AAF International PRU - Manual (Page 20)

Better Air is Our Business ®
AAF has a policy of continuous product research
and improvement and reserves the right to change
design and specifications without notice.
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ISO Certified 9001:2000
© 2007 AAF International
GPF-3-107 JUN ’07
SAAF TM Air Purification Systems Recirculation Unit (RU)
and Pressurization and Recirculation Unit (PRU)
with SAAF TM Cassette Gas-Phase Chemical Filters
Installation, Operation, and Maintenance Instructions
9.1.2 Issues to be Considered when Selecting the Fan: There
are a number of issues to be considered when determining the fan
design and selection criteria:
1. If the fan is selected for the maximum pressure drop shown
(4.06 in. w.g.), then it will have been selected for an operating
point that will rarely occur. This may be inefficient in terms of
energy usage (motor HP), physical fan size, and product cost.
2. If the fan is selected for the maximum pressure drop shown
(4.06 in. w.g.), then it will deliver far more than the required
airflow at the lower start-up pressure drop (1.90 in. w.g). Unless
the airflow is controlled by using balancing dampers or a variable
speed drive, this may result in discomfort from high discharge
velocities, high noise levels, high energy use, and inefficient filter
performance.
3. Conversely, if the fan is selected for the minimum pressure
drop shown (1.90 in. w.g.), then it will deliver far less than the
required airflow at the higher final pressure drop (4.06 in. w.g).
This may result in inadequate air supply. Alternatively, the filters
can be replaced before they reach their final recommended
pressure drop which results in higher filter replacement costs.
Other factors to be considered are:
4. The space available inside the cabinet. This limits the fan style
or size that can be accommodated within the cabinet.
5. The costs associated with providing non-standard fans or fan
or motor control systems.
6. The type of fan to be selected. For instance, will a non-
overloading fan wheel be required?
7. Power available in terms of voltage, phase, and frequency.
8. Special parameters such as special electrical, environmental,
explosion, or temperature requirements.
As is evident from this discussion, there are a number of factors to
be considered when selecting the appropriate fan.
9.1.3 AAF’s Policy Regarding the Provision of Internally Mounted
Fans: Items 1 & 2 are standard offerings, item 3 is an option:
1. When the size of the cabinet allows, AAF will supply a fan
capable of generating the design airflow at the average pressure
drop through the system. In the case of table 3, the average
pressure drop is 0.5 x (1.90 + 4.06) = 2.98 in. w.g. An external
pressure drop of 0.1 in. w.g. is assumed. A backward inclined
non-overloading fan wheel will be supplied unless otherwise
advised.
2. When the size of the cabinet limits the fan selection (normally
on special smaller RU sizes), AAF will advise the maximum
performance that can be supplied by a fan that will fit into the
space available. An external pressure drop of 0.1 in. w.g. is
assumed. A backward inclined non-overloading fan wheel will be
supplied unless otherwise advised.
3. When the customer specifies a fan performance that is
different from that based on the design airflow at the average
pressure drop through the system and the size of the cabinet is
not an obstacle, AAF will select a fan to meet the specified
requirements. This will normally occur when the external static
pressure drop exceeds the standard value of 0.1 in. w.g.
See AAF’s submittal drawing for the order in question for the details
of the fan supplied and its performance.