ASHRAE JOURNAL ashrae.org JULY 2016 52 COLUMN ENGINEER’S NOTEBOOK Steven T. Taylor It is common to see enclosed and underground parking garage exhaust systems consisting of g g extensive f f duct distribution systems with multiple exhaust inlets often ducted to near the floor. The California Mechanical Code (CMC), 1 for instance, includes this requirement: * Exhaust Inlet Distribution. To ensure proper exhaust of con- taminated air and fumes from parking garages, exhaust systems utilizing multiple exhaust inlets shall be designed so that exhaust inlets are distributed in such a manner that no portion of the park- ing garage is more than 50 ft (15 240 mm) from an exhaust inlet. Such exhaust inlets shall be installed so that the highest elevation of the exhaust inlet is no greater than 12 in. (305 mm) below the lowest ceiling level. Exception: Garage exhaust systems designed without distributed exhaust inlets may have their exhaust inlets designed based on the principles of engineering and mechanics and shall provide the minimum required exhaust rate in Table 4-4. The goal of this requirement is clear, but the extensive exhaust distribution system required is not necessary to meet this goal. It misses two key ventilation fundamen- tal concepts: 1. “You cannot suck out a match.” This is one of my favorite expressions because it makes this fundamental principle clear even to non-engineers. The idea is that exhaust inlets cannot capture pollutants unless they are generated right next to the exhaust inlet. Figure 1 (previously published in my February 2014 column “Restroom Exhaust Systems”) shows a computational fluid dynamics (CFD) simulation of a typical exhaust grille. Note the velocity vectors are only high right near the grille; more than 2 ft (0.6 m) or so away from the grille face, the velocity vectors are close to zero. This means that automobile exhaust emissions, which will almost always be more than 2 ft (0.6 m) from exhaust inlets, will not be captured by ducted exhaust inlets. Hence, the location of the inlets has essentially no impact on the source strength of the emissions into the space. 2. Pollutants are diluted by supply air, not exhaust air. It is the makeup air induced into the garage by the exhaust air that is diluting auto emissions. † So it is the makeup air distribution we need to pay attention to, not the exhaust distribution. In fact, the distributed exhaust inlets as mandated by the CMC can actually reduce ventilation efficiency and increase average pollutant concentrations depending on the location of the makeup air inlets relative to the exhaust inlets. BY STEVEN T. TAYLOR, P.E., FELLOW ASHRAE “Sweep” Parking Garage g g Exhaust Systems t t * This section was required in the 2010 CMC, forcing the use of the exception if alternative exhaust system layouts were to be used. The exception was interpreted by many code officials to mean that computational fluid dynamics had to be performed, such as that discussed in this article, to show alternative designs performed similarly. The 2013 version of the CMC includes this section as an alternative, so CFD is no longer required to justify alternative designs. † From the perspective of garage air quality, ventilation systems could directly supply outdoor air instead of inducing makeup air with ex- haust systems. However, the garage would then be positively pressurized, possibly pushing pollutants into adjacent occupiable spaces. Most codes, therefore, do not allow garages to be ventilated with supply air systems. FIGURE 1 CFD analysis of exhaust grille; velocity vectors. (Courtesy of Price Industries.) 400 300 200 100 0 fpm
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A S H R A E J O U R N A L a sh r a e . o r g J U LY 2 0 1 65 2
COLUMN ENGINEER’S NOTEBOOK
Steven T. Taylor
It is common to see enclosed and underground parking garage parking garage parking exhaust systemsconsisting ofconsisting ofconsisting extensive of extensive of duct distribution systems with multiple exhaust inlets oftenducted to near the floor. The California Mechanical Code (CMC),1 for instance,includes this requirement:*
Exhaust Inlet Distribution. To ensure proper exhaust of con-
taminated air and fumes from parking garages, exhaust systems
utilizing multiple exhaust inlets shall be designed so that exhaust
inlets are distributed in such a manner that no portion of the park-
ing garage is more than 50 ft (15 240 mm) from an exhaust inlet.
Such exhaust inlets shall be installed so that the highest elevation
of the exhaust inlet is no greater than 12 in. (305 mm) below the
lowest ceiling level.
Exception: Garage exhaust systems designed without distributed
exhaust inlets may have their exhaust inlets designed based on
the principles of engineering and mechanics and shall provide the
minimum required exhaust rate in Table 4-4.
The goal of this requirement is clear, but the extensive
exhaust distribution system required is not necessary to
meet this goal. It misses two key ventilation fundamen-
tal concepts:
1. “You cannot suck out a match.” This is one of my
favorite expressions because it makes this fundamental
principle clear even to non-engineers. The idea is that
exhaust inlets cannot capture pollutants unless they
are generated right next to the exhaust inlet. Figure
1 (previously published in my February 2014 column
“Restroom Exhaust Systems”) shows a computational
fluid dynamics (CFD) simulation of a typical exhaust
grille. Note the velocity vectors are only high right near
the grille; more than 2 ft (0.6 m) or so away from the
grille face, the velocity vectors are close to zero. This
means that automobile exhaust emissions, which will
almost always be more than 2 ft (0.6 m) from exhaust
inlets, will not be captured by ducted exhaust inlets.
Hence, the location of the inlets has essentially no
impact on the source strength of the emissions into the
space.
2. Pollutants are diluted by supply air, not exhaust
air. It is the makeup air induced into the garage by the
exhaust air that is diluting auto emissions.† So it is the
makeup air distribution we need to pay attention to, not
the exhaust distribution. In fact, the distributed exhaust
inlets as mandated by the CMC can actually reduce
ventilation efficiency and increase average pollutant
concentrations depending on the location of the makeup
* This section was required in the 2010 CMC, forcing the use of the exception if alternative exhaust system layouts were to be used. The exception was interpreted by many code officials to mean that computational fluid dynamics had to be performed, such as that discussed in this article, to show alternative designs performed similarly. The 2013 version of the CMC includes this section as an alternative, so CFD is no longer required to justify alternative designs.† From the perspective of garage air quality, ventilation systems could directly supply outdoor air instead of inducing makeup air with ex-haust systems. However, the garage would then be positively pressurized, possibly pushing pollutants into adjacent occupiable spaces. Most codes, therefore, do not allow garages to be ventilated with supply air systems.
FIGURE 1 CFD analysis of exhaust grille; velocity vectors. (Courtesy of Price Industries.)
400
300
200
100
0
fpm
J U LY 2 0 1 6 a sh r a e . o r g A S H R A E J O U R N A L 5 3
COLUMN ENGINEER’S NOTEBOOK
Steven T. Taylor, P.E., is a principal of Taylor Engineering in Alameda, Calif. He is a mem-ber of SSPC 90.1 and chair of TC 4.3, Ventilation Requirements and Infiltration.
For instance Figure 2 and Figure 3 show CFD predictions
of carbon monoxide (CO) concentration from a simple
garage with a center drive aisle with a continuous queue
of automobiles. The garage entry on the left side is the
source of makeup air. Figure 2 shows CO concentration
assuming multiple ducted exhaust inlets per the CMC,
while Figure 3 shows an unducted design with a single
exhaust inlet on the side opposite the entry. The exhaust
air draws makeup air from the entry across the garage
in a sweeping fashion; hence the name “sweep” exhaust
system.
The figures show that the sweep design has lower over-
all CO concentrations with maximum concentration
(~25 ppm) only at the very right side. The reason is that
the exhaust inlets on the left side of Figure 2 are extract-
ing air that has a low concentration of pollutants, wast-
ing this air and leaving less makeup air to dilute pol-
lutants generated on the right side of the garage. So the
sweep design in Figure 3 can provide better ventilation
than a fully ducted exhaust system, and it clearly will be
less expensive.
Example 1: One-Level GarageMany commercial and residential complexes have a
one-level underground garage below. Here is an exam-
ple of how we implemented a sweep garage exhaust
design on a 140,000 ft2 (13,000 m2) garage.
The garage entries will always be a source of makeup
outdoor air. So our first approach is to locate exhaust
points (tagged EA1, EA2, etc.) on the opposite side of
the garage so they can draw makeup air from the entry
down the drive aisles to the exhaust inlet, as shown in
Figure 4. This is the least expensive design. However, we
encountered some problems:
• The exhaust rate prescribed by the CMC (based on
ASHRAE Standard 62.1-20132) is 0.75 cfm/ft2 (3.7 L/s·m2)
for a total exhaust rate of 105,000 cfm (52,000 L/s).
The air speed through single garage entry would have
been ~2,000 fpm (10 m/s), which would be very notice-
able to people walking through and generate a higher
than desirable pressure drop. High velocity makeup
air also results in more stagnant areas caused by eddies
(discussed in the next example). Our experience has
been that the Standard 62.1-2013 garage exhaust rate
is extremely conservative; with fan speed controlled by
carbon monoxide (CO) concentration, as required by
ASHRAE Standard 90.1-20133 and California’s 2013 Title
24 Energy Standards4 for most garages, we find exhaust
rates seldom exceed half of the design rate and even
then only for short periods in the evening rush hour
(due to cold engine starts).
As hybrid, electric, and other low-emission vehicles
become more popular, the current Standard 62.1 garage
exhaust rate will become even more conservative. But
even half the 2,000 fpm (10 m/s) design air speed at
the entry seemed too high. So we decided we needed to
convert some of the exhaust points into additional sup-
ply air points. This increased first costs because the 0.75
cfm/ft2 (3.7 L/s·m2) exhaust rate still had to be main-
tained; the air that was previously exhausted at points
now used for supply had to shift to other exhaust points.
• The fact that we needed more supply air points
worked out well because two of our exhaust points, EA1
and EA2 in Figure 4, could not be made to work due to
architectural constraints. EA1 would have discharged air
into the ramp running down into the garage entry, caus-
ing exhaust air to recirculate. Converting it to a supply
air point solved that problem. EA2 was located near the