The mechanical systems usually decrease its performance after a certain period of time. So, these systems are to be optimized in order to get a maximum output from the systems. The optimization of the draft fans utilizes different kinds of steps like using series of fans on the shafts or using multiple shafts with series of fans.
Multiple fans in series or parallel
- Fans are combined in series or in parallel as an alternative to a single set of fan.
- A set of two smaller fans are less expensive than a single set of larger fan and they give better performance than the larger one.
- The operation and arrangement of this kind requires a special care and special operating arrangements.
- Fans are installed in series close to each other to increase pressure locally.
- Parallel arrangement is done for single or dual operation for varying air flow rate.
- Series and parallel arrangement has its own advantage.
- Series arrangement is preferable in systems that have long duct or large pressure drops.
- In draft systems either induced draft or forced draft, the fans in series can minimize the amount of pressure in a duct.
- In parallel arrangement high efficiencies are achieved in system air flow rate,
- In parallel arrangement lower noise is generated.
- Parallel configurations may be feasible for systems with large changes in air moving requirements.
- The fans are selected to match the maximum pressure and flow requirements of the system.
- The system may not require the operation at maximum conditions all the time.
- The motor selected for a drive system must be capable of supplying the fan with required driving power.
- The fans in the system are usually driven by alternative current.
- The motor selected for driving the fans is usually a squirrel cage induction motor, because of its durability,low cost,high reliability and less maintenance.
- These motors provide 3600 RPM when used a 2 pole 60 Hertz system and 1800 RPM when used a 4 pole 60 Hertz system.
- When there is a need of continuous range of speed variation, variable frequency drives are used to effectively vary the speed of the motor.
- In some cases when loading on the motors is not high, the predicted savings from varying frequency drives may not materialize because of the impact of the losses associated with variable drives.
- Care must be taken in situations where increased air flow rate causes an overload on the system and where increased system pressure caused by clogged air filters, causes an overload on the motor.
There are some small optimization steps such as controlling inlet vanes or use of multiple speed motors to drive fans which can make system effective.