A type of idealized tubular reactor that features no radial or axial mixing. All the components that flow through the reactor therefore possess the same residence time. A PFR may consist of either one long reactor or many short reactors as a tube bundle. The reactants flow through the length of the reactor during which the chemical reaction rate changes. They are usually used for gas-phase reactions requiring high temperatures. Used for both exothermic and endothermic reactions, heat transfer is effective through the tube walls.