Monday, September 20, 2021

Replacements and Improvements - Smelter Air systems

The September iron smelt proved problematic on several levels.


 The first contributing problem was the set up of the furnace, with the tuyere point set right above the extraction arch. This arrangement was dictated by the physical layout of the Hals sod cone build. Our normal build is to place the tuyere point at 90 degrees to the extraction arch. This places the air system well out of the way of the working area during extraction.

Air System in place at smelt start.

To provide clearance for the expected slag control steps, the pipe fittings used to conduct air to the tuyere were hung via chain off a metal bar, supported by a pair of uprights placed well to the sides of the gap in the sod structure. In addition to this long used element, a new section of plastic pipe had been added. This was for the introduction of a new air volume meter acquired by team second Neil Peterson.


The second thing was the level of damage after the first smelt in June. With a bottom extraction, especially given the thinner than our standard walls (at 4.5 cm), certainly a certain amount of breakage was expected. 

The bloom created was also much larger than expected, at almost 9 kg. As the mass was fairly spongy, that weight does include more slag than has been typical, but this actually increased the raw size of the mass that was pulled clear.

Then the 8 inch snapping turtle crawled in overnight, and broke away about a third of the front wall surface. 

The culprit, red line shows were piece seen to right was broken away

The Icelandic clay mix was expected to suffer more heat effects than our standard use of high fire EPK clay, and this certainly proved the case. There had been considerable erosion around the normal hot spot around the tuyere point. The original wall thickness reduced to as little as 1 cm at the location directly above the tuyere. (This can been seen in the lower wall section broken free by the turtle, laying to the bottom right in the image above.)

Furnace before start of repairs in August

In preparation for the Phase 3-B smelt, The lower section of the front of the furnace had to be significantly re-built. The extraction arch was framed up using the same side blocks and lintel piece as before. The area above this was built up of new clay mix. Another layer of clay was plastered over the eroded areas on the inside. 

The end result of all this was that there was a failure along the line between the fresh clay and the original wall surface (not entirely unexpected). Although certainly this did not prevent the full smelting cycle to proceed effectively, it did mean there was a major weakness along the edges seen above. 

The third thing was worker experience. It was decided (number of contributing factors) to let Rey Cogswell undertake the extraction. Although Rey had certainly observed the process several times, and assisted directly in the process at the June smelt, this would be Rey's first full attempt at an extraction. Understandably, Rey was tentative when working against the extreme temperatures and still uncertain about exactly how to manipulate the slag bowl to break the bloom free. Taken altogether, the work was not undertaken quickly enough, and the entire slag bowl and bloom complex had started to cool down. To be fair, the bloom was again high yield and spongy, meaning another excessively large mass.

Damage to the front of the furnace, right after extraction

This combination would result in breaking away the entire front bottom half of the furnace, pulling the tuyere and air system totally free, but still hanging from the supporting bar. There would also be a large fan of hot slag, furnace wall fragments and still burning charcoal pulled out of the furnace and into the slot in the sod structure. In the (normal!) haste to get the bloom mass over to the stump for the initial compaction hammering, this left the air system connections exposed to the concentrated heat from this debris.

Which is a long explanation of why it proved necessary to replace a number individual pieces of the long standing air system.

The fittings have long been 1 1/4 inch ID threaded pipe. This has allowed for simple modifications to the basic T format, with various different fittings available for the downstream end, to mate with the different tuyeres that have been used. Opposite this is a fitting that can quickly be unscrewed to allow for probing down the inside of the tuerye to clear any blocking slag. The original view port was a simple plexi disk, sealed on to the end of this fitting. (One of the casualties was this plexi, admittedly getting pretty scratched up from 15 + years of use.)

Tuyere to Port at bottom, air flow / pressure at left, pipe connects at right

Larger viewing glass towards air pipe diameter.

I had picked up a number of thick glass disks (from projector units) a number of years ago, each a bit over 2 1/2 inches in diameter. With a lot of picking and matching, I was able to get component pieces of plumbing fittings at my local hardware that let me mate up the glass disk to the air supply T. The result does include one rubber section and one plastic. In use to clear blockages with the customary 3/8 diameter rod, the whole end unit would screw free from the short double threaded section seen directly attached to the T connector above.

I also made up replacements for mounting either the straight section required for the probe of the new air flow gauge, or to include the nipple for the (older) air pressure meter. Either these, or the short double threaded pipe would be at right angles to to the tuyere / port combination. I was recently able to get a roughly 6 foot section of flexible solid metal piping (normally used to duct auto exhaust at repair shops) - Thanks to my old friend Lloyd Johnston. This pipe is about 3 inches ID, so the additional element allows a flexible coupling to a plastic piece that will fit nicely down the interior of that metal pipe. (One serious advantage, beyond durability, is that this metal pipe has a relatively smooth interior, and does not create the loud whistling noise from the corrugated plastic tubes in use up to recently!)

Blower side elements : sliding blast gate / Y to attach a second air supply.

 One of the major experimental elements to undertake is getting much more accurate data on both air volumes and pressures, not only for our standard high capacity electric blower, but also for the various human powered bellows used in the past. As detailed in an earlier posting : Mind the Blast the measurements recorded up to now should be at best considered approximate. The new flow gauge is capable of both measuring, and recording (via computer) extremely detailed data, over the progress of an entire smelt, minute by minute.

It would be quite valuable to our understanding of the dynamics with various human powered systems to also be able to generate comparison numbers. The main problem with this is not in instrumentation, but in the raw labour required to work an entire smelt (four to six hours!) using bellows. To that problem, I made up an additional element, which can be screwed in place downstream of the normal sliding plate air control. This has a one way (sump pump back flow) valve. Although not tested yet, the hope is that with this element in place, the bellows can be utilized for short periods, by using the blast gate to seal output from the electric blower. The one way valve should effectively seal air from the blower escaping through the bellows side when the blower is powering the air supply. 


My hope is to undertake at least one of these bellows comparison and recording tests for the next upcoming smelt...

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

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