Friday, May 24, 2013

Pushing (Hot!) Air...

 " My main question is what is the optimal air pressure and/or Volume. I realize too much air and you could have cast iron and erode the inside of the smelter, and not enough air, no Iron. "

Sauder & Williams have described their experience in a number of articles : 'A Practical Treatise on Bloomery Iron Smelting'
There are a couple of versions Lee wrote, ranging from a shortened verson for the Anvil's Ring, to a formal academic paper. Check Lee's web site (

They found - and my own work and that of the stronger voices here certainly supports - the use of higher volumes of air. The short value is to use 1.2 - 1.5 litre of air per minute for every square centimetre of furnace interior cross section at tuyere level.
Most of us are using furnaces in the 25 - 30 cm diameter range. That 'magic number' works out to something around 800 lpm. I have a table up on my web site : http://www.warehamfo.../flowrates.html

I have done some in line pressure readings. Forgive the ruggedness of the instruments (pretty much cobbled together!)
The measurements I have taken show air pressures in the range of 3 - 5 psi with my own successful smelts. This may be more a curiosity than a useful indicator.
Does anyone else even measure that? How?
Think of the difference in hand feel between a blacksmith's blower and something like a vacumm cleaner - or a hand powered bellows...

Remember that its always better to have TOO MUCH air - rather than not enough!
You can (and we all do) reduce the operating temperature of the working furnace by adding increasing ore quantities in with your charcoal additions.

In terms of easy practical measures, again most of us are using a standard 'bucket' for our additions of charcoal. Most of us in North America are using a standard 'galvanized pail', which holds roughly 2 kg of charcoal.  Timing how long it takes to add this consistent measure gives a very useful measure of 'burn rate'.
Again for furnaces in the size range indicated, we have found the ideal burn rate is roughly 8 - 10 minutes per bucket / 2 kg.
Europeans usually will record 'kg per hour'. Using the Sauder & Williams method, that would mean roughly 15 - 12 kg per hour.
(You will see that typical European methods use only half to a third that amount. The results are predictably smaller yields and much lacier blooms!)

Of course, the specifics of your furnace design, type of ore, even charcoal type, can all alter this advice. There is a quite complex interplay between a very large numbers of individual elements for a truly successful smelt. Those with experience will certainly agree there is 'More Art than Science' at work here!

(Preparing to teach this weekend's 'Introduction to Iron Smelting' course at Wareham...)
    Industrial blower with in line pressure and wind speed (volume) gages

1 comment:

David Robertson said...

As to your question of how to measure pressure in the air delivery line. the easiest low tech way is with inches of water. Simply use a clear hose filled with water plugged into a port on the side of the delivery tube. bend in an "U" shape so the water doesn't flow in to the delivery tube or out the end of the hose. Make a mark on the hose with no air pressure. Mark on the vertical side of the U with pressure turned on.

Accurately measure the vertical distance between the two marks and multiply by .0361 to get psi. A low tech way to measure pressure.

I think with the smelting that volume is much more important although pressure would give an indication of how deep the air penetrates into the chamber.


February 15 - May 15, 2012 : Supported by a Crafts Projects - Creation and Development Grant

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