01-02-2014, 03:20 PM
(01-02-2014, 02:22 PM)DaveH Wrote: Terry,
I have question
What makes it "fogless"?
DaveH
Here's my theory. Traditional misters use a venturi to draw in the coolant. The venturi has a high velocity section where the pressure is reduced which draws the coolant from an atmospheric tank. The coolant is admitted through a tiny orifice at the high velocity zone of the venturi. The air velocity is very high compared to the fluid velocity and tiny fluid "particles" or drops are sheared off as they exit the orifice. This is pretty much the same design as a small engine carburetor. It does a great job of "atomizing" the liquid. Some misters (such as the Mystic brand) have what is similar to a perfume atomizer right at the final nozzle. In any case, the droplets are micron to sub-micron size and they easily float around in the air creating a fog.
The fog-less designs all use a pressurized coolant tank to feed the coolant. Not a lot of pressure, 10 psi or so. The coolant is metered through a needle valve and then mixed with the air in a (relatively) low velocity section of the mixer so that the droplet sizes formed are much larger and are not capable of floating around in the shop atmosphere after leaving the nozzle.
I find that the mixer I built actually tends to "sputter" as it alternately blows air and then burps a small drop of coolant. I have described this sound to be similar to bacon frying (but you have to listen carefully to hear it). The coolant drops leaving the nozzle tend to hit the work piece and then "stick and flow" on the work piece. I am guessing the same thing is happening on the rotating cutter. The cutter will occasionally throw off some coolant, but I have been able to tune the flow rate so that most of the coolant hitting the cutter evaporates. Evaporating water consumes about 1000 btu per lb, and much of this heat is drawn from the cutter and the work piece which keeps them cool, and often below room temp based on my experience to date.
It's not a new idea. You can find designs on various forums that go back 10 years or more.
Apparently, back in the 1980's there were a lot of heath issues associated with the "fogging" version of misters. This was also the time when a lot of somewhat nasty chemicals were still used in coolant and cutting fluids. The "fog-less" versions were developed to get around the health issues. Coolant manufacturers like Cool Mist claim that their products are now safe and "OSHA Approved" for use in fogging misters. Still the fog floating around the machine slowly coats everything in a thin coating of slime. The fog-less version only slimes up the parts in the direct blast of the nozzle. You only have to clean up the work piece, the vice and the tool. The slime is the residual oil and anti-corrosion materials in the coolant. I find it wipes off easily with a shop towel.
I have not tried the fog-less mister yet on aluminum, but others have told me that Cool Mist doesn't work well on aluminum. I have been advised to load the mister tank with WD-40 when running aluminum. I'll try it next time I do some aluminum.
Terry S.