The load was 30 grains per standard cubic foot and the compressed air pressure was 85 psig. The flow was 200 ACFM. It had seven valves 3/4 inch diaphragm size. The complaint was the collector required pulsing every 4 seconds to maintain the pressure drop. Since the gas density was approximately 50% of atmospheric the pressure drop was equivalent of running at 11 inches w.g. at atmospheric with a filtering velocity of 1:1  when based on SCFM instead of ACFM. The collector on this basis should have run at about 0.2 to 0.3 inches of water pressure drop across the filter mounting plate. After an investigation it was discovered that six of the blow pipes failed because of a manufacturing error. When they were replaced the pressure drop dropped below 0.4 inches and maintained that pressure when the collector was cleaned every four to five minutes. It was an excellent demonstration of the pressure drop across a pulse jet collector when the density drops.

General
In designing several pneumatic conveying systems based on actual CFM on positive displacement systems pressure drop and capacities, the pressure drop is higher than predicted. The collector should be designed on the basis of SCFM as outlined above.
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