Over the past three years, I had a couple gas forges made with heavy refractory bricks, with the easily expectable bad results (no welding temps, tons of lost heat soaked into the bricks). This year I've decided to rebuild it entirely, using some proper castable refractory over kaowool, plus a ribbon burner.
Building the forge itself
The forge's enclosure was made out of a 10kg gas bottle I had lying around, which already had a long life in its past; it has been turned into a wood stove first, then into a small (too small!) barbecue, and finally a now gas forge. The bottle has been divided into four pieces, a bottom half with two "shelves" attached for easier stock placement, the top part and the two doors. I've insulated all the main parts with at least 1" kaowool (2" on the top and floor sides), rigidized them with some colloidal silica and then poured in the same castable refractory I used for the burner. The refractory I've used is a medium density, 1370°C (2500F) castable. It has proven to be a little bit low on the temperature range, but for the work I do it's been fine yet.
after casting all the pieces, and letting them cure naturally for a couple days covered in plastic, I've placed them all inside my pizza oven with a small linear burner to first dry off most of the moisture, then cook everything up to 200°C (400F) over a couple days. Oven's floor is square, and about 90x90 cm in size (3x3 ft) so I was able to fit everything inside.
Fuel choice, blower and burner configuration.
Many of my blacksmith friends usually run on propane, which is, when bought bottled, about 5-7 more expensive per unit of heat compared to my house supply's of natural gas. I've decided to go down the NG path, using a blower designed for condensing boilers that is able to pump pre-mixed air/gas fuel. Meaning, fuel is put into the air stream prior to the blower, eliminating any problem of backpressure from the blower towards the house's line (which is delivered at 20mBar, 1/4 psi).
This blower is made by EBM Papst, and is model RG128-3612/230V. It's a 120W brushless, up to 150m3/h (88cfm), 2550 PA (10.2" WC) design, able to be speed controlled via a simple PWM input. Literature is easily available, I have been able to create a simple circuit without hassle to control it through the output of a NE555 oscillator.
Due to the fuel being added prior to the blower, I've wired the fan's pwm output to enable a solenoid valve on the gas line, meaning if the fan doesn't move air no fuel will be allowed into the input manifold, avoiding any possibility of a fuel leak in case of a power loss or if the blower gets blocked by anything.
Running NG + forced air, I've decided in the beginning I wanted a ribbon burner for this forge. I've made out (see pics above) with a large plenum (80x80x220mm, about 2,5x2,5x9") and casted it out of the same refractory I used for the forge. It had 29 10mm ports, later reduced to 23 to prevent blowback at low output levels. air/fuel intake is 1 1/2 ". Fuel intake is 18x1mm copper (OD, about 5/8" US pipe), connected with a flexible hose to the house's gas line which is 22x1mm (~3/4" inside).
After assembling everything and placing it on wheeled stand, this was the result:
The forge is extremely quiet, and heats up quicky to an easily workable temperature.
First piece of work i've got out of it, a small pair of tongs drawn out of 20mm round steel. (anvil is a Fervi 50kg / 110Lbs casted steel)
I've recently got some borax and a few bits of a car's back spring (probably 1070), so I'm going to try bringing up to welding temps soon and do a few welds, perhaps a small san mai knife.
What are your ideas about the build? improvements I could do?
