Method and system for controlling suction of off-gases from electrolysis cells

The present invention relates to a method and a system for controlling the normal operational suction of off-gases from individual electrolysis cells for production of aluminium, where the cells can be of Hall-Héroult type, and further being provided with a hooding (CH) connected via a gas duct (GD) to a main gas duct (MGD) having means for generating a suction that transports the gas to a gas treatment centre (GTC), where the gas flow in said gas duct (GD) is controlled by a gas duct damper (GDD). One or more process variable/s such as pressure and/or temperature in the gas duct (GD) of each cell are measured and used as input signals to a controller (PLC) comprising a calculator where the controller calculates the actual mass flow in said gas duct (GD) based upon a pre-defined algorithm and produces an output set signal corresponding to a wanted flow rate, the signal is transmitted to an actuator (A) that regulates the position of the gas duct damper (GDD), and followingly the gas flow in the gas duct from each individual cell. The regulation of the flow in the gas ducts of the individual electrolysis cells is regulated towards an optimised minimum flow. This way of controlling the suction has shown to minimise the overall flow rate and savings can be achieved both in reduced effect from the fan but also less temperature loss from each individual cell.