Before beginning this chapter, it is worth noting that it contains more equations and calculations than any previous chapter in this book. This is not intended to be a textbook on thermodynamics or heat transfer. Rather, the measurements, calculations, and performance data presented here are used to evaluate how effectively the furnace performs its intended functions.
The goal of this chapter is to answer practical questions that builders and operators are likely to have. How quickly does the furnace heat up? How much electrical energy does it consume? How much heat is lost through the insulation? How efficiently does it melt aluminum? The calculations presented here provide a framework for answering these questions and for comparing the performance of this furnace to other designs.
Readers who are primarily interested in building and operating the furnace may choose to skim some of the mathematical details. However, the results and conclusions drawn from these calculations provide valuable insight into the performance of the finished equipment.
% Imagine a chapter containing these graphs:
% Temperature vs. Time
% Duty Cycle vs. Temperature
% Power Consumption vs. Temperature
% Temperature Gradient Through Furnace Wall
% Shell Temperature vs. Hot-Zone Temperature
% Cost of Operation vs. Temperature
\section{Test Equipment}
\section{Testing Equipment}
The following equipment was used to collect the performance data presented in this chapter.
\begin{description}
\item[PID Controller]
The Inkbird ITC-100VH PID controller was used to display and record furnace temperature through a Type K thermocouple installed in the lid of the heating chamber. The controller also signals when power is being supplied to the heating elements.
\item[Infrared Thermometer]
An inexpensive handheld infrared thermometer was used to measure the temperatures of the rear firebrick, furnace shell, lid, and other accessible surfaces during operation.
\item[Digital Ammeter]
A panel-mounted digital ammeter and current transformer were used to monitor heating element current during furnace operation.
\item[Digital Scale]
An inexpensive digital kitchen scale was used to measure the mass of aluminum used during melting trials and to verify the mass of completed castings.
\item[Stopwatch]
A stopwatch was used to record heat-up times, melting times, and other time-dependent measurements. Using the stopwatch was not very accurate and another method, discussed later in the chapter, was devised to capture duty cycle times.
\item[Type K Thermocouple]
A Type K thermocouple installed in the furnace lid provided temperature feedback to the PID controller and served as the primary temperature measurement device.
\item[Volt-Ohm-Milliammeter] A Fluke portable volt-ohm-milliammeter was used for troubleshooting and obtaining electrical circuit values.
\end{description}
The measurements presented in this chapter should be considered engineering estimates rather than laboratory-grade measurements. The primary objective of the testing was to characterize furnace performance and identify opportunities for future improvement rather than to produce highly precise scientific data.
% This establishes measurement credibility.
% \SI{240}{\volt}
% \SI{16.7}{\ampere}
% \SI{4000}{\watt}
% \SI{14.9}{\ohm}
% \SI{1000}{\celsius}
\section{Heat-Up Performance}
Measure:
Ambient temperature -- 25 \dg C \\
Time to 100 \dg C -- 7 minutes 20 seconds\\
Time to 200 \dg C -- 10 minutes 15 seconds\\
Time to 400 \dg C -- 17 minutes 5 seconds\\
Time to 600 \dg C -- 23 minutes 10 seconds\\
Time to 800 \dg C -- \\
Time to 1000 \dg C --
\begin{figure}[H] % a sample heat up plot
\centering
\begin{tikzpicture}
\begin{axis}[
width=1.0\textwidth,
height=0.5\textwidth,
grid=major,
minor tick num=1,
xlabel={Time (minutes)},
ylabel={Temperature (\dg C)},
title={Furnace Heat-Up Test},
]
\addplot[
color=red,
very thick,
mark=*,
]
table[
x=time_min,
y expr=(\thisrow{temperature_f}-32)*5/9,
col sep=comma
]
{data/furnace_heatup.csv}; % sample time-temp data
\end{axis}
\end{tikzpicture}
\caption{Measured furnace temperature during heat-up.}
\label{fig:heatupcurve}
\end{figure}
\section{Electrical Power Consumption}
Power consumption measurements were taken after the furnace had reached steady-state operation and the heating elements were operating at full power. Electric power consumption was measured using the installed voltage-current meter on the control panel.
Electric power consumed is calculated using either Equation \ref{eq:power1} or Equation \ref{eq:power2}. I elected to calculate the power both ways as a check.
\begin{equationbox}
\begin{equation}
P = I^2R
\label{eq:power1}
\end{equation}
\begin{equation}
P = IV
\label{eq:power2}
\end{equation}
\end{equationbox}
% This is where your \SI{14.9}{\ohm} element resistance becomes useful.
The furnace was designed to consume approximately 4000 watts. Using Equation \ref{eq:power1} and using the previously measured resistance of 14.9 ohms for the heating elements when instantaneous current displayed was 16.4 amps gave me a furnace power of 4008 watts. This is approximately 100\% of the design power level.
\begin{align}
P &= I^2R \\
&= (16.4\,\text{A})^2(14.9\,\Omega) \\
&= 4008\,\text{W}
\end{align}
Using Equation \ref{eq:power2} and using the displayed instantaneous current of 16.4 amps and the displayed voltage of 243 volts, I calculated that the furnace power was 3985 watts. This is approximately 99\% of the design power level.
\begin{align}
P &= IV \\
&= (16.4\,\text{A})(243\,\text{V}) \\
&= 3985\,\text{W}
\end{align}
The two calculated power values differ by less than one percent, indicating good agreement between the measurements and calculations. Several factors may contribute to this small difference.
First, the resistance value used in Equation \ref{eq:power1} was measured at room temperature with the furnace de-energized. The electrical resistance of Kanthal heating elements increases slightly as their temperature increases. Consequently, the actual operating resistance of the heating elements is somewhat greater than the value used in the calculation.
Second, the installed voltage-current meter is an inexpensive consumer-grade instrument intended for monitoring rather than precision measurement. Small errors in the displayed voltage and current values are therefore expected.
Considering these factors, the agreement between the two calculated power values is excellent and confirms that the furnace operates very near its intended design power of 4000 watts.
\section{Exterior Surface Temperatures}
Using the non-contact thermometer, I first confirmed that the workshop ambient temperature was approximately 29 \dg C by taking spot measurements of a dozen surfaces in the shop and averaging the result.
After the furnace came to equilibrium with a 1000 \dg C cavity temperature, I began taking spot surface temperature measurements on the shell (top, middle, bottom) at four locations 90\dg \, apart, at the point where the lid meets the body, four points on top of the furnace lid, the back of the control cabinet, and the top of the control cabinet.
Temperatures were taken with the shop exterior door closed and the room fan turned off.
\begin{table}[H]
\centering
\caption[Exterior Surface Temps]{Exterior Surface Temps at 1000 \dg C Cavity Temp}
\label{tab:surfacetemps}
\begin{tabular}{|l|c|}
\hline
\textbf{Location} & \textbf{Temperature (\dg C)} \\
\hline
Shell - Top (Location 1) & 35 \\
Shell - Middle (Location 1) & 31 \\
Shell - Bottom (Location 1) & 29 \\
\hline
Shell - Top (Location 2) & 36 \\
Shell - Middle (Location 2) & 32 \\
Shell - Bottom (Location 2) & 30 \\
\hline
Shell - Top (Location 3) & 35 \\
Shell - Middle (Location 3) & 32 \\
Shell - Bottom (Location 3) & 31 \\
\hline
Shell - Top (Location 4) & 34 \\
Shell - Middle (Location 4) & 33 \\
Shell - Bottom (Location 4) & 29 \\
\hline
Lid Surface (Point 1) & 37 \\
Lid Surface (Point 2) & 38 \\
Lid Surface (Point 3) & 39 \\
Lid Surface (Point 4) & 38 \\
\hline
Lid-to-Body Joint (Average) & 59 \\
\hline
Control Cabinet - Rear Surface & 31 \\
Control Cabinet - Top Surface & 33 \\
\hline
Ambient Workshop Temperature & 29 \\
\hline
\end{tabular}
\end{table}
The measured exterior temperatures demonstrate the effectiveness of the insulating firebrick and ceramic fiber blanket insulation. Even with a furnace cavity temperature of 1000 °C, the exterior shell remained cool enough to touch comfortably. The control cabinet temperatures remained near ambient, confirming that the electrical components were adequately isolated from the heat generated by the furnace.
The highest measured exterior temperature was 59 \dg C at the interface between the furnace body and the removable lid. This location was approximately 30 \dg C above the ambient workshop temperature and likely represents the largest source of heat loss from the furnace.
The measured temperatures compare favorably with those of Furnace Version 1.0 and indicate that the insulation system used in Furnace Version 2.0 is highly effective.
\section{A Few Words About Insulation}
One of the most effective ways to reduce heat loss is to increase the thickness of the insulation surrounding the hot zone. As a general rule, increasing insulation thickness reduces the rate of heat transfer through the wall. For simple conductive heat transfer, doubling the insulation thickness approximately halves the heat flow.
Consider a freezer with two inches of insulation through which 6000 BTU of heat enters each day. If the insulation thickness is increased to four inches, the heat transfer may be reduced to approximately 3000 BTU per day. Increasing the insulation thickness to eight inches may reduce the heat transfer further to approximately 1500 BTU per day.
See Figure \ref{fig:wallthickness} and notice that each additional doubling of insulation produces a smaller reduction in heat loss than the previous layer. Eventually a point is reached where the cost, weight, and space required for additional insulation are no longer justified by the reduction in heat transfer.
\begin{figure}[H]
\centering
\begin{tikzpicture}
\begin{axis}[
width=0.9\textwidth,
height=0.5\textwidth,
grid=major,
minor tick num=1,
xlabel={Insulation Thickness (inches)},
ylabel={Heat Gain (BTU per day)},
title={Insulation Thickness vs Heat Gain},
]
\addplot[
color=red,
very thick,
mark=*,
]
table[
x=thick,
y=BTU_day,
col sep=comma
]
{data/insulation_vs_conduction.csv};
\end{axis}
\end{tikzpicture}
\caption{Effect of increasing insulation thickness.}
\label{fig:wallthickness}
\end{figure}
Furnace Version 1.0 performed well with a thinner insulating wall. Because Furnace Version 2.0 incorporates approximately two additional inches of insulation, it is expected to exhibit lower heat loss, cooler exterior surface temperatures, and improved thermal efficiency.
\vspace{.5cm}
\begin{equationbox}
For flat walls the heat transfer can be approximated by:
\begin{equation}
q = \frac{k At \Delta T}{L} = \frac{kAt(T_{Hot} - T_{Cold})}{L}
\end{equation}
where:
\begin{compactitem}
\item[$q$] -- Heat flux (W or J/s)
\item[$k$] -- thermal conductivity (W / (m $\cdot$ K)
\item[$A$] -- Area of wall ($m^2$)
\item[$t$] -- time duration (seconds)
\item[$\Delta T$] -- Temperature difference (\dg C)
\item[$L$] -- Wall thickness (m)
\end{compactitem}
\end{equationbox}
Choosing an insulator with low thermal conductivity (k) reduces the wall thickness required to retain thermal energy. Because of their low thermal conductivities, insulating firebrick and ceramic fiber blanket were selected as the primary insulating materials for this furnace.
\begin{table}[H]
\caption[k-Values]{K-Values of Common Materials}
\label{tab:k-values}
\begin{center}
\begin{tabular}{l|c} \hline
\textbf{Material} & \textbf{k (W/m·K)} \\
\hline
Copper & ~400 \\
Aluminum & ~205 \\
Carbon steel & ~50 \\
Fireclay brick & ~1 \\
IFB & ~0.15 -- 0.30 \\
Ceramic fiber blanket & ~0.05 -- 0.15 \\
Ceramic fiber board & ~0.10 -- 0.25 \\
Air & ~0.026 \\
\hline
\end{tabular}
\par\medskip\footnotesize
Thermal conductivity varies with temperature, density, and manufacturer.
\end{center}
\end{table}
Although air has the lowest thermal conductivity listed in Table \ref{tab:k-values}, large air spaces are generally poor insulators because natural convection currents can transfer heat. Ceramic fiber blanket and insulating firebrick are effective because they trap countless small pockets of air, greatly reducing convective heat transfer while also limiting heat conduction.
\section{Rear Firebrick Temperatures}
This is one of the most interesting tests because you installed thermocouples specifically for this purpose.
Measure:
Rear brick \#1 \\
Rear brick \#2 \\
Rear brick \#3 \\
Rear brick \#4
This reveals:
Thermal gradients \\
Hot spots \\
Uniformity \\
\section{Controller Duty Cycle}
Because the PID controller cycles power to the heating elements, the furnace does not necessarily consume full rated power during the entire melt. Measuring duty cycle allows the average electrical power to be estimated more accurately. Better measurements of current, voltage, duty cycle, melt time, and charge temperature will produce a more realistic estimate of furnace efficiency.
Initially, I attempted to measure the furnace duty cycle by observing the heating element indicator LED on the PID controller, recording the elapsed time with a stopwatch, and manually noting the results. This method proved to be inaccurate because many of the heating cycles were very short and difficult to record reliably.
After discussing the problem with the OpenAI ChatGPT 5.5 language model, it was suggested that I record a video of the PID controller while the furnace was operating. A cellular telephone mounted on a tripod was positioned so that the indicator LED and the was visible during the recording.
The video was later reviewed frame-by-frame. Each time the heating element indicator illuminated, the corresponding video timestamp was recorded. By summing the total time that the indicator was illuminated and dividing by the total elapsed test time, an approximate controller duty cycle was determined.
This method proved to be significantly more accurate than direct observation and provided a permanent record that could be reviewed if necessary.
Recorded:
Duty cycle at 400 \dg C -- 32\%\\
Duty cycle at 600 \dg C -- 41\%\\
Duty cycle at 800 \dg C -- 56\%\\
Duty cycle at 1000 \dg C -- 65\%
We can see that the heater has to work harder to maintain higher temperatures.
\section{Aluminum Melting Trials}
This section will likely be of greatest interest to many readers because melting metal is the primary reason for building an electric resistance shop furnace.
The aluminum melting trial serves two purposes. First, it demonstrates the furnace's ability to melt metal under normal operating conditions. Second, the measured melting time and power consumption data can be used later to estimate furnace efficiency.
\begin{cautionbox}
Before beginning any melting trial, place an empty ingot mold near the furnace. If a furnace malfunction occurs, the molten metal can be safely poured into the mold before it solidifies. \\
Do not allow molten aluminum to solidify inside a fireclay crucible. As aluminum freezes it expands and may crack the crucible. Even if no visible damage is observed, the crucible may have been weakened and could fail during a subsequent firing.
\end{cautionbox}
\begin{warningbox}
Plan the pour before beginning the melt. Wear all required personal protective equipment, including:
\begin{multicols}{2}
\begin{compactitem}
\item Face shield
\item Heat-resistant gloves
\item Long-sleeved cotton shirt
\item Leather or rubber apron
\item Long cotton or denim pants
\item Safety shoes or boots
\end{compactitem}
\end{multicols}
Place spare firebricks or another suitable heat-resistant surface nearby before lifting the crucible. Position the mold on a non-flammable surface such as concrete or dry sand, and ensure that the area between the furnace and mold is free of obstacles and tripping hazards. \\
Practice the lifting and pouring motions before melting metal. Once the aluminum is molten, there is little time to decide where to walk, where to set the crucible down, or how the pour will be performed.
\end{warningbox}
To prepare for the test, weigh out 1 kilogram of clean aluminum and gather all required personal protective equipment, material handling tools, and molds.
Set up the furnace in an area free of combustible materials. Connect the furnace to the 240 VAC power supply, open the control cabinet, and close the 25 amp circuit breaker. The cooling fans should immediately begin operating. Once the circuit breaker is closed, components inside the control cabinet are energized. Close the cabinet door and leave it closed during operation.
Place a graphite saucer on the furnace floor and position the crucible on top of the saucer. Close the lid, set the PID controller setpoint to 750 \dg C, press the Start Pushbutton, and preheat the furnace and crucible.
When the furnace has reached operating temperature, load the 1 kilogram aluminum charge into the crucible and close the lid. Press the Start Pushbutton, and begin timing the melt.
Observe the furnace during operation and record any data required for later analysis, including heat-up time, melting time, shell temperatures, duty cycle, and power consumption. Once the aluminum has completely melted, proceed with the planned pour and record the total elapsed time.
Repeat the melt with 3 pounds and 5 pounds of aluminum. These additional datasets will be useful when you calculate furnace efficiency later.
\section{Energy Consumption Per Melt}
Determine:
kWh consumed for a typical melt.
\begin{equationbox}
\begin{equation}
E_{\text{kWh}} =
\frac{P_{\text{furnace}}Dt}{1000}
\end{equation}
where:
\begin{tabular}{>{$}l<{$}l}
P_{\text{furnace}} & -- furnace power when energized (kW) \\
D & -- duty cycle as a decimal \\
t & -- melt time (hours)
\end{tabular}
\end{equationbox}
Substituting values we measured and recorded:
\begin{align}
E_{\text{kWh}} &=
(4.0\ \text{kW})(0.60)\left(1\text{hr}\right) \\
&= 2.4\ \text{kWh}
\end{align}
Then calculate cost per melt using your local electricity rate. Here at Apex, NC we are charged \$0.12 per kWh. This calculated cost does not account for the energy used for pre-heating the furnace and crucible.
\begin{align}
\text{Cost} &= (2.4\ \text{kWh})(\$0.12/\text{kWh}) \\
&= \$0.29
\end{align}
It costs approximately \$0.30 per hour to operate this furnace. This is very economical when you compare it to the alternatives available to hobbyists.
\section{Furnace Efficiency Estimate}
To estimate the efficiency of the furnace, I conducted a series of runs at various setpoint temperatures to discover how widely the duty cycle varies during operation. Next, I calculated the calorimetry of completely melting one kilogram of aluminum metal and giving the molten aluminum about 40 \dg C of superheat.
Calculate:
\begin{compactitem}
\item{Energy required to heat metallic aluminum}
\item{Energy required for fusion}
\item{Energy required to heat liquid aluminum}
\item{Electrical energy consumed}
\end{compactitem}
Once we know how much energy is required to melt 1 kilogram of aluminum, we actually melt 1 kilogram of aluminum and compare the amount of energy used.
\begin{equationbox}
\begin{equation}
Q = mc\Delta T = mc(T_{Hot} - T_{Cold})
\label{eq:heat1}
\end{equation}
where:
\begin{compactitem}
\item[$Q$] -- Heat (J)
\item[$m$] -- Mass (kg)
\item[$c$] -- Specific heat capacity (J/kg·\dg C)
\item[$\Delta T$] -- Temperature change (\dg C)
\end{compactitem}
\end{equationbox}
To melt the aluminum, we have to heat it from room temperature ($T_{Cold}$) to the melting point ($T_{Hot}$). Assume that the room temperature is 20 °C. Aluminum melts at 660 °C. Assume we have 1 kilogram of aluminum (the mass, m) in the crucible.
The specific heat capacity of SOLID aluminum is 900 Joules/kilogram \dg C (c). Use Equation \ref{eq:heat1} to calculate the heat required to bring the 1 kilogram of aluminum to the melting point ($Q_1$):
\begin{align}
Q_1 &=
(1\,\text{kg})
\left(
900\,\frac{\text{J}}{\text{kg}\cdot{}^\circ\text{C}}
\right)
(660^\circ\text{C}-20^\circ\text{C}) \\
&=
(1\,\text{kg})
\left(
900\,\frac{\text{J}}{\text{kg}\cdot{}^\circ\text{C}}
\right)
(640^\circ\text{C}) \\
&= 576{,}000\ \text{J} \\
&= 576\ \text{kJ}
\end{align}
After the metal temperature reaches the melting point, additional heat must be added to cause the phase change from solid to liquid. This heat ($Q_2$) is called the latent heat of fusion ($L_f$), and for aluminum it is 390 kiloJoules per kilogram. \index{latent heat}
\begin{equation}
Q_2 = mL_f
\label{eq:heat2}
\end{equation}
\begin{align}
Q_2 &=
(1\,\text{kg})
\left(
390{,}000\,\frac{\text{J}}{\text{kg}}
\right) \\
&= 390{,}000\ \text{J} \\
&= 390\ \text{kJ}
\end{align}
The total amount of heat needed to melt 1 kilogram of aluminum is the heat needed to raise its temperature to the melting point ($Q_1$), plus the latent heat of fusion ($Q_2$) needed to make it melt and change phase to a liquid.
\begin{equation}
Q_T = Q_1 + Q_2
\end{equation}
\begin{align}
Q_T &= 576\ \text{kJ} + 390\ \text{kJ} \\
&= 966\ \text{kJ}
\end{align}
If additional heat is supplied to the aluminum to raise its temperature above the melting point for pouring ($Q_3$), we can use Equation \ref{eq:heat1} again to find out how much heat is required. The specific heat capacity of LIQUID aluminum is 1180 Joules/kilogram °C (c). Assume we heated the aluminum to 700 °C.
\begin{equation}
Q_3 = mc(T_{Hot} - T_{Cold})
\end{equation}
\begin{align}
Q_3 &=
(1\,\text{kg})
\left(
1180\,\frac{\text{J}}{\text{kg}\cdot{}^\circ\text{C}}
\right)
(700^\circ\text{C}-660^\circ\text{C}) \\
&=
(1\,\text{kg})
\left(
1180\,\frac{\text{J}}{\text{kg}\cdot{}^\circ\text{C}}
\right)
(40^\circ\text{C}) \\
&= 47{,}200\ \text{J} \\
&= 47.2\ \text{kJ}
\end{align}
\begin{equationbox}
\begin{equation}
Q_T = Q_1 + Q_2 + Q_3\
\label{eq:heat3}
\end{equation}
where:
\begin{compactitem}
\item[$Q_T$] -- Total amount of heat required. (J)
\item[$Q_1$] -- Heat to raise the metal to 660 \dg C. (J) -- Use Equation \ref{eq:heat1}
\item[$Q_2$] -- Heat to completely melt the charge. (J) -- Use Equation \ref{eq:heat2}
\item[$Q_3$] -- Heat to raise the liquid to 700 \dg C. (J) -- Use Equation \ref{eq:heat1}
\end{compactitem}
\end{equationbox}
To bring 1 kilogram of solid aluminum from room temperature to liquid at 700 \dg C, you add the heat required to raise the aluminum to the melting point ($Q_1$), plus the latent heat required to cause the phase change ($Q_2$), plus the heat added to the liquid to bring the temperature to 700 \dg C ($Q_3$).
Using Equation \ref{eq:heat3} and substituting our values:
\begin{align}
Q_T &= 576\ \text{kJ} + 390\ \text{kJ} + 47.2\ \text{kJ} \\
&= 1013.2\ \text{kJ} \\
&= 1.0132\ \text{MJ}
\end{align}
The first melt requires more energy than subsequent melts because the furnace materials and the crucible have to be heated up for that first melt. It is more accurate to pre-heat the furnace and crucible, then add the mass of room temperature aluminum for the test.
If the furnace were 100\% efficient, all of the energy supplied to it would go into melting the aluminum. The furnace supplies 4000 Joules per second. 966,000 Joules are required to completely melt 1 kilogram of aluminum. It would take 966,000 Joules divided by 4000 Joules per second, or 242 seconds -- about 4 minutes, to melt 1 kilogram of aluminum.
Of course it took much more time to actually melt the metal.
The efficiency of the furnace ($\eta$) is the amount of energy required to melt the aluminum divided by the amount of energy supplied to the furnace multiplied by 100\%. The actual amount of energy is found by multiplying the rated power of the furnace by the duty cycle and time (in seconds). See Equation \ref{eq:eta_furnace} below.
\begin{equationbox}
\begin{equation}
\eta = \Bigg(\frac{Power Output}{Power Supplied}\Bigg) \times 100\%
\label{eq:efficient1}
\end{equation}
\begin{equation}
\eta =
\left(
\frac{Q_{\text{required}}}
{P_{\text{furnace}} D t}
\right)
\times 100\%
\label{eq:eta_furnace}
\end{equation}
where:
\begin{tabular}{>{$}l<{$} l l}
Q_{\text{required}} & = & \text{heat required to melt and heat the aluminum, (J)} \\
P_{\text{furnace}} & = & \text{furnace power when energized, (W)} \\
D & = & \text{duty cycle expressed as a decimal fraction} \\
t & = & \text{elapsed melt time, (s)}
\end{tabular}
\end{equationbox}
Assume it takes 40 minutes to completely melt 1 kilogram of aluminum and the furnace duty cycle is 60\%.
\begin{align}
\eta &=
\left(
\frac{966{,}000\ \text{J}}
{(4000\ \text{J/s})(0.60)(2400\ \text{s})}
\right)
\times 100\% \\
&= 16.8\%
\end{align}
I pre-heated the furnace and crucible, added 1 kilogram of room temperature aluminum to the crucible, and then timed how long it took for the charge to completely melt. I had to track the duty cycle of the heater to calculate the actual amount of energy supplied. There is some uncertainty in this measurement, but it gives me an idea of how much of the supplied energy escapes or is wasted.
So, about 17\% efficient. This is a rough measurement, but it shows that there is room for improvement. One improvement that was identified during this testing was that heat appears to be leaking from the lid-to-body interface. Improved sealing here seems to be needed.
\section{Comparison With Propane Furnace}
At the time of this writing my propane furnace is 14 years old. It is a very simple device and it continues to give me good service. Because I have now operated the propane furnace and the electric resistance furnace, I can do a side-by-side comparison of both.
\begin{table}[H]
\caption[Comparison]{Propane versus Electric Shop Furnace}
\label{tab:compare}
\begin{center}
\begin{tabular}{|l|c|c|} \hline
\textbf{Criterion} & \textbf{Propane} & \textbf{Electric} \\
\hline
Noise & Very loud & Silent \\
Heat-up time & Rapid & Slower \\
Operating cost & About \$4 per hour & About \$0.30 per hour \\
Convenience & Some setup & Plug-in and go \\
Indoor use & Dirty - Outdoors only & Clean - Indoor use \\
Temp Control & Poor & Good \\
\hline
\end{tabular}
\par\medskip\footnotesize
The propane furnace continues to be useful for cleaning dirty scrap metal.
\end{center}
\end{table}
The propane furnace must be used outdoors because the combustion of propane generates poisonous carbon monoxide. Outdoor use cannot occur on rainy days due to the risk of a steam explosion. The electric furnace generates no smoke or fumes and it can run indoors anytime.
The propane furnace makes a roaring sound during operation. This generates unwanted attention from neighbors and curious neighborhood children. The electric resistance furnace is completely silent and is unseen indoors.
The propane furnace heats up very quickly, but controlling the temperature of the charge is difficult. I have boiled the zinc out of brass and zamak alloy because I was unable to limit and control the temperature. The electric resistance furnace has very tight temperature control and this makes it useful for melting alloys and heat treatment of metals.
The propane used in the propane furnace costs about \$4.00 per hour of operation. I use a 20-pound steel storage tank, and if I exhaust the tank I must drive to the hardware store to have the tank refilled. The electric furnace uses power supplied by my local utility. I am charged \$0.12 per kWh, and I estimated that the electric furnace costs about \$0.30 per hour to operate.
I continue to use my propane furnace to melt down and clean dirty scrap metal - metal that is oily or painted. After pouring this cleaned metal into ingots it is suitable for use in the electric resistance furnace. These two furnaces compliment each other.
\section{Summary}
The testing performed in this chapter confirmed that Furnace Version 2.0 operates very near its intended design power of 4000 watts and is capable of reaching useful operating temperatures within a reasonable time.
Temperature measurements demonstrated that the insulating firebrick and ceramic fiber insulation system is highly effective, keeping most exterior surfaces only slightly above ambient temperature even when the furnace cavity is at 1000 \dg C. The highest exterior temperatures were observed at the lid-to-body interface, indicating that this area is the primary source of heat loss.
Duty cycle measurements showed that the heating elements must operate for an increasing percentage of time as furnace temperature rises.
Aluminum melting trials demonstrated that the furnace is fully capable of supporting hobby casting operations while maintaining precise temperature control.
Energy consumption and operating costs were found to be low, making electric resistance heating an economical alternative to propane. Efficiency calculations indicated that only a portion of the supplied electrical energy reaches the aluminum charge, revealing opportunities for future improvement.
Overall, the furnace met its design goals and proved to be a practical, economical, and effective tool for metal melting and heat-treating applications.
In the next chapter we will explore how these capabilities can be applied to practical metal casting projects.
.jpg "Zadig EDM from 1968")
