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.
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
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
8. Keep all drain valves open after the system has been
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.
can be equipped with the following accessories to purge and protect hose lines
in cold weather:
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.
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.
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.
only two ways to operate a spray system continuously in freezing temperatures:
- The system must
be housed in a heated and insulated enclosure and output lines and spray
bars are wrapped with heat tape and insulated.
- 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,
Taping and Insulation
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.
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.
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:
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:
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.
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.
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.
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
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.