Centrifugal Fans In Series Or Parallel Operation

When a single centrifugal fan within the system cannot deliver sufficient airflow and pressure or if the fan is too large to install in the desired space, then multi-stage fan systems are used as an alternative for single fan system. Two or more separate centrifugal fans can be operated either in series or in parallel arrangements to serve specific requirements of industrial applications. When an increased volume or pressure becomes necessary for a process applications, multiple fans systems can be used either in series or parallel operation rather than simply replacing the original fan with larger capacity. Using centrifugal fans in multiple fan arrangements offer greater flexibility in unit sizing and also reduce the system noise.

Centrifugal fans in series operation
In series operation, fans are installed in series close to each other like in push-pull arrangement, so that the first fan in series supplies air pressure into the inlet of the second fan. If the fans have the same system resistance, then together they produce a greater pressure difference. In series operation, each fan will have different inlet pressures. Since the first fan pressurizes the gas before supplying it to the inlet of the second fan, there is an increase in the specific weight of the gas at the inlet of the second fan. Accordingly, the second fan in series will experience greater pressure differential across the system and draws greater shaft power than the first fan. The air pressure doesn’t simply double when two fans with the same capacity are operated in series arrangement. However, by arranging two fans in series, the static pressure capability of a given airflow can be increased. Since the individual performance of each fan is not the same, the fans will handle the same mass flow of air but not volumetric flow rate. Series arrangement is suitable to high resistance systems, that have long ducts or large pressure drops across system components. When fans are coupled in series, it is not advisable to run one fan with the other off as the pressure drop across it will make the other fan inefficient.

Centrifugal fans in parallel operation
Unlike in series operation where typically two fans are involved, parallel operation use multiple fan systems arranged together side by side. In parallel arrangement of two fans, each fan is selected for half of the design flow rate.
Parallel configurations are feasible for systems with large changes in air / gas moving requirements. When fans are in parallel, the combined performance of fans will result in increase in the volume of airflow.
The higher the system resistance, the lesser will be the gain in airflow rate. Hence, parallel configurations are generally not recommended for high-resistance systems. Parallel arrangements are suitable for applications where fans can operate in a low resistance almost in a free delivery condition.
To achieve better efficiency, fans in parallel system must be properly sized, should start simultaneously and brought up to speed at the same rate. For controlling the airflow requirement, fans in parallel can utilize additional equipment such as dampers, variable speed drives or variable inlet vanes.