Saaf side access housings (sah) – AAF International SAH User Manual
Page 20
SAAF Side Access Housings (SAH)
with SAAF Cassette Gas-phase Chemical Filters
Installation, Operation, and Maintenance Instructions
A typical pressure drop profile for an SAH might be as shown
in Table 3 below:
9.1.1 continued
Table 3: Nominal Pressure Drop (∆P) @ 250 fpm Nominal Airflow Velocity
Pressure Component
Initial ∆P (in. w.g.)
Final ∆P (in. w.g.),
Recommended or Actual
External pressure
1.0
1.0
loss from inlet and
outlet ducts
2” PerfectPleat
0.11
1.0
HD Cassette with
0.73
0.73
SAAF Oxidant
HD Cassette with
0.73
0.73
SAAF Oxidant
After-filter
0
0
M-Pak Filter
0.23
1.5
TOTALS
2.80
4.96
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
(4.96 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
(4.96 in. w.g.) then it will deliver far more than the required
airflow at the lower start-up pressure drop (2.80 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 (2.80 in. w.g.) then it will deliver far less than the
required airflow at the higher final pressure drop (4.96 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 may limit the fan
style or size that can be accommodated within the cabinet. This
may require the use of an external stand-alone fan.
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 the table above the
average pressure drop is 0.5 x (2.8 + 4.96) = 3.88 in. w.g. An
external pressure drop of 0.1 in. w.g. will be 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 smaller SAH sizes such as size 102 through 204) 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. will be 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.
See AAF’s submittal drawing for the order in question for the details
of the fan supplied and its performance.
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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Customer Service 888.223.2003
Fax 888.223.6500
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