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 In the past
ten or so years we have seen a large growth in the use of multiple fans
installed in a parallel airflow arrangement using direct driven plenum
fans. These type applications are commonly
called FAN ARRAYS.
There are three main criteria that justify
using fan arrays in lieu of a single fan - Redundancy,
Sound and Unit Length. If an
application is deemed to need a fan array then the first step is to select the
proper number and size of the fans and motors, the second step is to select the
most efficient and cost effective method of controlling the performance of the
fan array and the final step is to address the isolation of a failed fan.
As an
example we have selected a blow through supply fan section in an air handling
unit that requires 30,000 CFM at 6.0 "
TSP with a control set point of 1.0 " WG and requires a minimum of 95%
redundancy when one fan fails. In this example we have elected to look at six
options:
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Option A: 12 - 15.0" diameter direct driven
plenum fans each with a 5 HP - 3600 RPM motor.
Option B: 6 - 18.25" diameter direct driven
plenum fans each with a 7.5 HP - 1800 RPM motor.
Option C: 4 - 22.25" " diameter direct driven
plenum fans each with a 15 HP - 1800 RPM motor
Option D: 2 - 27.0" diameter direct driven plenum
fans each with a 25 HP - 1800 RPM motor
Option E: 1 - 40.25" " diameter direct driven
plenum fans with a 50 HP - 1200 RPM motor
Option F: 1 -33.0" diameter belt driven DWDI
housed centrifugal fan with a 50 HP - 1800 RPM motor.
Note: All of the above fan options will be
controlled by variable frequency drives.
Efficiency
is not a criterion for using a fan array, but if a fan array is the best option
for a particular application then the efficiency of the fan array is important. The following table shows the design brake
horsepower and the redundancy (when one fan fails) for the above six options at
30,000 CFM @ 6.0" TSP.
____________Option
A____Option B____Option C____Option
D____Option E____Option F
Design BHP __45.24______40.68______38.78_______38.56______37.11______39.10
Redundancy _100%______100%_______96%_______75%________0%________0%
Notes:
1. The
BHP for Option F reflects the blow thru arrangement and the v-belt drive
losses.
2. The redundancy of options D, E and F do
not meet the stated requirements, but their selection
data will be of interest in future blogs.
When you take a quick look at the
efficiency and redundancy of the above options it is clear - more smaller fans -
are not necessarily the best option.
CR4 would like to thank Holcombe Kelley of Air Handling Solutions LLC for contributing this blog entry.
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