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Dust Control Mining and Aggregate Plant Dust Control Products

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.

OR

  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.


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