ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 26

15.0
(11.2)

5.0
(1,244)

12.5
(9.3)

4.0
(995)

10.0
(7.5)

3.0
(747)

7.5
(5.6)

2.0
(498)

5.0
(3.7)

1.0
(249)
0.0
(0)

2.5
(1.9)

300AF

0
2,500 5,000
(0.00) (1.18) (2.36)

7,500 10,000 12,500 15,000 17,500 20,000
(3.54) (4.72) (5.90) (7.08) (8.26) (9.44)

Power,
bhp (kW)

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

17.5
(13.0)

7.0
(1,742)
6.0
(1,493)

0
2,500 5,000
(0.00) (1.18) (2.36)

0.0
(0.0)
7,500 10,000 12,500 15,000 17,500 20,000
(3.54) (4.72) (5.90) (7.08) (8.26) (9.44)

Flow, cfm (m 3/s)

Flow, cfm (m 3/s)

FIGURE 2. Flow-vs.-pressure curve.

FIGURE 3. Flow-vs.-power curve.

static pressure. The seven selections are for one model

increases.2 This type of power curve is known as "non-

type, GI Cent., and one wheel type, AF. A manufacturer's

overloading." If the driving motor power is greater than the

selection program may yield many more options.

peak power on the curve with allowances for temperature

All of the selections in Table 1 are capable of achieving

and drives, etc., the fan should not overload in operation.

the duty point in our example.

The power curves of many axial propeller fans and
ventilators will show power decreasing as ﬂow increases

Fan Performance Curve

to the point of free delivery.2

The ﬁrst point on our graph is the duty point: 12,000 cfm (5.66
m3/s)

Though flow-vs.-power curves can be shown on a

at 5.0-in.-w.g. (1,244 Pa) static pressure. Adding data

separate graph, typically they are superimposed with

showing fan ﬂow and pressure at points other than the duty

ﬂow-vs.-pressure curves on the same graph, with pressure

point produces the ﬁrst curve, shown in black in Figure 2.

along the left axis and power along the right (Figure 4).

To read the curve, draw or imagine a line extending

Figure 4 includes additional information necessary

vertically from the required ﬂow on the x-axis up to and

on a fan-curve graph: fan model, fan size or diameter

slightly through the curve (the yellow dashed line in

(both included here), revolutions per minute (RPM), air

Figure 2). From that intersection of line and curve, draw

density, and outlet area.2 Outlet velocity may be provided

or imagine a line extending horizontally to the y-axis

in lieu of outlet area.

to determine the pressure produced by the fan (the blue

In the case of most industrial exhausters, pressure

dashed line in Figure 2). This should be the required

and combustion blowers, and forward-curved fans, the

duty point at the desired ﬂow and pressure, as required.
Typically, the power required by the fan also is
displayed. A power curve can be shown on the left or
Figure 3, draw or imagine a line extending from 12,000
cfm (5.66 m3/s) on the x-axis straight up to the red power
curve. From there, read horizontally to the right to determine the power required by the fan. In this case, the
required power is 11.7 bhp (8.7 kW).
Generally, the power required by a fan increases as
ﬂow through the fan increases. With many airfoil and
backward-curved or backward-inclined fans, however,
power reaches a peak value and then decreases as ﬂow

26

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

7.0
(1,742)
6.0
(1,493)

Model: GI Cent.; size: 300AF; diameter: 30;
RPM: 1,366; density: 0.075; outlet area: 5.09

17.5
(13.0)
15.0
(11.2)

Flow vs. pressure

5.0
(1,244)

12.5
(9.3)

4.0
(995)

10.0
(7.5)

3.0
(747)

7.5
(5.6)

2.0
(498)

Flow vs. power

1.0
(249)
0.0
(0) 0
2,500 5,000
(0.00) (1.18) (2.36)

5.0
(3.7)

Power, bhp (kW)

the right axis and should be read at the required ﬂow. In

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

pressure. In Figure 2, the fan ﬂow curve intersects the

2.5
(1.9)
0.0
(0.0)
7,500 10,000 12,500 15,000 17,500 20,000
(3.54) (4.72) (5.90) (7.08) (8.26) (9.44)

300AF

Flow, cfm (m 3/s)

FIGURE 4. Superimposed ﬂow-vs.-pressure and ﬂowvs.-power curves.

w w w. a m c a .o r g
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ASHRAE Journal Supplement - AMCA InMotion - October 2020

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