ESDU 78018 gives calculation procedures for the calculation of three types of problem. They are the calculation of the pressure change in a system of known geometry@ with known mass flow rate and heat transfer distribution@ the calculation of the mass flow rate in a system of known geometry@ with known inlet and outlet pressures and known heat transfer distribution@ and the calculation of the diameter of a constant bore system of known length@ with known mass flow rate and known inlet and outlet pressures. Worked examples illustrate the use of the procedures in all three cases.