The disclosure relates to a flange for a process tube in an apparatus for processing substrates, e.g., a vertical furnace. The flange may be provided with an opening for in use giving access to the process chamber of the process tube and a cooling channel for allowing a cooling fluid to flow there through and cool the flange. A material with a heat conductivity between 0.1 and 40 W/m K may be at least partially provided in between the cooling fluid and the rest of the flange.