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| Commercial Kitchen Exhaust and Makeup Air Systems
Design |
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Building and/or health codes typically
provide basic construction and
materials requirements for exhaust hoods, as well
as prescriptive exhaust rates
based on appliance duty and length of the hood (cfm
per linear ft.) or open face
area of the hood (cfm per ft2). Codes usually recognize
exceptions for hoods
that have been tested against a recognized standard,
such as Underwriters Labo-
ratories (UL) Standard 710. Part of the UL standard
is a “cooking smoke and
flair up” test. This test is essentially a cooking
effluent capture and containment
(C&C) test where “no evidence of smoke or
flame escaping outside the exhaust
hood” must be observed. Hoods bearing a recognized
laboratory mark are
called listed hoods, while those constructed to the
prescriptive requirements of
the building code are called unlisted hoods. Generally,
an off-the-shelf listed hood
can be operated at a lower exhaust rate than an unlisted
hood of comparable style
and size over the same cook line. Lower exhaust rates
may be proven by labora-
tory testing with specific hood(s) and appliance lineup
using the test protocol
described in ASTM Standard F-1704, Test Method for
Performance of Commercial
Kitchen Ventilation Systems. This process is sometimes
referred to as “custom-
engineering” a hood. |
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Side Panels and Overhang
Side (or end) panels (as represented in Figure 3) permit
a reduced ex-
haust rate in most cases, as all of the replacement
air is drawn across the front
of the equipment, which improves containment of the
effluent plume generated
by the hot equipment. They are a relatively inexpensive
way to improve C&C
and reduce the total exhaust rate. Another benefit of
end panels is to mitigate
the negative effect that cross drafts can have on hood
performance. It is impor-
tant to know that partial side panels can provide almost
the same benefit as full
panels. Although tending to defy its definition as an
“island” canopy, end panels
can improve the performance of a double-island or single-island
canopy hood.
An increase in overhang should improve the ability of
a canopy hood
to capture because of the increased distance between
the plume and hood
edges. This may be accomplished by pushing the appliances
as far back under a
canopy hood as practical and/or by increasing the side
length. Although this
improves C&C performance, for unlisted hoods under
a local jurisdiction refer-
encing the Uniform Mechanical Code (UMC), this would
require increase in the code-required exhaust rate.
Larger overhangs are recommended for appliances
that create plume surges, such as convection and combination
ovens, steamers
and pressure fryers. This was the driving argument for
converting the code-
specified exhaust rates from a “cfm/ft2”
to a “cfm/linear ft.” basis in the cur-
rent edition of the International Mechanical Code (IMC).
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Hood Geometry
The ability of a hood to capture and contain cooking
effluent can often
be enhanced by adding passive features (e.g., angles,
flanges, or geometric flow
deflectors) or active features (e.g., low-flow, high-velocity
jets) along the edges
of the hood or within the hood reservoir. Such design
features can improve
hood performance dramatically over a basic box-style
hood with the same
nominal dimensions. |
Cross Drafts
Cross drafts can have a detrimental affect on all
hood/appliance com-
binations. Cross-drafts affect island canopy hoods
more than wall mounted
canopy hoods because they have more open area allowing
drafts to push or pull
effluent from the hood. For example, a pedestal fan
used by staff for additional
cooling can severely degrade hood performance, may
make capture impossible, and may spill the plume into
the kitchen. Location of delivery doors, service
doors, pass-through openings and drive-through windows
may be sources of
cross drafts due to external and internal air pressure
differences. Cross drafts
can also be developed when the makeup air system is
not working correctly,
causing air to be pulled from open drive-through windows
or doors.
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| Commercial
Kitchen Exhaust and Makeup Air Factor |
| Kitchen
Exhaust And Makeup Systems Design Procedure |
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| Commercial
Kitchen Exhaust and Makeup Air Systems Design |
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