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| Commercial Kitchen Exhaust and Makeup Air Systems |
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The Makeup Factor
The layout of the heating, ventilating, and air-conditioning
(HVAC) and
makeup air (MUA) supply air outlets or diffusers can
affect hood performance.
These can be sources that disrupt thermal plumes and
hinder C&C. Safety fac-
ors are typically applied to the design exhaust rate
to compensate for the effect
hat undesired air movement within the kitchen has on
hood performance.
Air that is removed from the kitchen through an exhaust
hood must be
eplaced with an equal volume of outside replacement
(makeup) air through one
or more of the following pathways:
1. Transfer air (e.g., from the dining room)
2. Displacement diffusers (floor or wall mounted)
3. Ceiling diffusers with louvers (2-way, 3-way, 4-way)
4. Slot diffusers (ceiling)
5. Ceiling diffusers with perforated face
6. Integrated hood plenum including (see Figure 4):
• Short circuit (internal supply)
•
Air curtain supply
• Front face supply
• Perforated perimeter supply
• Backwall
supply (rear discharge)
• Combinations of
the above
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Makeup Air
To operate any fan safely, the air that is exhausted
from the inside to the outside must be replaced. This
is why the Uniform Mechanical Code (UMC) requires makeup
air for commercial kitchens. Makeup air is air that
is intentionally pushed or pulled into the building
in an amount more or less equal to the amount of air
that is exhausted by the kitchen fan. Makeup air prevents
fuel spillage and backdrafting from vented combustion
appliances, such as gas water heaters or fireplaces,
within the restaurant. Another purpose of requiring
makeup air is to ensure that the fan can actually exhaust
at its rated capacity. If the fan isn't running at its
rated capacity, it does not maintain sufficient air
velocity to trap the various emissions from the cooking
surface. A building's tightness and exhaust duct configuration
will affect the amount of air that fans inside it can
exhaust. The tighter the building, the greater the pressure
the fan has to operate against and the less air the
fan can move. The amount of air that a fan can move
against any given pressure is called its fan curve.
Get too low on this fan curve, and a 1,200 CFM fan might
exhaust only 1,000 CFM.
Makeup air is usually tempered (heated or cooled to
offset the outside temperature) to ensure the comfort
of kitchen staff. Installing makeup air devices also
ensures that large amounts of air from the eating
area of the restaurant are not drawn into the kitchen.
This can create drafts that affect the comfort of
the diners.
In many jurisdictions, the owner of a commercial
building must pass a performance-based test that certifies
that the makeup air is sufficient for the fan and
grease filter to work safely. Testing and balancing
companies typically administer these tests. They measure
the flow of both the exhaust and supply fans and check
to make sure the restaurant is not depressurized.
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| Commercial
Kitchen Exhaust and Makeup Air Systems Design |
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