Limits on foundry particulate emissions — now 0.015 grains per dry standard cubic foot (dscf) — will soon tighten substantially, to 0.006 grains per dscf for existing cupolas. Installing all-new emissions equipment is one answer, but recent tests at a foundry in Defiance, OH, and elsewhere, have shown that the new regulations can also be satisfied by retrofitting existing primary equipment, whether wet scrubber, electrostatic precipitator, venturi or baghouse, with a more economical tail gas scrubber.
Such a system has been developed by Tri-Mer Corp., Owosso, MI. Called the CCS, or Cloud Chamber, the system was originally developed for the microelectronics industry. Increasing levels of silica and ammonia-based particulates in exhaust streams, along with various gas phase species, had created a demand for an alternative to packed towers. The goal was a system that did not use packing, and thus would not have the attendant problems of fouling and biological growth management.
The operating principle for CCS, surprisingly, is based on weather phenomena, specifically the way lightning and rain clean the air.
Electrically Charged Raindrops
Lightning begins with an electrically charged raindrop, which becomes distorted by a charge from the cloud’s electric field. Clouds form when water condenses on particles of dust or salt. Those particles naturally coagulate with larger ones — a phenomenon called Brownian motion. In nature, the process is only about 1% efficient. The key to collecting particulate efficiently is to improve upon the action of the cloud: to “grow” particulates by increasing humidity.
At 80 percent relative humidity, for example, a .1 micron particle will, in seconds, double in size. At 100 percent humidity, it grows to about .5 microns. Increase relative humidity to 101 percent —- the point of slight super-saturation — and that particle becomes readily removable from the gas flow.
The technical challenge became to create a water droplet, particulate, and electrical charge equation that allowed particulate to be collected efficiently — and that was the genesis of CCS.
H2O Droplets as Particle Collectors
Inside the system’s collection chamber, billions of droplets form, causing droplets and particles to move continuously in relation to each other. As any particle passes within 10 or 20 microns of a water droplet, electrical attraction causes the particles to enter the water droplets. Each water droplet therefore becomes a “collector” of thousands of particles, constantly re-energizing with each pass through the cloud chamber. Since the charged droplets are the collectors, there is no need for fibrous filters, collector plates, venturi throats, layered pads, bags, or cartridges.
Low Operating Pressure Minimizes Operating Costs
The total system operating pressure is 0.75 in. w.g. (water gage) per stage — a small fraction of that required by traditional collection devices. This is possible because the system design is void of interferences, so clouds form as they do in the sky. The chamber of the CCS actually generates zero pressure; the 0.75 in. w.g. is produced by the mist eliminator droplet remover, and connecting duct. This low pressure drop minimizes the costs of fan operation.
Also minimizing costs is the requirement of less than 1 in. of pressure drop per CCS stage under full load operation. By contrast, Venturi systems typically require 40 in. - 70 in. of pressure drop; diffusion candles typically require 16 in. - 20 in. of pressure drop.
Most notable from a cost standpoint is that the Cloud Chamber scrubber primary charge modules require just 600 W of power, less than 1/100th the level of a typical ESP (electrostatic precipitator) power draw of 60,000 W or more.
Cloud Chamber Versatility
The Cloud Chamber will scrub fumes, gases, and odors simultaneously with particulate. If a foundry wants to remove SO2, it can run caustic through the addition of NaOH, or it can run acid to remove ammonia. Other fumes and gases removed by CCS include HCl, HF, HNO3, H2SO4, Cl2 and NH3. The system is highly adaptable to individual foundry requirements, and does not require changes in air handling configurations.
Tests indicate the most efficient particle range for a CCS is 0.1 to 5 microns; the system can also handle much larger particulate. Removal efficiencies of 99.99% and higher for foundry particulate of 0.1 microns to 300 microns are consistently achievable. CCS handles inlet grain loadings in excess of 1.5 grains per dscf, reducing loads to below 0.003 grains per dscf.
At the foundry in Defiance, the inlet reading into the CCS was 0.0156 dscf. In successive one-hour (standard) tests, the CCS unit, performing as a tail gas scrubber and interfacing with the existing wet scrubber, achieved readings of 0.0030, 0.0020 and 0.0030 grains dscf, respectively. Operation was at less than 0.5 in. of static pressure.
The CCS unit engineered for foundry use has three stages, and can be adapted for use as either a primary or tail gas scrubber. It will remove SO2 and ammonia in addition to particulate.
Systems are manufactured to customers’ specifications in polypropylene, FRP, PVC, 304 and 316 stainless steel, and Hastelloy.
Edited from information supplied by Tri-Mer Corp., Owosso, MI