compressed air on burner?

[Fredi Vilina]

has anybody ever made a burner that uses compressed air along with natural gas for a small glory hole? Some flameworking setups have the option to add compressed air. I thought I read something here once but can't find anything on it.

[Hugh Jenkins]

If you have a venturi, you can put compressed air in through what is usually considered the gas input. Using a good regulator, you will have infinite range for the air setting. Then run the natural gas into a T as usual and you can control both gas and air very well. The low pressure of natural gas is what limits the power of venturis with that fuel source. Think of the venturi with compressed air as a very adjustable blower.

[Pete VanderLaan]

I am trying to visualize this. I can see the air going in where the orifice normally is for the gas input. Where specifically is the "T" for the gas and where does it attach? Or are you suggesting that the air and gas are premixed and then attached thru the opening normally for the orifice?

[Fredi Vilina]

could you do a t connecting to the orifice stub but also adding a check valve on the gas line as it comes into the 90deg side and the air going straight through pulling the gas from the t but still mixing and pulling in air through the venturi as it would for propane? maybe a check valve on the air side also?

[Wallace Venable]

Originally posted by Pete VanderLaan
..... I can see the air going in where the orifice normally is for the gas input. Where specifically is the "T" for the gas and where does it attach? ...

In an automobile carburetor (you remember them, Pete) the fuel jets are at the throat, right?

One arrangement is to put your gas inlet T at the narrowest point of the venturi. According to someone's aerodynamic theory (a very well proven one) velocity and pressure are inversely related, so the restriction is where velocity is highest and pressure is below ambient.

Other points could be used with higher pressure gas, but I think that is the preferred point. You can use a tube L brazed into the wall which injects gas parallel to the flow as well. (Some carburetors do that, too.)

[Hugh Jenkins]

If you were using a blower, where would you plumb in the gas? At a T down stream from the blower, and before going into the burner. Now, remove the blower, put a venturi in its place and use compressed air to drive the air stream. The gas comes in exactly the same.

If you have low pressure gas, it will not push through a check valve into a higher pressure air line. I would not try that anyway. The arrangement above allows for any pressure of gas appropriately metered to be used with accurately adjusted high pressure air to run a burner. It adds the advantage of having an air tank as a buffer against power outages. The larger the tank the more time you have.