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VAV-PRC011M-EN 147

Application Considerations

VP = TP - SP (All units are expressed in inches of water)

The amount of air traveling through the inlet is related to the area of the inlet and the velocity of

the air:

AIRFLOW = AREA (square feet) x AVERAGE VELOCITY (feet per minute)

Accuracy

The multiple, evenly spaced orifices in the flow ring of the VariTrane terminal unit provide quality

measurement accuracy even if ductwork turns or variations are present before the unit inlet. For

the most accurate readings, a minimum of 1½ diameters, and preferably 3 diameters, of straight-

run ductwork is recommended prior to the inlet connection. The straight-run ductwork should be

of the same diameter as the air valve inlet connection. If these recommendations are followed, and

the air density effects mentioned below are addressed, the flow ring will measure primary airflow

within ±5% of unit nominal airflow.

Air Density Effects

Changes in air density due to the conditions listed below sometimes create situations where the

standard flow sensing calibration parameters must be modified. These factors must be accounted

for to achieve accuracy with the flow sensing ring. Designers, installers, and air balancers should

be aware of these factors and know of the necessary adjustments to correct for them.

Elevation

At high elevations the air is less dense. Therefore, when measuring the same differential pressure

at elevation versus sea level the actual flow will be greater at elevation than it would be at sea level.

To calculate the density at an elevation other than standard conditions (most manufacturers choose

sea level as the point for their standard conditions), you must set up a ratio between the density

and differential pressure at standard conditions and the density and differential pressure at the new

elevation.

Since the data from the manufacturer is published at standard conditions, this equation should be

solved for the differential pressure at standard conditions and the other quantities substituted to

determine the ratio for the differential pressure measured at the new conditions.

Duct Pressure and Air Temperature Variations

While changes in these factors certainly affect the density of air, most operating parameters which

VAV systems need keep these effects very small. The impact on accuracy due to these changes is

less than one half of one percent except in very extreme conditions (extreme conditions are defined

as those systems with static pressures greater than 5 in. wg (1245 Pa) and primary air temperatures

greater than 100°F (37.8°C)). Since those types of systems occur so infrequently, we assume the

effects of duct pressure and air temperature variations to be negligible.

Figure 14. Air pressure measurement orientations

ΔPS dardConditionsταν

DENSS dard Conditionsταν

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ΔPNew Conditions

DENSNew Conditions

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