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How much air does a forge require?


Glenn
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Sometimes the answer to a question is found in the old reference books. The answer is also related to the forges and work being done at that period of time (before 1945), by blacksmiths that WORKED 10 hours a day, and earned their living by what and how much they produced.




12th ed Machinery's Handbook 1945

Air persssures and pipe sizes for forges

Blacksmith's forges require air pressures varying from 1-1/2 to 6 ounces per square inch. Small forges with the blower close to them are adequately supplied with 1-1/2 ounce pressure. If the blower is some distance away and a long discharge pipe with many bends leads to the forge, even through the latter be small, it may be necessary to carry 3 ounces pressure or more, to overcome the friction of the air ducts. Large forges usually require from 3 to 6 ounces pressure. The table "air pressures and pipe sizes for forges" gives the diameters of discharge mains for various tuyere sizes and numbers of forges.

For a one forge set up the numbers are diameter forge tuyere in inches vs diameter discharge main at the blower in inches.

3/4 forge tuyere......... 1-1/2 diameter discharge main at the blower in inches
1 forge tuyere............ 1-1/2 diameter discharge main at the blower in inches
1-1/4 forge tuyere...... 2 diameter discharge main at the blower in inches
1-1/2 forge tuyere...... 2 diameter discharge main at the blower in inches
1-3/4 forge tuyere...... 2-1/2 diameter discharge main at the blower in inches
2 forge tuyere............ 3 diameter discharge main at the blower in inches
2-1/4 forge tuyere ......3 diameter discharge main at the blower in inches
2-1/2 forge tuyere...... 3-1/2 diameter discharge main at the blower in inches
2-3/4 forge tuyere...... 4 diameter discharge main at the blower in inches
3 forge tuyere ........... 4 diameter discharge main at the blower in inches
3-1/2 forge tuyere..... 4-1/2 diameter discharge main at the blower in inches
4 forge tuyere ........... 6 diameter discharge main at the blower in inches

The book goes on to give forge tuyere sizes and diameter discharge main at the blower for up to 10 forges connected to the same air supply.


I saw nothing related to CFM of air required for the forge, only ounces of pressure.

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The discussion so far has NOT covered the size or configuration of the forge and how much coal is actually been dumped into the forge and onto the table.

I found the following in reference in some of my notes. The source was not listed.



For machine blacksmithing forges should be from 36 to 42 inches in diameter and from 26 to 30 inches high, the top of the tuyere being from 4-1/2 to 7 inches lower than the top of the forge. As there are no standards or definite data for guideance in determining the size of the opening for tuyeres or the depth at which they should be placed below the level of the hearth. The table below gives what the writer considers to be proper dimensions for work varying from i inches to 10 inches in diameter, when the blast is delivered at a pressure of 8 ounces per square inch or over. Work over 10 inches in diameter can be more uniformly and economically heated in a furnace.

Table of sizes and arrangement of the tuyeres.

Size of the opening in tuyere..................................3/4 inch................1 inch
Distance between top of tuyere and top of forge ...4 inch .................5 inch
Size of the supply pipe...........................................1-3/4 inch.............2 inch
Size of the work to be done.....................................1/4 to 1 inch........1 to 2 inches


The forge is 3 to 3-1/2 FEET (a meter) in diameter. The top of the tuyere is lower than the top of the forge by 4-5 inches with additional coal on top of that. The air blast is 8 ounces per square inch or greater.

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I was always taught that the best way to test for the correct amount of air into a forge was to drop a ping pong ball just above the tuyere and it should just hover above it about 100 mm or 4 inches. Too much air and it flies off and too little and it drops down. This test is done with forge emptied of any fuel. Coke in my case.
I have a fan blower which is controlled with a speed controller which I turn back between each heat to conserve fuel and crank up higher when extra heat is required. The ping pong test really only enables you to get the optimum lighting discharge of air. The rest will come with experience and the task at hand.

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