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2 8 A S H R A E J o u r n a l a s h r a e . o r g J a n u a r y 2 0 0 5Standard 62-2001 Addendum 62nSingle-Path Multiple-Zone System DesignBy Dennis Stanke, Member ASHRAEAbout the AuthorDennis Stanke is a staff applications engineer with Trane, La Crosse, Wis. He is vice chair of SSPC 62.1.NSI/ASHRAE Standard 62-2001, Ventilation for Acceptable Indoor Air Quality,1 as modifi ed by Addendum 62n,2 prescribes new minimum breathing-zone ventilation rates and new calculation proce-dures to fi nd intake airfl ow for different ventilation systems. Previous articles3,4 discussed the design of “simple” ventilation systems (single-zone, 100% outdoor-air, and changeover-bypass VAV) in compliance with Addendum 62n requirements. Here, we examine the design of a more complex set of ventilation systems, namely single-path, multiple-zone recirculating systems.zone, recirculating ventilation systems. For instance, constant-volume systems with terminal reheat, traditional constant-volume multizone systems, single-duct VAV systems, and single-fan dual-duct VAV systems all provide ventilation from a single source or path. (A single-fan, dual-duct system supplies air to each space using two different ducts, but the air in each duct contains the same fraction of outdoor air, because one fan—a single source—delivers the same air mixture to each duct.) Other systems have multiple ventilation paths, including dual-fan, dual-duct VAV systems and VAV systems with fan-powered or induction terminal units. Single-duct VAV systems with series fan-powered boxes are always dual-path ventilation systems, but those with parallel fan-powered boxes are single-path with the local fan off and dual-path with it on. Although any of these HVAC systems may be used in vari-Although the Ventilation Rate Pro-cedure in Standard 62 has required specifi c calculations (Equation 6-1) for multiple-zone systems since 1989, the calculation procedure was sketchy at best; consequently, it was widely misun-derstood and largely ignored by design-ers. Addendum 62n includes a detailed calculation procedure for multiple-zone system design. Use of this procedure is expected to increase consistency among designers and reduce the tendency to de-sign multiple-zone systems—especially VAV systems—that provide inadequate ventilation for some fully occupied zones. Addendum 62n also includes operational control options that can be used to modu-late ventilation capacity as ventilation load and/or effi ciency varies, but these options are left to a future article. The following discussion covers only design calculations.Many HVAC systems are confi gured as “single-supply” or single-path, multiple-ANSI/ASHRAE Standard 62-2001, ANSI/ASHRAE Standard 62-2001, The following article was published in ASHRAE Journal, January 2005. © Copyright 2005 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE.J a n u a r y 2 0 0 5 A S H R A E J o u r n a l 2 9outdoor air in its primary airstream. In the past, many designers simply added the zone outdoor airfl ow requirements and set the intake airfl ow to match this sum, which resulted in a very low outdoor-air fraction and many underventilated zones. Some designers went to the other extreme, fi nding the highest fraction of outdoor air needed by any zone in the system and setting the intake airfl ow to provide this fraction at all times. This approach considers only fi rst-pass outdoor air, giving no credit for unused recirculated outdoor air, and results in a very high outdoor-air fraction and overventilation in all zones. Proper design in compliance with Addendum 62n calculation procedures strikes a balance between these extremes, appro-priately accounting for both critical-zone needs and unused, recirculated outdoor air.Let’s look at an example offi ce building (Figure 2Let’s look at an example offi ce building (Figure 2Let’s look at an example offi ce building (). We as-sumed that thermal comfort can be achieved using only eight VAV thermostats, with each thermostat control-ling one or more VAV boxes. We considered each of these “comfort zones” (or “HVAC zones” per ASHRAE Standard 90.1-2001) as a separate “ven-tilation zone.” According to Addendum 62n, a ventilation zone is “one occupied space or several occupied spaces with similar occupancy category, occupant den-sity, zone air-distribution effectiveness, and zone primary airfl ow per unit area.” Most (but not all) HVAC zones qualify as ventilation zones. The area and population for each zone in this example were selected to help illustrate the calculations rather than to refl ect typical zone sizes or popula-tion densities.To comply with Addendum 62n, our design calculations begin by fi nding the ventilation needs at the zone level and conclude by determining the required intake airfl ow at the system level.Zone Ventilation CalculationsFollowing the procedure under “zone calculations” in Sec-tion 6.2.1, we found zone outdoor airfl ow (Voz) for each zone (Figure 3(Figure 3():1. Referring to Addendum 62n, Table 6.1 (not shown), look up the prescribed minimum people outdoor-air rate (R (R (p) and the prescribed minimum building area outdoor-air rate (R(R(a). In our example offi ce building, each zone needs 5 cfm/person and 0.06 cfm/ft². Using these values, along with the design zone popula-tion (P (P (z) and zone fl oor area (A (A (z), fi nd the minimum breathing-zone outdoor airfl ow by solving Equation 6-1 (VbzVbzV = Rbz = Rbzp × Pz + Ra × Az). Either peak or average expected occupancy may be ous building types, we narrow our discussion to a single-duct VAV system, with throttling VAV boxes for interior zones and reheat VAV boxes in perimeter zones, applied in an example offi ce building.Demonstrating Compliance by ExampleOur example system (Figure 1Our example system (Figure 1Our example system () includes a central air handler, with a modulating outdoor-air damper that may be controlled as an economizer; a variable-volume supply fan to deliver primary air; cooling-only, throttling VAV


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Stanford CEE 215 - Single-Path Multiple-Zone System Design

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