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154 VAV-PRC011M-EN

Application Considerations

scheduling units much easier. Contact the local sales office or the Trane C.D.S.™ department for

more details on this program.

Design Methods

The two most widely used supply duct design methods—equal friction and static regain—are

discussed below.

Equal Friction – Using this method, ducts are sized at design flow to have roughly the same static

pressure drop for every 100 feet of duct. Static pressures throughout the duct system can be

balanced at design flow using balancing dampers, but are no longer balanced at part load flows.

For this reason, equal friction duct designs are better suited for constant volume systems than for

VAV systems. If the equal friction method is used for the VAV supply duct design, the terminal units

usually require pressure-independent (PI) control capability to avoid excessive flow rates when

duct pressures are high.

In VAV systems, the ducts located downstream of the terminal unit are usually sized for equal

friction. The advantage of this design method is its simplicity. Often, calculations can be made

using simple tables and duct calculators. Drawbacks include increased higher total pressure drops

and higher operating costs.

Static Regain – In the static regain method, ducts are sized to maintain constant static pressure in

each section, which is achieved by balancing the total and velocity pressure drops of each section.

In other words, static pressure is “regained” by the loss of velocity pressure. Since the static

pressures throughout the duct system are roughly balanced at design and part load flow, static

regain duct designs can be used successfully for either constant volume or VAV systems. When the

static regain method is used for VAV systems, the system is roughly pressure balanced at design.

Advantages of the static regain method include reduced total pressure drops, lower operating

costs, and balanced pressures over a wide range of flows. The drawback of this design is the time-

consuming, iterative calculation procedure and for large systems, it is essential to have a duct

design computer program.

Best Practices

Common Mistakes

Some of the most common system or installation errors are discussed below.

Reducers at Unit Inlet

This problem is a very common issue that is seen in applications of VariTrane products. It is often

mistaken by those in the field as an unacceptably large static pressure drop through the unit. It is

also sometimes mistaken as a malfunctioning flow ring, pressure transducer (if DDC or analog

electronic controls are present) or PVR (if pneumatic controls are present).

This problem is sometimes unknowingly encountered because of the capability of the VariTrane

unit to allow greater airflow for a specific size duct than other terminal units. For example, a project

engineer specifies an 8" (203 mm) round take off from the main duct trunk to the VAV terminal unit.

The person supplying the VAV terminal unit checks the required airflow and finds that a VariTrane

unit with a 6" (152 mm) inlet will provide the specified terminal unit performance. The terminal unit

supplier submits, receives approval, and orders the 6" (152 mm) inlet unit. While this is happening,

the installing contractor has run the connecting duct from the main trunk to the terminal unit in the

specified 8" (152 mm) round. The unit arrives at the job site, and the installer notices that the 8" (203

mm) duct and the 6" (152 mm) terminal unit inlet do not match. To get the unit installed, an 8- to

6-inch reducer is placed at the inlet to the terminal unit air valve.

The reducer will cause a phenomenon called flow separation at the unit inlet. Fluid dynamics

analysis can present a detailed technical explanation of flow separation, but the characteristics

important to this discussion are the production of pressure loss and turbulence. The reducer will

have a significant static pressure drop associated with it since the air velocity is increased (i.e.,

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