Control of the concentration of solvents in a dryer

A dryer for drying printed lengths of carrier materials that have been printed with an ink containing an evaporable solvent comprising:

• a chamber, through which the lengths of carrier material are fed;
• a supply channel connected to said chamber for supplying a heated gas mixture to said chamber;
• a venting channel connected to said chamber for venting the gas mixture from the chamber having executed the drying process;
• a heater for heating the gas mixture connected to said supply channel and said venting channel, said chamber, said supply channel, said venting channel and said heater forming a venting circuit;
• and
• means for venting a part of the gas mixture from the circuit having a control element for controlling the amount of gas mixture vented from the circuit as a function of the concentration of the evaporated solvent, wherein said heater is a fuel supplied burner and said means for venting a part of the gas mixture from the circuit determines the concentration of the solvents by measurement of the temperature and measurement of the flow rate of the gas mixture supplied to the burner, by measurement of the temperature of the gas mixture heated by the burner, measurement of the flow rate of the fuel supplied to the burner, and thereby calculating the burning heat supplied to the burner by the fuel, calculating the heat supplied to the gas mixture containing evaporated solvents, and calculating from the difference of the burning heat supplied by the fuel to the burner and the heat supplied by the burner to the gas mixture containing evaporated solvents the heat value of the evaporated solvents.