For many years, vacuum furnace designers have tried to improve hot-zone designs with the hope of minimizing power losses to reduce operating costs. Unfortunately, new and more efficient designs have been very slow in reaching the marketplace.
The task of finding better materials and designs to produce a significantly more efficient hot zone was undertaken by the Solar Manufacturing Research and Development team. The preliminary results of these efforts are outlined below.
Materials and Testing Equipment
Most furnace hot zones use a graphite foil “hot face” backed by multiple layers of ½-inch graphite felt, all supported in a ring structure. Many standard furnaces are built with this design, using four layers of graphite felt.
In order to get a baseline for our initial studies, we tested this basic insulation configuration in one of our laboratory furnaces by modifying the hot-zone cover assembly to represent this design. Test thermocouples were located on and within the cover assembly to establish a temperature profile across the insulation and, most critically, to record the support-ring outer surface temperature.
The modern vacuum furnace uses a double-wall design with cooling water continually flowing between the walls. This provides an inner/outer chamber wall temperature that is typically at 100-130°F even when the internal hot-zone temperature is running at 2400°F (1343°C).
Solar Manufacturing has determined that the true losses in a furnace are defined as the radiating energy between the support-ring outer surface temperature and the inner chamber wall of the furnace. This can be calculated using the Stefan-Boltzmann Law of radiating surfaces where the difference in the two temperatures is raised to the fourth power (in degrees Kelvin). The results of this calculation highlight the need to keep the support-ring outer surface temperature as low as possible. The equation is:
P = e σ A(T4 – Tc4)
where P = net radiated power; e = emissivity of the radiating surface; σ = Stefan’s Constant – 5.6703 x 10-8 watt/m2K4; A = radiating surface area; T = radiating surface temperature (support-ring outer surface); and Tc = surrounding surface temperature (inner chamber wall).
All of our tests were based on an empty furnace with a cycle that brought the furnace to three different temperatures (1750°F, 2000°F and 2250°F) with a hold time of one hour at each temperature to allow for complete stabilization. Utilizing available thermo-couple (T/C) ports, three work T/Cs were used with the prime T/C bolted to the outside plate to indicate the support-ring outer surface temperature. This was the temperature that we hoped to reduce with our new approaches, and it was the investigation’s focus temperature. Although the other two temperatures located within the insulation were interesting, they only reflected relative infor-mation within each different design.
Our first test established a base for subsequent tests by utilizing the most commonly used insulating package, which was a graph-ite-foil hot face and four layers of ½-inch graphite felt, all supported in a stainless steel ring or plate. In this Test 1, the results of the outside ring temperature are seen in Figure 2.
Due to the physical furnace dimensions, we were limited as to how we could modify the insulation package to reduce this sup-port-ring outer surface temperature. Prior studies have indicated that adding one or two layers of graphite felt will reduce losses in the area of 12-25%. Graphite felt is an expensive material and will absorb moisture when the furnace door is open, however, making it harder to rapidly achieve good vacuum levels. Therefore, we proceeded to find an alternate solution.
Our first attempt to reduce the support-ring outer surface temperature was to consider a molybdenum sheet placed between the first and second layer of graphite felt. In all-metal-shielded furnaces, molybdenum provides excellent reflective characteristics, and we wanted to see what impact it might have on the felt layers. This test was identified as Test 1A.
The results proved to be very interesting, with calculations showing an improvement in radiation losses of approximately 20% over the base hot zone of Test 1. Molybdenum is somewhat expensive and more difficult to work with, however, so we tested other alternatives.
Our next attempt to reduce the support-ring outer surface temperature was to replace the molybdenum sheet with a sheet of graphite foil. Graphite foil has reflective surfaces, and we thought it would be interesting to see how it would perform between graphite felt layers. This had not been done before, to our knowledge, in production vacuum furnaces. This graphite foil was inserted between the first and second layer of felt. Results of the support-ring outer surface temperature for Test 2 are shown in Figure 4.
The results were very encouraging and indicated that we were on the right track. In terms of losses based on the radiation equations, this represented an improvement in the area of 22-23% over the base hot-zone design of Test 1.
Based on Test 2, we decided to insert a second sheet of graphite foil between layers two and three of the graphite felt. This resulted in further improvement (Test 3 results are shown in Figure 5).
These numbers indicated that we had now reduced the losses by 44-46% over the base hot-zone design. Although there is an added cost for the foil layers, it becomes somewhat insignificant over the long term with excellent payback after the first 6-12 months and provides an additional advantage of extending the life of the hot zone.
Building on the success of Test 3, we proceeded to put a third foil layer after the third felt layer. Results are shown in Figure 6.
This reduced power losses by a factor of 61-64%, depending on the hold temperature. These numbers become most important when the furnace cycle requires holding at high temperature for extended times. Figure 7 summarizes the results of Tests 1-4.
The data of Figure 7 is presented in Figure 9 and Figure 10.
The photo in Figure 8 shows a new furnace hot zone built with this more-efficient design. This leads us to conclude that additional layers of foil or other foil arrangements could possibly further impact the results. We tried different combinations with results which are summarized in Figure 11.
None of these results showed an improvement over Test 4, leading us to conclude that foil interspersed between each layer provides an excellent barrier to conduction through the graphite felt.
PAN vs. Rayon Graphite Felt
All of the above tests were performed using PAN-type (Polyacrylonitride) graphite felt. Claims have been made that rayon-type graphite felt is much more efficient than PAN graphite felt, although it is priced higher. In order to compare the two felt types, we repeated Test 4 (including the foil interspersed) using Rayon graphite felt with the comparative results shown in Figure 12. This data shows a loss improvement in the area of approximately 20% and should be considered on future applications based on types of cycles and length of holding times.
Other Hot-Zone Concerns
The aforementioned studies are concentrated on reducing power losses through the hot-zone insulation. However, this is only one of the overall concerns of the vacuum furnace designer.
Most vacuum furnace hot zones have various penetrations to retain or support certain hot-zone members. These include insula-tion retainers, element supports, hearth pins and gas cooling nozzles. All of these components provide heat conduction losses. Since the insulation retainers, element supports and gas cooling nozzles are all connected to the supporting ring assembly, they will also influence the final ring temperature. Finding improved ways of isolating these members to minimize thermal conduction must be addressed. This will be discussed in a future technical paper. Thermal losses through the hearth support pins will also be evaluated.
Based on testing to date, we conclude the following:
• Graphite foil, when inserted between each graphite-felt layer, provides an excellent barrier to hot-zone insulation losses.
• Placing a graphite-foil layer between graphite-felt layers in a production vacuum furnace hot zone will reduce insulation power losses by as much as 65-70% when properly configured.
• Although graphite foil adds some cost and weight to a graphite-felt hot zone, the payback in power savings means that the original cost is easily recovered over the first 12-18 months of operation. We also expect that this arrangement will extend hot-zone life by 2-3 years.
• With the continual escalation of electrical power cost, any significant reduction in overall power usage is most important to all production vacuum furnace users.
• Rayon graphite felt is approximately 20% better than PAN graphite felt regarding losses at temperature.
The concept of inserting graphite-foil layers between graphite-felt layers has been named the Solar PowerSave Hot Zone by Solar Manufacturing Inc. This design is protected under license of U.S. Patent Number 7,760,992 covering the vacuum furnace industry. The author appreciates the contributions of William R. Jones and Trevor Jones of Solar Atmospheres Inc. to these investigations and this manuscript. IH
For more information: Contact Reál J. Fradette, senior technical consultant, Solar Manufacturing, Inc., 1983 Clearview Rd., Souderton, Pa. 18964; tel: 215-721-1502 x 560; fax: 215-723-6460; e-mail: firstname.lastname@example.org; web: www.solarmfg.com