The Myfordboy Waste Oil Burning Furnace
Introduction
My propane fired furnace has been used for many years and still works well but I wanted to see if I could make an easily operated and clean to use oil burner. The advantage of this would be that I can get a free supply of used motor oil to burn and also the higher temperatures available would make melting iron possible.
I decided that I would only switch to an oil burner only if the following criteria could be met:
- It had to be clean to use. I did not want dirty used oil dripping over my garage floor or clouds of smoke filling the air.
- It had to be easy to light and control. Some designs use propane, oily rag or wood to pre heat the furnace before switching to oil. I wanted to be able to eliminate the pre heat and just light the oil.
Research showed there are two popular ways of making an oil burner, drip feed and siphon feed.
A drip feed burner is the simplest and basically consists of a steel tube with air blown into the end and a smaller tube positioned inside which drips oil into the airstream. As the oil hits the hot sides of the furnace it vapourises and ignites. The oil flow is controlled by a needle valve and the oil is gravity fed or can be pressurised. The blown air supplies the combustion air. The furnace needs pre heating before the oil is turned on.
This method met neither of the above criteria so my choice was the siphon burner.
The siphon nozzle burner needs a compressed air supply as well as a blower for combustion air. It seemed a lot of equipment to use but I already had a small compressor to use.
Whilst it is quite possible to make a nozzle I decided to go for a ready made one used by other oil burning home foundries. The nozzle used is specifically designed for waste oil burners and produces a very fine mist of oil which can be ignited, no need for it to be hot inside the furnace on start up.
This method met neither of the above criteria so my choice was the siphon burner.
The siphon nozzle burner needs a compressed air supply as well as a blower for combustion air. It seemed a lot of equipment to use but I already had a small compressor to use.
Whilst it is quite possible to make a nozzle I decided to go for a ready made one used by other oil burning home foundries. The nozzle used is specifically designed for waste oil burners and produces a very fine mist of oil which can be ignited, no need for it to be hot inside the furnace on start up.
Construction
Below is a video showing how the base was made.
The Base
The completed furnace would be too heavy to lift so a strong wheeled base was required.
I used some material that already had in stock. This included some 2mm steel sheet and some 30 x 60 box section. The parts were assembled with Pop rivets but they could have be welded. I later replaced some of the rivets on the top with countersunk screws and nuts to make a flat surface for the furnace bottom.The dimensions are shown in the drawing below.
Below is a video showing how the base was made.
There are 2 legs at the rear that sit the ground. The wheels only make contact with the ground when the furnace is tipped, using a steel tube inserted in the front leg. It can then be wheeled around. Later I added another bracket in the centre of the base so all the weight is not on the front leg alone. The front bracket extends above the top of the base and is used for securing the furnace.
Furnace Design
Many furnace designs start with finding a suitable metal container for the outer casing but my approach was a little different.
The refractory was cast into shuttering formed from MDF and lined with thin transparent acrylic sheet. The transparent sheet allowed the rammed refractory to be inspected as it was added so voids could be eliminated.The cured refractory was then wrapped in 2" of ceramic blanket for insulation and sheet steel was wrapped around the outside to for the casing.
This construction allows the furnace to be made to the size I wanted and is modular construction. The size chosen is suitable for an A10 crucible. If the refractory should ever need replacing all the other parts of the furnace can be reused.
I decided not to add a drain hole to the furnace. On my GAS FIRED FURNACE I added a 3/4" drain hole in the bottom so any spillage would drain out. I was using stainless steel crucibles back then and they do spring leaks after a few melts.The fist time I had a leak the metal froze in the drain hole and I was never able to get it out. If you want to add a drain hole it needs to be larger than what I used.
Furnace Design
Many furnace designs start with finding a suitable metal container for the outer casing but my approach was a little different.
The refractory was cast into shuttering formed from MDF and lined with thin transparent acrylic sheet. The transparent sheet allowed the rammed refractory to be inspected as it was added so voids could be eliminated.The cured refractory was then wrapped in 2" of ceramic blanket for insulation and sheet steel was wrapped around the outside to for the casing.
This construction allows the furnace to be made to the size I wanted and is modular construction. The size chosen is suitable for an A10 crucible. If the refractory should ever need replacing all the other parts of the furnace can be reused.
I decided not to add a drain hole to the furnace. On my GAS FIRED FURNACE I added a 3/4" drain hole in the bottom so any spillage would drain out. I was using stainless steel crucibles back then and they do spring leaks after a few melts.The fist time I had a leak the metal froze in the drain hole and I was never able to get it out. If you want to add a drain hole it needs to be larger than what I used.
Now I am using graphite crucibles there is less chance of a leak, Sometimes I might spill a bit over the side of the crucible but when the furnace cools it can just be lifted of the furnace floor as the floor is smooth.
Exploded view of furnace construction
Green - Outer casing
Grey - Refractory walls
Blue - Refractory base
Orange - Ceramic fibre blanket
Yellow - Compressed ceramic fibre board
Grey - Refractory walls
Blue - Refractory base
Orange - Ceramic fibre blanket
Yellow - Compressed ceramic fibre board
Making the shuttering and casting the refractory base was cast can be seen in the video below.
The 3D image at the beginning of the video shows a ring of compressed ceramic fibre to centre the furnace on the base. I omitted this on the final design as the weight of the refractory is sufficient to keep it in place.
The castable refractory is rated at 1700°C and was bought from artisanfoundry.co.uk
Here is the LINK to the product I used. 2 x 25 Kg bags are required and there is just enough left over to form a couple of crucible stands.
Click on the plan above to see a larger image or if you would like a free PDF send me an email.
A ring of steel was rolled for the cover and the refractory cast directly into in. A strip of steel was riveted to the inside to form a ridge to hold the refractory in place. two layers of ceramic blanket were used to insulate the cover.
The refractory was left to dry for a several weeks and then a 100w light bulb was suspended inside for a further few days.
The refractory was now a white colour and I heated it up slowly with my gas burner, before fitting it to the base. This would allow any further moisture to escape.
The video below shows the refractory being cast. A vibrating tool was made from an orbital sander. As the refractory was rammed into the forms the inside was filled with sand to prevent the tube collapsing. This is important and was not made clear in the video.
The cover was made as shown the video below.
A disc of compressed ceramic fibre board was placed on the base and the refractory casting placed on top. 2 layers of 25mm ceramic fibre were wrapped around the refractory and the outer casing rolled from 1mm steel sheet. This is held in place with 2 steel straps.
The ceramic fibre board and blanket were again supplied by artisanfoundry.co.uk
The bottom strap is drilled an tapped at the front and is fixed to the upward protruding front leg of the base and is also attached to the base at the rear with a threaded eye fitting.
The top strap has a latch and stop added for the cover.
A steel strip was added at the rear of the furnace which connects the top and bottom straps together and it is also attached to the base so that the weight of the open cover is transferred to the base.
The cover lifting mechanism works well. Some home furnace builders use a foot pedal to lift the lid but it still needs to be swung out of the way by hand which make it a 2 two hand operation, well, one hand and one foot! My design is easily operated with one hand.
It is important that the pivot point for the cover is level with the furnace top for correct operation.
A thin steel tube was rolled for the tuyere. The tuyere is the correct name for the pipe through which air is blown into a furnace. It needs to be a close fit to the for the burner tube to give a reasonable seal. I wrapped some thin card around the burner tube and made the inside diameter of my tuyere a close fit to this. I silver soldered a rolled strip to the end to strengthen the tube and to improve the appearance.
A curved plate was silver soldered on to it to attach it to the furnace outer casing. The tube passes though the ceramic insulation and stops just short of the refractory. Pop rivets were used to attach the tuyere assembly.
