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Dust Control Mining and Aggregate Plant Dust Control Products

Sunday, September 18, 2016

Maintaining Dust Control Equipment

Preventive Maintenance is the Key to Compliance.

By: Mark Kestner, Ph.D, President, NESCO

            Maintaining dust control equipment is the key to compliance.  But these days it’s tough enough to maintain production equipment much less dust controls.  Poor maintenance is the largest contributing factor to non-compliance and the major cause of citations and fines.

            Dust control equipment for aggregate processing can be divided into three categories: enclosures to contain it, spray systems to suppress it, and baghouses to collect it.   These deserve the same level of preventive maintenance as your crushers, screens and conveyors. If you think your spray system isn't a piece of production equipment, read your operating permits.  They stipulate that the plant cannot be operated unless dust controls are in good operating condition.  Operating with a broken down spray system or baghouse will put health and safety at risk and expose the plant to fines up to $25,000 per day.   If you think regulators will let you slide, just ask the owners of a major east coast quarry that was shut down for a month  this year until dust control equipment was restored to good operating c­ondition. 

            Dust Enclosures
            Dust enclosures include chutes, covers, skirts, curtains and the like. Maintenance of dust enclosures is critical to protect employees from respirable silica-containing dust.  Dust and spillage that leaks has to be cleaned up.  Workers who have to sweep, shovel or bobcat it can get a nose full of dust so make sure workers have access to respirators and use them.  Fabric masks offer little or no protection against respirable dust.   Whenever possible, use hoses or vacuums to flush down or pick up spillage.

            Leaks in chutes, worn skirting, and torn curtains need to be repaired promptly.  Much of this work can be done with in-house labor and materials.  Old conveyor belting can be used for curtains, scrap steel for patches and covers.

            Spray Systems

            Water spray systems are the principal control device at most quarries and are pretty simple to maintain.   The two most important maintenance items are:

Spray nozzles.  Nozzles should be inspected daily.  Nozzles that are easy to see and easy to reach are easy to maintain.  Nozzles that can't be seen or are hard to reach don't get inspected and cleaned.  Nozzles can plug from the inside-out due to suspended or dissolved solids (scale) in the water supply or from the outside-in due to deposition on spray bars.

Water filters.  Water filters and strainers need to be inspected regularly and cleaned or replaced as necessary.   The spray system should be equipped with a pressure gauge.  A decline in spray pressure indicates a dirty strainer and will provide maintenance personnel with a metric for frequency of inspection.

Most conventional spray systems use centrifugal pumps are that self-lubricating and do not require any regular maintenance.  If you have a "fog" system that uses a positive displacement pump, they will require regular oil changes and are much more susceptible to wear.

It's also a good idea to have a spare pump and motor on hand in case of a mechanical failure or freeze damage. 

Solenoid valves can be another sore point, particularly when the water supply is dirty or contaminated with sand.  Dirt and grit that builds up on the valve seat can prevent it from closing properly.  If spray nozzles keep on running or dripping after they have been shut off, that’s a good indication that the solenoid is dirty and needs to be cleaned or replaced.

If you are thinking about installing a spray system, here are some guidelines to reduce spray system maintenance:

1. Use heavy duty components designed to operate in an abusive environment.

2. In a cold weather climates, house the pump in a heated enclosure.

3. Use the cleanest water possible,  Using dirty pond or river water will make nozzle, filter and pump maintenance a nightmare.

4. Install drain valves at all the low points.  Frozen lines can delay ­start-up on cold morning and that means lost production.

5. Put nozzles where they can be seen and are easy to access.  Out of sight is out of mind.

6. Consolidate all moving parts in a central location with the pump.  Solenoids and filters out in the plant are more likely to be damaged or freeze and take more time to inspect and maintain.

7. Most nozzle pluggage occurs from the outside-in due to deposition of wet fines that blow back onto the nozzle – particularly at crusher discharges.  Install nozzles so that they are out of the way of this “blowback” or put a dust curtain in front of them with a small hole that nozzles can spray through.

8. Keep a list of all nozzles and their location to facilitate ordering and replacement


            Most modern baghouses are “pulse-jet” that periodically and sequentially pulse reverse air through the filter media to keep them clean.

            The differential pressure across the filter bags as measured by a magnehelic or photohelic gauge is the best indicator of performance.  High differential pressures indicate that bags are caked up or blinded.  Low differential pressure can indicate that bags are torn or missing.

            If visible dust emissions are observed at the baghouse exhaust, this is also a pretty good indicator that bags are torn or missing.

            Inspect the hopper discharge for any material build-up.  If not discovered in time, dust can fill a hopper to its inlet and plug the unit.  Whether a rotary valve, screw or pneumatic conveyor is used to empty the hopper, it should be inspected frequently.

            Exhaust fans should be inspected semi-annually.  Loose or worn belts or an imbalanced impeller, will reduce the fan's performance.  Any time unusual vibration or squealing is observed, the unit should be thoroughly inspected.

            The filter media is the most important element of the baghouse and periodic inspection should be mandatory.  Inspect the clean air side of the baghouse for any leaks or tears in the filter media.

            Ductwork should also be inspected for any leaks or deposition.  If the exhaust fan is not able to maintain the air velocity in the duct of at least 3500 fpm, dust will settle out and restrict air flow.  Eddies in sharp bends can also result in deposited dust that restricts air flow.

            If you are going to install or refurbish a baghouse, here are some guidelines to keep maintenance costs down:

1. Make sure they are designed for heavy-duty service. 

2.Specify materials of construction that are corrosion resistant.  Using cheap metal will result in corrosion that eats holes in ductwork.

3. Avoid sharp bends in ducts that will quickly wear through.  This is particularly true if the plant processes hard, abrasive rock like sandstone or granite.

4. Select filter media that will hold up to your specific stone.  Again, abrasive dust will require more resistant media.  If the rock contains any sulfides that can produce acidic corrosion when exposed to moisture, you’ll need a filter media that is up to the task.

5. Avoid long-multiple intake ducts.  To save money, many plants install a single big baghouse with tentacles of ductwork that extend to multiple sources.  It is almost impossible to balance air flows with such an arrangement and you’ll be much better off installing multiple baghouse with fewer and shorter intake ducts.

            Preventive Maintenance - Don't wait '­­til it breaks!

            Whether you use wet suppression systems, dry collectors or a combination of both, institute a routine program of preventive maintenance and make sure that the plant pursues it with the same alacrity that they do for production equipment.  The program should include a daily inspection recording spray system pressure and/or baghouse differential pressure as well as a log of any maintenance or repair work.    Pollution control equipment may not put more tons into trucks, but it does protect the health and safety of your employees that do.

Wednesday, September 30, 2015

10 Good Reasons Not to Build Your Own Spray System for Dust Control

10 Good Reasons Not to Build Your Own Spray System

I have often fantasized about running an ad that read, "Here's the cheap hunk of junk spray system you always wanted".  Why, because the last thing in the world most plants want to spend their money on is dust control.  I've had a plant manager rip up my literature and a quarry owner ­­storm out a meeting because of their anger and frustration over regulation that requires them to protect the environment. Our biggest competitor at NESCO is the plant that decides to build and install their own spray system.  I've seen a few that are well-designed, but hundreds that aren't. 

If NESCO built that cheap spray system, we would've been out of business years ago.  Why?  Because when that system didn't control your dust, got your stone too wet and broke down all the time you wouldn't remember that's the system you wanted but only that I was the guy that sold it to you.

Thankfully, there are plenty of mines and quarries that see the value in quality equipment that is designed to last in an abusive mining environment. 

But, if you're still thinking about building your own system, think twice.  Here are 10 good reasons not to do it:

1.  If you do it right, you won't save any money.  If you build a well-designed, high pressure spray system you won't save any money compared to commercial system.  NESCO builds systems with quality components designed to withstand an abusive mining environment.  If you use the same components and figure in all the labor that went in to sourcing the components, purchasing the materials,  constructing the pump module and installing system, the savings just aren't there.

2. If you do it wrong, it'll cost you production.  If you build a cheap, low pressure system you'll save money compared to a commercial system but you'll pay for it in lost production.  For lack of pressure and adequate controls, they add so much water that they blind screens, cause belt carryback and throw products out of spec.  Shutting a 500 tph plant down for an hour or two to clean screens could cost you $1000 to $2500 a day!  That will  wipe out the savings in your do-it-yourself spray system in a few weeks.

3. Whose going to build it?  Most plants these days barely have enough manpower to keep production equipment operating and don't have the manpower to spare to design, build and install a spray system. 

4. Whose going to fix it?  When your DIY system breaks down - and it will - there's no manual, there's no spare parts list, there's no replacement parts in inventory.  The odds are the employee who got stuck designing and building the system has moved on and no one knows where to go for parts or service.

5.  It is rocket science.  I've been listening to people tell me for 30 years that building a spray system isn't rocket science.  Well, guess what - designing and installing a spray system that will knock down the dust and won't screw up production is just as much rocket science as designing and installing a crusher or any other piece of production equipment.  It doesn't take any talent to dump enough water on the stone to kill the dust.  Making sure you've got the right nozzles in the right spot at the right pressure with the right kind of controls that won't cost production does.   

6.  There's no guarantee it will get you into compliance.  If you're building a new plant, you've got 180 days after the plant achieves full production to pass a visible emissions (VE) test.  What if your system doesn't pass that test?  What are you going to do then - just dump more water onto the stone?  You'll pass the test but you won't sell those off-spec products.  NESCO guarantees compliance by offering an Emissions Control Warranty with every DustPro System.

7. Make dust control our problem - not yours.  Who gets the blame if your DIY system turns out to be a disaster that gets the rock so wet that you can't screen it or make spec products?  You do.  If we screw up, it's on us. 

8. Regulators don't trust you.  They'd rather see a commercial system installed by experts.

9.  You've got better things to do.  Time spent on designing and building a spray system is time lost on projects that improve production.

10. Support small business - when you imitate, we can't innovate.  Companies like NESCO that­­­ sell dust control aren't General Motors.  We are all small or even, micro, business that sell into a niche market.  Mining companies intent upon driving costs down at the expense of small businesses dry innovation up.

Wednesday, October 30, 2013

Operating Spray Systems in Cold Weather

Winter is right around the corner and using spray systems to control dust in freezing weather can be a real challenge.  As a practical matter, NESCO spray systems can be operated at temperatures as low as 25 0F as long as the water is moving.  Below 25 0F nozzles and other exposed metallic parts will begin to freeze even if the system is on-line. 

Here are some guidelines for cold weather operation:

1. Install a thermometer to alert the operator to freezing temperatures.

2. Inspect spray nozzles daily to make sure that they are not plugged or frozen.

3. Inspect the pump daily.  Verify that the heater is working and that the water inlet line is not frozen.

4. Minimize the number of spray lines in service.  In freezing weather it will not be possible to use spray lines at feed hoppers where ice can form on sidewalls.  Use only essential spray nozzles.

5. Make sure that nozzles are properly targeted.  Nozzles should not spray on steelwork or conveyor belting.

6. Drain connecting hose or pipe immediately after they are taken out of service.  Do not allow water to sit in hose lines and spray manifolds.

7. Verify by visual inspection that no water remains in the line.

8. Keep all drain valves open after the system has been drained to prevent any residual water from freezing and cracking the valve.

9. Make sure that water lines run straight and true and that drain valves are located at all low points.  Bends, loops, and kinks in hose lines will make lines more difficult to drain in cold weather.

NESCO systems can be equipped with the following accessories to purge and protect hose lines in cold weather:

Air Purge System: This system uses compressed air to blow water out of connecting hose and spray nozzles after the spray line has been shut off.  When the operator uses the remote panel to switch a spray line from the "Run" to the “Purge” position, that line will automatically purge for a preset period of time that depends on the length and diameter of the hose or pipe. The purge cycle can be repeated as often as necessary.

Anti-Freeze Injection System: This system operates in conjunction with the air purge and is designed to prevent any water remaining in the hose, plumbing, or spray nozzles from freezing.  We developed this system because once the lines are blown out with compressed air, a small amount of water may still remain in drain valves or spray nozzles.  It uses a metering pump that automatically dispenses glycol into output hose lines after they have been blown out with air.  When the operator turns any output line to the “Purge” position, three separate timed operations occur. First, the air purge blows out the line. Second, the metering pump injects glycol (0.2 to 0.5 gal.) into the line. Third, the controller turns the air purge back on to blow the glycol through the connecting hose and out to spray nozzles. This coats the interior of the line with glycol so that any water that collects in a drain valve or nozzle will not freeze.

However, the air purge and anti-freeze injection systems do not permit continuous operation in freezing temperatures.  They are designed solely to purge and protect hose lines and spray in cold weather.

There are only two ways to operate a spray system continuously in freezing temperatures:

  1. The system must be housed in a heated and insulated enclosure and output lines and spray bars are wrapped with heat tape and insulated.


  1. The system must be housed in a heated and insulated enclosure and the pump is supplied with an aqueous solution of a non-toxic glycol or corrosion inhibited, brine-based anti-freeze.

Heat Taping and Insulation    

Heat tape can be wrapped around the pipe or hose to prevent it from freezing.  Heat tape is sold in various wattages and we generally recommend a 5 watt per foot self-regulating heat tape. This type of heat tape is thermostatically controlled and should be set to power on when the temperature approaches 32 0F.  The heat tape is wrapped around the water line and number of wraps or turns per foot of heat tape will depend upon the lowest temperature the system will be exposed to.   The heat tape also requires an independent power supply.  For example, 1000 feet of 5 watt/ft heat tape will require 5KW of power.   Heat tape and insulation could run anywhere from $15 to $30 per foot depending on the temperature the water lines will be exposed to and the type of heat tracing and insulation used. Since the heat tracing is subjected to an abusive mining environment, we recommend a high quality fiberglass insulation that is steel-jacked to protect it from falling rock or other items that might damage it. 

The economics of heat taping favor large mineral mines, like gold, silver or copper that have to run 24/7 in any kind of weather that can justify the expense.   For a typical 500 tph aggregate crushing and screening plant with 1500 ft of pipe or hose employing 3000 ft of tape, total installed costs could range anywhere from $45,000 to $90,000.  That’s a lot to spend up front, but it may be the most cost-effective option over the long haul.

Here are a couple of links where you’ll find information about the design and installation of heat tape:

It is also possible to purchase heated hose which is pre-fabricated with insulation and coated with a durable plastic. Technical Heaters, Inc. is one manufacturer and here is a link to their website for more information: 

Anti-freeze Solutions

There are two types of anti-freeze solutions that could be used to replace water when the system is operating in freezing weather - glycols and brines.

In the U.S. the only glycol approved for use as anti-freeze is propylene glycol because it has low toxicity and low volatility.  Here is a link to the freeze point curve of propylene glycol:

This diagram shows that you need about 20% glycol in solution with water to reduce the freeze point to 20 0F (-7 0C).  We use a 60% glycol/40% water solution for our glycol injection system.  Propylene glycol is commonly sold as a 60/40 mix because that is about as concentrated as you can get before viscosity starts to pose pumping problems.  It sells in bulk for about $3 /gallon depending on purity so you could be looking at treatment costs on the order of $1 to $2 per ton depending on the concentration required.  The high cost of propylene glycol has inhibited its widespread use. 

There are many other glycols available including waste materials and you should check with your environmental agency to see what is approved.  You might also check with chemical companies that sell freeze conditioning agents, also known as ice crystal modifiers, to the coal industry where they are widely used to prevent coal from freezing in railcars.  These are usually solutions of salts like sodium acetate or urea with a bit of glycol and water mixed in. 

Calcium and Magnesium chloride brines are very effective anti-freeze agents which are much less expensive than glycols.   However, they may be corrosive to spray system components.  We have a couple of customers using brine without incident but our systems use brass, bronze or stainless steel plumbing.  If your spray system uses black iron or galvanized components, they are going to corrode unless the brine solution contains a corrosion inhibitor. 

A 20% solution of calcium chloride in water, for example, will depress the freeze point to about -4 0F (-20 0C). Concentrated liquid brines (32%) sell for anywhere from about $0.70 to $0.90 per gallon - about a third the price of propylene glycol.  It may be possible to find a waste brine that is a byproduct of gas drilling that is even less expensive.  For a 20% commercial brine solution to operate at -4 0F (-20 0C), treatment costs would run about $0.20 per ton assuming it was sprayed at an addition rate of 0.2% (about 0.5 gal/ton).  For a 500 tph aggregate plant producing a total of 30,000 tons over 60 days of winter operation, treatment costs for anti-freeze could range from $6,000 to $12,000.  For a plant like this, the anti-freeze option looks much more cost-effective than heat taping. 

But remember that anti-freeze agents can alter the chemical properties of aggregate.  Glycols are set-retarders in concrete and brines are set accelerators.  At mineral processing plants, the anti-freeze could potentially upset downstream processes like froth flotation or waste water treatment.  It is unlikely that the small concentration of anti-freeze agent present in the stone or ore could cause a problem but better to be safe and sorry and take a hard look at potential side effects.

Monday, January 21, 2013

How to Save Dollars on Dust Control

            Times are tough and cost control is the key to profitability.  Air quality standards are a fact of life and quarries have little choice but to spend money on control measures that will keep them in compliance.  But why spend more than you have to?  .

            What’s the difference between a million dollar and a ten thousand dollar race horse?  Does the former run one hundred times faster than the latter?  Of course not.  The difference is the nose that gets across the finish line first.  In other words, it’s the little things that count.  If you want to save money on dust control, do the little things that will add up to big savings. 

            Here ten money-saving tips to control dust from roads, piles and crushing plants.
No. 1 – Slow Down

            The slower a vehicle travels the less dust it produces.  On most roads, vehicles that travel at 15 mph do not produce appreciable dust emissions. A speed limit of 10 mph is even better. Is a 15 mph practical for haul routes?  If trucks can get stone up to the primary crusher fast enough at 15 mph, it makes sense.  If they can’t, calculate the minimum speed required to the maintain production rate and set speed limits accordingly.  I’ve seen haul trucks race to a primary crusher at 40 mph in a cloud of dust only to sit there for 10 minutes waiting to dump – that’s what you want to avoid.

No. 2 - Enforce Speed Limits

            Posted speed limits won’t reduce dust if they aren’t enforced.  That can be tough if truck drivers aren’t your employees.  Speed bumps can help trucks to observe speed limits but banging tailgates can cause a noise problem.  Rumble strips are more likely to slow trucks down without causing gates to bang.  Posting and rigorously enforcing speed limits is the best way to reduce dust emissions and the frequency of treating unpaved roads.  Plant management has to set the example.  If you blast around the plant in your pick-up, don’t expect anyone else to slow down.

No. 3 - Shorten Routes

            Shorten traffic routes as much as possible.  Fewer vehicle-miles traveled means less dust in the air and less road to water or chemically treat.  Use flags or other markers to delineate traffic areas and install concrete barricades to make sure trucks do not wander off of designated routes. Keep paved roads clean by restricting access to them from unpaved roads. 

No. 4 – Improve Road Structure

            Poor construction, bad drainage and lack of maintenance all aggravate dust emissions and track-out.  Inspect roads to make sure they have a proper crown, a good mix of fines and aggregates and a well-compacted surface.  When surface material has more than 30% silt, dust control measures will work but will not be cost-effective.  Put some chips or other coarse aggregate down to reduce the amount of silt on the surface.  This will reduce the frequency of treatment and the amount of water or chemical used per square yard.

No. 5 - Use Road Dust Suppressants

            A wide variety of chemical additives are used as road dust suppressants.  Just about every sticky, gooey or gummy chemical you can imagine has been used to control road dust.  They can reduce the frequency of treatment to the point where they are less costly than routine watering especially in dry, arid environments where water is hard to come by.  Vendors may be willing to provide a free trial to let you gauge effectiveness of chemicals and determine cost savings.

            If you are currently using chemicals to control road dust, try to use a waste product for this purpose.  For example, waste brine produced from oil and gas wells can be just as effective as more refined and expensive products.  Pulp and paper plants, steel mills, oil refineries and chemical plants may all be sources of waste or off-spec materials that can be used on unpaved roads.

No. 6 – Replace Tankers with Sprinklers

            Install a stationary sprinkler system instead of using a mobile tanker for road dust control.  It can cost in excess of $100,000 annually in capital and operating costs to employ a mobile tanker to water roads in a large quarry.  Stationary sprinkler systems can significantly reduce these costs depending on the distance that requires watering.  Simple computer controls can be used to optimize the frequency and duration of treatment.  Just make sure that the sprinklers are protected from errant trucks and can be drained in cold weather.
No. 7 – Train Loader Operators in Control Measures

            Loaders are the most active vehicles in quarries.  Train operators to take personal responsibility for dust control.  That means not overfilling buckets and spilling stone as they load trucks.  Avoiding sharp turns and rapid accelerations also helps to keep dust down.  Loading out from the lee side of a pile on windy days can also make a big difference.  In most cases, dumping quickly can help to reduce the amount of dust that gets into the air. 

No. 8 - Choke feed crushers

            Good operating practice can not only reduce dust but also improve production.  Choke feeding crushers is one example.  Crushers, particularly horizontal or vertical shaft impactors produce less airborne dust when they are operating at capacity because they are moving less air.  Some plants I’ve seen are a Frankestein mix of mis-matched equipment slapped together with screens or belts to small to keep up with their crushers.  Dust produced from impactors that are operating at less than 50% of capacity is almost impossible to control.

No. 9 - Avoid Interruptions

            Avoiding interruptions in process flow also reduces emissions and keeps production rates up.  Crushers that are “windmilling” an put ten times the dust into the air as they do when they’re loaded.  The most common reason for intermittent operation is the lack of sufficient haulage to sustain primary plant operation.  Stone runs through the plant in a burst of production followed by a burst of dust. 

No. 10 – Enclose Dust Sources

            Enclosing dust sources is one of the most cost-effective ways to reduce emission levels and your reliance on more expensive controls like wet suppression systems or baghouses.  The EPA estimates that partial enclosure of a dust source like a transfer point or hopper on three sides with a cover has a control efficiency of 70%.  Adding a simple rubber dust curtain at discharge points also helps to contain emissions.  Make sure that belt skirting is in good shape and there is enough idler support at load points to prevent dust and spillage.  Workers have their greatest exposure to respirable dust when they have to shovel or bobcat spillage caused by leaking skirts.  Much of this work can be accomplished using scrap steel, old belting and a little elbow grease.
Conclusion – Implement the least expensive control measures first.

            A cost-effective dust control plan designed to comply with air quality standards at the lowest possible cost requires the use of good operating and engineering practices combined with a proper wet suppression (or dry collection) system.  Good operating practice means training operators to take personal responsibility for minimizing dust.  Training them to do so is barely a blip in the budget and can have a big payoff.  Good engineering practices that emphasize the containment of dust sources can often be implemented with in-house labor and material.  Do the least expensive things first.  You’ll find that the savings that result will go a long way in reducing the costs of more expensive controls like wet suppression and dry collection systems.

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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