ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 28

Flow vs. pressure

35.0
(26.1)

6.0
(1,493)

30.0
(22.4)

5.0
(1,244)

25.0
(18.6)

4.0
(995)

20.0
(14.9)

3.0
(747)

15.0
(11.2)

2.0
(498)

10.0
(7.5)

1.0
(249)

5.0
(3.7)
440AHIE
0.0
(0.0)
2,500 5,000 7,500 10,000 12,500 15,000 17,500 20,000 22,500 25,000
(1.18) (2.36) (3.54) (4.72) (5.90) (7.08) (8.26) (9.44) (10.62) (11.80)

0.0
(0) 0

Flow vs. power

System static-pressure loss, in. w.g. (Pa)

7.0
(1,742)

Power, bhp (kW)

System static-pressure loss, in. w.g. (Pa)

Model: industrial exhauster; size: 440AHIE;
diameter: 44.5; RPM: 750; density: 0.075; outlet area: 3.59

GI Cent. ﬂow-vs.-pressure curves for duty point
17.5
(4,355)
15.0
(3,733)
12.5
(3,111)
10.0
(2,488)
7.5
(1,866)
5.0
(1,244)
2.5
(622)
0.0
(0)

0
(0.00)

300AF
330AF
360AF

200AF

400AF

240AF
270AF
5,000
(2.36)

10,000
(4.72)

15,000
(7.08)

20,000
(9.44)

25,000
(11.80)

30,000 35,000
(14.16) (16.52)

Flow, cfm (m 3/s)
Flow, cfm (m 3/s)

Performance certified for installation Type D: ducted inlet/ducted outlet. Power
ratings (bhp) do not include transmission losses. Performance ratings do not
include the effects of appurtenances (accessories).

FIGURE 5. Industrial exhauster with increasing power.

FIGURE 6. Flow-vs.-pressure curves.

power curve increases as ﬂow increases to the point of

Static- and Total-Pressure Curves

free delivery.2 Figure 5 is the fan-curve graph for an indus-

Corresponding to average air velocity at a speciﬁed fan

trial exhauster selected for our example duty point. This

outlet, fan velocity pressure is added to the fan static pres-

selection is much less efﬁcient at 14.4 bhp (10.7 kW). The

sure at each ﬂow point to develop the fan-ﬂow-vs.-total-

peak power on the power curve is about 20.5 bhp (15.3

pressure curve.3 Fan total pressure is equal to the sum of

kW). The graph includes additional notes regarding how

fan static pressure and fan velocity pressure. In Figure

the fan was tested and the performance and power losses

7, the total-pressure curve is shown by the dashed blue

that are included. In this case, the test was ducted inlet

line. In this example, the outlet velocity for the 300AF

and ducted outlet. The fan installation should be similar

fan is 2,359 fpm (12 m/s), the velocity pressure is 0.35 in.

to the test setup to avoid system effects. The power ratings

w.g. (86 Pa), and the total pressure is 5.35 in. w.g. (1,331

do not include drive losses; V-belt-drive losses will have

Pa) at the ﬂow point of 12,000 cfm (5.66 m3/s).

to be added to the power requirement for the motor.

The required system velocity may be determined based

A quick review of the fan curves of the seven fan selec-

on the approximate duct size for the ﬂow. Lower veloci-

tions in our example narrows the ﬁeld of acceptable options.

ties require larger ducts, which, of course, take up more

In Figure 6, the ﬂow-vs.-pressure curve of each selection is

space. Higher velocities require smaller ducts, which

superimposed on a single graph. Generally, a fan will exhibit

often produce more noise. Other factors, such as cost,

some degree of ﬂow and pressure instability to the left of

space, and conveying velocity, are important consider-

peak pressure. A fan's operating point should be to the right

ations when determining velocities.3

of peak pressure. Using 95 percent of peak pressure as a
guide for selection in this case eliminates the 400AF and

Fan Efﬁciency

360AF selections. Most manufacturers' software will iden-

Flow-vs.-static-efficiency or flow-vs.-total-efficiency

tify, or not select, fans to the left of or near peak pressure.

curves also are included on many performance graphs.

Smaller fans operating at lower efﬁciencies can be

Scaling the display of efﬁciency-curve values to match

found operating closer to free delivery, farther to the right

the displayed power- or pressure-scale value can be

than other selections, on the fan curve. In this case, the

challenging. In Figure 8, the static-efﬁciency curve is

240AF selection is operating to the right, while the 200AF

the light-blue line. At 12,000 cfm (5.66 m3/s), the static-

selection is operating far to the right.

efﬁciency value is 81.1 (8.11, 811% × 0.1) percent (scaling

28

2020 A M CA i n m o t i o n

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http://www.amca.org
ASHRAE Journal Supplement - AMCA InMotion - October 2020

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