Temperature Control and Uniformity in Industrial High-Temperature Atmosphere/Vacuum Furnaces
|Fig. 2. Placement of survey thermocouples inside the furnace|
Most high-temperature vacuum furnaces are set up such that the power input to the furnace and thus the temperature in the furnace is controlled by one thermocouple. Does that mean the temperature in the entire furnace is what the thermocouple is reading?
AMS 2750D is an Aerospace Materials Specification that specifies temperature uniformity inside a furnace and how it should be measured. It specifies six furnace classes and defines the temperature uniformity for them. Table 1 shows the temperature uniformity levels for these furnace classes. Temperature uniformity is defined as follows:
- For conventional furnaces (non-retort) it is the temperature variation expressed in ± degrees within the qualified work zone with respect to a setpoint temperature.
- For retort furnaces, where a sensor in the retort is used to control the temperature, it is the temperature variation with respect to the sensor in the retort and not the furnace set temperature.
Batch furnaces are popular because of their flexibility. You run them only when you need them, and you can run different temperature profiles for different materials as you wish. These advantages work out when the processor has relatively small lot sizes of many different products.
Product quality, on the other hand, depends on the parts being processed seeing the “same” temperature irrespective of their position in the furnace. As furnaces get larger it becomes more difficult to know and define the “sweet spot” within a furnace, because when a thermocouple reads a certain temperature it does not mean that the whole furnace is at that temperature. This is especially true for a large batch furnace heating up with a full load when there is a large temperature gradient between the outside of the load and the center of the load. Hence, temperature uniformity inside a furnace is a necessity for obtaining quality products for all thermal processing.
So, how do you assure the temperature inside a furnace is uniform? First, you break up the heater zones from one to six so you have six zones and control them individually. Then, you put a thermocouple at the center of the furnace, monitor the temperature at the center and use this thermocouple to time the soak periods. Then you take the errors of each of the individual thermocouples to obtain the correct temperature. This paper shows how each of these factors affects the temperature inside a furnace.
|Fig. 1. Schematic diagram for atmosphere and/or vacuum furnaces|
Figure 1 shows a schematic diagram for typical atmosphere or vacuum furnaces, which may be horizontal or vertical. The details of the furnace used for testing are as follows:
- A horizontal furnace with a square hot zone with molybdenum heat shields and tungsten heaters with six-zone temperature controls.
- It has a 4.5-foot3 molybdenum retort that holds 44 8-inch x 12-inch load shelves.
- The retort atmosphere is either partial pressure with laminar gas flow of argon, nitrogen, hydrogen or vacuum.
- The intended use for the furnace is to sinter stainless steels or titanium-based alloys.
|Fig. 3. Schematic diagram showing the position of the survey thermocouples on shelves|
Per AMS 2750D, furnaces with work-zone volumes larger than 3 foot3 but less than 225 foot3 must have eight temperature uniformity survey (TUS) sensors located in the eight corners of the work zone and one TUS sensor located in the center. This has been described in an earlier paper.
Hence, eight survey thermocouples were placed at the eight corners of the furnace – four in front and four in back. Two more survey thermocouples were placed near the center thermocouple that is used to control the time in the furnace. These were placed on the right-rear corner of the front left tray and the front left corner of the back right tray at the 6th level from the bottom. These temperature readings were recorded on a different computer from the furnace computer that recorded the temperatures in the six points of the heating elements and the center thermocouple. All surveys were performed with calibrated K-type thermocouples with AccuTemp corrections. The temperature setpoint for all furnace runs was 1200°C. All seven furnace thermocouples belonged to different manufacturing lots and had different correction factors
Figure 2 shows the positioning of the survey thermocouples at the front of the furnace. Figure 3 shows the placement of all the survey thermocouples. To simulate single-zone control, all the heater zones were controlled by the one at the upper-middle position in the furnace. AccuTemp corrections were not activated for the seven thermocouples. Readings were taken under vacuum only.
In addition to the single-zone simulation, four sets of temperature measurements were taken using the standard six-zone temperature control:
1.) Without AccuTemp in 0.040 mbar vacuum
2.) With AccuTemp in 0.040 mbar vacuum
3.) Without AccuTemp in 400 mbar hydrogen
4.) With AccuTemp in 400 mbar hydrogen
The trend screen shots from the furnace computer show the setpoint and all controls reading 1200°C (2192°F), except for the one for the single-zone control. Since the screen shot looks similar to the survey thermocouple, this data has been presented in Table 2. The control thermocouple reads the same as the setpoint, but the other thermocouples show a wide variation.
The trend screen shots for all the experiments from the survey thermocouple computer look very similar, even after zooming in at the corner before cool down. Figures 4 and 5 show examples of these shots.
The data showing the differences between the experiments is shown in table 3 and 4. Table 3 shows the temperature readings as measured in the locations given in Figure 3 for the different experiments performed. Since the center thermocouple in the furnace always reads at a level below that of the control thermocouples, these values have been added to this table.
Table 4 shows a summary of uniformity data. Here is an explanation of the data presented:
- Survey T/C uniformity is the total variation between all the survey thermocouples.
- Maximum deviation from setpoint refers to the maximum difference in the survey thermocouples from the setpoint, which may be positive or negative.
- Setpoint deviation to middle is the maximum deviation of the two middle survey thermocouples with respect to the setpoint.
- Setpoint deviation to center T/C is the deviation of the center thermocouple from the setpoint.
The results show that for single-zone temperature control in vacuum, we have the largest variations in the temperature readings. All the furnace thermocouples in the heating zones, other than the control thermocouple, show significantly lower readings than the setpoint. The variations, as shown in Table 4 for this case, are the greatest.
Using a six-zone control narrows the difference and results in a significant improvement over the single-zone control system. Correcting the readings of the individual thermocouples by AccuTemp, in conjunction with a six-zone temperature control, results in the least variation. A hydrogen atmosphere results in a larger variation than that in vacuum because the high thermal conductivity of hydrogen causes greater heat loss.
|Fig. 4 (top). Trend screen shot of six-zone control with AccuTemp in hydrogen; Fig. 5. Zoomed-in trend screen shot of six-zone control with AccuTemp in hydrogen|
Experiments on temperature uniformity per AMS 2750D in a 4.5 feet3 work-volume furnace with molybdenum hot zones and a molybdenum retort showed that:
- A single-zone temperature control produced a wide variation in temperature readings.
- Changing to a six-zone temperature control resulted in a significant decrease in the temperature variation.
- Correcting the temperature readings of the individual furnace thermocouples by using AccuTemp in conjunction with the six-zone temperature control resulted in the least variation in the temperature readings.
- The use of a hydrogen atmosphere results in a larger temperature variation in comparison to vacuum because of the greater heat loss due to the high thermal conductivity of hydrogen. IH
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