b. **Dimensioning of the furnace masonry and thermal calculus**

The furnace has a cylindrical shape. Its disposal is vertical, and on its walls it chose a configuration with multiple layers, considering the temperature, as shown in **Figure 14**:

Let it be the exterior temperature of the refractory coat of 1200°C.

### c. **Resistors**

The choice of the material from which the resistors are made should be considered so that the maximum working temperature exceeds almost 2–10% of the maximum temperature of the furnace. Knowing that the temperature within the furnace can reach 1600°C, Kanthal is chosen for resistors. Kanthal resistors are like spiral wires.

#### **Figure 14.**

*The diagram of furnace wall: 1 – Refractory coat made of chromium magnesite with a thickness of 15 mm; 2 – Thermal insulation made of 700 diatomites treated with binders with a thickness of 30 mm; 3 – Exterior shell made of steel plate with a thickness of 2 mm.*

In each chamber of the furnaces, 27 resistors are placed in the channels of the refractory material. They are disposed symmetrically on vertical and placed over the crucible so that we have a uniform temperature in the entire chamber of the solar furnace.

The length of each Kanthal spiral is 105 mm. The equivalent resistance on each chamber is 51.5 Ω. **Figure 2** shows the disposal of all 27 resistors.

The installed power of the furnace is calculated by the relation:

$$P\_i = k \cdot P = k \cdot \frac{Q\_i}{t\_i} \qquad [W] \tag{10}$$

where k = 1.1÷1.5 is the safety coefficient that takes into account the possibility of forcing heating regime of the cold furnace, the possibility of decreasing network tension toward its nominal value, the possibility of decreasing time of the thermal insulation properties, the possibility of heating elements aging – that determine a high strength than the one initially calculated and in addition a smaller developed power (**Figure 15**).

The furnace with small dimensions, two chambers with a crucible, and the volume of 0.5 liters for each active chamber will be the first lab furnace. The reason the furnace is constructed with two working chambers is for the optimization of the working time.

We choose for a chamber furnace with crucible, because of its simple construction, the possibility of using it for different processes (for example, melting, burning, and purifying) as the possibility of realizing some different thermal regimes into the furnace, in essence, is what we propose to accomplish.

The resistor furnace (in the future, we will extend our research on an induction furnace too) has an alternate functioning regime because of the following functioning cycle:

**Figure 15.** *Chamber disposal of the resistors.*

*Assessment of Solar Energy Potential Limits within Solids on Heating-Melting Interval DOI: http://dx.doi.org/10.5772/intechopen.104847*


The furnace will have the possibility of fitting in all working temperatures that are considered to be classifying criteria in heating electric heating technology:

