Construction Experience of "White" Upper Chamber Bakeoven Option for Contraflow Heater. 1996

(Supplementary Images 2005)

by Marcus Flynn

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I first became aware of white upper chamber bakeovens about two years ago, but, not having access to the necessary technical information, never offered it as an option to my clients. Also contributing to my apathy towards white ovens was my belief that their only advantage over the standard black ovens was that the bake chamber remained clean. My clients would never inquire about the white oven option and I would even as a general rule try to disuade them from opting for the black oven on the grounds that it was primarily a novelty feature which most clients seemed to abandon using once their initial enthusiasm had worn off. I have nevertheless built several "black" upper chamber ovens.

Last season I received a couple of inquiries from prospective clients who had first been in touch with my principal competitor, who I knew built only white ovens. It seemed that at least in terms of bakeoven technology, some of my clients were better educated than myself.

Eventually the situation arose where a client said " OK, you've got the job as long as you build me a white oven." Knowing that the white oven my competitor used had been developed by Norbert Senf in Shawville, Quebec, I contacted Norbert and asked if he would be willing to offer any information regarding the white oven layout, etc. I was promptly faxed a set of working drawings and together with some over the telephone backup just before starting, built my first heater with the white oven option. My construction experience was as follows:

Figure 1

After building a standard firebox I began corbelling straight off the angle iron lintel over the firebox opening, using the bond indicated in Norbert's plan (Figure 1, Figure 2). The lower end corners of the bricks, corbelling up to form the firebox ceiling, were not cut back at an angle as is standard. This gave a "stepped" rather than a smooth angled firebox ceiling. I was initially concerned about the potential drag that this stepped firebox ceiling might produce. After firing the heater, however, it became evident that any drag or confluence was negligible. The stepped ceiling may even be of benefit in that it promotes mixing of the smoke, flame and secondary air as it rushes up towards the throat. The corbelling was stopped approximately 7 1/2" from the rear wall of the firebox which had been laid straight up (ie., no corbelling off the rear wall).

Figure 2

The next course (Figure 3) formed the portion of the smoke path beneath the oven hearth slab. This course has one slot or opening of approximately 3" at the rear of each side of the core. These slots allow the gases which flow under the oven hearth slab to exit into the side channels. The two bricks forming the front course were cut back, allowing the smoke path to extend as far forward under the hearth slab as possible.

The oven's side walls were then laid up four courses high and a conventional vaulted oven ceiling cast in place. A 1/8" gasket board was used to prevent bonding of the cast vault to the upper surfaces of the bricks forming the side walls.

At this point a slab was cast (separately, on the floor) 3" thick by 27" wide ( width of core) and just slightly higher than the highest point of the extradus at the rear end of the cast oven vault. This slab would form the rear wall of the oven chamber. It also bridges the throat, deflecting approximately 25% of the gases under the hearth slab.

The hearth slab was also cast at this time (also separately on the floor). Nobert had said that precautions against leakage had to be taken with white ovens, so I set out to make the bake chamber as tight and well gasketed as possible. For this reason the hearth slab was cast exactly to the square of the oven's side and rear walls (less 1/8" all round to allow space for gasketing). Here an angle finder was used to account for any possible variation of square between the rear and side walls. As the rear wall slab had just been cast a piece of wood was temporarily inserted in its place for measuring purposes. The molds for the two slabs were neatly lined with clean plastic. The bottom surface of the slabs, i.e. that which was cast against the plastic on the bottom of the molds, would be the surface visible inside the oven chamber. This gave a smooth, shiny, easily cleanable surface texture on the hearth and rear wall.

Figure 3

Figure 4

Once cured, the slab for the oven rear wall was set in place dry (Figure 4), and the line of the extrados of the cast arch marked off against it. After being taken outside, the slab was cut down with a grinder to the line marked. The top rear edge of this slab was ground round to improve smoke flow over the oven vault.

The slab then had a 1/8" role board gasket inserted between it and the rear surface of the side walls and the rear edge of the vault. The rear face of the slab was then gasketed where the side walls of the core would push up against it. The rear wall of the core was then laid up along with the side walls which are at this point only as long as the throat is wide. These were laid up to the capping slab level. The two short portions of the side walls brace the rear slab tightly against the rear faces of the vault and side walls. Bringing the side walls of the core up to capping slab height ensures that the gases flow over the vault before entering the side channels rather than pouring straight over the side of the throat into the channels and bypassing the vault.

For the portion of the front wall of the core above the oven vault, a precast refractory lintel was used rather than bricks, in an attempt to relieve some of the pressure of the core's front wall (the portion above the vault) from the middle of the vault.

The last step was the insert the hearth plate. The side and rear inside surfaces of the the bake chamber were lined with 1/8" role board gasket up to the thickness of the hearth slab (about 2 ½"). The gasket was glued in place with the smooth side outside, to reduce its tendency to tear as the hearth slab was slid in. Before the hearth slab was slid in, about 1/64" was ground off its lead conrners and its edges checked for lumps, etc. The slab was then carefully slid into place, paying attention not to scuff or tear the gasket and thus destroy the seal.

One thing concerning me before starting the job was the extra time it would take. I found though that the extra time needed to build the white oven against the simpler black oven was made up for by the simplicity of the bond for the firebox ceiling corbelling, and the fact that I didn't have to make angle cuts on the ceiling bricks as I had always done.

Besides the inside of the white oven not becoming coated with carbon deposits and fly ash, it does have other considerable advantages over its black counterpart:

  • the oven can be used during combustion.
  • once the gasketed oven door is closed, there are no openings in the bake chamber. The black ovens always have a slight draw through the bake chamber, even when the damper is closed. This results in much of the cherished "baking vapours" being drawn out through the throat in the rear of the bake chamber's vault.
  • no buildup of carbon deposits on the glass inspection panel in the oven door means that the oven contents can be judged "cooked" without having to check by opening the oven door, causing sudden temperature drop in the bake chamber.
  • as secondary combustion does not occur in the bake chamber there is much less stress on the oven's door and the surrounding brickwork.

Figure 5

After an appropriate number of curing fires, the heater was brought up to temperature with two large fires. The hearth and rear wall of the oven became too hot to touch even momentarily without getting burnt. Since it is now the end of April, I will have to wait until the next heating season for a full assesment of the heater's performance.

Figure 6

Potentially worrying observations were:

  • the section of the smoke path beneath the oven's hearth may become blocked with fly ash. A small cleanout door could easily be placed beneath the oven door, giving access to the under-hearth channel - although I relied on the velocity of the gases keeping this clear.
  • it seems that there is a concentration of heat towards the rear of the heater due to the core's smoke path running through the rear, rather than the centre of the mass. Whether this will affect the overall efficiency of the heater or not, I don't know.
  • the oven's bake chamber is about 7" to 8" shorter than the standard black oven.

This season I am enthusiastically offering the white oven as a low cost, practical option! Comments and feedback are welcome.

NOTE : This document is only available in english.

Pyromasse, Montréal, April 1996. (Marcus Flynn)

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