Production Process » Firing the Kiln
This kiln, previously fired with propane, was
converted to burn waste oil in November, 2005 when I modified a pair of residential oil furnace burners by changing the nozzles and installing a heated filter that facilitates in the combustion process while also removing impurities that can clog the nozzles. I have fired with just about every kind of oil, including kerosene, motor oil, transmission fluid and even vegetable oil reclaimed from the food industry.
Waste oil is collected in a 60 gallon air
compressor tank that was converted to a vacuum tank by installing a small 12 volt vacuum pump
on top. The tank is evacuated to about 20" of vacuum, either at home with a 110 volt vacuum pump or on the road with the 12 volt one
shown.
A hose coming out the bottom is then inserted into a barrel of waste oil and a ball valve is opened, allowing the vacuum in the tank to pull the oil into it with quite a bit of force. At home, a small amount of compressed air is then pushed into the tank and the ball valve opened again, this time forcing the oil out the same hose into the barrel shown in the picture below.
Filling the tank takes only a few minutes, and it works in cold weather when oil is thick and nearly impossible to collect with any kind of 12 volt fuel pump.
The 55 gallon barrel on wheels was obtained for free
at a car wash. I inserted a small electric hot water heater element in through one of the top openings and secured it with the length of PVC
pipe shown coming out the top. This allows me to warm the oil up a little to make it easier to filter, which is especially important in cold
weather.
From there, the oil is pumped through a special filter, designed originally for hydraulic return lines, with an aluminum housing and stainless steel filter element which is cleanable, by means of a household water pump. I scavenged the pump from an old water filtration system that was upgraded. These centrifugal pumps are very common and can be found used for reasonable prices, and are especially useful for this application because they are fast and also tend to help emulsify any moisture present in the oil so that runs through the burners gradually instead of setting out and running through all at once. This filtration system is practially automated, requiring very little time to implement. The only problems i have had were in the coldest weather, when temps were in the single digits, and the oil in the pump had solidified causing it to stall when i turned it on - but i solved the problem by applying the heat of a plumber's torch to the pump for 5 - 10 minutes and that got it flowing again.
The small pneumatic pump on top of the barrel is just used for transferring when needed and is not an essential part of the system.
Now the kiln is ready
to burn a far more economical source of fuel than gas and with more BTUs per gallon. Shown on the right is my own variation of a
gravity-feed supply tank system. The blue barrel is the main supply tank and the two black 15 gallon barrels feed directly to the burners.
The siphon nozzles I use on the burners rely on a combination of gravity and compressed air to draw the oil and then atomize it into a fine mist, which ignites as it passes a spark igniter. The lower tank is used to begin the firing as it allows for a slower flow of fuel through the nozzles, and thus a more gradual increase in temperature.
After an hour or so I then switch to the higher tank. Because it is a full three feet above the nozzle, it delivers fuel to the burners with substantially more pressure, allowing for greater flow of fuel through the nozzles and therefore more BTUs.
Ball valves are used to determine which tank the pump sends fuel to. Each tank has a float switch that controls the supply barrel pump, keeping the oil at a constant level. Each float switch is controlled by a separate wall switch to determine which one will be used. The amount of oil pressure to the burners is determined by the distance between the top level of the supply tanks and the burner nozzle.
To the left is a close-up of the float
switch. There are other ways to accomplish the same thing, but I went with this system because I had the barrels and a lot of the pipe
fittings on hand already and found the float switches for a few dollars each.
Commercial waste-oil burners use a pump instead of a gravity-feed system. This would have some advantages with a kiln, but would be more expensive to implement.
The picture on the right shows a modification that
most likely completes the permanent system for supplying fuel to the burners. The blue pump is a 12 volt Holley automotive fuel pump that
runs off a car battery attached to a battery charger. With this pump and a few ball valves, I can now switch from a
gravity-feed system to a pressure-feed system in the middle of the firing for a more rapid rise in temperature.
Pumping the fuel directly to the burners pushes considerably more oil through the nozzles - in fact, much more than the kiln can even burn cleanly. Some of the pressure needs to be bled off via a return line that goes back to the pump inlet, and this part is controlled by another ball valve. I have gradually phased out the pressure-feed system since it really is not needed and this particular pump really is not the best one for the application. Mostly i just use it for the initial start up of the burners in the morning when a little more pressure than just gravity feed is sometimes needed to clear out the line going to the nozzle.
Another major improvement was the addition of an
in-line filter mounted just below each of the burner nozzles. They are just like the one used in the pre-filter setup described above but smaller, made by also Lenz with a part number of "DH-750-100 VITON" and element number "5062-100W/VITON" —
Even though the oil in the supply tank is filtered there is still enough fine debris that can accumulate and clog the nozzles, plus the oil-preheaters can get slowly polymerize some of the oil that passes through them that will break free and get into the nozzles. Even with this system i still have to swap out a nozzle in the middle of a firing once in a while but it is very rare. I keep an extra pair nozzles soaking in kerosene or biodiesel on hand so i can put them together and replace a clogged nozzle quickly if need be.
A thermostatically controlled band heater warms the oil enough to make it pass easily through the filter, an especially important accommodation for winter firings when the oil can be nearly solidified.
Here is a closer look at the
filtering setup, showing the hot water heater element plumbed in. The 55 gallon drum is not quite enough for the entire firing, so when it
gets low I filter another 30 gallons into it, which takes less than five minutes with this system.
The input hose on the pump is braided poly with a foot valve at the end to keep the system primed for each use and to prevent oil from spilling out of the hose. I can't say for sure that this particular pump will last a long time for this purpose — some water pumps do fine with vegetable oil and others don't. I hope it does last because it makes the filtering process very easy!
The main compressed air line that runs
throughout the studio is powered by a heavy-duty, 60 gallon shop compressor which is capable of supplying enough air to run them but it would be pushing it, so instead i use these, which use anywhere from seven to 30 psi, depending on
what stage of the firing they are in.
Instead, a pair of dual-head continuous-duty air compressors, pictured at left, are plumbed into the main compressed air line. These smaller compressors provide most of the air needed for the burners. I can boost the air pressure as needed by drawing from the shop compressor, without needing to run it constantly and risk damaging it.
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