Shortridge Instruments AIRDATA ADM-880C, Руководство по эксплуатации

Страница: 56 / 73

ADM-880C 07/20/09 51
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 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.
12.5 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