CFM-88L 07/20/0922
11.0 SPECIAL BALANCING PROCEDURES
11.1 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.
11.2 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.
11.3 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.
11.4 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 controller responds to the additional
backpressure and maintains the preset air flow. If the controller is slow acting, the FlowHood must be held in place long
enough for the controller to re-attain the set point.
Master and slave, volume controlled outlets often do not interact favorably with the FlowHood backpressure compensation
function. Air delivery measurement for this type system should be performed using a nonbackpressure compensated
measurement.
11.5 LINEAR SLOT DIFFUSERS
Linear slot diffusers deliver supply air in a sheet or air curtain that tends to follow the side of the cloth transition as it flows
to the FlowHood base. This can result in an extremely uneven velocity distribution across the flow sensing grid when the
flaps are open. The air flow is more evenly distributed across the flow sensing grid when the flaps are closed.
Linear slot diffuser readings at less than 100 cfm per lineal foot of diffuser should be taken with the FlowHood flaps
closed. The AirData FlowMeter will permit flaps closed, nonbackpressure compensated readings up to 500 cfm. In most
cases, the FlowHood backpressure effect on linear slot diffusers is not significant. Accurate readings can normally be
obtained with flaps open readings on linear diffusers sized for more than 100 cfm per lineal foot.
The tendency of a sheet of air to cling to, or follow the side of the FlowHood top, can be reduced by using a top width
dimension that is as close as possible to the outlet width. Special transition tops can be constructed of corrugated cardboard
and duct tape. Custom dimension, cloth and frame top sizes may be ordered from our factory. A commonly used size is
5.25" x 47" with a 4.25" x 46" inside frame dimension.
11.6 SIDEWALL REGISTERS
The FlowHood may be used to measure air delivery of supply sidewall registers using the standard 30" length, 2'x2' top or
larger top sizes. It is best if the top dimensions closely match the outlet size to assure even air flow across the flow sensing
grid. Exact matching of hood size to the outlet size is not practical in most cases.
The standard 30" length top sizes may be used with equal accuracy on supply or return air outlets. However, small supply
outlets at high jet velocity, "punkah" diffusers or nozzle type outlets may cause an extreme concentration of air velocity on
portions of the flow sensing grid. The FlowHood readings will be inaccurate under such conditions.
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