On April 17-19 2024, TAV VACUUM FURNACES participated as a speaker at the 4th MCHTSE, Mediterranean Conference on Heat Treatment and Surface Engineering.

Our speech focused on the energy aspects of vacuum heat treatment, a subject towards which all of us within the industry need to pay attention for reducing the carbon emissions aiming at a zero net emissions future.

We have already analyzed the essential role that vacuum furnaces will play in this transition, with a focus on the optimization of energy consumption in our previous article; with this new presentation instead, we wanted to emphasize how selecting the right vacuum furnace configuration for specific processes may impact the energy required to perform such process. For doing so, we compared two different furnace designs, i.e. single chamber VS dual chamber vacuum furnaces, detailing all of the components energy consumption for a specific process.

As a sneak peek into our presentation, we will summarize below how the main features of the two vacuum furnaces design are affecting their energy performance.

Let’s start by introducing the protagonist of our comparison: a single chamber, graphite insulated vacuum furnace, model TAV H4, and a dual chamber furnace TAV DC4, both having useful volume 400-400-600mm / / 16”x16”x24” (WxHxD). 

 

TAV DC4, dual chamber vacuum furnace for low pressure carburizing and gas quenching

 

In a single-chamber vacuum furnace, as the TAV H4, the entire process is carried out with the load inside the furnace hot zone. This represents a highly flexible configuration that allows to perform complex heat treatment recipes with a multiple sequence of heating and cooling stages and to precisely control the temperature gradients at each stage.

The TAV DC4 dual-chamber vacuum furnace is instead equipped with a cold chamber, separated from the hot zone, dedicated for quenching. Despite the greater complexity of this type of vacuum furnace, the dual-chamber configuration allows for several benefits.

 

Configuration of the TAV DC4 dual chamber vacuum furnace

 

In first instance, the graphite insulated hot chamber is never exposed to ambient air during loading and unloading of the furnace; for this reason, the hot chamber may be pre-heated at the treatment temperature (or at a lower temperature, to control the heating gradient).

On single-chamber vacuum furnaces instead the hot zone must always be loaded and unloaded at room temperature to avoid damages due to heat exposure of graphite to oxygen. 

This will result in both faster heating cycles and lower energy consumption on the dual chamber furnace, as a substantial amount of energy is required to heat up the furnace hot zone. This advantage obviously will be more relevant in terms of energy savings the shorter the time between subsequent heat treatments.

In addition, since the quenching phase is performed in a separated chamber, in dual-chamber vacuum furnaces the hot zone insulation can be improved by increasing the thickness of the graphite board without compromising the cooling performances. This translates into a significantly lower heat dissipation, to the extent that at 1100°C the power dissipation per surface unit (kW/m2) is reduced by 25% compared to an equivalent single chamber vacuum furnace.

 

View of the cold chamber of the TAV DC4 dual chamber vacuum furnace

 

Additionally, quenching in a dedicated cold chamber allows to obtain higher heat transfer coefficients and higher cooling rates compared to a single chamber vacuum furnace; firstly, because the cold chamber is dedicated solely to the quenching phase and it can be designed for optimizing the cooling gas flow only, without the need to accommodate all the components required for heating. All things considered, the heat transfer coefficient achievable in the TAV DC4 can be, all other things being equal, even 50% higher compared to a single chamber vacuum furnace. Secondly, since the cold chamber remains at room temperature throughout the whole process, only the load and loading fixtures need to be cooled down; as a result, the amount of heat that needs to be dissipated is significantly less compared to the single chamber counterpart.

 

CFD simulation showing a study on the cooling gas speed in a section of the cooling chamber for the TAV DC4 dual chamber vacuum furnace

 

For heat treatments requiring high cooling rates, it is then possible to process significantly higher loads on the dual chamber furnace compared to the single chamber model; translated into numbers, the dual chamber model can effectively quench as much as double processable in a single chamber furnace, depending on the alloy grade, load configuration and overall process. The savings in terms of energy consumption per unit load (kWh/kg) achievable on the dual chamber furnace, for such processes are remarkable and can be as high as 50% compared to the single chamber furnace.

In the end, the aim of the speech was to highlight how the energy efficiency of vacuum furnaces is highly dependent on the machine - process combination. Choosing the right vacuum furnace configuration for a specific application, instead of relying solely on standardised solutions, will improve significantly the energy efficiency of the heat treatment process and drive the return on investment.

To find out more about which solution is the most effective solution for your vacuum heat treatment process, contact us at info@tav-vacuumfurnaces.com.