step by step guide to building a gas fired furnace

Step by step guide to building a gas fired furnace.

I have been using my furnace for nearly 6 years and I have improved the performance to the point where I am now regularly melting the refractory lining each time I fire it up.

It is now time to rebuild the refractory lining and the following text is the step-by-step method that I used, having some past experience on what not to do.

I have so far built two furnaces, all gas fired, and the last is by far the best but it now needs repair / rebuild / improvement.

The first furnace had the refractory lining cast directly into a steel drum, this worked ok, but radiated a lot of heat, and because of this it was dangerous to use, it also took a long time to reach melting temperature.

The second furnace and slightly bigger, had and initial backing insulation using dry sand, this worked well, but gradually collected moisture which increased the thermal mass of the sand to a point where there was steam coming out of little holes and again took a long time to heat, it also took a very long time to cool.

I next replaced the sand with Perlite

Perlite, (also called Pearlstone) is a water-containing volcanic rock of an acidic composition. At an abrupt heating to temperatures of 1000 to 1200C, perlite sand swells to many times its initial volume, producing a highly porous friable material with a fine cellular structure, chemically inert, and biologically stable, heat- resistant, non toxic, odourless, water-insoluble, mould-repellent, offering exceptional insulating and filtering properties, Perlite is a favourite with gardeners as it aerates the soil, the up side of which means that it is available at most garden centres at a moderate cost.

Perlite, because of the way it is expanded has a fraction of the thermal mass of sand, and therefore does not absorb heat, unlike dry sand, it has an appearance similar to polystyrene beads, and when you plunge your hand into the bag, it feels warm, using this insulation helps to retain the heat in the furnace and enables the refractory lining to attain a higher temperature in a shorter time, by not letting the heat radiate or conduct away,

by capping off the perlite with a lightweight refractory concrete, helps to trap the heat within the furnace container, and prevents the perlite from blowing away, it also speeds the heating time. This step proved to be very successful as the time to melt a 20 Kg charge was instantly halved.

The only down side about this insulation is that it has no structural component, and remains looks and moves like polystyrene beads, another insulation very similar to Perlite is vermiculite,

Vermiculite is basically hydrated phlogopite mica, which has the remarkable ability to expand to many times its original volume when heated a property known as exfoliation.

This can be treated in the same way as perlite and will also form a lightweight concrete when mixed with cement. Both form good quality backing insulations when mixed into a lightweight concrete or refractory with other additions but be vigilant not to add too much bonding or it will start to conduct heat. Next I made a power burner with a fan, this together with the increased insulation enabled a satisfactory cast iron melt, unfortunately the operating temperature of the refractory has now been exceeded, and is now melting each time the furnace is fired.

A dust mask should be worm when using these insulations as an irritating dust will fill the air.

The Design.

The furnace is to be designed so that individual pieces can be replaced at a later date without causing excessive financial ruin,

the furnace will have a thin high temperature wall of refractory concrete cast from a commercial grade 1700ºC refractory which will be approximately 60~70mm thick, A base, which will sit on a layer of backing insulation, and will only be 50mm thick, this will be big enough to support the furnace wall, and enable a seal of fire cement to be pushed into the gap between the wall and the bottom. The lid will also be cast from 1700ºC refractory, and will be about 60~70 mm thick. The lid being the only component that will have any form of metal reinforcement and will have a metal band clamped around the rim to prevent fracture by expansion, this band will also to enable a lifting mechanism to be fitted to the lid.

All of these components with exception of the lid will have a homemade backing insulation to support them.

The backing insulation as described on Lionels excellent site

Lionel's Laboratory

will consist of quick setting cement (1.5 parts), silica sand (2 parts), perlite (1.5 parts) and fireclay (2 parts).

I find that quick setting cement is best in this application as it has a higher alumna content, and generally will withstand extended temperatures expected with this type of use, it will mean that every thing must be made ready before hand, as the mix sets surprisingly fast.

Having decided the size cavity you require, the next step is to construct the mould into which the refractory lining is rammed, the last mould I made I used hardboard, rolled around an inner wooden frame to form the shape and a large cardboard bin to form the outside, this was very difficult to make as hardboard will resist bending in tight corners and requires at least four hands!!

This method worked, but it proved to be impossible to remove the inner former with out burning it out, this process over heated the green concrete and formed cracks in the lining, which has proved troublesome ever since.

This time I am using wooden staves cut to form a barrel, these were sawn from cheap grade timber into trapezium shaped pieces which, when fitted together will hopefully form a reasonably circular shape, with open ends.

The staves are held together with rope, flexible cramps, or luggage straps (the ratchet type) I used Long jubilee clips, the type that tighten with a screw, I only used them because I found them in an earlier skip diving trip, an alternative is to use parcel strapping, but you would need the crimping machine here.

This procedure only requires one and one half people to build, (only 3 hands!!!) as the staves generally like to lie in a horizontal fashion, and very briefly in the required vertical! but with the extra hand it is relatively easy

The end pieces are fashioned out of shuttering plywood, I found the easiest way of cutting these are to drill a centre hole and use an anchored router see photo this will enable a reasonable circle to be cut.

These end pieces are pushed into the centre of the staves, which then forces them out helping to form a reasonable circle, taking out the necessity of perfectly cutting each stave to fit. The piece that is left over when the circle is cut, can be used to fit over the top of the mould to hold the outside together.

When the bottom is fitted, the router is again anchored into the centre and a circle is cut to the diameter required to form the inner of the furnace, the pieces left over are cut to fit inside the inner form. The top of the inner form again seems to want to go anywhere but centre so a couple of scrap pieces of ply are cut to form arcs which fit the section where the concrete is being rammed and can be moved around as necessary. It is possible to move the inner former to the required position after some refractory concrete has been rammed into place

An important not here is to ensure that the staves for the inner form must be longer than required to enable them to be held together with the rope / jubilee clip.

The hole that will form the burner quarl (yes this is the correct name) is fashioned out of a short piece of 4 inch plastic soil pipe with the ends cut into the arcs to fit the radiuses between the inner and outer sides, this will enable the flame will run around the furnace bottom, This is a critical part and extreme car must be taken or you will not be able to attain the full melting potential of the furnace.

This method of making the mould has the advantage that very little pressure is placed on the refractory concrete when the mould is removed form the furnace casting, by removing the rope / jubilee clips they can be removed by simply tapping out one the rest will fall out, this has the advantage of allowing the cast refractory to stand and dry slowly, without heating, and hopefully prevent cracking,

As a form of diversionary tactic, the used moulds can be used later, with a little care and artistic flare, as a plant pot on the patio!!!!!!!

(This will hopefully keep you’re wife happy thinking, that you are not spending (wasting) lots of time and money) as well as not being totally selfish.

The mould for the bottom is a simple disk shape, but I ran a few turns of Ceramic / asbestos rope in a spiral fashion around the outside rim to form a sealing cushion, This can be tacked into place with a little water-based glue or wax just to hold it together whilst it is being fitted, the refractory can then be troweld over the top this will then hold the rope into position until the two components are fitted together.

The general method of construction will be to cast a refractory tube with separate top and bottom, disks of refractory cast to form the top lid and base, the joint between the walls of the tube and the base disk can be later filled with a little fire cement.

Mixing the refractory concrete

Unlike ordinary concrete, refractory concrete will rapidly loose its thermal stability as the amount of water in the mix increases as this washes out the finer components. Generally the correct method of testing water content is to take hold of a handful mould into a ball about the size of a Tennis ball, and throw this up into the air, Here the idea is to catch it if you can, if it slaps back down it is Too wet, if it breaks or cracks open then it is too dry but further mixing may correct this, if it stays in a ball, then it is just right.

Ramming the refractory concrete

When the mixing as above has bee achieved you should ram the mix into the mould giving it a good stiff poke With a wooden stick about an inch square to ensure that every little gap is filled and is solid

This photo shows an attempt to wind Asbestos / ceramic rope onto the wet refractory concrete to form a seal, this was a last minute thought and failed dismally!!

It does however show the mould rammed full.

When the refractory components are set, gently remove the moulds and leave to dry for as long as you can possibly bear.

Construction

BE WARNED THESE PARTS WILL BE HEAVY

First place supporting bricks into the base of the oil drum or container and loosely fill with perlite insulation just over the tops of the bricks, Next the base disk is gently lowered the loose perlite insulation, and positioned to the best level central position possible. Next the tube is gently lowered onto the base and so that the coil of ceramic / asbestos rope gives just a little cushion, See "design Photo above"

The burner quarl is now fitted,

This is simply a tube of ceramic refractory moulded to fit the hole in the side of the furnace with a similar hole to suite the burner tube, the purpose of this device is to prevent the steel section of the burner from being over heated by extending it into the furnace

At this point it would be a good idea to let the furnace stand for a week to allow the refractory to dry you can always re-engineer the burner during this time.

this is as good an excuse to prove to her indoors that you really do use all of the tools that you have been collecting and jealously grading, but be careful, this can easily turn into a full time DIY horror.

rebuild mess

The burner should be fitted before the refractory backing poured in to remove the last traces of water from the refractory a short firing would be a good idea, when cool, you then can gently ram the refractory lining around the tube ensuring that the positions are not moved.

Being very careful not to make the mix too wet, as this water will have nowhere to escape and may well pop the refractory when it is turned into super heated steam on later a firing. Whilst the refractory backing is drying the lid, and operating mechanism of your choice can be fitted and adjusted.

When all is dry and a short test firing has been completed the burner attitude needs to be adjusted so that the flame runs correctly around the inside wall of the furnace, the reason for this is that where the gas/air blast is coming into the furnace it will form a cool spot, if it is allowed to cut across the furnace and onto the crucible, this will increase the melt time as this blast will remove heat from the crucible. If necessary a small piece of refractory cement can be used to deflect the gas / air blast into the correct direction. It may be necessary to very slightly moisten the refractory wall where you require adding this deflector in order to gain adequate bonding.

Igniting the Furnace Simple Huh, just toss in a match!! Oh yeah, lets just see it?? Ok, what you have now created by combining a load of gas and air is a contained fuel air explosive!! and the last thing you want to do is to ignite it carelessly, if it cannot vent quick enough through what ever opening there is, it will make one with ease. so lets not blow up a lot of hard work, and a few windows as well.......Besides tossing in a match just wont work, as the blast of fuel/air will blow it out.

There are three sure ways of igniting the flame, with out causing an explosion.

dampen some card with fuel, light, and place into furnace under the crucible and turn on air/gas, yes this works but a lot of smoke will be made.
ignite with a spark, probably the best method, but not the easiest.
The best, and simplest, is to grab a hank steel wool, put a flame to it, or place a battery across it, drop into furnace, and turn on, no smoke instant ignition, and no ash.

I will be posting plans on this page for a constant spark wand purposely designed for this application so watch this space.

A secondry advantage to placing a card under the crucible is that it will leave a layer of ash / carbon between the crucible and the stand, this will help prevent it sticking to the stand. This would be good practice to follow regardless which method of ignition you choose..

My furnace in operation A short movie by Triston My No1 son

The building and use of this design involves high temperatures and the use of a variety of hand and power tools, including electric welders, also many of the items used are very heavy.

(No responsibility whatsoever will be accepted for your stupidity)

If you think that you will have difficulty in building this item THEN DON’T TRY!!!

Home