An example that has already proven itself in practice explains the procedure for designing the electric heating system for a roasting machine with a batch size of 5 kg. This was equipped with a modern electric air heater. The functionality is visualized in the image below.
The drum roaster is controlled via the parameters of temperature, the air volume and drum speed. The software of the roasting machine regulates the heating output and the exhaust fan in such a way that the roasting process exactly follows the speci ed temperature pro le of the roasting curve. The two blowers operate in a “master/slave” mode to ensure that there is always a negative pressure or a controlled draft in the roasting machine. The exhaust air blower works in master mode. The bean temperature, which is measured directly at the bean in the drum via a sensor, is decisive for temperature control. In addition, the outlet temperature is also monitored. This is measured before the exhaust fan. The hot air inlet temperature is also checked and measured on the air heater. In order to prevent overheating of the heating element even in the case of faulty operation, it is recom- mended to install an air ow meter or pressure sensor in front of the air heater. This ensures that su cient air- flows through the roasting machine before switching on the air heater. An important prerequisite for the feasibility of the process is the precise control of the air volume or the blower and the temperature of the air heater. Blowers and air heaters must therefore be selected that can be precisely controlled and regulated and matched to the speci c design of the roasting machine in terms of performance and dimensions.
Mathematical Design of the Heating System
In order to de ne the appropriate electrical heating system, the required heating output must be calculated in advance. The following variables are required as a basis for the calculation:
Design,structureanddimensionsoftheroastingmachine
Insulationoftheroastingmachineandtypeofinsulationmaterial,itsthicknessanditsthermal conductivity λ [W⁄m*K]
Batchsizeinkilograms[kg]
Maximumheatingtemperature(dependingonthedesignoftheroastingmachine)indegreesCelsius[°C]
Temperatureontheoutputfanaftertheheatingphaseoftheroaster
Maximumairvolume owV̇ instandardliterperminute[slpm]
Approx.roastingtimeinminutes[min] The following graphic illustrates this example calculation:
