ASHRAE Journal - September 2020 - 66

COLUMN ENGINEER'S NOTEBOOK

18,000
16,000
14,000
12,000
10,000
8,000
6,000
4,000
2,000
0

target dilution criteria (20+ ft height with higher y-axis resolution).
3,500

Single Fan (10,060 cfm)
Single Fan (13,000 cfm)

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Exhaust Stack Height (ft)

Airflow moving over the edge of a building tends to
create a "wake zone" of turbulent airflow downstream of
the building edge. Exhaust stacks that terminate within
a wake zone experience a substantial exhaust plume
dispersion performance penalty as the turbulent air will
locally recirculate the exhaust plume back toward the
surface of the roof. Significant exhaust plume momentum is required to overcome the forces exerted by the
turbulent air within the wake zone. The single stack, single fan scenario results demonstrate the influence of the
building wake zone. Lower stack heights within the wake
zone recirculation region require significantly higher
exit velocities than taller stack heights that terminate
above the wake zone recirculation region.
While it may be possible to compensate for lower
stack heights by increasing exhaust plume momentum through increased exit velocity, the designer will
eventually need to consider increasing mass flow rate
at the stack discharge. The stack discharge exit velocity tends to be practically limited to "near" 3,000 fpm
(15.2 m/s) due to sound power emitted from the stack
and fan power considerations. Both sound power and
fan power tend to escalate quickly, as exhaust stack pressure drop is primarily a function of velocity squared.
Our team tested adding 1,000 cfm (472 L/s) to the discharge flow rate to the 20 ft (6 m) stack scenario and
found that increasing the flow rate from 13,000 cfm
(6136 L/s) to 14,000 cfm (6607 L/s) decreased the minimum required exit velocity by a very modest 55 fpm
(0.3 m/s) (Table 5).

Performance Design: Multiple Stack Operation
Our team's wind tunnel validation testing was able to
quantify the benefit to plume dispersion performance
66

FIGURE 10 Single Fan Operation: Stack height vs. exit velocity required to achieve

Stack Exit Velocity (fpm)

FIGURE 9 Single Fan Operation: Stack height vs. exit velocity required to achieve
target dilution criteria.

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Single Fan (10,060 cfm)
Single Fan (13,000 cfm)

3,000
2,500
2,000
1,500
1,000

22
23
Exhaust Stack Height (ft)

TABLE 4 Stack height vs. reduction in exit velocity required to achieve target dilu-

tion criteria for a single stack in operation.
STACK HEIGHT

AVERAGE VELOCITY REDUCTION PER
STACK EXIT AIRFLOW RATE
FOOT INCREASE

10 ft to 20 ft

1,351 fpm per ft

13,000 cfm

20 ft to 21 ft

474 fpm per ft

10,060 cfm

21 ft to 22 ft

375 fpm per ft

10,060 cfm

22 ft to 25 ft

184 fpm per ft

10,060 cfm

TABLE 5 Airflow rate increase vs. exit velocity reduction to achieve target dilution

criteria for a single stack in operation.
STACK HEIGHT

AVERAGE VELOCITY REDUCTION PER
1,000 CFM INCREASE

STACK EXIT AIRFLOW RATE
RANGE TESTED

20 ft

55 fpm per 1,000 cfm

13,000 to 14,000 cfm

that was achieved by clustering the exhaust stacks
together (Figure 11). The data show that it is possible to
achieve a reduction in volumetric airflow required at
the exit of each stack when clustering stacks as a design
strategy. In this case, two clustered stacks running at
6,250 cfm (2950 L/s) each with a stack height of 20 ft
(6 m) require an exit velocity of 2,865 fpm (14.6 m/s)
compared to the 3,040 fpm (15.4 m/s) needed for a
single stack running at 13,000 cfm (6136 L/s) at the same
stack height of 20 ft (6 m).
Our analysis of the two fans in operation scenario
evaluated stack exit airflow rates of less than the Day
1 minimum airflow load to determine how low each
stack could operate. This was used to inform the fan
staging strategy to optimize fan power performance
(Figure 12).

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ASHRAE Journal - September 2020

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