In earlier discussions, we explored how various design elements can influence the efficiency of vacuum furnaces concerning their heating energy needs.
While heating indeed forms the major part of vacuum heat treatments, numerous auxiliary systems inside these furnaces—like water-cooling systems, gas quenching systems, and vacuum pumps—also contribute substantially to energy usage.
Though essential for the proper functioning of a vacuum furnace, continuous operation of these auxiliary systems can lead to significant energy consumption.
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Optimizing Energy Usage in Vacuum Pumps
Vacuum pumps enable a sealed furnace to reach the desired vacuum levels before heat treatment begins. The common setup for low and medium vacuum operations (1*10-3mbar to a few millibars) typically includes a mechanical pump—either oil-sealed or dry vacuum—coupled with a roots blower to expedite evacuation.
For high vacuum operations (down to 1*10-6mbar), options vary: Although turbomolecular pumps are favored for special applications requiring a clean environment, oil diffusion pumps are generally more cost-effective and robust, making them common choices.
Detailed information on vacuum pump functions and selection can be found in this related article.
For medium-sized horizontal hardening furnaces (like the TAV H8 model, 800x800x1200mm), the mechanical and roots pump motors can require approximately 5-6 kW and 4 kW, respectively. Larger furnaces (TAV H15 model, 1500x1500x1500mm) may see power needs up to 15 kW for the mechanical pump and 11 kW for the roots blower.
Additionally, oil diffusion pump heating power for the boiling and vaporizing working fluid is about 18 kW for the TAV H8 model and 24 kW for the TAV H15 model. Approximating the pumps' operational power to their peak values results in a total of about 28 kW for the H8 and 50 kW for the H15 vacuum furnaces.
Regular maintenance operations for roots blowers are crucial.
Smart energy management can significantly reduce energy consumption by modulating or switching off vacuum pumps when idle or during heat treatment stages that do not require chamber evacuation. Automated management systems for the pumping units offer these capabilities.
Modern TAV furnaces include various energy-saving features aimed at minimizing vacuum pump energy consumption.
Firstly, automatic pump shutdown occurs when they are not needed—not only during cooling but also during convection heating and loading/unloading between batches. Additionally, using a roots pump equipped with an inverter helps modulate power usage, reducing consumption when ultimate vacuum isn't necessary.
In terms of diffusion pumps, a control loop featuring a thyristor controller and a thermocouple can adjust power output based on the cycle phase (stand-by, heating up, holding), rather than always delivering maximum output. Using silicon oils as the working fluid, this adjustment can save around 17% during normal operations with full pumping capacity. Lastly, TAV furnaces use a holding pump to maintain the required vacuum for the diffusion pump, minimizing the need to frequently activate the main mechanical pump—which is particularly energy-intensive for medium to large furnaces.
Energy-Efficient Gas Quenching Systems
In vacuum furnaces, gas quenching involves circulating high-pressure inert gas using a fan through dedicated gas-guiding systems, cooling the load before returning it to a water-cooled heat exchanger.
The motor power required for the fan varies with furnace size, maximum allowable pressure, targeted gas velocity, and gas density.
Generally, motor power for a fan in larger furnaces ranges between 50kW to 250kW, though specific applications might require higher powers.
Maximum power isn't always needed for all heat treatment processes. For example, in steel hardening, during the low-temperature cooling phase below the martensite finish temperature, the heat generated by the fan motor might outweigh that extracted by the water-cooled heat exchanger, making maximum power both uneconomical and inefficient.
Therefore, an inverter to control the cooling fan motor can regulate its power, reducing energy consumption during cooling phases. In such setups, fan motor speed can be reduced to 30% of its maximum, decreasing absorbed power by 37 times (since absorbed power is proportional to the cube of the rotational speed).
TAV vacuum furnaces can automatically lower the fan motor speed when the furnace or load temperature reaches a certain threshold through "furnace interlock" and "load interlock" functions. Moreover, a "controlled cooling" function self-regulates the fan motor speed to follow a predefined cooling rate.
Water Cooling System Energy Management
Commonly, vacuum furnaces feature a cold-wall design, with the external vessel containing a double wall acting as a cooling water jacket. These systems circulate water through a heat exchanger and back to the furnace using one or more pumps, maintaining the appropriate temperature.
Water pumps are another energy consumption source. Thus, TAV furnaces have an energy-saving mechanism that automatically switches off water pumps once the furnace and diffusion pump have cooled sufficiently.
For closed-loop recirculating systems, water cooling might be achieved using a dry cooler or chiller with a refrigerated circuit. Ideally, the cooling water temperature should be kept below 25°C; slightly higher temperatures can be tolerated for the furnace vessel, but consistent cooling water temperature is essential, particularly for the diffusion pump.
Therefore, a combined water cooling system incorporating a dry cooler and a chiller is efficient.
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A dry cooler efficiently dissipates heat with minimal energy by allowing hot water to flow through cooler tubes, exchanging heat with the ambient air. During warmer seasons, fans can be activated to enhance heat exchange. These fans can be activated at various temperature thresholds, running only when necessary.
If ambient temperatures require it, a small chiller can exclusively manage the diffusion pump's cooling needs. Alternatively, in warmer climates, a larger chiller serving the entire water cooling system might run during hotter months.
Diffusion pump maintenance with its dedicated chiller setup can further optimize cooling efficiency.
If you're interested in mastering your water cooling system's operation and maintenance, be sure to check our detailed article.
Additionally, TAV furnaces feature real-time energy consumption monitoring for each batch, displaying electrical absorptions and cooling fan speed on a specialized "Energies" page. This function allows operators to track process parameters impacting energy consumption and address any deviations from historical data.
Overall, effectively managing auxiliary systems can substantially reduce the energy requirements of your vacuum furnace, lowering operating costs and minimizing the environmental impact of your vacuum heat treatments.
For further details on TAV VACUUM FURNACES and their energy-saving features, contact us at: info@tav-vacuumfurnaces.com
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