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FAN FUNDAMENTAlS
133
Example
Solution
SOUND LEVELS
In many cases, the sound generated by a fan must be
considered. For the fan industry, a common unit for
expressing sound pressure level is the sone. In practical
terms, the loudness of one sone is equivalent to the sound
of a quiet refrigerator heard from five feet away in an
acoustically average room.
Sones are a linear measurement of sound pressure levels.
For example, a sound level of 10 sones is twice as loud as
5 sones.
Refer to the Suggested Limits for Room Loudness chart to
determine the acceptable sone range for the application.
As a general guideline, choose a fan that has a sone value
within the range specified.
NOTE: Rooms with a hard construction (concrete block,
tile floors, etc.) reflect sound. For these rooms, select
fans on the lower end of the range. Rooms with soft
construction or those with carpeting and drapes, etc.,
absorb sound. For these rooms, fans near the higher end
of the range may be selected.
For example, an exhaust fan for an
office in the “Suggested Limits for Room
Loudness” chart below says that offices
should have a loudness range from 4 to 12
sones. Comparing a GB-141, GB-161 and
GB-180 fan for approximately 3100 cfm at
0” Ps only the GB-180 has a sone value of
less than 12. Therefore, the GB-180 is the
best selection for this application.
Introduction to FAN SELECTION
FAN LAWS
]
In a steady-state system, as the fan rpm changes, cfm, Ps and BHp (horsepower) also change. The equations
below, known better as fan laws, show the relationship between these performance parameters.
NOTE: A 25% increase in rpm results in a 95% increase in horsepower. Considering this, initial fan selections
should be sized with motor horsepowers greater than necessary if any increase in fan rpm is likely in the future.
ADJUSTING FAN PERFORMANCE
There is a direct relationship between cfm and rpm within a
system. Doubling the fan rpm will double-the cfm delivered.
Sample: The example at the right shows a fan curve at 700
rpm which had an operating point of 1000 cfm at 0.25 in. Ps.
What rpm is required to move 2000 cfm through the same
system?
Solution: Within a system, cfm is directly related to rpm.
Therefore, the new rpm (rpm
2
) can be determined from the
following equation:
Referring to figure at right, this results in sliding up the
system resistance curve from 700 rpm to 1400 rpm.
Notice that as we doubled our airflow from 1000 cfm to
2000 cfm, the Ps went up from 0.25 in. to 1.0 in. It must
be kept in mind that we are not changing the system, only
increasing fan speed. Therefore, we must remain on the
system resistance curve. Within a system, Ps varies as the
square of cfm. Since cfm and rpm are directly proportional,
an equation relating Ps and rpm can be derived as follows:
For our example,
This verifies the operating point on the 1400 rpm curve
(2000 cfm at 1.0 in. Ps). With this example, it should be
clear how cfm, rpm and Ps tie together in a steady-state
system.
Varying Operating Points
This equation relates horsepower to rpm. The change in
horsepower can be determined when the rpm is increased by
25%. This is shown below:
Bhp
New
= (1.25)
3
x Bhp
Old
= 1.95 x Bhp
Old
cfm
New
=
(
rpm
New
)
x
cfm
Old
rpm
Old
Bhp
New
=
(
rpm
New
)
3
x
Bhp
Old
rpm
Old
Ps
New
=
(
rpm
New
)
2
x
Ps
Old
rpm
Old
rpm
2
= rpm
1
x
(
cfm2
)
cfm1
= 700 rpm x
(
2000 cfm
)
=
1400 rpm
1000 cfm
Ps
2
= Ps
1
x
(
rpm2
)
²
rpm1
Ps
2
= 0.25 in. x
(
1400 rpm
)
²
= 1.0 in.
700 rpm
SUGGESTED LIMITS FOR ROOM LOUDNESS
Sones DBA
1.3-4 32-48 Private homes (rural and suburban)
1.7-5 36-51 Conference rooms
2-6 38-54 Hotel rooms, libraries, movie theatres, executive offices
2.5-8 41-58 Schools and classrooms, hospital wards, and operating rooms
3-9 44-60 Courtrooms, museums, apartments, private homes (urban)
4-12 48-64 Restaurants, lobbies, general open offices, banks
5-15 51-67 Corridors and halls, cocktail lounges, washrooms and toilets
7-21 56-72 Hotel kitchens and laundries, supermarkets
12-36 64-80 Light machinery, assembly lines
15-50 67-84 Machine shops
25-60 74-87 Heavy machinery
From AMCA Publication 302 (Application of Sone Ratings for Non Ducted Air Moving Devices with
Room-Sone-dBA correlations).