5.15 Air Distribution Systems

Air Flow Diagram, Atrium, Phoenix Courthouse
Air Flow Diagram, Atrium,
Phoenix Courthouse

Variable Air Volume (VAV) Systems
The VAV supply fan shall be designed for the largest block load, not the sum of the individual peaks. The air distribution system up to the VAV boxes shall be medium pressure and shall be designed by using the static regain method. Downstream of the VAV boxes the system shall be low and medium pressure construction and shall be designed using the equal friction method. Sound lining is not permitted. Double wall ductwork with insulation in-between is permitted in lieu of sound lining. All VAV boxes shall be accessible for maintenance. Ducted return shall be utilized at all locations. VAV fan-powered box supply and return ducts shall have double wall ductwork with insulation in-between for a minimum distance of 5 feet.

Underfloor Air Distribution Systems. Provide plenum zones both for perimeter and interior in order to control the underfloor variable volume dampers or boxes with separate plenum barriers between perimeter and interior zones. The underfloor plenum shall be air tight and compartmentalized with baffles. Provisions shall be provided for cleaning the plenum space.When underfloor supply air distribution is used, the ceiling plenum shall be used for the distribution of the ducted return air. The perimeter and interior underfloor zones shall be clearly separated in order to maintain proper pressurization, temperature and humidity control. Zoning of the underfloor air distribution systems shall be in accordance with descriptions presented elsewhere in this chapter. Perimeter wall below the raised flooring system shall be provided with R-30 insulation and vapor barrier below the raised floor. All VAV boxes that are part of an underfloor air distribution system for both perimeter and interior systems shall be located below the raised floor. The floor area used for an underfloor system shall have the slab provided with a minimum of R-10 insulation and vapor barrier from below. This shall incorporate the entire slab area used for the underfloor system.

Volume Control. Particular attention shall be given to the volume control. VAV systems depend on air volume modulation to maintain the required ventilation rates and temperature set points. Terminal air volume control devices are critical to the successful operation of the system and shall be provided. Zone loads must be calculated accurately to avoid excessive throttling of air flow due to oversized fans and terminal units. Diffusers shall be high entrainment type (3:1 minimum) to maximize air velocity at low flow rates. If ventilation air is delivered through the VAV box, the minimum volume setting of the VAV box should equal the larger of the following:

1. 30 percent of the peak supply volume;
2. 0.002 m3/s per m2 (0.4 cfm/sf) of conditioned zone area; or
3. Minimum m3/s (cfm)to satisfy ASHRAE Standard 62 ventilation requirements. VAV terminal units must never be shut down to zero when the system is operating.Outside air requirements shall be maintained in accordance with the Multiple Spaces Method, Equation 6-1 of ASHRAE Standard 62 at all supply air flow conditions.

Airside Economizer Cycle. An air-side enthalpy economizer cycle reduces cooling costs when outdoor air enthalpy is below a preset high temperature limit, usually 15 to 21°C (60°F to 70°F), depending on the humidity of the outside air. Airside economizers shall only be used when they can deliver air conditions leaving the air handling unit of a maximum of 10°C (50°F) dew point and a maximum of 70 percent relative humidity. Enthalpy economizers shall operate only when return air enthalpy is greater than the outside air enthalpy.

All air distributions systems with a capacity greater than 1,416 LPS (3,000 CFM) shall have an air-side economizer in accordance with ASHRAE 90.1, unless the design of the air handling systems preclude the use of an airside economizer.

Ductwork. Ductwork shall be designed in accordance with ASHRAE: Handbook of Fundamentals, Duct Design Chapter, and constructed in accordance with the ASHRAE: HVAC Systems and Equipment Handbook, Duct Construction Chapter, and the SMACNA Design Manuals. All ductwork joints and all connections to air handling and air distribution devices shall be sealed with mastic— including all supply and return ducts, any ceiling plenums used as ducts and all exhaust ducts. Energy consumption, security and sound attenuation shall be major considerations in the routing, sizing and material selection for the air distribution ductwork.

Supply, Return and Exhaust Ductwork
Ductwork Pressure. Table 5-3 provides pressure classification and maximum air velocities for all ductwork. Ductwork construction shall be tested for leakage prior to installation. Each section tested must have a minimum of a 20 ft. length straight-run, a minimum of two elbows and a connection to the terminal. The stated static pressures represent the pressure exerted on the duct system and not the total static pressure developed by the supply fan. The actual design air velocity should consider the recommended duct velocities in Table 5-4 when noise generation is a controlling factor. Primary air ductwork (fan connections, risers, main distribution ducts) shall be medium pressure classification as a minimum. Secondary air ductwork (run outs/branches from main to terminal boxes and distribution devices) shall be low pressure classification as a minimum.

Supply, return and exhaust air ducts shall be designed and constructed to allow no more than 3 percent leakage of total airflow in systems up to 750 Pa (3 inches WG). In systems from 751 Pa (3.1 inches WG) through 2500 Pa (10.0 inches WG) ducts shall be designed and constructed to limit leakage to 0.5 percent of the total air flow.

Pressure loss in ductwork shall be designed to comply with the criteria stated above. This can be accomplished by using smooth transitions and elbows with a radius of at least 1.5 times the radius of the duct. Where mitered elbows have to be used, double foil sound attenuating turning vanes shall be provided. Mitered elbows are not permitted where duct velocity exceeds 10m/s (2000 FPM).

Table 5-3 Ductwork Classification

Static Pressure
 
Air Velocity   Duct Class
250 Pa (1.0 in W.G.) < 10 m/s DN < (2000 FPM DN) Low Pressure
500 Pa (2.0 in W.G) < 10 m/s DN < (2000 FPM DN) Low Pressure
750 Pa (3.0 in W.G.) < 12.5 m/s DN < (2500 FPM DN) Medium Pressure
1000 Pa (+4.0 in W.G.) < 10 m/s DN > (2000 FPM UP) Medium Pressure
1500 Pa (+6.0 in W.G.) < 10 m/s DN > (2000 FPM UP) Medium Pressure
2500 Pa (+10.0 in W.G.) < 10 m/s DN > (2000 FPM UP) High Pressure

Table 5-4 Recommended Duct Velocities


 
Controlling Factor Noise Generation
(Main Duct Velocities)
Application m/s (fpm)
Private Offices
Conference Rooms
Libraries
6 (1,200)
Theaters
Auditoriums
4 (800)
General Offices 7.5 (1,500)
Cafeterias 9 (1,800)

Sizing of Ductwork. Supply and return ductwork shall be sized using the equal friction method except for ductwork upstream of VAV boxes. Duct systems designed using the equal friction method place enough static pressure capacity in the supply and return fans to compensate for improper field installation and changes made to the system layout in the future. In buildings with large areas of open plan space, the main duct size shall be increased for revisions in the future. Air flow diversity shall also be a sizing criterion. 80 percent diversity can be taken at the air-handling unit and decreased the farther the ductwork is from the source until air flow diversity is reduced to zero for the final portion of the system.

Ductwork Construction. Ductwork shall be fabricated from galvanized steel, aluminum or stainless steel sheet metal depending on applications. Flex duct may be used for low pressure ductwork downstream of the terminal box in office spaces. The length of the flex duct shall not exceed the distance between the low pressure supply air duct and the diffuser plus 20 percent to permit relocation of diffusers in the future while minimizing replacement or modification of the hard ductwork distribution system. Generally, flex duct runs should not exceed 3 m (10 feet) nor contain more than two bends.

Joint sealing tape for all connections shall be of reinforced fiberglass backed material with field applied mastic. Use of pressure sensitive tape is not permitted.

Kitchen Ventilation Systems. Products of combustion from kitchen cooking equipment and appliances shall be delivered outside of building through the use of kitchen ventilation systems involving exhaust hoods, grease ducts and make-up air systems where required. Commercial kitchen equipment applications shall be served by a Type I hood constructed in compliance with UL 710 and designed in accordance with code having jurisdiction. Grease ducts shall be constructed of black steel not less than 0.055 inch (1.4 mm) (No. 16 gauge) in thickness or stainless steel not less than 0.044 inch (1.1 mm) (No. 18 gauge in thickness).

Make-up air systems serving kitchen exhaust hoods shall incorporate air-side heat exchange to recover energy from the exhaust stream to be used for heating the supply air stream.

Ceiling Plenum Supply. Ceiling plenum supply does not permit adequate control of supply air and shall not be used.

Raised Floor Plenum Supply. In computer rooms, underfloor plenum supplies are appropriate. As a general application in other areas (e.g. open offices), underfloor air distribution/displacement systems are appropriate. Where raised floor plenums are used for supply air distribution, the plenums shall be properly sealed to minimize leakage. R-30 insulation with vapor barrier shall be provided for perimeter of raised floor walls.

Plenum and Ducted Returns.With a return plenum care must be taken to ensure that the air drawn through the most remote register actually reaches the air-handling unit. The horizontal distance from the farthest point in the plenum to a return duct shall not exceed 15 m (50 feet). No more than 0.8 m3/s (2,000 cfm) should be collected at any one return grille. Figure 5-2 illustrates an example of an open ceiling plenum with return air ductwork. Return air plenums should be avoided. When deemed necessary for economic reasons, plenums shall be sealed air-tight with respect to the exterior wall and roof slab or ceiling deck to avoid creating negative air pressure in exterior wall cavities that would allow intrusion of untreated outdoor air. All central multi-floor-type return air risers must be ducted.

Other less flexible building spaces, such as permanent circulation, public spaces, and support spaces, shall have ducted returns. Where fully ducted return systems are used, consider placing returns low in walls or on columns to complement ceiling supply air.

Return air duct in the ceiling plenum of the floor below the roof shall be insulated. Double wall ductwork with insulation in-between shall be used in lieu of sound lining for a minimum of the last 5 feet before connecting to the air handling unit or a return air duct riser.

Figure 5-2 Ceiling Return Plenum with Minimal Return Ductwork

Figure 5-2 Ceiling Return Plenum with Minimal Return Ductwork

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Last Reviewed 2016-08-02