The objective of this paper is to quantify the power requirements resulting from the use of a pump driven by a variable frequency drive (VFD) in a water loop heat pump system. The operation of such a system is first reviewed and the required pumping power is calculated with a proper account of the various inefficiencies in the pump, the electric motor, and the VFD. Nondimensional power curves are then derived and are presented as a function of nondimensional flow rate for various ratios of DP _s / DP _t (where DP _s is the differential pressure switch setting and DP _t is the total head requirement at the nominal operating conditions). These general curves can be used for any specific project. The generation of these nondimensional power curves has led to interesting results. For instance, it is shown that, for a given value of DP _s / DP _t , the nondimensional power curves are essentially independent of the slope of the pump curve and of pump efficiency. Furthermore, it is shown that the required pumping power variation is significantly different from the classic pump law which states that power varies with the cube of the flow rate. The usefulness of the nondimensional power curves is then demonstrated by looking at a practical application.

Units: Dual