Considerations for selecting vacuum furnace hot zone ...

When choosing the appropriate design for a vacuum furnace hot zone, several critical factors must be taken into account to ensure optimal performance and efficiency.

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Over the years, variations in hot zone insulation designs have been numerous for vacuum furnaces, encompassing a myriad of configurations and materials. This article breaks down these designs and their respective performance results with regards to vacuum and power efficiency.

Configurations include:

• a: Graphite foil facing backed by 2"-3" Kaowool insulation.
• b: Graphite foil facing backed by 2" PAN graphite felt.
• c: Graphite foil facing backed by 2" Rayon graphite felt.
• d: Standard graphite board (2") with foil facing.
• e: New GMI graphite board (2") with foil facing.
• f: All-metal (3 molybdenum, 2 stainless steel shields).
• g: All-metal (5 molybdenum shields).

Key Factors for Selection

1. Desired Ultimate Vacuum Level

Processes requiring "soft vacuum" levels within the mm Hg or 10-3 to 10-4 Torr range can typically be achieved with any of the aforementioned designs, particularly configurations (a) through (e). However, processes demanding super-clean environments with vacuum levels in the 10-5 Torr range or higher necessitate specific designs like (b) through (e). For processes needing 10-6 Torr or better, all-metal hot zones (f) and (g) are required.

2. Maximum Processing Temperature

Each hot-zone configuration comes with distinct maximum operating temperatures, and while some hot zones might be driven above their normal ranges, sticking to recommended normal processing temperatures is advised.

3. Thermal Efficiency and Power Losses

Hot zone efficiency is determined by the power losses from the outer support ring to the inner cold wall of the furnace. Key factors include the ring's temperature, which significantly impacts power loss calculations. The outer support ring temperature varies depending on the insulating materials used.

Analyzing different materials reveals that:

• a: Rayon graphite felt proves more efficient than PAN graphite felt.
• b: The GMI board material stands out as the most efficient, improving with rising temperatures.
• c: All-metal hot zones are the least efficient.

At 2,000°F, based on the HFL-3648 Furnace, the data suggests that Rayon Felt hot zones are approximately 12.8% more efficient than PAN hot zones, while the GMI Board [1] hot zones boast 61.8% higher efficiency compared to Rayon and 66.7% better than PAN.

4. Overall Cycle Time

Cycle times are influenced by various elements such as:

• a: Vacuum pumping capacity.
• b: The insulation’s moisture absorption rate.
• c: Insulation’s efficiency in overcoming recurring losses.
• d: The mass and thickness of the workload.
• e: The gas cooling system’s capabilities.
• f: The type of insulation used.

5. Minimal Part Surface Contamination

Moisture, water, and pick-up are significant concerns with felt-insulated furnaces, leading to oxygen contamination. Comparative testing between PAN and Rayon felt stacks against all-metal shields demonstrated that PAN insulation is more susceptible to moisture pick-up, highlighting the superiority of the all-metal hot zone in maintaining cleaner environments.

6. Conclusions

To summarize, selecting the right hot zone design involves evaluating multiple factors such as vacuum levels, processing temperatures, thermal efficiency, cycle times, and contamination risks. Cost is also a notable consideration, where understanding the expense associated with each configuration aids in making an informed decision.

More detailed information can be found about Continuous Brazing Furnace.

For additional insights, explore The advantages of Vacuum heat treatment.

Recently, in Hermitage, Pennsylvania, Solar Atmospheres of Western Pennsylvania completed a hot zone replacement on one of its larger horizontal car bottom vacuum furnaces. This marks the second rebuild over 17 years of operation.

The furnace, featuring a work zone of 54"W x 54"H x 144"L and a load capacity of 50,000 pounds, has been in continuous service since 2003. Built by Solar Manufacturing, the furnace boasts a maximum operating temperature of 2800°F and performs various daily cycles ranging from 500°F to 2750°F.

The hot zone comprises three ring assemblies that support graphite felt insulation and circular graphite heating elements, with additional insulation and heating elements on the loading cars for uniform bottom heating.

The recent rebuild included insulation replacement on rings and doors, as well as new heating elements throughout the furnace length, effectively restoring it to "new condition" for another 17 years of service.

For more information on the Continuous Vacuum Furnace, feel free to reach out to us.