Engineered Systems - March 2008 - (Page 62)

VAV Systems solutions are available through a nationwide network of Vykon Distributors. ActiveLogix Charlotte, NC 704-578-9776 vykonsales@activelogix.com Advanced Power Control, Inc. Newark, DE 302-368-0443 vykonsales@adv-power.com BITS Waukesha, WI 877-334-9333 vykonsales@envsysinc.com Cochrane Supply Madison Heights, MI 248-588-9260 vykonsales@ cochranesupply.com Controlco Automation Distributors Concord, CA 925-602-7728 vykonsales@controlco.com Control Consultants, Inc. Weymouth, MA 781-335-8353 vykonsales@ controlconsultantsinc.com E.C. Smyers Co. Pittsburgh, PA 412-471-3222 vykonsales@smyers.com Industrial Control Distributors Wanamassa, NJ 732-918-9000 vykonsales@icdmail.com Jackson Control Company, Inc. Indianapolis, IN 800-772-9859 vykonsales@jacksoncontrol.com Long Building Technology Englewood, CO 303-975-2100 vykonsales@long.com Minvalco, Inc. Minneapolis, MN 952-920-0131 vykonsales@minvalco.com Temperature Control Systems, Inc. Dallas, TX 214-343-1444 vykonsales@tempconsys.com Universal Supply Group, Inc. Hawthorne, NJ 973-427-3320 vykonsales@usginc.com • The control system must know how much outdoor air is required at each space, and how much air is actually being delivered. When the amount of outdoor air required (Voz) at each space equals the computed ventilation requirements from Table 6-1 (Vbz) divided by the zone air distribution effectiveness (Ez) (Voz = Vbz / Ez). Voz may be a fixed quantity, or dynamically reset based on scheduled occupancy levels, direct sensing of occupancy such as card readers or infra-red and/or ultrasonic sensing technologies, or through indirect measurement of occupancy indicators such as CO2 sensing5. • The zone primary outdoor air fraction (Zp) is determined by dividing the amount of outdoor air actually required (Voz) by the amount of total air delivered at the air terminal unit (Vpz) (Zp = Voz /Vpz). As airflow rates to the space decline, the value of Pz will increase. When the value of Pz exceeds unity (1.00), it means that less total air volume is being delivered to the space than required. • The system air distribution effectiveness (Ev) is the lowest value of Evz computed for all zones served by the system (Ev = min Evz). Evz should be computed for each zone where Evz = 1+Xs – Zd where Xs is the primary outdoor air fraction at the air handling unit. Those familiar with Standard 62-1989 should recognize the above equation as the denominator from the multiple spaces equation. What this essentially means is that instead of making this computation once at the system level, Standard 62.1-2004 now requires it to be computed for each zone. This essentially gets us to the same end-point, although via a more roundabout and torturous route. • For design purposes with VAV systems, this value must be computed at the minimum flow settings at the VAV box. Section 6.5.2.1 of Standard 90.1 limits the amount of air that can be reheated to no more than the greater of: • The volume of outdoor air required to meet the requirements of Standard 62.1 • 0.4 cfm/sq ft • 30% of the design cooling load of the system Since the 30% parameter usually varies, the value of Zd is going to typically be at least 3.33 times higher than for a constant volume system at design. For example, for a Standard 90.1 compliant VAV reheat system a critical zone with a Zd value of 0.25 at full flow, Zd would actually equal (= 0.25 / 0.30) 0.833. This value is off the scale for using Table 6-3, and the designer must then determine the amount of outdoor air on the system (Vot) by dividing the diversified amount of outdoor air for the system (Vou) by system air distribution effectiveness (Ev). So, the design outdoor air fraction on a system with a diversified outdoor air fraction of 20% would actually have to process (= 0.2 / (1 + 0.2 -0.833) = 55% outdoor air, almost three times the outdoor air actually required to meet the sum of the ventilation requirements of the space. • The zone air distribution effectiveness (Evz) for each zone on the system must be continuously monitored to determine which space is critical and adjust the outdoor fraction at the AHU accordingly. Exceeding the exceptions to Section 6.5.2.1 of Standard 90.1 requires the consent of the governing code official, and only then if the design professional can demonstrate that employing more reheat actually reduces energy use. While this can be done in limited cases where relatively few spaces drive the rest of the system, it cannot be accomplished where the value of Zd is relatively consistent from zone to zone. The exceptions to Section 6.5.2.1 are listed in Standard 90.1 in general order magnitude, lowest first. The significance of these limitations is that Standard 90.1 prefers to increase the outdoor air fraction before increasing overall system volume. This is not a mistake or a misinterpretation. It is the result of careful modeling by SSPC 90.1. What makes this counter-intuitive for most HVAC engineers is that increasing the outdoor air fraction permits reductions in flow, thereby reducing energy use. As a rule-of-thumb, the design outdoor air fraction of a VAV reheat system for cooling purposes can be approximated by dividing the uncorrected outdoor air fraction by 0.3. If the resultant outdoor air fraction exceeds 0.7 at either design extreme, the system must be designed using air-to-air energy recovery to comply with Section 6.5.6 of ANSI/ASHRAE/IESNA Standard 90.1-2004 CONCLUSIONS The key to making VAV work is recognizing the fact that there are two variables which degrade the IAQ performance of these systems: Varying the amount of outdoor air introduced into the system, and varying the airflow to the space. To avoid control nightmares, one of those variables must be reduced to a constant. One general approach to accomplishing FREE INFO: 59 62 ESM05074TridLH.indd 1 En gi neer ed S y stem s March 2008 4/13/07 10:20:23 AM

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Engineered Systems - March 2008 Contents Editor’s Note Back2Basics HVAC Challenge Case In Point Commissioning Building Automation Energy Wiz HVACR Designer Tips Application Checklist Exemplary Design = Elementary Success The Modern Unit Ventilator On A Mission VAV Systems And Green Design – Part II Issues & Events Products Glossary Classifieds Advertiser Index Tomorrow’s Engineer

Engineered Systems - March 2008

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