This chapter describes the construction of the furnace heating chamber, including the insulating firebrick hot face, heating element grooves, furnace floor, ceramic fiber insulation, and initial testing before final installation.
\section{Overview of the Heating Chamber}
Dave Gingery made the heating chamber of Lil Bertha from castable refractory cement and he molded the element groove into the sides of the chamber with rubber hose. The refractory was held in place with a temporary internal form and the metal sides formed by galvanized metal. \index{Gingery, David} \index{Gingery, David!Lil Bertha} \index{refractory, castable}
He made the furnace in three parts - the base standing on short pipe legs, the body, and the lid. He attached his range control to the side of the body in a small box. There were advantages to this set up: you could stack multiple bodies for a taller heating chamber, you could remove the base and use the furnace for melting out wax.
Dan Hartman quickly learned how difficult it was to form the groove, to remove the temporary form from the interior, and how unreliable a range control would be for temperature control. \index{Hartman, Dan}
Dan improved on Gingery's design by using commercially available HVAC duct for the shell of the base, the body, and the lid. Dan overcame the problem of the molded element groove crumbling and being difficult to form by cutting grooves into insulating firebrick arranged in a hexagon around the heating chamber. He made his own homemade refractory material out of furnace cement mixed with perlite. Dan designed and built his own electronic controller for heater control.
On Version 1.0 of my shop furnace, I incorporated Dan's idea of using firebrick for the hot face, but I also realized that I needed to keep the furnace mass low to reduce heat transmission. I used a commercially available steel drum for the body shell and I placed it on a drum dolly to make it portable in the shop. \index{steel drum} \index{drum dolly}
The heating chamber has a groove cut into the side and the heating element coil is installed in the groove. In the Gingery design the heating element spirals around the heating chamber twice. In Dan's design, the element circles around the heating chamber six times, but it does not follow a spiral: he cut ramps in the terminal brick that guides the element from level to the next. At the top and bottom of the terminal brick there are holes drilled for the heating element leads to pass through. \index{terminal brick}
On Version 1.0 of my shop furnace, the heating element groove follows a continuous incline on the faces of the eight firebricks. There is a terminal brick and it has holes at the top, bottom, and midlevel. The top and bottom holes connect the heater to the power supply. The middle hole is used to connect the two coil segments to each other in series.
\section{Materials Used}
I do not use castable refractory at all in my furnace design. I use Vulcan refractory furnace cement to bond the firebricks together and seal the joints. The vertical firebricks are bonded to each other at the edges and they are clamped together with encircling worm drive band clamps. \index{refactory cement} \index{band clamps}
The vertical heating chamber walls are not bonded to the floor. The vertical part of the chamber floats on the floor so that it can be disassembled for element replacement. The chamber is kept centered by bolts screwed in and supported by the furnace shell.
The face of the firebricks are painted with ITC-100 HT to protect the firebrick. This coating is rated up to 5000 \dg F. This coating reflects heat back into the furnace which improves heat up times and raises furnace efficiency. \index{refractory coating} \index{ITC-100 HT}
I used insulating ceramic fiberboard on the lid and below the firebrick floor between the firebrick and the bottom of the steel drum. \index{fiberboard, insulating}
The space behind the vertical firebricks is loosely filled with ceramic fiber insulation blanket. \index{ceramic fiber blanket}
\subsection{Insulating Firebrick}
The 2600 \dg C insulating K26 firebrick (IFB) that I used are made by Morgan Advanced Materials. I purchase these locally at Joe Moore Company, a boiler and furnace materials supplier, to save the cost of shipping. \index{firebrick} \index{Joe Moore Company} \index{Morgan Advanced Materials}
The IFB are soft and can be easily cut with wood working tools. Care must be taken because the corners can chip easily and if you are not careful the brick can crack and break. If the break is clean, the two halves can be cemented back together with refractory cement. \index{firebrick!broken}
The IFB have the following dimensions: 9 inches tall, 4.5 inches wide, and 3 inches thick. \index{firebrick!dimensions}
\subsection{Ceramic Fiber Board}
The ceramic fiberboard that I use below the firebrick floor is rated up to 2300 \dg F. It has a density of about 20 pounds per cubic foot, it is one-inch-thick, and it is easily cut with a knife. It has low thermal conductivity and is almost impervious to thermal shock. \index{fiberboard, insulating}
\subsection{Ceramic Fiber Insulation}
The ceramic fiber insulation blanket is rated up to 2400 \dg F. This material is easily cut with a knife. It has low thermal conductivity making it a great insulator. \index{ceramic fiber blanket}
\textbf{Safety Note: } This material is asbestos-free, but the fibers can be hazardous when they are airborne, so you must wear gloves and an N100 particulate mask when working with this material in a well-ventilated space.
Use the vacuum cleaner - not the broom - to clean up loose fiber after cutting. Wear long sleeves -- this material will irritate your skin just like fiberglass insulation. \index{ceramic fiber blanket!hazards} \index{cleaning up}
This material is more hazardous after it has been exposed to high temperatures because the fibers change their geometry and become needle-like lodging deeply in the lungs. When disassembling the furnace for maintenance it is crucial that the worker use respiratory protection.
\begin{figure}[htbp]
\centering
\includegraphics[width=0.8\textwidth]{images/firebrick-arch}
\caption{Firebrick geometry used to form the heating chamber.}
\label{fig:firebrickarch}
\end{figure}
\begin{figure}[htbp]
\centering
\includegraphics[width=0.8\textwidth]{images/element-grooves}
\caption{Heating element groove as it transitions from brick to brick.}
\label{fig:element-grooves}
\end{figure}
\section{Preparing the Firebrick}
Dan Hartman cut both inside corners off of his firebricks to make an arch around his heating chamber. I was playing around with the geometry and I discovered that only one corner needs to be cut off of each firebrick to create the brick arch. That is what I did. I cut off the corners at a 45 \dg angle using my table saw starting about 0.75 inch from one side. See Figure \ref{fig:firebrickarch}. \index{firebrick!cutting} \index{firebrick!arch}
I created a cardboard template for the heating element grooves and traced the groove path onto the face of the bricks.
\subsection{Cutting Firebrick}
The insulating firebrick are very soft and easy to cut. They can be cut with a wood saw, but the bricks are very abrasive and they will quickly dull your saw. I purchased an inexpensive hand saw that will be sacrificed for this job. \index{firebrick!cutting} \index{saw, sacrificial}
\subsection{Routing Heating Element Grooves}
After drawing the heating element groove outline onto the face of the firebricks with pencil, I used a router bit in a small trim router running along a wooden fence clamped over the brick to cut the groove. The groove is about 3/8 inches wide and 3/8 inches deep.
The firebrick are very abrasive and they will ruin your router bit. I used an inexpensive router bit that was sacrificed for cutting the heater groove. After the groove was cut I smoothed the edges and transition from brick to brick with a round file. See Figure \ref{fig:element-grooves} \index{router bit, sacrificial}
\section{Heating Element Design}
The heating element is formed from coiled resistance wire and it follows a spiral groove around the heating chamber so that heat is radiated from the chamber walls uniformly. I am using two elements joined in series. \index{heating element!design}
\subsection{Required Resistance}
For this project I require a heater with about 14.4 ohms electrical resistance (See Equation \ref{eq:current1}). Each element should have about 7.2 ohms of resistance. I gently stretched the element out so that the coils were not touching each other and checked the resistance again with my Fluke portable volt-ohm meter. \index{resistance, electrical}
The element has a length of uncoiled wire on each end. This is not included in my resistance measurement -- only the coiled section is measured for resistance.
The element has a resistance of 9 ohms before it is shortened. The resistance of the element is proportional to the length of the resistance wire. Since the coil is manufactured from wire of uniform diameter and composition, the required element length can be estimated using the ratio of resistance to wire length: \index{electrical resistance!ohms per inch}
\begin{equation}
\frac{R_1}{R_2} = \frac{L_1}{L_2}
\label{eq:RLratio}
\end{equation}
\begin{equation}
\frac{9\ \Omega}{7\ \Omega} = \frac{15\ inches}{L_2\ inches}
\label{eq:newlength}
\end{equation}
Solving for $L_2$ yields:
\begin{equation}
L_2 = 11.7\ \text{inches}
\label{eq:L2}
\end{equation}
The new length is about 3.3 inches shorter. I cut the wire off one inch at a time checking the resistance with my volt-ohm meter after each cut. \index{heating element!cutting}
The element wire has about 3 inches of uncoiled wire on each end. I bend this over to double it on itself (and reduce its resistance) and twist this together tightly. This will be the wire segment that passes through the hole in the terminal block and is connected to the electrical supply.
\begin{figure}[htbp]
\centering
\includegraphics[width=0.8\textwidth]{images/initial-test}
\caption{The first reduced power test of the heating assembly.}
\label{fig:initial-test}
\end{figure}
\subsection{Stretching the Elements}
I measured the heating element groove length by pressing a cotton rope into the groove, marking the end with a black marker. I pulled the rope out and measured the length. I found it to be 130 inches long. I have two elements to stretch out and fill the entire 130 inches of groove. The elements have been shortened to 11.7 inches. \index{heating element!stretching}
Each element has to be stretched to 65 inches long.
I put the uncoiled tail into my bench vise and I put the end of my tape measure into the vise jaw next to it. I slowly back up while pulling and stretching out the coil as the tape measure extends out. Care must be taken to stretch the element uniformly so that the spacing between adjacent turns remains reasonably consistent. Doing this will prevent hot spots from being created. I stop when I have stretched the element to 65 inches long. I repeat this process for the second element. \index{heating element!hot spots}
\section{Heating Element Installation}
I push the doubled up wire end of the first element through the lower terminal brick hole and then I begin to carefully push the coil into the groove and work my way up to the halfway point (about 3 times around the heating chamber). At the halfway point, the other doubled up wire end is pushed through the middle hole in the terminal brick. \index{heating element!installation}
I push the doubled up wire end of the second element through the middle hole in the terminal brick and then carefully push the coil into the groove and work my way up to the top. At the top the doubled up wire end is pushed through the top hole.
In the middle of the terminal brick where the two element wire ends are pushed through the brick, the two elements are joined with a stainless steel bolt, washers, and nut.
Where the element crosses the firebrick corners it tends to lift up out of the groove -- the groove sections are linear, but the element tends to form a circular shape. To ensure that the element remains in the groove, I pin the element in place with U-shaped pins of Kanthal wire. \index{heating element!pinning}
\section{Heating Chamber Testing}
Before final installation into the furnace body, the completed heating chamber assembly was subjected to an initial low-power test. The assembly was placed on two insulating firebricks to elevate it above the work surface and minimize heat transfer. \index{testing!low power}
The heating element was temporarily connected to a 120 VAC power source. Since the heating elements were designed for operation at 240 VAC, this reduced-voltage test limited the power output and allowed the assembly to be heated gradually. The purpose of the test was to remove any remaining moisture from the refractory materials, verify the integrity of the heating elements and electrical connections, and identify any obvious construction defects before the furnace was fully assembled. See Figure \ref{fig:initial-test}. \index{testing!purpose}
During the test, the heating element reached a dull red color and was energized for approximately one hour. No damage to the heating elements, refractory cement, or firebrick was observed. This preliminary cure cycle provided additional drying of the refractory materials before the furnace was placed into service at full operating power.
\textbf{NOTE:} After the initial firing, the Kanthal heating element will have been exposed to elevated temperatures and will no longer possess the same ductility it exhibited during installation. The element becomes progressively more brittle with use and may fracture if bent sharply or handled roughly. If maintenance or repairs require the heating element to be moved, it should be handled carefully to avoid accidental breakage. \index{Kanthal} \index{heating element!used}
\section{Heating Chamber Floor Construction}
The furnace floor was constructed by cementing several insulating firebricks together along their edges to form a single assembly. After the refractory cement had cured, the assembly was cut into a circular shape using a saw. A coat of ITC-100 HT was then applied to the surface exposed to the heating chamber. \index{firebrick} \index{ITC-100 HT} \index{heating chamber!floor}
Two layers of one-inch-thick ceramic fiberboard were cut into circles and placed in the bottom of the steel drum. The completed floor assembly was lowered into the drum using lifting straps. After the floor was positioned, the straps were carefully withdrawn from beneath the brick and removed from the furnace shell. \index{fiberboard, insulating}
\begin{figure}[htbp]
\centering
\includegraphics[width=0.8\textwidth]{images/completed-HC}
\caption{The heating chamber and insulation blankets before being closed.}
\label{fig:completedHC}
\end{figure}
\section{Completed Heating Chamber}
The vertical firebrick assembly was carefully lowered into the drum and arranged on the floor. Carriage bolts were screwed in from the side walls so that their ends were in contact with four of the bricks. These bolts would keep the heating chamber centered in the shell. \index{heating chamber!assembly}
The heating elements were connected to high temperature hook-up wire with stainless steel bolts, washers, and nuts. The high temperature hook-up wire passes through the furnace shell and is routed to the control cabinet where it is connected to the SSR and the contactor. \index{high-temp hook-up wire}
The lid safety interlock switch was mounted to the steel furnace shell and connected to the safety chain in the control cabinet using 16 AWG wire. The switch is wired using its normally open (NO) contacts and closes only when the lid is fully closed. \index{lid interlock}
Two anti-tilt switches were also mounted to the furnace shell. These switches are wired in series with each other and with the lid safety interlock switch. Any switch opening will interrupt the safety chain and de-energize the contactor, removing power from the heating elements. \index{anti-tilt switches} \index{safety chain}
To support future performance testing and data collection, four thermocouple junctions were cemented to the rear faces of selected firebricks. Their wires terminate at connectors mounted on the side of the furnace shell, allowing a portable thermocouple reader to be connected when rear-face brick temperatures are to be measured. \index{thermocouple!static devices}
After these connections were completed, ceramic fiber insulating blanket was cut and carefully inserted into the annular space between the heating chamber and the steel drum. The insulation was installed loosely enough to avoid excessive conductive heat transfer, yet densely enough to prevent significant air circulation and convective heat loss. See Figure \ref{fig:completedHC}. \index{ceramic fiber blanket!installation}
Finally, a wide ring of ceramic fiberboard was cut and fitted to the top of the body to seal off the annular internal space. This ring seals the annular insulation space, helps keep the ceramic fiber blanket clean, and reduces the release of loose fibers during operation and maintenance. \index{fiberboard, insulating}
\subsection{Initial Electrical Checks}
Before connecting the heating element assembly to the 240 VAC power supply, several electrical checks were performed. These checks were intended to verify the integrity of the heating elements and confirm that no unintended electrical paths existed between the heater and the furnace shell. \index{electrical testing}
Using a Fluke portable volt-ohm meter, the resistance of the assembled heater was measured at the terminal connections. A resistance of 14.9 ohms was obtained, compared to the design value of 14.4 ohms. The small difference is likely attributable to the additional conductor length introduced by the uncoiled element leads and measurement uncertainty. \index{resistance, electrical!measurement} \index{heating element!resistance}
This value was considered acceptable (about a 3.5\% difference). A reading of OL (open line) would have indicated a broken heating element or an open connection. Conversely, a reading near zero ohms would have indicated a short circuit within the heater assembly. \index{resistance, electrical!continuity}
Next, the resistance between one heater terminal and the grounded steel furnace shell was measured. The meter indicated OL, corresponding to infinite resistance. This confirmed that no electrical connection existed between the heating element and the furnace body. \index{resistance, electrical!ground}
Having verified that the heater assembly was electrically sound, the heating elements were connected to the SSR and contactor in preparation for the initial low-power test.
With the heating chamber complete and initial testing successfully concluded, the project was ready to proceed to final assembly. The following chapters describe the integration of the heating chamber with the furnace body, lid, control system, and supporting hardware. Detailed performance testing, operating data, and efficiency measurements will be presented later in this document.
