“sweep” parking garage exhaust systems · exception: garage exhaust systems designed without...
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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
air inlets relative to the exhaust inlets.
BY STEVEN T. TAYLOR, P.E., FELLOW ASHRAE
“Sweep” Parking Garage Parking Garage ParkingExhaust SystemsExhaust SystemsExhaust
* 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
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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
FIGURE 2 Multiple inlet garage exhaust system. FIGURE 3 Sweep garage exhaust system.
Exhaust Fan
Garage Entry
CO 0.00 4.55 9.09 13.64 18.18 22.73 27.27 31.82 36.36 40.91 45.45 50.00 ppm
Exhaust Fan
Garage Entry
CO 0.00 4.55 9.09 13.64 18.18 22.73 27.27 31.82 36.36 40.91 45.45 50.00 ppm
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 4
main entry to the campus, and we could not find an ex-
haust design that met code (e.g., 10 ft [3 m] above grade
and from building openings) and that was also archi-
tecturally acceptable. Converting this exhaust point to
a supply point solved that problem due to less stringent
code limitations on air intakes.
It also improved ventilation on the left drive aisle
between EA2 and EA3, circled in green in Figure 4, where
a stagnant‡ spot would occur with the original concept.
The final design concept is shown in Figure 5.
Example 2: Two-Level GarageMy second example is a 510,000 ft2 (47,000 m2) two-
level garage. The sweep design with two-level garages
can be made to work using transfer fans that can elimi-
nate stagnant spots on both levels without using any
ductwork. The garage is shown in Figure 6. The exhaust
fans are all located on one corner of the garage in a loca-
tion that is not architecturally sensitive. There are two
entries on the upper level through which all makeup air
is drawn.
The design consists of the following:
• Four exhaust fans totaling 110,000 cfm (55,000 L/s)
on upper level (shown as red squares in Figure 6);
• Ten exhaust fans totaling 275,000 cfm (140,000 L/s)
exhaust on lower level (shown as red squares in Figure 6);
and
• Five transfer fans, each 24,000 cfm (12,000 L/s)
drawing air from the upper level and discharging to the
lower level (shown as yellow squares in Figure 6).
The split in exhaust rate between the upper and lower
level was determined by experimenting with the CFD
model.
All fans are propeller fans with low-noise blades. Even
with the special blades, the fans are not very quiet at
full speed (35 sones), but they never get to full speed
when controlled off CO concentration (as noted earlier)
and garages are not acoustically sensitive spaces. Where
noise is a concern, e.g., if the area where the fans dis-
charge is acoustically sensitive, mixed flow fans can be
used; they are much quieter and somewhat more effi-
cient, but also much more expensive.
A CFD analysis was performed to justify the design
using the CMC code exception shown earlier. Both the
proposed sweep design shown in Figure 6 and a fully
ducted system compliant with the CMC were modeled.
The results, shown in Figure 7 for both upper and lower
levels, indicate that both designs work acceptably—they
both result in predicted CO concentrations well below
the 50 ppm CMC limit—but the sweep design results in
fewer stagnant areas caused by eddies with CO concen-
trations above 30 ppm.
Advantages of the sweep design over a fully ducted
CMC design include:
• Much lower mechanical costs, about $1.2 million
savings in this case, from $3.75/ft2 ($40.36/m2) down
FIGURE 4 One-level garage—initial concept. FIGURE 5 One-level garage—final concept.
‡ We used to call these “dead” spots but this did not go over well with clients concerned about CO poisoning.
COLUMN ENGINEER’S NOTEBOOK
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 5
FIGURE 6 Two-level garage exhaust design. (Courtesy of CPP, Inc.) FIGURE 7 CFD CO concentration sweep vs. CMC design. (Courtesy of CPP, Inc.)
to $1.25/ft2 ($13.45 m2), one-third the cost. On other
projects, we have seen mechanical cost savings as high as
$4/ft2 ($43.06/m2).
• Lower floor-to-floor height due to the elimination
of ductwork. The cost savings of reduced floor-to-floor
height can exceed the mechanical savings.
• Generally, lower fan energy costs. The connected
power for the sweep design with propeller fans in this
example was ~100 kW vs. 125 kW for a CMC ducted ex-
haust system with mixed flow fans, 25% lower despite
the less efficient propeller fans and the added fan
energy of the transfer fans. This is due to the much
lower pressure drop of unducted systems. The energy
savings, however, can degrade if there is a large vari-
ance in emissions throughout the garage; the speed
of all fans must be controlled based on the highest
CO reading, which can cause overventilation in other
areas. A ducted system, if provided with multiple fans
serving discrete areas, may be able to be controlled at
different speeds depending on local CO readings. But
our experience has been that, for office buildings at
least, fans usually run at minimum speed (20% or 0.15
cfm/ft2 [0.74 L/s·m2] per California’s Title 24 Energy
Standards) almost all the time with fairly uniform
increases in CO concentrations during evening rush
hour.
• Improved architectural appearance due to the
elimination of ductwork.
• Fewer exhaust discharge locations. For most sweep
designs, garage exhaust fans can be located in one or two
locations. Ducted systems usually require many more
discharge locations to limit duct sizes to minimize floor-
to-floor height. Exhaust discharge locations are difficult
to coordinate architecturally due to code minimum
separation distances (e.g., 10 ft [3 m] above grade and
from openings into the building) and architectural resis-
tance to exhaust stacks, louvers, etc., which can be very
large for large underground garages.
ConclusionsCode requirements and standard practice in many
areas include fully ducted garage exhaust systems with
multiple intakes distributed around the garage. These
designs are unnecessary for minimum garage air qual-
ity and can even result in reduced air quality while
increasing first costs 300% or more and increasing
energy costs 25% or more compared to sweep garage
exhaust system designs. Sweep designs can be made
to work for almost any garage architectural layout,
including multi-level garages with transfer fans used
to prevent stagnant areas by moving air from one level
to the next. In most cases, the sweep system can be
designed without modeling the system using computa-
tional fluid dynamics, as in Example 1, but CFD can be
a valuable design tool for more complex garage layouts,
such as Example 2.
References1. California Code of Regulations, Title 24, Part 4 California Me-
chanical Code, California Building Standards Commission.2. ASHRAE Standard 62.1-2013, Ventilation for Acceptable Indoor Air
Quality.3. ASHRAE Standard 90.1-2013, Energy Standard for Buildings Except
Low-Rise Residential Buildings.4. 2013 Building Energy Efficiency Standards for Residential and
Nonresidential Buildings, Title 24, Part 6 CEC-400-2012-004-CMF.
Transfer FanTransfer
Fan
Transfer Fan
Transfer Fan
Transfer Fan
EntryEntry
Exhaust Fans
50.0046.4342.8639.2935.7132.1428.5725.0021.4317.8614.2910.717.1433.5710.000
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Sweep Design CMC Design ppm
COLUMN ENGINEER’S NOTEBOOK