The prescriptive mechanical code exhaust
rate requirements must be
conservative because the AHJ (authority having jurisdiction)
has no control
over the design of an exhaust hood or the positioning
and diversity of appli-
ances placed beneath that hood. However, in cases where
the CKV system de-
sign and appliance configuration has been optimized,
the code-specified exhaust
rate may be significantly greater than what is required
for effective capture and
containment of the cooking plume. The code-based safety
factor (which may
be necessary for unlisted systems) can place an energy
cost burden on the CKV
system through its demand for more heated and cooled
makeup air.
One strategy, called “internal compensation,”
was to introduce the
makeup air directly into the hood reservoir. This is
more commonly known as
“short-circuit”
makeup air. Although short-circuit hoods have been installed
and operated with as much as 80% of replacement air
being introduced inter-
nally, field and laboratory investigations have shown
that these hoods fail to
capture and contain effluent adequately.
The second industry strategy was to test hoods under
laboratory condi-
tions according to a test protocol specified by Underwriters
Laboratories, Stan-
dard 710, Exhaust Hoods for Commercial Cooking Equipment.
This UL Standard
covers materials and construction of exhaust hoods as
well as C&C perform-
ance. The C&C performance is based on testing a
single appliance under a rep-
resentative hood at one or more of three cooking temperature
operating set
points (400°F, 600°F, or 700°F). The UL
listing reports the minimum C&C rate
determined under this laboratory test.
Another national standard, ASTM Standard F-1704-1999,
Test Method
for Performance of Commercial Kitchen Ventilation Systems,
covers exhaust hood cap-
ture and containment performance as well as heat gain
from hooded appliances.
The current version of ASTM F-1704 also does not address
dynamic condi-
tions, but there are amendments under consideration
to add a dynamic test that
would quantify a safety factor. The capture and containment
tests in UL 710
and ASTM F-1704 are similar.
While the exhaust rates shown in Table 1 are minimum
mandatory
rates for unlisted hoods, the rates in Table 2 reflect
the typical range in design exhaust rates for listed
hoods. The values in this table may be useful for estimat-
ing the “cfm” advantage offered by listed
hoods over unlisted hoods for a given
project. But in the final stage of design, exhaust rates
may be adjusted to ac-
count for:
|
Appliance Duty Classifications
From ASHRAE Standard 154
Light Duty
• Gas and electric ovens (including
standard, bake, roasting, revolving,
retherm, convection, combination
convection/steamer, conveyor,
deck or deck-style pizza, and pas-
try)
• Electric and gas steam-jacketed
kettles
• Electric and gas compartment
steamers (both pressure and at-
mospheric)
• Electric and gas cheesemelters
• Electric and gas rethermalizers
Medium Duty
• Electric discrete element ranges
(with or without oven)
• Electric and gas hot-top ranges
• Electric and gas griddles
• Electric and gas double-sided grid-
dles
• Electric and gas fryers (including
open deep-fat fryers, donut fryers,
kettle fryers, and pressure fryers)
• Electric and gas pasta cookers
• Electric and gas conveyor (pizza)
ovens
• Electric and gas tilting skillets
/braising pans
• Electric and gas rotisseries
Heavy Duty
• Electric and gas underfired broilers
• Electric and gas chain (conveyor)
broilers
• Gas open-burner ranges (with or
without oven)
• Electric and gas wok ranges
• Electric and gas overfired (upright)
broilers
• Salamanders
Extra Heavy Duty
Appliances using solid fuel such as
wood, charcoal, briquettes, and mes-
quite to provide all or part of the heat
source for cooking.
Calculation of Exhaust Rates Us-
ing Duty Ratings
The rule for unlisted hoods is to
apply the duty rating for the highest
duty appliance to the length of the
entire hood (or separate section of
hood served by an individual ex-
haust fan).
For listed hoods, the same rule may
be applied if little is known about
the expected cooking operations. If
details of the cooking operation are
known, rates for each appliance
may be applied and added up to
determine the total exhaust rate. |
1. Diversity of operations (how many of the appliances
will be on at the
same time).
2. Position under the hood (appliances with strong
thermal plumes, lo-
cated at the end of a hood, tend to spill effluent
more easily than the
same appliance located in the middle of the hood).
3. Hood overhang (in combination with appliance push-back).
Positioning
a wall-mounted canopy hood over an appliance line
with an 18-inch
overhang can dramatically reduce the required ventilation
rate when
compared to the minimum overhang requirement of 6
inches. Some
manufacturers “list” their hoods for a
minimum 12-inch overhang,
providing an immediate advantage over unlisted hoods.
4. Appliance operating temperature (e.g. a griddle
used exclusively by a
multi-unit restaurant at 325ºF vs. 400ºF
surface temperature) or other
specifics of appliance design (e.g. 18-inch vs. 24-inch
deep griddle sur-
face).
5. Differences in effluent from menu selections, such
as cooking ham-
burger on a griddle versus on a charboiler, or using
a charbroiler to
cook chicken versus hamburger.
6. Operating experience of a multi-unit restaurant
can be factored into the
equation. For example, the CKV system design exhaust
rate (for the
next new restaurant) may be increased or decreased
based on real-world
assessments of the CKV system in recently constructed
facilities. |
