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Dust Control Mining and Aggregate Plant Dust Control Products

Monday, August 20, 2012

Fog Dust Suppression

I get a lot of calls and e-mails from customers looking for a ”fog” system because they have this idea that fog systems are able to control dust without adding any moisture to the process.  That’s true, but there are a lot of practical difficulties and hidden costs.

First, what exactly is a fog?  By definition, a fog consists of droplets that have an average mean diameter of 10 microns. If the spray system is not capable of producing droplets with a mean diameter of 10 microns, it cannot be termed a fog system. There are commercially available systems which represent themselves as "fog" systems which do not meet this criteria so you have to be careful. Unfortunately, “fog” is a term that has been used very loosely.
There are only two types of commercially available equipment that are capable of producing a fog. The first uses a postive dislacement pump to produce pressures in excess of 1000 psi and nozzles with an orifice about the width of a human hair. The tremendous shear that occurs at the nozzle tip produces droplets in the 10 micron range. The second is air atomization in which compressed air and specially designed nozzles are used to shatter water into 10 micron droplets.
Fog systems are designed to suppress only airborne dust and do not add any detectable moisture to the substrate. Because excessive moisture can decrease screen efficiency, aggravate crusher wear and cause wet handling problems like belt carryback, aggregate processing and other material handling plants have every incentive to keep moisture addition rates for dust control to a minimum.  This is why the term "fog" is so often used as an effective marketing tool.
However, while I’ve designed and built a lot of fog or air atomizing systems I don’t recommend them for use in quarry and mining environments for the following reasons.
1.     They require a high degree of enclosure for optimal performance. The enclosure is used to create a "dead air" space in which the fog droplets and dust particles are able to collide, agglomerate and drop out of the air. For example, a truck hopper should be completely enclosed on three sides and covered for a fog system to effectively control dust. Without the benefit of an enclosure, even the slightest wind will blow the fog off target.
2.     Because they are designed to suppress airborne dust and add no detectable moisture, they do not have any "residual effect" that helps to prevent dust from downstream operations. For this reason, nozzles must be installed at every drop or transfer point.  This means more nozzles and can  significantly increases maintenance.
3.     Fog systems require a very clean (city) water supply. Fog and air atomizing nozzles have very fine orifices and are much more susceptible to pluggage than conventional spray nozzles. In the case of fog systems which use positive displacement pumps, any suspended particulate in the water supply will aggravate pump wear.
4.     4. In the case of air-atomizing systems, we have seen instances where the injection of large amounts of compressed air required to produce fog droplets pressurize the enclosed source area causing mist and wet fines to blow out of the area and actually aggravate emissions.
5.     Positive displacement pumps used to produce fog require periodic (monthly) lubrication. The vertical stage centrifugal pumps we use in lower pressure misting systems is self-lubricating and requires no regular maintenance.
6.     Air atomizing systems require significantly more maintenance than a conventional water spray system. Air atomizing systems use regulated water and air flows and pressures to produce an atomized fog spray. With all the clanging, banging and vibration in a typical plant, it is not unusual for these settings to drift. Too much air and the nozzle fails to atomize enough water to maintain the spray, too little air and the spray becomes too wet and droplet size increases.
7.     Air atomizing nozzles are significantly more expensive than conventional and fog nozzles. Prices for fog nozzles range from $8 to $15. The typical flat spray nozzles we use in a conventional water spray system in the same price range. Air atomizing nozzles are much more highly machined and cost 5 to 10 time as much. The fact of the matter is that nozzles get damaged and wear out and air atomizing nozzles have a much higher replacement cost.
8.      Fog systems require the use of more expensive hose and hydraulic fittings. The conventional water spray systems we’ve designed operate at 200 psi because that is about as high as you can go with conventional pipe and hose fittings. At pressures in excess of 300 - 400 psi, hydraulic hose and fittings are required.
9.     Fog systems in particular and air-atomizing systems to a lesser extent are not designed for use in an abusive mining environment. Both systems were originally developed for evaporative cooling applications and that remains their major market. Air atomizing systems were the first to be adapted to dust control in the '70's. Raring Corporation introduced the "Sonic" system in the U.S. and installed many of these in quarries (all of which have been abandoned - I know of no quarry in the U.S. using an air atomizing system).  We, in fact, were contracted to install a Sonic system in 1990 at a quarry in New Jersey. They were termed "sonic" nozzles because these air atomizing nozzles were equipped with a resonator tip that promoted fog droplet atomization. After one week in service, every resonator tip had been damaged by errant stone. In our 23 years in business we have installed three or four air atomizing systems of our own design (no resonator tips). All of these have been used to produce a curtain of mist at an enclosed truck hopper.
As for fog systems, they were also adapted to dust (and odor) control in the early '90s and their major market in this regard has been enclosed trash transfer stations where arrays of fog nozzles are installed in the ceiling to settle dust and deisel exhaust on the tipping floor. We have used fog systems in warehouses for the same purpose and recently completed a large installation for  Pacific Coast Materials near Sacramento, CA - a large clay pipe manufacturer. We also use a fog system in our line of SpreadKleen products that are designed to suppress dust from trucks spreading lime, ash or portland cement for soil stabilization. They work well in enclosed applications but we do not recommend them for quarry applications because they just don't hold up. Positive displacement pumps simply don't last because they have to operate in a dirty environment with poor water quality where production machinery is always the first priority of maintenance.
I have no axe to grind with fog and air atomizing systems technologies - they work well in the proper application but there’s no getting round their principal problems of high maintenance, the need for clean water and poor reliability. 
Now, conventional spray systems (and I don’t mean a garden hose stuck in a crusher) that are designed to operate at 200 psi and use conventional spray nozzles will produce an atomized mist with droplets distributed within a 50 to 500 micron range. This is also the size range for most of the visible dust produced in a crushing and screening plant. As a result, the system affords an excellent match between dust and droplet sizes that enables the system to suppress a wide range of particulate. Now, we can go to impingement nozzles which will consistently produce droplets with a mean diameter in the 40 to 60 micron range (mayber 5-10% of droplets in the 10 micron range) but that's about as fine as we can get.
Conventional spray systems like NESCO’s DustPro are designed to control dust in two ways:
1.     It produces a higly atomized mist that suppress airborne dust and
2.     adds from 0.2% to 0.5% by weight water to the process to prevent dust emissions from downstream operations.
In the instances where we have been able to track water addition rates, we find our customers are operating in the 0.2% to 0.3% range. The addition of moisture to the process reduces the dustiness of the stone and eliminates the requirement for nozzles at every drop point. We have found empirically that the addition of less than 0.5% moisture rarely leads to reduced screen efficiency, aggravated crusher wear or wet handling problems. The only exceptions to this have been at quarries processing a very soft, chalky limestone or materials contaminated with large amounts of fine clay. In both cases, we have been able to resolve these problems by installing smaller nozzles and improving operator training.
When I began NESCO in 1989, I had the benefit of previously working as a consultant to mining companies and much of this work was designing water spray systems. At the time, (early '80s) there was no manufacturer of water spray systems for quarries. There were chemical systems that were widley used in the coal industry, air-atomizing and fog systems for special applications but, surprisingly, no company sold a simple, reliable water spray system. Many plants simply built their own systems.
At that time, the main problems with these water spray systems were:
1.     Low pressure - most systems operated at 40 - 80 psi and had to add as much as 1% to 1.5% water to acheive the desired level of dust control. Such low pressure systems could not suppress fine airborne dust because droplet size was too large. It was these low pressure water spray systems that gave water spray systems a bad name - the garden hose in the crusher approach.
2.     Manual control - operators had to leave their production tasks to open and close valves and were unable to adjust the amount of water in response to dust levels. Multi-million dollar, highly automated production plants were saddled with rudimentary spray systems that either added too much or too little water for dust control because operators could not respond fast enough to changes in the production process or material properties.
3.     Cheap construction - This was a big problem - companies were reluctant to invest in quality pollution control equipment because it did not contribute to production. This kept the accountants happy but maintenance personnel had to deal with constant breakdowns.
So we designed the DustPro to overcome these deficiencies. First, we used a 200 psi pump so that we could improve droplet atomization and increase penetrating power. This enabled us to reduce moisture addition rates to values that would not affect screen performance or crusher wear.
Secondly, the DustPro is electronically controlled. The operator can remotely switch nozzles in independent spray zones on or off in response to dust levels. The same man who controls production equipment also controls the spray system. This enables the operator to maintain an optimum moisture additon rate - just enough to control the dust but not so much as to cause screen blinding etc.
Thirdly, our equipment is built to last. All the plumbing is brass or bronze and the pump has stainless internals. The system uses the best components we can find because it has to hold up in a very abusive environment. I visited a LaFarge plant outside of Chicago last month where we installed a system in 1992 that was still in excellent operating condition.
I believe that conventional water spray systems represent the best compromise between performance, ease of operation, and reliability in an abusive mining environment.  If you have to have a fog system make sure that you have the benefit of an enclosed source area, are prepared for higher maintenance costs, have a very clean water supply and use only heavy-duty components.

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