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VAV-PRC011M-EN 155

Application Considerations

static pressure is given up for increased velocity pressure). The pressure loss is sometimes

mistaken as a loss due to the function of the terminal unit. The turbulence is at its greatest just

downstream of the reducer. Unfortunately, this is the location of the flow ring at the air-valve inlet.

The reducer will cause the flow ring to give an inaccurate and inconsistent reading because of the

turbulent air.

The solutions to this situation are:

• Locate the reducer upstream of the terminal unit at least three duct diameters to eliminate flow

separation and turbulence at the unit inlet and to improve the airflow measurement accuracy.

• Consider proper sizing of the terminal unit in the duct design and account for the pressure loss

of the reducer in the central fan selection if a reducer is required. Be cautious of “oversizing”

a VAV terminal. It is good practice to make sure that the inlet duct velocity at the minimum

airflow setting is no lower than 500 feet per minute.

Improper Use of Flexible Ductwork

While flexible ductwork has many benefits, improper use can cause numerous problems in a VAV

system. Flexible ductwork causes turbulent airflow and relatively large static pressure drops.

Flexible ductwork at a primary damper inlet (i.e., the flow sensor location) may cause flow accuracy

and repeatability problems due to turbulence. The use of flexible ductwork should be primarily

limited to the downstream side of the terminal units in a VAV system. Use of flexible ductwork

upstream of terminal units should be kept to an absolute minimum. All runs of flexible ductwork

should be kept as short as possible. While most know these guidelines, the ease of installation

which flexible ductwork provides is always an enticement to push the limits of what are acceptable

practices.

Static Pressure Measurement Errors

Improper measurement techniques for static pressure can lead many to mistakenly believe that the

terminal unit is causing a large pressure drop in the system. The chief error made here is taking a

static pressure measurement in turbulent locations such as flexible ductwork or near transitions.

This produces invalid static pressure readings. Another error commonly made is trying to read the

static pressure at the same point as the flow sensing device. The inlets to VAV terminal units

produce turbulence and will give poor readings. Flow sensors with their multiple-point averaging

capability are best equipped to deal with this type of flow, while a single-point static probe is not.

Another common error is the incorrect orientation of the static pressure probe. The static pressure

is correctly measured when the probe is oriented perpendicular to the direction of airflow. The

probe, or a part of it, should never be facing the direction of airflow, because the total pressure will

influence the reading of the probe.

Unit Conversions

Table 52. Conversions of length and area

To convert From To Multiply by

Length

In.

0.0254

0.3048

39.3701

3.28084

Area

In.

Ft2

0.00064516

0.092903

1550

10.7639

1 2 ... 150 151 152 153 154 155 156 157 158

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Trane Round In/Round Out Lista de partes Pagina 155

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