​Parallel and Series Operation of Centrifugal Fans

Parallel and Series Operation of Centrifugal FansCurrently, while various specifications and models of ventilators are available for system selection, in certain situations where a single ventilator cannot meet operational requirements, multiple units must be operated in combination. Centrifugal fans can be configured for either parallel or series operation. This arrangement is rarely used in standard ventilation systems but is frequently employed in specialized systems. Theoretically, when a single centrifugal fan's characteristics double the pressure at identical volume and flow rates, the performance curve of two fans operating in series can be obtained. In practice, however, the volume flow rate slightly decreases due to increased air density in the later stage of the series-connected fans. The flatter the characteristic curve, the less pronounced the series-connected effect becomes. The performance curve for two centrifugal fans operating in series represents the sum of the total pressure corresponding to each fan at the same flow rate. Therefore, on the same flow rate chart, the corresponding points on the performance curve during operation can be obtained by adding the total pressures of both centrifugal fans. The curve formed by these points constitutes the combined performance curve for the two centrifugal fans operating in series. If centrifugal fans operate in series within a network with steep curves, the operating point in series is at point A. Each fan will then operate at points A and B respectively. When centrifugal fans operate in series within a network with flat curves, the operating point is at point C. Its flow rate and pressure are identical to those of the first fan operating in this network, rendering the second fan inactive. When centrifugal fans are connected in series in a network with a flat curve R1, the operating points are at points B1 and B2, and the fans operate independently within this network. Clearly, the pressure and flow rate after series connection are lower than those of the first fan operating alone. The second fan will impede the operation of the first fan, producing a counteracting effect. In summary, when two centrifugal fans are connected in series, they should operate in the network with higher resistance. When two or more centrifugal fans are connected in series within a single network, the pressure curves of the fans and the resistance curve of the network must be plotted on the same coordinate system. After analysis and comparison, a decision should be made regarding series operation; otherwise, adverse consequences may occur. 2. Parallel Operation When two centrifugal fans operate in parallel within a network, only the suction or discharge pipe can be utilized based on actual conditions. During parallel operation, the flow rate within the suction and discharge pipes equals the sum of both fans' flow rates. The network resistance is overcome by the combined pressure of both fans. The performance curve for two parallel fans is obtained by adding the flow rates at the same pressure point from the individual fan performance curves. If centrifugal fans operate in parallel within a flat-curve piping network, the flow rate and total pressure at point B will be higher than those at points B1 and B2 when the two fans operate independently within the network. Therefore, this configuration represents an effective parallel operation.