# Calculation Methods - Fans and Compressors

The performance curves for fans and compressors are usually expressed as simple polynomials, as a series of cubic splines or in some instances as Chebyshev polynomials. FluidFlow uses this approach to store the performance data. For a simple polynomial the expression is ΔP = C1 + C2Q + C3Q2 + C4Q3 ... The performance data in situ is adjusted according to any or all of the following factors. 1. A change in operating speed or impeller size, from that provided in the fan database. If this occurs, the fan performance data is adjusted according to the affinity laws: Q1/Q2 = (N1/N2) (D2/D1) and H1/H2 = (N1/N2)2 (D2/D1)2 and Ph1/Ph2 = (N1/N2)3 (D2/D1)3 2. The suction density also affects the fan or compressor performance. The volumetric flow does not change, but the stagnation pressure developed varies directly with the density ratio. P2 = P1 (ρ 2/ρ 1) and the shaft power requirement varies with the same ratio: Ph2 = Ph1 (ρ 2/ρ 1) Fan Shaft Power is calculated from the hydraulic power and the fan efficiency at the duty point. This is given by the formula: Ph = Q ρ g H / η If Adiabatic compression is selected as the heat loss model then FluidFlow estimates the output temperature from the expression T2 = T1(P2/P1)n where pressures and temperatures refer to absolute static conditions and n is (Cp/Cv-1)/(Cp/Cv) ratio See the Nomenclature section for the symbol definitions.