2 2 A S H R A E J o u r n a l a s h r a e . o r g N o v e m b e r 2 0 0 4Standard 62-2001 Addendum 62nAVentilation for Changeover-Bypass VAV SystemsBy Dennis Stanke, Member ASHRAEAbout the AuthorDennis Stanke is a staff applications engineer with Trane, La Crosse, Wis. He is vice chair of Standing Standards Project Committee 62.1.NSI/ASHRAE Standard 62-2001, Ventilation for Acceptable Indoor Air Quality,1 prescribes ventilation rates for commercial and insti-tutional buildings. Historically, Standard 62 required both zone- and system-level calculations for the design of multiple-zone ventilation systems, such as single-path constant volume and VAV systems. Unfor-tunately, without clear calculation procedures, system-level calculations frequently were misinterpreted or ignored by designers. As a result, many multiple-zone systems were underventilated. Standard 90.14 as areas “with heating and cooling requirements that are suffi -ciently similar so that desired conditions can be maintained throughout using a single sensor.” Each comfort zone has its own thermostatically controlled VAV damper and ideally, all comfort zones associated with an air handler have similar thermal loads (that is, all zones need some level of heating or some level of cooling, like interior spaces or southern perimeter spaces). In practice, however, many systems include comfort zones with dissimilar loads; some zones need heating at the same time that other zones need cool-ing. In either case, each comfort zone usually is considered as a “ventila-tion zone,” defi ned by Addendum 62nas an area with “similar occupancy cat-egory, occupant density, zone air distri-bution effectiveness, and zone primary airfl ow per unit area.”In an effort to avoid underventilation and increase calculation consistency among designers, Addendum 62n2 up-dates prescribed ventilation rates and the calculation procedure for zone ventilation airfl ow and for system in-take airflow for different ventilation systems. A previous article3 introduced Addendum 62n rates and described the design of single-zone and 100%-outdoor-air ventilation systems. This article discusses the “62n-compliant” design of a very specifi c multiple-zone ventilation system: changeover-bypass VAV, also called “variable volume and temperature” (VVT).A changeover-bypass VAV system (Figure 1) uses a constant-volume air handler (often a packaged rooftop unit or split DX system) to ventilate and cool or heat a large area within a build-ing. The area served comprises many “comfort zones” or “HVAC zones,” def ined by ANSI/ASHRAE/IESNA Although Addendum 62n shows ventilation rates in both IP and SI units, this paper uses IP, except in selected specifi c calculations. This is because 62n uses rational conversions, not mathematical.Copyright 2004, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. This posting is by permission of ASHRAE Journal. This article may not be copied nor distributed in either paper or digital form without ASHRAE’s permission. Contact ASHRAE at www.ashrae.org.N o v e m b e r 2 0 0 4 A S H R A E J o u r n a l 2 3the outdoor-air intake fl ow and then calculating the required settings for minimum zone primary airfl ow.Based on thermal loads, our example building area is divided into eight comfort zones, each with its own thermostat and VAV damper. We consider each of these comfort zones as a separate ventilation zone.The following zone-level calculations must be performed, re-gardless of which approach to system design is used (Figure 3gardless of which approach to system design is used (Figure 3gardless of which approach to system design is used ():1. Look up the minimum people outdoor air rate (R (R (p) and the minimum area outdoor air rate (R (R (a) in Addendum 62n, Table 6.1, for each zone. Using peak zone population (P (P (z) and zone fl oor area (A (A (z), fi nd the design breathing-zone outdoor airfl ow using Equation 6-1 (airfl ow using Equation 6-1 (airfl owVbzVbzV = bz = bzRp×Pz + Ra×Az). (We could have used average zone population for Pz, but then we probably could not have taken credit for occupant diversity. See Step 6.)For example, the south offi ces in our example building require VbzVbzV = 5 × 20 + 0.06 × 2000 = 100 + 120 = 220 cfm.bz = 5 × 20 + 0.06 × 2000 = 100 + 120 = 220 cfm.bz2. For each zone, look up zone air-distribution effectiveness(E(E(z) based on the air-distribution confi guration and the default values in Table 6.2 (see excerpt in Figure 4).In our example, all offi ces use overhead diffusers and ceiling returns, and receive 58°F (14°C) primary air during cooling and 95°F (35°C) during heating. Effectiveness changes from 1.0 when cooling to 0.8 when heating, demonstrat-ing how operating mode can affect zone ventilation performance. 3.Find zone outdoor airfl ow using zone outdoor airfl ow using zone outdoor airfl owEquation 6-2 (Voz = VbzVbzV/bz/bzE/E/z) for both cooling and heating operation.For our example south offi ces, Voz = 220/1.0 = 220 cfm during cooling, and Voz = 220/0.8 = 275 cfm dur-ing heating. Therefore, the system must be designed to deliver at least Voz = 275 cfm, either continuously oz = 275 cfm, either continuously ozor on average, to ensure adequate ventilation in both modes. (During operation, the example VAV damp-ers may be allowed to close to deliver less than 275 cfm when the primary air temperature doesn’t match the zone thermal requirements, provided system controls are designed to ensure that average minimum primary airfl ow is at least 275 cfm. See “Design for Short-Term Conditions.”) Addendum 62n doesn’t explicitly require calculation of Voz at both cooling and heating design, but it is implicit in the require-ments for system ventilation effi ciency and in the “variable load conditions” section, which states that required ventilation rate must be delivered under all load conditions.Each ventilation zone in our example is assigned to one of two air handlers, forming two multiple-zone ventilation systems (Figure 2(Figure 2(). The “interior” system includes all interior zones and the north zones, which are largely adjacent to a conditioned warehouse space; these zones
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