A system arranged in sequence such that the flow through each part is the same. In electric circuits in which the circuit elements are arranged in sequence, the same current flows through each in turn where the total resistance is equal to the sum of the individual resistances. In heat transfer, the temperature drop across a wall comprising several materials is the sum of the temperature drops for each material:
For pipe flow, the total pressure drop across several pipes linked in series is the sum of the pressure drops for each pipe: