during a manufacturing process, hot air flows through a duct carrying fine particles of a glutinous substance. With time, those particles stick to the walls of the duct and create a thick layer on them. This layer reduces the cross-section area of the duct, changing the velocity of the flow. To remove this layer, the flow has to be stopped. Any ideas of how to remove the layer without having to stop the flow? Or to prevent the buildup of the layer?