Indoor Air Quality

ASHRAE’s latest standard, 62-1989, specifies minimum outside air quantities. Maintaining a minimum ventilation rate is a challenge in any VAV system, since the supply airflow decreases as the load decreases. Minimum flow settings are specified to insure that outdoor air reaches the space. On projects where indoor air quality is a concern, accurate airflow measurement and modulation is imperative, especially at reduced cfm. Airflow modulation should be linear and pressure independent, and use the finest flow sensors and transducers in the industry. (At minimum cfm to zone, the accuracy is more vital to ensure a constant amount of fresh air to the zone.)

In some cases, insulation that is exposed to supply air is objectionable from an indoor air quality standpoint. Trane offers foil faced and double wall units to minimize the exposure of the airstream to the insulation. Typical applications calling for those units include hospitals and labs. Some manufacturers supply an inner lining of perforated metal. This would not be recommended due to insulation fibers being exposed to the airstream.

Other concerns include the mixing of the supply air with the air existing in the zone. The linear slot diffuser, if applied correctly, using its Coanda effect, will mix supply air with zone air even at low cfm. The Coanda effect also provides better comfort by reducing the feeling of being dumped on by the diffuser.

VAV Duct Design

Designing cost effective variable air volume duct systems is challenging. Not only must the supply ducts deliver air to each terminal unit at flow rates ranging from design flow to minimum flow, but the inlet pressure to each terminal unit must remain roughly constant throughout this flow range. Some duct design methods result in better pressure balance than others.

Design Methods

The two most widely used duct design methods, equal friction and static regain, are discussed next. A new optimizing method, the T-Method, is also mentioned.

Equal Friction

Using this method, ducts are sized so that, at design flow, each 100 feet of duct has roughly the same total pressure drop. 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 VAV supply duct design, the terminal units require pressure independent (Pi) control capability to avoid excessive flow rates when duct pressure is high.

Equal friction duct designs are most often used for constant volume systems with low pressures, low velocities, and rectangular duct. The advantage of this design method is its simplicity: calculations can be made using simple tables and duct calculations. Drawbacks include increased total pressure drops, higher operating costs, large duct sizes and poor pressure balance at part load flow.

Static Regain

Using this method, ducts are sized so that the static pressure drop in each duct section is balanced by the velocity pressure drop in that section. (In other words, static pressure is "regained" by a loss in velocity pressure.) Since the static pressures throughout the duct system are roughly static pressure balanced, both at design flow and at 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 terminal units may not need pressure independent control capability, since the system is roughly pressure balanced by design.

Static regain duct designs are most often used for VAV systems with medium pressures, medium velocities and round duct. Advantages of this design method include reduced total pressure drop, lower operating costs, smaller ducts and balanced pressures at all flows. The drawback is the time consuming, iterative calculation procedure; for large systems, a duct design computer program is essential.