Greenheck Fan QEI Design & Performance, Mixed Flow Axial/Centrifugal Hybrid, Sound and Efficiency

Design & Performance

Mixed Flow: Axial/Centrifugal Hybrid

A mixed flow wheel is a hybrid between an axial propeller and a centrifugal wheel. The result is a design that combines the best axial

and centrifugal properties: highly efficient air movement, increased	Pressure
pressure capabilities, extremely low sound levels, and a steep fan
performance curve. To further enhance the fan’s performance,	Static
straightening vanes are incorporated in the housing. These serve
to convert swirling airflow into a straight axial flow with a resulting
static pressure rise and energy savings.

Centrifugal

Mixed Flow

Axial

CFM

Airflow Profiles

			Axial Fans: Straight through
			nearly linear airflow.
			Centrifugal Fans: Two 90°
			deflections, before airflow
			exits the fan.
			Mixed Flow Fans: Slight
Axial	Centrifugal	Mixed Flow	airflow deflection from
			straight through.

Sound and Efficiency

Sound power levels and fan efficiency are two very important factors when selecting a fan. There is a noticeable difference between impeller types. The table and chart below compare the sound power (LwA) and the static efficiency of the belt-driven mixed flow fan wheel versus tubular centrifugal and vane axial inline fans. Over a wide range of operating points, the mixed flow fan provides the quietest and most efficient selection.

	Mixed Flow	Sound Power (LwA)				Static Efficiency
Performance
	Fan Size	Mixed Flow	Inline		Mixed Flow		Inline
				Vane Axial				Vane Axial
		(QEI-L, QEI-I/II)	Centrifugal		(QEI-L, QEI-I/II)		Centrifugal
15,000 cfm	24	90 dB	100 dB	103 dB	43%		17%	34%
@ 1.0 in. wg
25,000 cfm	36	86 dB	93 dB	102 dB	68%		45%	56%
@ 2.0 in. wg
50,000 cfm	49	92 dB	98 dB	111 dB	71%		60%	59%
@ 4.0 in. wg

Model comparisons based on similar outer tube diameters.

Lower Sound Power, Better Sound Quality

Sound power levels shown at each frequency:

Typical	Typical	QEI-L
Tubular Centrifugal	Vane Axial	QEI-I/II

	100
dB	90
Sound Power,	80
	70
	60
	62	125	250	500	1000	2000	4000	8000

The sound quality of the belt-driven mixed flow fan line is as beneficial to low sound design as is the reduced overall sound power. The sound chart compares units of similar outer tube diameters at an operating point of 20,000 cfm with

1.5inches wg of static pressure (Ps). Tubular centrifugals (green) have dominant tones in the 63 Hz through 250 Hz octave bands, while vane axials (red) have more mid to high frequency sound. The mixed flow fan (blue) does not have a dominant tone. A bystander would hear a more bland sound that is quieter than a tubular centrifugal or vane axial.

Frequency, Hz

6