T-Method

The T-Method optimizes duct sizes based on predetermined cost factors. It produces duct systems that are low in cost and balanced for both velocity and static pressure. Since system static pressures are balanced, both at design and part load flow, the T-Method can be applied to either constant volume or VAV systems. However, since its calculation procedure is iterative and time consuming, a duct design computer program is essential.

Duct Material and Shapes

Most VAV system ducts are metal. Sheet metal ducts are relatively easy to manufacture and install, and have the mechanical strength needed for medium pressure, medium velocity VAV systems. Near the diffusers, downstream of the terminal units, where pressures and velocities are lower, fibrous and/or flexible ducts are often used. Table A-1 shows typical static pressure losses through flex ducts with varying duct diameters at different flows.

Ducts come in three popular shapes rectangular, round and oval.

Rectangular ducts are used when velocities are below 2500 fpm and static pressures are less than two inches wg. Rectangular ducts become noisy at high velocities and expensive at high pressures. Rectangular ducts are not as "flow efficient" as round or oval ducts, and are more difficult to seal.

Round ducts are used when velocities exceed 2500 fpm and static pressures range from two inches wg to 10 inches wg. Although prefabricated round duct may cost more than rectangular duct, round duct is almost always less costly to install and may provide a lower first cost. Round ducts are also quieter, and easier to seal. For large systems, however, the required round duct diameters may be larger than the available ceiling space.

Oval ducts and fittings are used when ceiling height restrictions preclude the use of round ducts, and are usually more expensive than round ducts and fittings.

Duct System Layout

In general, duct systems should be kept as simple and as symmetrical as possible. The first duct section should be treated with duct lining or a duct silencer to attenuate fan noise. Main trunk runs, as well as branch runs and terminal units, should be placed above hallways and other "unoccupied" areas. This placement improves installation and maintenance convenience, and helps attenuate radiated sound. Balancing dampers should be placed upstream of diffusers and in all noncritical branches.

Detailed duct layout considerations are presented in SMACNA 1990 "HVAC Systems — Duct Design."

Room Air Distribution

When a variable air volume system is used in a building, air is delivered to each zone at either constant or variable flow depending upon the type of VAV terminal unit used and whether cool or warm air is delivered.

Series fan powered terminals with continuous fan operation and constant volume dual duct terminals deliver both cool air and warm air at constant flows. Shutoff terminals with reheat and parallel fan powered terminals with intermittent fan operation deliver cool air at variable flow and warm air at constant flow. Shutoff terminals without reheat, bypass terminals, and VAV dual duct terminals deliver both cool air and warm air at variable flows.

When constant flow delivery is used for both cooling and heating, zone air diffusers are selected as they would be for a constant volume system. Ceiling or wall-mounted grilles or diffusers are used, and are sized and positioned to insure proper operation at design velocities. To avoid uncomfortable drafts, dense cool air must not "dump" to the floor without mixing; similarly, to avoid uncomfortable stratification, buoyant warm air must not "float" near the ceiling without mixing. Therefore, each diffuser must have sufficient throw capability to mix zone air during both heating and cooling operation.

When variable flow cooling and constant flow heating are combined in a zone, the zone air diffusers are usually selected at partial cooling load to insure proper operation. Ceiling mounted linear slot diffusers are recommended since they perform well over a wide range of velocities. Cool air delivery takes advantage of the "Coanda" effect, whereby cool air discharged through a linear ceiling slot hugs the ceiling before descending, insuring proper operation over a wide range of flows without dumping. When delivering heating air, the constant flow velocities must be high enough to insure that the warm air reaches the floor. To prevent stratification, the warm air temperature should not be more than 20 F above the zone air temperature.

When variable flow delivery is used for both cooling and heating, linear slot diffusers are also recommended. Again, linear slot diffusers can effectively exploit the Coanda effect when delivering low velocity cool air. When delivering warm air, a minimum flow rate must be maintained to insure that the warm air reaches the floor. To avoid stratification, the warm air temperature should not exceed the zone air temperature by more than 20 F. In addition to choosing the correct diffuser type, the designer must properly size and place each diffuser in the zone to minimize noise and diffuser pressure drop while maximizing the Air Diffusion Performance Index for the zone. Details regarding slot configurations, throw distances, collision velocities, acoustic performance, diffuser placement and so on, are available from each diffuser manufacturer.

Table A-1 - Typical Static Pressure Losses Through Low Pressure Flexible Duct (inches wg)