Air Distribution Systems

Air Distribution Systems

A. Ventilation: Outdoor air ventilation shall be introduced into each occupied building space in accordance with the current revision of ASHRAE Standard 62 and approved addenda.B. Energy Conservation: Exhaust airflow and associated outdoor air makeup airflow shall be minimized to conserve energy. However, the applicable requirements of ASHRAE Standard 62 (referenced above) as well as the American National Standard for Laboratory Ventilation (ANSI/AIHA Standard #Z9.5, referenced in the Wet Laboratories section within these General Guidelines) shall not be violated. The conditioning of outdoor air is the most significant energy consumer on campus. Fan energy consumption shall also be minimized. This requires that careful attention be given to the selection of the most efficient fan type(s) and size(s) for each application. This will also go a long way toward minimizing fan-generated noise. Air distribution ductwork and accessories shall be sized generously (within appropriate limits) and configured so as to minimize the total pressure requirement at each fan, thus minimizing fan brake horsepower requirements.

C. Energy Recovery Systems: See the Energy Recovery Systems section within these General Guidelines for specific air distribution/exhaust system requirements related to energy recovery systems.

D. Economizers: Each air distribution system with a design recirculated airflow rate of 3000 CFM or greater shall be equipped with an airside economizer to allow the system to take advantage of opportunities for free cooling.

E. Return Fans: Each air distribution system that is equipped with an airside economizer shall incorporate one or more dedicated return fans.

F. VAV Systems: Each air distribution system with a design recirculated airflow rate of 5,000 CFM or more shall be a variable air volume (VAV) type system. Each fan motor in each VAV system shall be served by a variable frequency drive (VFD). Non-wet lab applications with capacity ranging from 5000 CFM to 10,000 CFM may not require heat recovery as long as the Project complies with the current revision of ASHRAE 90.1 and approved addenda as well as the current versions of the International Building and Mechanical Codes. Fan-powered VAV terminal units shall never be used.

G. Outdoor Air Intakes: Each outdoor air intake opening shall be located at least 3 ft. above the adjacent ground or roof level. When an outdoor areaway is used for air intake purposes it shall be sized such that the air velocity within the areaway does not exceed 500 FPM. A 2-3 ft. deep snow retention basin shall be provided at the bottom of the areaway to allow airborne snow to accumulate without entering the intake louver. If possible, the intake louver(s) shall be inset into the building away from the areaway a few feet to minimize the potential for entrainment of dry powdery snow. This is especially important with air distribution systems that utilize a high percentage of outdoor air. When locating outdoor air intake openings it is essential that sources of odors/contaminants be avoided. Vehicle exhaust from streets and loading dock areas as well as exhaust from emergency generators are common culprits for indoor air quality problems. Outdoor air intake louvers shall be sized generously to prevent the carryover of rain and snow into the associated ventilation systems. Serious consideration shall be given to the installation of bird screen on the exterior rather than the interior of all louvers in order to prevent the roosting and nesting of pigeons. This has become a serious problem at numerous locations across campus. (See the Pest Control section within these General Guidelines.)

H. Relief Air Openings: Each relief air opening shall be located a minimum of 10 feet from the nearest outdoor intake opening. Consideration shall be given to the prevailing wind direction, building configuration, intake and relief opening orientations, etc. when locating outdoor air and relief air openings to minimize the potential for recirculation of relief air back into the intake air stream. Relief air shall be ducted directly to the outdoors rather than being discharged into a mechanical equipment room or other interior space. As addressed above regarding outdoor intake louvers, bird screen shall be installed at each relief air louver, preferably on the exterior of the louver.

I. Mixing of Air Streams: The outdoor air ductwork and return air ductwork as well as the actual mixing area upstream of each air-handling unit shall be configured so as to ensure that the air streams mix thoroughly prior to entering the unit. This typically requires that a significant amount of the mechanical equipment room be utilized for this purpose. This prevents coil freeze-up problems as well as nuisance trip-outs of freeze protection thermostats. This has been a very serious repeated problem on campus in recent years. Air handling units in VAV systems are especially susceptible to this problem. When employed, special air blending devices located within air handling units or mixed air plenums shall be treated as a secondary rather than a primary means of achieving thorough mixing of air streams. In other words, although the use of air blending devices is allowed and even encouraged as insurance to ensure adequate mixing under the most extreme operating conditions, each air distribution system shall be designed such that it does not rely upon such a device as an essential component in order to achieve this.

J. Filtration: As a general rule, unless specific requirements dictate otherwise, supply airflow serving standard building environments shall be filtered by a 30% efficient prefilter followed by a 65% efficient final filter. Supply airflow serving special building environments (e.g. laboratories where cleanliness is more critical) shall be filtered by a 30% efficient prefilter followed by a 95% efficient final filter. Air distribution systems of less than 3000 CFM total capacity may be treated as exceptions to the above filtration requirements. It is acceptable to filter supply airflow through these smaller systems with a 30% efficient filter with no additional secondary filtration. (Filter efficiencies are dust spot efficiency ratings per the current revision of ASHRAE Standard 52.1)

K. Plenum Fans / Discharge Plenums: A plenum fan shall be utilized in each application where a fan discharges into a plenum rather than discharging directly into a connected duct. Therefore, when an air handling unit utilizes a discharge plenum, plenum supply fan(s) shall be used in conjunction with it. Thus, a discharge plenum shall only be utilized in conjunction with an air handling unit when the total static pressure requirements are within the conservative operating limitations of a plenum fan (i.e. 5 to 6 of total static pressure). Housed fan(s) shall be used in applications that require higher static pressures. In these applications a discharge plenum shall not be utilized, but rather ductwork shall be attached to the outlet connection of each fan. Since most air handling units incorporate a return plenum, the installation of plenum return fan(s) within the return plenum of an air handling unit should be considered and is encouraged.

L. System Effect: When ductwork is attached to the outlet connection of a housed fan, a length of straight duct shall be provided prior to the first fitting or transition as required to minimize system effect, maximize efficiency and minimize generated noise. The minimum length of this section of duct shall be calculated using the formulas provided within the chapter entitled Duct Design in the current edition of the ASHRAE Fundamentals Handbook. Typically, the application of these formulas yield a minimum straight duct length that is equal to 2 to 3 times the diameter of the equivalent round fan outlet opening. This typically requires that a significant amount of the mechanical equipment room be utilized to accommodate the duct configuration that is required to achieve this. The inlet of each fan, whether it be ducted or unducted, shall also be configured so as to ensure that air enters the fan uniformly so as to minimize system effect. Again, this typically requires that additional space be dedicated for this purpose.

M. Vibration Control: All fans shall be supported by spring type vibration isolators to minimize the transmission of vibration into associated equipment and building structures. Larger fans shall be equipped with inertia bases as required to control undesirable movement and vibration transmission. Flexible duct connections shall be installed at all points of connection between fans and associated air distribution ductwork to allow movement of the fan relative to fixed ductwork and to minimize the transmission of vibration into and throughout the duct system.

N. Ductwork: Refer to Section 15810 for additional detail. Ductwork is generally classified in three categories with each category possibly having three pressure classifications. No Supply / Return duct system of any type shall be designed for an air velocity exceeding 2400 fpm (High Pressure >= 6 inches w.c.), or 2000 fpm Medium Pressure (between 3 inches and 6 inches w.c.), or 1200 fpm for Low Pressure (under 3 inches w.c.) static. Supply ductwork downstream of VAV or Constant Volume boxes shall be designed to minimize sound generation and should generally fall in a range between 500 and 900 fpm. Examples of exceptions include a high velocity discharge stack that serves a fume exhaust system and the possible exception of an exhaust system that is required to transport particulate within the air stream. (Installing a VAV system to conserve energy does not result in decreased fan energy consumption unless air velocities and the resultant total pressure requirement of the fan(s) are kept within modest boundaries.) Ductwork shall also be designed and constructed so as to minimize leakage. This is important for low pressure as well as higher pressure duct classifications. Excessive duct leakage has been a problem on campus in recent years.

O. Fibrous Lining: Fibrous interior lining shall not be installed within any duct or any other air distribution system component (including air handling units, sound attenuators, VAV terminal units, etc.) such that it is in direct contact with the air stream. It may, however, be installed within ductwork and other system components if it is completely sealed within a Tedlar film enclosure and is separated from the air stream by a perforated metal barrier.

P. Sound Control: Sound control as it relates to air distribution systems should be given focussed attention. Without the direct application of fibrous interior lining, adequate sound control is difficult but not impossible to achieve. The use of sound attenuating devices within supply and/or return air duct systems is often necessary. However, the best way to control noise is to not create it in the first place. When a high priority is place on maximizing the efficiency of air distribution systems, noise becomes much less of an issue. As stated elsewhere, the allowable HVAC-related background noise level for a given type of occupancy shall not exceed the guideline criteria provided within the chapter entitled Sound and Vibration Control in the current edition of the ASHRAE HVAC Applications Handbook.Q. Terminal Zoning: Air distribution systems should be configured such that each occupied space can be controlled as a separate zone with regard to temperature and/or airflow. In other words, one terminal control unit (e.g. constant volume reheat coil or VAV unit with reheat coil) should be provided for each occupied space.R. Diffusers and Grilles: The use of sidewall supply grilles shall be avoided where possible. They are notorious for causing uncomfortable drafts. Ceiling supply diffusers are preferred. Ceiling supply plenums shall not be used. Supply diffusers and grilles shall be selected and installed so as to direct a greater percentage of the airflow primarily toward the source of the heating/cooling load (e.g. toward exterior walls). Supply diffusers and grilles shall be selected and installed so as not to create undesirable turbulence, particularly within laboratories that contain sensitive equipment (e.g. near the face of fume hoods).

S. Return/Exhaust Air Systems: In a VAV system, the installation of a return or general exhaust VAV terminal unit to serve each space, in conjunction with the supply VAV unit, is required when serving areas where relative space pressurization is critical (e.g. chemical laboratories, animal facilities, etc.). This type of system is preferred but not required when serving areas where relative space pressurization is less critical. This approach allows for the continuous positive control of the quantity of return or exhaust air from each space as compared to the quantity of supply airflow. The control of the static pressure within this type of return/exhaust system shall be accomplished similarly to that within the supply system. The use of a static balancing damper in the branch return/exhaust ductwork serving each space in conjunction with some form of fan tracking or overall airflow monitoring control to control return airflow is inferior to this approach and often insufficient in maintaining appropriate air balance. With these approaches, as overall airflow changes in the system, the return-side pressure gradient changes, resulting in return-side air balance changes. In any case, the installation of return air ductwork is preferred over the use of a ceiling plenum as a path for return air.

The use of exit passageways, stairs, ramps, and other exits as a part of a supply, return, or exhaust air system serving other areas of the building is prohibited in any new structure or in major renovations of existing buildings, except as permitted in the International Building Code and the International Mechanical Code. For the purpose of this section, a building will be considered to undergo a major renovation if either the existing air distribution system, or the interior space arrangements and/or structure are modified to an extent that the renovation costs exceed 25 percent of the replacement cost of the distribution system or structure. Existing air distribution systems that utilize any of the exit elements of a building may not be altered in any way that would reduce the level of life safety to users of the facility. Alteration proposal for existing, non-compliant systems will be reviewed by DEHS and must clearly demonstrate that the safety of the users will not be reduced by the proposed modifications. In addition to creating more stringent requirements for the materials of construction above the ceiling level (e.g. plenum rated communication cables), use of a ceiling plenum as a return air path results in the deposition of a substantial amount of air-born dust on the upper surfaces of the suspended ceiling system with the passage of time.

T. Restroom Exhaust Systems: Multiple restrooms shall be served by a central exhaust system. Individual exhaust systems shall be avoided. Exhaust air from restrooms shall be discharged in an appropriate manner at the roof level to minimize the potential for the recirculation of exhaust air back into an air intake opening.

U. Makeup Air: Appropriately conditioned makeup air shall be directly ducted from an air distribution system into each space from which air is exhausted. The control of makeup air and exhaust air quantities shall be coordinated. At any time, the quantity of makeup airflow shall be approximately equal to that of the exhaust airflow.

V. Wet Laboratories: See the Wet Laboratories section within these General Guidelines for specific air distribution / exhaust system requirements related to chemical laboratories.

W. Fume Exhaust Systems: See the Wet Laboratories and the Laboratory Equipment sections within these General Guidelines for specific fume exhaust system requirements related to chemical laboratories. Filters and access to them for replacement will be required. Specific types of filters can vary depending on the application. The degree of filtration should be established in conjunction with the User, O&M, and the Architect/Engineer.

X. Manifolded Vs. Dedicated Systems: See the Wet Laboratories and the Laboratory Equipment sections within these General Guidelines for specific guidance regarding manifolded versus dedicated fume exhaust systems.

Y. Exhaust Filtration: Final filters shall not be installed in exhaust systems, with the possible exception of systems in which high hazard biological contaminants or highly radioactive isotopes are used. In these cases, DEHS shall be contacted for guidance.

Z. Animal Facilities: See the Animal Facilities section within these General Guidelines for specific air distribution / exhaust system requirements related to animal facilities.

AA. Sterilizing Equipment: See the Animal Facilities and Wet Laboratories sections within these General Guidelines for requirements related to exhaust systems that serve sterilizing equipment.

AB. Odor Control: See the Animal Facilities and Wet Laboratories sections within these General Guidelines for relative air pressurization requirements to achieve odor control.

AC. Smoking Prohibited: Campus policy dictates that smoking is prohibited in any campus building.