Blowers/005 Blowers Application of twin plenum blowers.pdf

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Subject:           (5)Applicationof Twin Plenum Blowers

Date:               October 26, 2007

 

There are typically two applications for twin plenum blowers.  Twin plenum blowers are used for large airflow designs where a single blower is not feasible, or where the customer is asking for redundant operation.  Twin plenum blowers are most commonly used for supply air applications, but may be used for return, exhaust, etc. 

Anytime twin plenum blowers are used, one of the blowers must be ordered with clock-wise (CW) rotation and the other must be ordered with counter clock-wise (CCW) rotation.

If redundant operation of the blowers is not required, it is not required to provide a separate VFD for each blower motor.  If Sales requires separate VFD’s, this must be specified in the factory order. 

When Sales specifies twin blowers for redundant operation, Engineering must provide a means to prevent recirculation of air through either blower when only one blower is in operation.  This is typically accomplished by providing a heavy duty backdraft damper (such as an Arrow model 655) at the inlet to each blower.  When selecting this damper, the damper manufacturer’s maximum velocity through the damper must be considered and the damper size should never be smaller than the inlet opening size to the blower.  Also, with redundant operation, each blower must be provided with a separate VFD to allow operation of only one of the blowers.

For redundant operation, Sales must specify the level of redundancy required.  For 50% redundancy, each blower is selected to provide half of the total supply airflow at the full design static pressure.  This is the standard selection procedure for any twin blower application.  If Sales requests 100% redundancy, each blower must be selected to provide 100% of the total supply airflow at the full design static pressure.  The project engineer must verify that the blower operation will be stable at 50% flow when both blowers are running and 100% when only one blower is running.  For 75% redundancy, each blower must be capable of providing 75% of the total airflow.  In this case, since the total airflow for the unit is reduced, the static pressure at 75% flow will be less.  The engineering specifications will show the normal operation (both blowers operating at 50% flow).  The submittal should include a blower curve for the normal operation and a blower curve showing the redundant blower operation.

NOTE:

If a separation wall is provided between the blowers, two blowers of the same rotation (CW or CCW) may be used.  This method should only be used when necessary due to lead time issues, etc. because it will add additional labor (cost) and weight to the unit.