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146 VAV-PRC011M-EN

Application Considerations

Flow Measurement and Control

The differential pressure signal output of the flow ring provides the terminal unit controller a

measurement of the primary airflow through the inlet. The terminal unit controller then opens or

closes the inlet damper to maintain the controller airflow setpoint

Flow Measurement

Most VAV terminal units contain a differential pressure airflow measurement device, mounted at

the primary air inlet, to provide a signal to the terminal unit controller. Numerous names exist for

the differential pressure measurement device—flow sensor, flow bar, flow ring. The differential

pressure measured at the inlet varies according to the volumetric flow rate of primary air entering

the inlet.

The total pressure and the static pressure are measurable quantities. The flow measurement

device in a VAV terminal unit is designed to measure velocity pressure. Most flow sensors consist

of a hollow piece of tubing with orifices in it. The VariTrane air valve contains a flow ring as its flow

measuring device. The flow ring is two round coils of tubing. Evenly spaced orifices in the upstream

coil are the high-pressure taps that average the total pressure of air flowing through the air valve.

The orifices in the downstream ring are low-pressure taps that average the air pressure in the wake

of flow around the tube. By definition, the measurement of static pressure is to occur at a point

perpendicular to the airflow. The low-pressure taps on the VariTrane flow ring measure a pressure

that is parallel to the direction of flow but in the opposite direction of the flow. This “wake pressure”

that the downstream ring measures is lower than the actual duct static pressure. The difference

between the “wake pressure” and the static pressure can be accounted for so that the above

relationship between flow and differential pressure remain valid. The difference also helps create

a larger pressure differential than the velocity pressure. Since the pressures being measured in VAV

terminal box applications are small, this larger differential allows transducers and controllers to

measure and control at lower flow settings than would otherwise be possible.

The average velocity of air traveling through the inlet is expressed in the equation:

Where:

Often, the density is assumed to be a constant for dry air at standard conditions (68°F (20°C)) and

sea level pressure of 14.7 psi (101.4 kPa)). These conditions yield the following commonly used

equation:

The velocity pressure is defined as the difference between the total pressure in the duct and the

static pressure in the duct:

One of the most important characteristics of a VAV terminal unit is

its ability to accurately sense and control airflow. The VariTrane

terminal unit was developed with exactly that goal in mind. The

patented, multiple-point, averaging flow ring measures the

velocity of the air at the unit primary air inlet.

FPM = Velocity of air in feet per minute

1096.5 = A constant

VP = The velocity pressure of the air expressed in inches of water

DENS = The density of the air expressed in pounds per cubic foot

FPM 1096.5

DENS

------------------=

FPM 4005 VP=

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