Fundamentals of Kitchen Exhaust
Hot air rises! An exhaust fan in the ceiling could remove
much of the
heat produced by cooking equipment. But mix in smoke,
volatile organic
compounds, grease particles and vapor from cooking,
and a means to capture
and contain the effluent becomes necessary to avoid
health and fire hazards.
While an exhaust hood serves that purpose, the key question
becomes: what is
the appropriate exhaust rate? The answer always depends
on several factors: the
menu of food products and the type (and use) of the
cooking equipment under
the hood, the style and geometry of the hood itself,
and how the makeup air
(conditioned or otherwise) is introduced into the kitchen.
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The Cooking Factor
Cooking appliances are categorized as light-, medium-,
heavy-, and extra
heavy-duty, depending on the strength of the thermal
plume and the quantity of
grease, smoke, heat, water vapor, and combustion products
produced. The
strength of the thermal plume is a major factor in
determining the exhaust rate.
By their nature, these thermal plumes rise by natural
convection, but they are
turbulent and different cooking processes have different
“surge” characteristics.
For example, the plume from hamburger cooking is strongest
when flipping the
burgers. Ovens and pressure fryers may have very little
plume until they are
opened to remove food product. Open flame, non-thermostatically
controlled
appliances, such as underfired broilers and open top
ranges, exhibit strong steady
plumes. Thermostatically controlled appliances, such
as griddles and fryers have
weaker plumes that fluctuate in sequence with thermostat
cycling (particularly
gas-fired equipment). As the plume rises, it should
be captured by the hood and
removed by the suction of the exhaust fan. Air in
the proximity of the appliances
and hood moves in to replace it. This replacement
air, which must ultimately
originate as outside air, is referred to as makeup
air.
Cooking that produces smoke and grease requires liquid-tight
construction with
a built-in fire suppression system (Type I hood),
while operations that produce
only heat and moisture do not require liquid-tight
construction or a fire sup-
pression system (Type II hood).
Menu items may produce more or less smoke and grease
depending on
their fat content and how they are cooked. Higher
fat content foods tend to re-
lease more smoke and grease regardless of the type
of cooking process. Testing
under an ASHRAE sponsored research project at the
University of Minnesota
confirmed that hamburger cooked on a charbroiler releases
finer smoke parti-
cles and more grease vapor and particles than hamburger
cooked on a griddle.
The percentage fat content of hamburger also contributes
to differences in the
amount of grease and smoke released in cooking. Chicken
breast, which has
less fat compared to hamburger, releases less particulate
and less grease during
cooking on a charbroiler or on a griddle compared
to hamburger. |
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The Hood Factor
The design exhaust rate also depends on the hood style
and construc-
tion features. Wall-mounted canopy hoods, island (single
or double) canopy
hoods, and proximity (backshelf, pass-over, or eyebrow)
hoods all have differ-
ent capture areas and are mounted at different heights
and horizontal positions
relative to the cooking equipment (see Figure 1). Generally,
for the identical
(thermal plume) challenge, a single-island canopy hood
requires more exhaust
than a wall-mounted canopy hood, and a wall-mounted
canopy hood requires
more exhaust than a proximity (backshelf) hood. The
performance of a double-
island canopy tends to emulate the performance of two
back-to-back wall-
canopy hoods, although the lack of a physical barrier
between the two hood
sections makes the configuration more susceptible to
cross drafts. |
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