Forced Air and Pressure


You don't need very high pressure because you don't need much force to get it to the other end of the burner. You *do* need a lot of volume, enough to keep the fire burning.

(And don't forget that it can take 10HP to burn 20CFM up to 100PSI, whereas 1/2HP can churn out well over 500CFM at maybe 1/4PSI on a good day.)

A muffin fan isn't going to net you too much burn. My first furnace (burning charcoal and wood) had a similar 6" fan on it (pulled from a microwave ), and it just couldn't get stoked. Enough to melt aluminum but nothing else.

Static pressure = pressure at standing conditions... this could mean the average pressure coming out of a fan, for instance. CFM = Cubic Feet per Minute, a measure of volumes per time. Generally speaking, this is directly proportional to how much fuel you can burn, and thus what temperature you can reach. Inches of water is a measure of pressure. You connect a bit of clear tubing with some water in it, hook one end to the pressurized area to be measured (the other end stays open to air) and measure the distance the water is pushed by the pressure. Same goes for inches of mercury, or any other fluid for that matter. (Note that heavy fluids like mercury weigh heavily: consider that atmospheric pressure is often quoted at 32" Hg, while it's more like 33 feet of water!)

You usually want a centrifugal fan (so called because the spinning impeller throws air out radially into the housing which then directs it out a tube) because it's easy to get an adequate amount of pressure and CFM. Considering burners often have restrictions as small as 1" or less, you'll need at least a little pressure. If it's straight open like my trashcan furnace (3" tuyere!), you won't need as much *TECHNICALLY* -- but the charcoal restricts it a bit, so you still need a little.

If you can find a muffin fan that works (within design limits!! ) at like 5 or 10,000RPM, it should have a goodly bit of pressure and CFM going. I doubt a computer fan will deliver though.

--Tim tmoranwms@charter.net


Air pressure is the force of the air, volume is the mass of the air. Look at it this way you can have a very tiny hole shooting a very tiny amount (volume) of air out with tremendous force (pressure), say 100 pound per square inch, say out of the end of air hose. You could try to put your finger over the hole but because of the force you can not stop the air from coming out due to the pressure. Even though the air is coming out with great force there is not a lot of volume. For example you where trying to inflate a large balloon that was ten feet square 10 x 10 x 10 = 1000 cubic feet of volume. I would take 1000 CFM of air flow (volume) to fill the balloon in one minute. Even though it has a lot of pressure your hose would take hours to fill the balloon due to the volume coming out of the tiny hole at the end of your air hose. Now try some sort of blower, say a hair dryer. place your hand over the end and try and stop the air, you can, because it has very low pressure (force). But it has High volume, and could inflate your balloon in only minutes.

Air pressure is the force the air has at a give moment in time. Air volume is the amount of air something can hold. "CFM" cubic feet per minute is the amount of air, volume, that passes a point over a period of time. If you are sitting on the sofa at sea level you are sitting in air that is at about 15 PSI, (pounds per square inch) if the room you are sitting in is 10 feet square with a seven foot ceiling (10*10*7=700) you are sitting in a volume of 700 CFM. If you had a fan that can move 700 "CFM" it could replace all the air in the room in one minute.

If you had a an unpressurized air tank that was one foot square it would contain one cubic feet of air at normal atmospheric pressure (15psi) if you compressed it to 15psi above atmospheric pressure it would contain two cubit feet of air at atmospheric pressure. If you raised it to 30psi above it would have 3 cubit feet of air or 3 atmospheres. you see what is happening here, by compressing the air you are taking a volume of air, in this case 3 cubic feet of air, and packing it in the space of only one cubic foot thus increasing its pressure. So your tank would contain 3 cubic feet at 30 pounds per square inch. If you released this air at a rate of one "CFM" it would take two minute to empty. Too much??? too simple ????? Enough ????

--Waygat

You need air to burn something (volume).
You need to constantly replace the air you just burned with the fuel in order to burn the new fuel (volume flow rate, cfm).

--Don krag@kragaxe.com

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