Industrial thermoprocessing plants often still offer a high potential for optimisation in terms of cost and energy efficiency as well as the product quality achieved. Optimisation approaches concern the kiln insulation, setting plans, kiln furniture used as well as the temperature cycles and kiln atmospheres - the latter concerning composition and gas flow. Optimisation can be much more targeted if the kiln is not used as a black box in input-output analysis, but if detailed data on the heat treatment are available.
For this reason, a mobile furnace measuring stand was put together at Fraunhofer-Center HTL, which can be used on-site at the industrial furnace without having to interrupt the furnace operation. The following measurements can be carried out with the mobile furnace measuring stand:
- Quantitative measurement of heat flows on the outer shell
- Uninterruptible flow measurement for cooling water
- Uninterruptible power measurement of electrical consumers
- Measurement of temperature distribution in the useful volume
- Measurement of the composition of the furnace atmosphere
- Measurement of gas flow in the furnace
- Analysis of the kiln exhaust gases
3D thermography and the measurement of heat flows are used to identify heat leaks and to evaluate the resulting losses. Measurements of the cooling water and electrical consumers are used to draw up energy balances. The quality of the heating process can be assessed via the measurements of the furnace atmosphere and temperature distribution.
The composition of the furnace atmosphere can have a major impact on the result of the heat treatment. For example, the slightest impurities in the atmosphere of inert gas furnaces can already lead to rejects. The HTL has electrochemical/optical sensors with which critical gas species (e.g. CO, O2, CO2, SO2, NO2, CxHy) in the furnace exhaust gas can be analysed. Up to temperatures of 1200°C, furnace gases can also be extracted directly from the useful volume through a gas sampling lance. Gas flows can be measured directly in the furnace atmosphere or the exhaust gas with a differential pressure lance developed at the HTL (up to 1200°C) or with a vane anemometer (up to 600°C).
Homogeneous temperature distribution is crucial for optimal product quality in industrial furnaces. The HTL offers temperature distribution measurements of varying accuracy. The basis is provided by the commercially available temperature measuring rings or temperature measuring rings developed at the HTL for particularly high temperatures. The temperature measuring rings are conditioned and calibrated at the HTL before use. This allows the scattering of the measurement results to be reduced and the interpretation of the shrinkage data to be quantitatively converted into temperature differences. The temperature measuring rings are positioned at different points in the furnace chamber and evaluated at the HTL after the heating process. The ThermoOptic Measuring (TOM) equipment at the HTL is used to calibrate the temperature measuring rings.
With the mobile furnace measuring stand, potential analyses are carried out by the HTL on industrial furnaces, which already show optimisation possibilities. Depending on the level of detail of the potential analysis, a simplified finite element (FE) model of the furnace system can be created from the values measured at the furnace (heat flow, exhaust gas temperatures, atmospheric flows, cooling water temperatures, etc.). Using this model, changes to insulation, flows or heating parameters can be visualised and their effects on the furnace operation and the product can be investigated. Heat management in the useful volume of the kiln can also be optimised using FE simulation. By combining the furnace simulation with the simulation of the process kinetics in the material to be heated (drying, debinding, sintering, melt infiltration) developed at HTL, an overall optimisation with regard to product quality and energy efficiency can be carried out for many heat treatment processes.
The potential analysis as well as the subsequent FE simulations show different optimisation possibilities. Changes to the process parameters can often be implemented without having to make investments. Even repairs to the kiln insulation do not require large investments. Changes in the kiln furniture or the kiln insulation, on the other hand, are usually associated with larger investments or downtimes. In addition to the kiln analysis, the HTL also carries out estimates of the service life of refractory materials and kiln furniture. This allows the customer to estimate the effects of the improvements and to carry out a cost-benefit analysis.
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