A supply duct is provided for supplying a gaseous fluid at a constant flow rate per unit length of the duct. The supply duct includes a main compartment, at least one nozzle disposed in the main compartment, and a plurality of restrictors extending from an inner wall of the main compartment. The main compartment of the supply duct has a generally constant cross-section. Each nozzle in the main compartment defines an outlet to allow a gaseous fluid to flow out from the main compartment. Each restrictor is configured to produce a flow rate through the nozzle at the first end of the main compartment that is substantially equal to the flow rate through the nozzle at the second end of the main compartment. The restrictor may be, for example, a restrictor plate. A method of sizing restrictor plates is also provided.