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In Part 1 and Part 2 we
looked at the redundancy, sound levels, unit length and efficiency. Now we will look at fan isolation, variable
frequency drive redundancy and some adjunct issues.
Fan Isolation
 If redundancy is
one of the reasons for selecting a fan array then there are several questions
that need to be answered because some portion of air will flow backward through
the failed fan.
- What is the minimum redundancy that a
particular application requires?
- How quickly do you need to have this
minimum redundancy?
- Are sound levels also a consideration?
If the
application requires instant minimum redundancy then some type of fan isolation
device needs to be installed on all the fans in the fan array, but if it is
acceptable for the application to operate at some reduced capacity, for say 30
to 60 minutes until the maintenance force can blank off the failed fan inlet,
the fan array can operate more efficient and produce less noise.
The most common
device used to automatically isolate a failed fan is either a gravity or a
motor operated backdraft damper. There
are several issues that need to be considered when selecting isolation dampers.
- Inlet dampers add noise and additional air
pressure drop. Offsetting the dampers
from the inlet bulkhead would reduce these negative effects.
- Gravity dampers on high pressure fan
arrays tend to self destruct. If gravity
dampers are desired then a counter balanced heavy duty damper is recommended.
- Dampers on the fan outlets reduce the
ability to access to the fan and motor and also add air pressure drop.
Fan isolation
dampers should only be installed when the desired redundancy must be automatic
and instant.
Variable Frequency Drive Redundancy
The most common
and most efficient method of controlling the desired airflow on a fan array is
variable frequency drives and if one of the reasons for the fan array is system
redundancy then we need to address VFD
redundancy. In the HVAC industry there
are three approaches to providing VFD redundancy.
- One large VFD to control the speed of all
the fans and a second large VFD as backup.
In this system each fan motor must have its own motor overload
protection.
- One VFD per fan. In this system the fan redundancy and VFD
redundancy is the same and each VFD provides motor overload protection to its
motor.
- Multiple VFDs with each VFD controlling
more than one motor. The VFD redundancy
would be less than the fan redundancy and each motor would need its own
overload protection.
No matter which
VFD approach is selected the air handling unit should be delivered to the job
site with a single point of electrical connection for each fan array.
Adjunct Issues
Motors - Larger motors are more efficient than
smaller motors:
| 5 HP | 3600 RPM | ODP | FL Eff. | = | 88.50% |
| 7.5 HP | 1800 RPM | ODP | FL Eff. | = | 91.00% |
| 15 HP | 1800 RPM | ODP | FL Eff. | = | 91.70% |
| 25 HP | 1800 RPM | ODP | FL Eff. | = | 93.60% |
| 50 HP | 1200 RPM | ODP | FL Eff. | = | 94.10% |
Larger motors
have a longer life than smaller motors. If motor replacement is a concern then
the air handling unit should be provided with a lifting aid such as a monorail.
Even Airflow - Fan arrays provide better airflow across
both upstream and downstream components which could reduce the overall air
handling unit length.
Summary -
The fan array that provides the
redundancy, sound levels and unit length required with the fewest number of
fans will be the most efficient and lowest cost.
Editor's Note: CR4 would like to thank Holcombe Kelley, of Air Handling Solutions LLC, for contributing this blog entry.
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