The range of applications of industrial furnaces is highly diverse. A distinction is made here between numerous different applications in which the products to be heat-treated are subjected to specific temperatures depending on the application. A wide variety of materials (such as steel, aluminium, plastics, rubber, silicone, carbon, composites, composite materials, glass, ceramics etc.) in their various, sometimes complex, component geometries with parameters specific to the product are not simply heated in the industrial furnace, but sometimes follow different time/temperature levels for the application in question. In addition, depending on the customer requirements, application product and filling level the industrial furnace, there are further parameters to be considered or met, such as heating and cooling rates, temperature homogeneity, the safe handling of any combustible materials escaping from the product and the measurement, recording and documentation of the process.
At Airtec, the focus is always on implementing the specific and necessary process and customer requirements. The many years of material and application specific know-how, in connection with the application of continuously developed and flexible Airtec technology, guarantees the optimum implementation of all heat treatment and heat recovery processes for the benefit and success of our customers.
The industrial furnace and heat recovery plants from Airtec are used for the production of a wide range and sometimes highly complex range of products in different industries. All procedures required in terms of the heat process technology are applied here a targeted way in line with the product to be treated.
A distinction is primarily made between the following heat treatment applications:
tempering, ageing, heating/warming, hardening, solution heat treatment, tempering, drying
The following descriptions show, without claiming to be exhaustive, the variety of applications as well as what needs to be considered when mapping the process.
Tempering Process
In the tempering process, the product to be treated is heated to a temperature below its melting point over an extended period of time.
The product is heated to a temperature above the lower flash point and kept at this level until the product is uniformly heated through.
Subsequently, the product is cooled down in a targeted manner, whereby a distinction is made between two basic cooling modes (slow, fast).
While slow cooling frees the product to be treated from internal stresses, e.g. to avoid unwanted deformations, and increases the toughness of metal products depending on the duration of the tempering process, fast cooling uses the “freezing” of the structural matrix to increase the resistance, i.e. the thermal and mechanical strength of the material.
After steel hardening or welding, for example, heat treatments such as tempering are necessary to achieve the required material properties (toughness, yield strength).
At temperatures of generally 200 °C to 550 °C, the heat treatment can last from minutes to several hours.
During tempering, of springs, for example, the springs are also partly pre-tensioned, i.e. loaded on the block, in order to prevent subsequent setting.
Ageing:
With the ageing process at temperatures of generally 100 °C to 240 °C, strength values can be increased or adjusted via temperature and time by means of tensions in the crystal lattice of a steel.
Related processes include, for example, soft annealing for applications of vehicle parts that are relevant in crash tests or further processing after homogenisation.
Heating/heat maintenance process
Many chemical components have to be tempered or heated before processing, as they are exposed to the weather conditions, e.g. from transport or outdoor storage. During the processing of liquid products, drums are often stored in industrial furnaces to be able to supply them at the right temperature for the production process.
For the assembly of mechanical assemblies, parts are also preheated before shrinking, or tools such as casting tools, rolling tools, forging tools and moulds for processing PU or PA must be preheated before installation in the production plant and kept warm until the time of use.
For dipping preheated workpieces into liquid coating materials, e.g. PVC, with subsequent gelling process to smooth the surface, the workpieces must be heated continuously or in batch operation to the melting temperature of the coating material.
Hardening process
In the hardening process, the material matrix is subjected to a targeted structural transformation in the product to be heat-treated by the action of temperature in the hardening furnace. The microstructure of the material matrix is further developed until the desired or final material structure is achieved.
The aim of this process is to increase both the dimensional stability and the mechanical resistance of the product. While this property is achieved in plastic products by the reaction of the binding systems responsible for binding the material matrix, metal products require rapid cooling after the tempering process, during which the microstructure is “frozen” to the required extent.
If further-reaching changes in the properties of the product are required, these can be achieved by a subsequent annealing or tempering process in the annealing furnace.
The hardening of phenolic resin compounds, for example in the production of resin-bonded abrasives or discs, is thus also referred to as hardening.
This also applies to the processing of components made of carbon or glass fibre reinforced plastics.
Numerous applications can also be found in the curing of epoxy resins, for example, in electrical engineering, such as in the insulation of the copper windings of electric motors.
Silicone tempering is also regarded as a curing process.
In order to allow the reaction partners in the plastic injection moulding part to bond, especially in the case of high-quality products such as silicone, more than one component is provided for reaction or polymerisation.
Especially with silicone products such as babies’ dummies or medical products, it is therefore essential to expel the components remaining from the reaction. In what is referred to as the silicone tempering process, these components are bonded to the supplied oxygen in the furnace at high temperature, and removed.
The silicone manufacturers specify a reference value of less than 10 m³/h fresh air for each kilogram of silicone in the furnace. The high proportion of fresh air to be introduced into the industrial furnace thus also means the introduction of a relatively high heating capacity.
In order to keep this energy requirement as low as possible, heat exchanger systems can be provided here to preheat the fresh air quantity with the exhaust air, which often pay for themselves after a few months. At the same time, the polluted exhaust air must be safely discharged from the process area of the industrial furnace.
In order to prevent condensation from forming in the industrial furnace in the above examples, which can lead to high levels of contamination and even to furnace fires, it is also necessary to ensure consistent sealing at all points that can be opened, to avoid thermal bridges on the housing and to avoid return flow from exhaust air pipes.
Also to reduce cost-intensive press time during the forming or vulcanising of rubber parts and rubber-steel compounds, parts are removed from the press after they have retained their shape and then placed in a post-vulcanising furnace for curing, i.e. final structural strengthening.
Solution annealing
During solution annealing, including quenching of aluminium alloys, the atoms of the workpieces are completely dissolved at temperatures of up to 540 °C.
For this purpose, the temperature uniformity in the solution annealing furnace must be ensured in such a way that no coarse particles remain in the workpiece due to a too low temperature or a too short heat treatment time, which are not dissolved, or which can cause melting processes to occur due to a too high temperature.
The subsequent quenching process serves to “freeze” the dissolved structure.
In order to achieve this, rapid transport to the quenching process in a few seconds must be ensured. The quenching medium is usually cold or slightly heated water, polymer solutions or, increasingly for large parts, air.
When quenching with air, close cooperation with the customer or even trials are necessary, since the uniformity of the quenching process and the quenching rate within a very short time is critical.
Tempering
Tempering is the heat treatment of different materials to compensate for mechanical properties, e.g. the relaxation of the material matrix after the production of glass, thermoplastics such as silicone plastics, but also thermosets.
Some annealing processes also have the capacity to ensure the dimensional accuracy of the workpiece. After galvanic coating for corrosion protection, for example, an annealing process must be carried out downstream to prevent hydrogen embrittlement.
Drying
Drying is the most widespread topic for industrial furnace applications.
This refers to the drying of components that are made of water-soluble components for production but which must be dry at a later stage.
Drying is also used for solvent-containing coatings (lacquers) on the surface in accordance with DIN EN 1539, or after cleaning industrial cleaning systems.
In the drying process, liquid is extracted from the product to be heat-treated by vaporisation or evaporation in order to either simply dry a product or to cause the binding of resin systems.
Critical in the drying process by convection is the vapour pressure of the liquid to be expelled at a certain temperature and pressure. If the vapour pressure is identical or lower than the liquid content in the atmosphere, drying is not possible.
However, if the vapour pressure is higher than the liquid content in the atmosphere, the speed of the drying process depends on the gas exchange (convection surface, air flow).
As the vapour pressure usually increases at higher temperatures and thus favours drying, the drying process of the product to be treated is designed to the parameters required for this purpose, depending on the type of liquid to be expelled.
When drying paint for surfaces, uniform air flow is also required.
To avoid burning in contamination in the paint layer, fresh air filters are used, as well as circulating air filters.
A special application in paint drying is the drying of bulk materials, such as screw clips or clamps for the automotive industry, which have been coated in painting centrifuges with e.g. zinc lamella coatings in masses of approx. 6 t/h and have to be evaporated and baked on belts or trays.
All paint baking processes must be carried out in accordance with the DIN EN 1539 standard (dryers and furnaces in which flammable substances are released), which is used, among other things, to determine the quantity of exhaust air in order to prevent explosive atmospheres of the solvents used in the furnace.
In plants for impregnating varnish drying of motor windings, since the introduction of water-soluble, but also thinner resins, care must be taken to ensure that the resin is prevented from flowing off, e.g. by rotating the workpiece in the dryer or by effective ventilation to achieve a high heat transfer rate, and is heated quickly and polymerised before the resin can flow.
After washing processes in cleaning plants, or after galvanic coating, workpieces must also be dried. For spotless drying, or when drying adhesive water in gaps or in blind holes, effective special ventilation or the use of dried air is recommended.
Continuous or discontinuous dryers are also used for dissolved or mixed masses in the chemical or process engineering industry.
Material properties according to customer requirements
The customer product, i.e. the targeted achievement of the material properties desired by the customer, is the focus of every heat treatment process. This is a central component of the actions of the entire Airtec team. Based on the specific product requirements, our engineers and technicians develop tailor-made process solutions and, together with our specialists, convert these into modern and energy-efficient industrial furnace and cooling technology, or a combination of both. In this way, system solutions are created from the cross-industry wealth of experience, from which our customers benefit to the full extent. Implementation examples can be found on our product sub-pages
If you have further questions about the processes of our heat treatment or cooling plants, please do not hesitate to contact us. We will be happy to advise you.
