Note that the previous expression holds true only if the kinetic energy flow rate is conserved between system inlet and outlet. Otherwise, it has to be included in the enthalpy balance. During steady-state operation of a device (see turbine, pump, and engine), the average ⁠ d U / d t ⁠ may be set equal to zero. This yields a useful expression for the average power generation for these devices in the absence of chemical reactions: