ADM-860C 04/14/0848
12.0 SPECIAL BALANCING PROCEDURES
12.1 PROPORTIONAL BALANCING
Backpressure compensated readings should be taken during the preliminary survey of the entire system with all
dampers fully open, and also during the final reading after balancing is complete. Nonbackpressure compensated
readings require less time and are usually adequate for the preliminary balancing of outlets.
12.2 LARGE RETURN AIR GRILLES
Fan systems such as package air conditioning units or fan coil units may have only one main return air inlet. This
is common in residential air conditioning systems ranging from two tons to 7 ½ tons of cooling capacity. The
rated air delivery range is 800 to 3000 cfm. The FlowHood may be used to measure the return air flow if special
attention is paid to avoiding excessive restriction of the air flow. The flow restriction is kept to a minimum by
taking readings in two or more segments. For instance, if the return air grille is 30" x 24" and rated flow is 2400
cfm, read each half of the grille (15" x 24") using backpressure compensation. The sum of the backpressure
compensated readings taken on each half is the total air flow through the return air grille. Keep in mind that this
flow measurement represents only the flow at the return air grille. The total air flow at the fan includes return air
duct leakage and possible outside air intake.
The reading for each portion of the grille would be even less restrictive and thereby more accurate if the 1'x 4'
top were used on the FlowHood. The air flow could then be measured in three segments along the 30"
dimension, with the excess length of the 48" top overlapping onto the ceiling. The 30" dimension of the grille
could be broken into 9", 12" and 9" segments using the 12" width of the 1'x 4' FlowHood top. This procedure is
also usable on large supply or discharge grilles and diffusers.
12.3 KITCHEN EXHAUST HOODS
Exhaust fan delivery of room temperature air in kitchens is reduced substantially when the cooking surfaces,
ovens, broilers and fryers are heated to normal working temperatures. This is caused by the significant reduction
in air density which occurs during the change from cool set-up to heated working conditions.
A fan which was set to deliver 5000 cfm at 75
/ F intake air will actually deliver only about 4610 cfm of the 75/ F
room air when the air is heated to 140
/ F in the exhaust hood. It is very important that a kitchen system balance
be confirmed by measurements for air flow, room pressure, and actual makeup air, while the cooking devices are
at their normal working temperatures.
12.3.1 RANGE EXHAUST FILTERS AND GREASE EXTRACTORS
IMPORTANT: See Section 6.1 VELOCITY CORRECTION FACTORS
The VelGrid may be used to measure the range exhaust washable filter and grease extractor face velocities as
described below. NOTE: The VelGrid is not designed to be used without the standoffs.
Begin by determining the gross face area of each filter or grease extractor to be tested. In this case, the nominal
filter size marked on the filter should be used to calculate the full gross face area. For example, a 20" x 25" filter
is 500 inches square overall (or 3.47 feet square overall). Measure the face velocity of each filter or extractor with
the VelGrid positioned at the center of the filter face only. The 1.5" standoffs must be pressed against the face
of the filter to hold the VelGrid 1.5" from the filter face. Determine the Kv factor for this type of filter as described
in Section 6.1 VELOCITY CORRECTION FACTORS. Multiply the center face velocity by the calculated Kv factor
to obtain the corrected face velocity for that filter or extractor. The air flow in cfm or l/s is calculated by multiplying
the gross face area of the filter or extractor by the corrected face velocity.
Most commercial grade exhaust hoods leak to some degree between the filters and along the filter support tracks.
The sum of the filter air flows obtained in the manner described above is generally sufficiently accurate for
properly constructed kitchen exhaust hoods. Hoods which have been installed with excessive clearances and
leakage will require correction for leakage.
12.4 CONSTANT VOLUME CONTROLLERS
The backpressure compensation process requires special attention when used to measure air flow at individually
controlled constant volume air terminals. The nonbackpressure compensated and the backpressure
compensated air flow readings should be essentially equal for fast acting constant volume air terminals. The
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