MANUFACTURING I THERMAL
In our opinion, measurement of absolute temperatures makes more sense for the ascertainment of machine capability coefficients. A firing system offers numerous options for acquiring temperatures. Figure 6 depict the temperature curves generated by temperature measuring systems integrated into the machine.
Figure 5: Calculation of the mean value in dual lane systems
the difference between maximum temperatures is 14.5° C. We take a critical view of acquiring temperature differences for the purpose of calculating machine capability coefficients. This is discussed briefly below. The temperature difference between two measuring points, as shown in figure 3 is noteworthy.
As a prerequisite, the wafer must be very well prepared with several thermocouples. Differences in thermocouple mounting result in changes to the thermal mass at the measuring point, which has a critical influence on data acquisition. The position of the test wafer on the conveyor belt influences thermal mass as well.
Furthermore, the results only reflect the temperature difference on a single wafer (e.g. ôL-Rô), which depends more upon wafer preparation than on interaction with the firing systems temperature profile. Table 1 lists the statistical values associated with figure 3. The requirement for a capability coefficient of Cm > 1.67 for a tolerance width of ± 7 K is fulfilled using temperature difference on the wafer as a characteristic, although this is not the case when absolute temperature on the same test wafer is used as the characteristic.
For this reason, we haven’t made use of this temperature difference for the capability analysis. The acquisition of temperature differences between lanes is used for dual and multi-lane systems
Standard deviation of all measuring points on the test board for both the right and left-hand lanes is used as a basis for calculation in figure 4. Heat transfer tolerance over the entire width of the profile is ascertained in this way. Due to the fact that only one test board is available as a rule, the lanes have to be measured one after the other with the same board. This, too, is not in the spirit of the capability analysis. The calculation based on the procedure depicted in figure 5 provides us with the mean temperature difference between the lanes. In this case as well, two test boards which run through the oven simultaneously are really required.
Mean value Standard deviation Machine capability Cm Table 2: Statistical Values (peak zone Tmax) Associated with Figure 6 24
www.solar-international.net I Issue IV 2014
The thermocouples designated as controllers are incorporated into the firing system’s temperature control loop, whereas the so-called watchdog thermocouples measure temperature in proximity to the conveyor belt independent of the control system. Table 2 lists the corresponding statistical values. Machine capability Cm is calculated here with a tolerance width of ± 5 K.
If a controller thermocouple is used to determine machine capability coefficients, the required value of Cm > 1.67 is easily complied with, whereas a dramatically worse value results from the data supplied by the watchdog thermocouple. This comparison demonstrates that it makes little sense to use data from thermocouples which are integrated into the firing system in order to calculate machine capability coefficients.
Measuring equipment and tolerance width For this reason, it’s advisable to ascertain absolute temperatures on standardized test boards, which are independent of the firing system, when determining machine capability coefficients. These can be either calibrated test setups on specified solar wafers (as shown in figure 7a) or standardized measuring devices consisting of, for example, defined metal sheets to which thermocouples are attached (see figure 7b). The latter have the advantage of being significantly more rugged, and are thus able to withstand the procedure which involves multiple repeat measurements.
As opposed to mechanical systems, the fact that there’s direct interaction between the firing system and the measuring device (test board) must be taken into consideration in the real thermodynamic world. Fundamentally, firing systems are radiation systems, i.e. heat transfer is subject to the conditions set forth in the Stefan-Boltzmann law. Emittance specifies the amount of radiated power emitted/absorbed by the respective body. Actual emittance depends to a great extent on the respective material (color and finishing quality of the surface), and usually lies within a range of 0.012 to 0.98.
Controller 905.4 0.9
1.78
Watchdog 886.6 9.8
0.17
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