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| An effective commercial kitchen
ventilation (CKV) system requires balance air balance
that is. And as the designer, installer or operator
of the kitchen ventilation system, you may be the
first person called upon to perform your own “balancing
act” when the exhaust hood doesn’t work.
Unlike a cooking appliance, which can be isolated
for troubleshooting, the exhaust hood (a non-functioning
sheet metal box by itself) is only one component of
the ventilation system. To further complicate things,
the CKV system is a subsystem of the overall building
heating, ventilating and air-conditioning (HVAC) system.
Fortunately, there is no “magic” to
the relationship between an exhaust hood and its requirement
for replacement or makeup air (MUA). The physics are
simple: air that exits the building (through exhaust
hoods and fans) must be replaced with outside air
that enters the building (intentionally or otherwise).
The essence of air balance: “air
in” = “air out!”
Background If the replacement air doesn’t
come in, that means it doesn’t go out
the exhaust hood and problems begin. Not only
will the building pressure become too “negative,”
the hood may not capture and contain (C&C)
cooking effluents due to reduced exhaust flow.
We have all experienced the “can’t-open-the-door”
syndrome because the exhaust fan is sucking
too hard on the inside of the restaurant. |
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The mechanical design may call for 8000 cubic feet
per minute (cfm) of air to be exhausted through the
hood. But if only 6000 cfm of outdoor air is able
to squeeze in through closed dampers on rooftop units
and undesirable pathways in the building envelope,
then only 6000 cfm is available to be exhausted through
the hood. The exhaust fan creates more suction (negative
pressure) in an unsuccessful attempt to pull more
air through the hood. There is no piece of equipment
that generates more controversy within the food service
equipment supply and design community than the exhaust
hood in all its styles and makeup air combinations.
The idea that by not installing a dedicated |
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| makeup air supply, the operator
is going to save money (in both first cost and operating
cost) is short sighted. It may be okay if, by design,
all of the makeup air can be provided through the
rooftop HVAC units (this strategy has been adopted
successfully by several leading quick-service chains).
However, in full-service and institutional kitchens
with larger exhaust requirements, it may not be practical
to supply 100% of the replacement (makeup) air through
the building HVAC system.
The solution is to specify an independent makeup
air (MUA) supply. But, once MUA has been added to
the system, the challenge becomes introducing this
air into the kitchen without disrupting the ability
of the hood to capture and/or without causing discomfort
for the kitchen staff. Kitchens are not large and
dumping 7000 cfm of MUA, for example, in front of
a cook line does not go as smoothly in practice as
it does on the air balance schedule! This design guide
presents strategies that can minimize the impact that
the makeup air introduction will have on hood performance,
kitchen environment, and energy consumption.
An exhaust fan in the ceiling could easily remove
the heat produced by cooking equipment. But mix in
smoke, volatile organic compounds, grease particles
and vapor from cooking, a means to capture and contain
the effluent is needed to avoid health and fire hazards.
While an exhaust hood serves that purpose, the key
question is always: what is the appropriate exhaust
rate? The answer always depends on 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. 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 and smoke produced. The strength of the
thermal plume is a major factor in determining the
exhaust rate. By their nature, these thermal plumes
are very 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 charbroilers 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 by natural convection, it is captured
by the hood and removed by the suction |
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| This replacement air, which originates
as outside air, is referred to as makeup air (MUA).
The design exhaust rate also depends on the hood style.
Wall-mounted canopy hoods, island (single or double)
canopy hoods, and proximity (back shelf, pass-over,
or eyebrow) hoods all have different capture areas and
are mounted at different heights relative to the cooking
equipment . Generally, a single- island canopy hood
requires more exhaust than a wall-mounted hood, and
a wall-mounted hood requires more exhaust than a proximity
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. Lastly, the layout of the HVAC and
makeup air distribution points can affect hood performance.
These can be sources that disrupt thermal plumes and
hinder capture and containment. Location of delivery
doors, service doors, and drivethrough windows can also
be sources of cross drafts. Safety factors are typically
applied to the design exhaust rate to compensate for
the effect that undesired air movement within the kitchen
has on hood performance. |
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