Engineered Systems - March 2008 - (Page 64)

VAV Systems believing that we have never had a problem in the past, but whether we want to acknowledge it or not, the truth is that we probably have had problems. VAV systems introducing outdoor air through the mixed air path have always achieved energy use reduction at the direct expense of ventilation and by virtue of physical configuration and underlying concept, are incapable of either efficiently processing or effectively managing the introduction of outdoor air through the mixed air path. You can provide adequate ventilation, but only at great energy expense. ASHRAE Standards 62.1 and 90.1 are inexorably pushing designers in the correct direction toward more efficient and effective solutions. Those who have dedicated themselves to preserving the “tried and truly awful” strategies of the past are going to find it increasingly difficult and engineering intensive to justify continuing to use obsolete strategies. In doing so, they will handicap their clients with costly and inefficient designs, and watch their professional liability exposure go through the roof. Those who are smart enough to religiously go through the above effort will inevitably reach the conclusion, “There has to be a better way!” When they start looking for a better way, they will find to their delight that there are, in fact, not one, but many better ways. Energy recovery and avoidance is green. The enemy is ignorance and the thermodynamic triple point. He lays in wait for careless and unsuspecting engineers. His tactics are known, but poorly documented. He can be outsmarted, but he should be considered to be armed, dangerous, unpredictable and particularly unforgiving of both design and installation error. The principles are not hard, but the devil is in the details. There is a long learning curve ahead, and designers need to take them one step at a time. ES If the reader is inclined to pursue this approach, there is a whole new world of performance potential waiting to be discovered, and that performance can be simply dazzling. On the other hand, while energy recovery is most effective and economical when used aggressively, aggressive use of energy recovery can become a technological minefield for the careless and unwary. this objective is the use of a technique known as “dual path” ventilation, which independently supplies outdoor air to each space. This permits the use of shutoff VAV for cooling. A second, 100% DOAS is used to meet minimum ventilation requirements. This technique substantially reduces heating and cooling capacity requirements, can functionally eliminate the need for reheat, saves significant energy, and reduces the delivery of outdoor air to each occupied space to a constant. On the other hand, it increases the total amount of air that must be delivered at peak flows and requires a minimum of two duct systems. It carries a small cost premium, which is quickly returned with improved life cycle cost characteristics. The other approach is to make the VAV system 100% outdoor air and build it around full-range energy recovery. There are multiple options, and while some are vastly better than others, no single solution is correct in all applications. This approach requires designers to think outside the box. If the reader is inclined to pursue this approach, there is a whole new world of performance potential waiting to be discovered, and that performance can be simply dazzling. On the other hand, while energy recovery is most effective and economical when used aggressively, aggressive use of energy recovery can become a technological minefield for the careless and unwary. If any readers of this article were able to follow the torturous logic of Standard 62.1-2004’s Ventilation Rate Procedure, which SSPC 62.1 has euphemistically described as a “simplified” multiple spaces equation, they would probably qualify for sainthood. And, for those inclined to think that this is some form of vast conspiracy to take away your favorite HVAC system, you may be onto something. It is the author’s position that VAV reheat is an obsolete strategy with grievously serious problems achieving compliance with both Standard 62.1 and 90.1. The IAQ problems with these systems have been known for almost two decades, and there are legitimate reasons for them. Vastly superior strategies which have been tested and proven over the past 20 years are available. As individual designers, we can delude ourselves by Lentz is president of Lentz Engineering Associates (Sheboygan Falls, WI). He is a graduate of the University of Wisconsin, Madison, with a Bachelor of Science in Mechanical Engineering, and a frequent speaker at ASHRAE Annual and Winter Meetings and has been very active with respect to ANSI/ASHRAE Standard 62.1. He is a member or corresponding member of six ASHRAE Technical Committees, and has served as chairman of ASHRAE Technical Committee TC 9.8- Large Building Air Conditioning Applications, and vice chairman of ASHRAE TC 5.7-Evaporative Cooling. He has been nationally recognized by the DOE for having successfully developed, tested, and proven several advanced HVAC system strategies designed to exceed the performance requirements of ANSI/ASHRAE/IESNA Standard 90.1 – Energy Standard for Buildings Except Low-Rose Residential Buildings, while meeting or exceeding the requirements of ANSI/ASHRAE Standard 62.1 - 2004, Ventilation for Acceptable Indoor Air Quality. CITED WORKS 1. American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), “Ventilation for Acceptable Indoor Air Quality,” ANSI/ASHRAE Standard 62.1-2004, 2004. 2. ASHRAE, “Energy Standard for Buildings Except Low-Rise Residential Buildings,” ANSI/ASHRAE/IESNA Standard 90.1-2004, 2004. 3. International Code Council (ICC), International Mechanical Code, Section 4, 1998: 27-30. 4. ASHRAE, 1992 ASHRAE Handbook — Systems and Equipment, Chapter 2, “All-Air Systems, Outside Air Requirements,” 1992:2.5 5. Mumma, Dr. Stanley, “Transient Occupancy Ventilation by Monitoring CO2,” ASHRAE IAQ Applications, 2004: 21-23. 64 En gi neer ed S y stem s March 2008

Table of Contents Feed for the Digital Edition of Engineered Systems - March 2008

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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