ASHRAE Journal - January 2015 - 73

COLUMN HVAC APPLICATIONS

data actually shows that the larger
FIGURE 1A (LEFT) Series unit. FIGURE 1B (RIGHT) Parallel unit.
the capability of the ECM motor,
the more efficient it becomes at
reduced rpms. Analysis of several
manufacturers' units, of several
sizes, indicates that they all operate in a similar manner. As seen
in Figure 2, if there are three units
capable of 3,500, 3,000, and 2,500
cfm (1652, 1416, and 1180 L/s) at
100% flow, turning each down to
2,000 cfm (944 L/s) would result in
an energy consumption of 301, 407,
526 W, respectively. In other words,
the largest unit would operate at
approximately 9% wattage of its full flow potential,
FIGURE 2 Simplified performance evaluation of ECM fan-powered terminal unit energy use.
whereas the smallest unit would operate at roughly 21%.
1,800

Power at
Maximum Airflow

2,500 cfm

1,200
1,000
800
600

2,000 cfm

760 W
526 W
407 W

400

341 W

200
0

Part Load Curves

1,000

2,000

3,000

4,000

5,000

Fan Airflow (cfm)

Of course, documentation of these particular measurements (W/cfm) is not often seen as a requirement
in specifications, but it can certainly allow design engineers to better calculate and predict building energy
use. By late 2015, we should see the results of this study
available in several building energy models (through an
ongoing program at AHRI).

Research Project 1515, conducted in California last year,
supply airflow rates as low as 0.25 cfm/ft2 (1.27 L/s·m2) can
provide a highly acceptable environment, provided that
the air-distribution outlets provide uniform temperatures and are free of unwanted drafts.

Reducing Fan Airflow in Proportion to Space Loads

Provides Controllability Using Existing Technology

Provides Acceptable Air Distribution with Turndown

Most ECM motors used to power fan terminal units are
programmed to be "pressure independent." This means
that they will deliver a desired airflow without regard to
the external pressure on the fan. Even if there are variations in the supply airflow rate, which can have a significant effect on fan cabinet pressures, the fan will adjust
its rpm to compensate. In fact, this generally holds true
within 5% of desired flow rate. Because it is important
that the fan in a series unit always deliver more air

Series units have traditionally been used as constant
airflow/variable temperature devices, providing relatively
constant sound and airflow to a given space. Contrary to
popular belief, it does not ensure constant air motion in
the space. Even with properly selected air-distribution
components, air motion at a point in the room is much
more a result of load-driven convection currents than it
is of air supply rates. As was shown in the recent ASHRAE

JAN UARY 2015

ashrae.org

ASHRAE JOURNAL

(Courtesy of Dennis L. O'Neal, Ph.D., P Dean of
.E.,
Engineering and Computer Science, Baylor University)

Results Integration

3,000 cfm

1,400
Fan Motor Power (W)

As we look at the equivalent series unit with a PSC
motor and an SCR speed controller, it uses nearly the
same amount of energy at reduced flow as it does at
full flow. This means that the W/cfm actually increase
as the fan is turned down. PSC motors also use a sleeve
bearing, which limits the turn down to about 600 rpm,
whereas ECM motors with ball bearings can operate as
low as the motor has been programmed.

3,500 cfm

1,600

PSC Data

ASHRAE Journal - January 2015

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