Greenheck Fan 129 Fan Laws, Adjusting Fan Performance, Sound Levels, Varying Operating Points

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.

cfmNew

=

rpmNew

x

cfmOld

This equation relates horsepower to rpm. The change in

( rpmOld

)

horsepower can be determined when the rpm is increased by

(

)

PsNew

=

rpmNew

2

PsOld

25%. This is shown below:

x

rpmOld

Bhp

New

= (1.25)3

x Bhp

Old

= 1.95 x

Bhp

Old

(

)3

BhpOld ]

BhpNew

=

rpmNew

x

rpmOld

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 (rpm2) can be determined from the
following equation:			(	cfm2	)
rpm2 =	rpm1	x
				cfm1
				2000 cfm			1400 rpm
=	700 rpm	x	( 1000 cfm )			=

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:

Ps2	=	Ps1	x	(		rpm2		)²
						rpm1
For our example,				(			) ² = 1.0 in.
Ps2	=	0.25 in.	x		1400 rpm
					700 rpm

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

Example

Solution

Sound Levels			Refer to the Suggested Limits for Room Loudness chart to
In many cases, the sound generated by a fan must be			determine the acceptable sone range for the application.
			As a general guideline, choose a fan that has a sone value
considered. For the fan industry, a common unit for
			within the range specified.
expressing sound pressure level is the sone. In practical
terms, the loudness of one sone is equivalent to the sound			Note: Rooms with a hard construction (concrete block,
of a quiet refrigerator heard from five feet away in an			tile floors, etc.) reflect sound. For these rooms, select
acoustically average room.			fans on the lower end of the range. Rooms with soft
			construction or those with carpeting and drapes, etc.,
Sones are a linear measurement of sound pressure levels.			absorb sound. For these rooms, fans near the higher end
For example, a sound level of 10 sones is twice as loud as			of the range may be selected.
5 sones.				For example, an exhaust fan for an
Suggested Limits for Room Loudness				office in the “Suggested Limits for Room
Sones	DBA			Loudness” chart below says that offices
1.3-4	32-48	Private homes (rural and suburban)		should have a loudness range from 4 to 12
1.7-5	36-51	Conference rooms		sones. Comparing a GB-141, GB-161 and
2-6	38-54	Hotel rooms, libraries, movie theatres, executive offices		GB-180 fan for approximately 3100 cfm at
2.5-8	41-58	Schools and classrooms, hospital wards, and operating rooms		0” Ps only the GB-180 has a sone value of
3-9	44-60	Courtrooms, museums, apartments, private homes (urban)		less than 12. Therefore, the GB-180 is the
4-12	48-64	Restaurants, lobbies, general open offices, banks
				best selection for this application.
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).
					F A N F U N D A M E N T A L S	133