Consulting-Specifying Engineer - March 2009 - (Page 46) Systems design and performance tips for packaged rooftop units (RTUs) BY RICHARD L. KRONICK, Freelance writer and MICHAEL IVANOVICH, Editor-in-Chief Manufacturers and engineers offer a variety of perspectives on RTU energy efficiency, sizing, refrigerants, acoustic/seismic concerns, dampers, and duct connections. A currently fashionable adage is “Where you stand depends upon where you sit.” That certainly applies to attitudes about direct-expansion unitary rooftop HVAC units (RTUs or “packaged air conditioning units”). Not surprisingly, manufacturers’ representatives speak of RTUs as the workhorses of the industry, as though they are proud Budweiser Clydesdale horses lined up on building roofs. One senior engineer of a manufacturer extolled the long, consistent improvement in the efficiency ratings of RTUs—and tipped us off that the newest version of ASHRAE 90.1, due in 2010, will raise the bar again. He pointed to the current system of three tiers that allows engineers to balance a unit’s efficiency level with its price. He concluded by claiming that, while it is commonplace that chilled water systems are more economical than RTUs, when you add in the pumping power, the air handler, and the cooling power, chilled water systems are not as efficient as everyone thinks. Other industry leaders would probably compare RTUs to a different species—say, the goat. David Sellers is a Portland, Ore.based engineer for Facility Dynamics Engineering with decades of experience in HVAC design, commissioning, and building man- Based on research and interviews with a wide range of industry leaders, here are some answers Size matters When asked for the No. 1 mistake made What should consulting engineers know about designing with RTUs for optimal lifecycle performance? agement. He agrees that you need to look at the whole-building system and not just the efficiency of the RTU, but maintains that the compactness that makes RTUs attractive often results in energy inefficiency. This article will not pass judgment on whether RTUs should be seen as workhorses or goats. In view of the fact that RTUs account for at least 50% of HVAC tonnage in the United States (some estimates are as high as 60%) and are here to stay for the foreseeable future, the more pressing questions are: What should consulting engineers know about designing with RTUs for optimal lifecycle performance? And what can we expect from manufacturers in the near and not-so-near future? Of course, “where you stand depends on where you sit” applies to factors such as climate, local contractor expertise for installation and service, what’s first-cost affordable, and a variety of other factors. in specifying RTUs, several experts pointed to oversizing. For example, Glenn Hourahan, vice president of research and technology for the Air Conditioning Contractors of America (ACCA), said, “No one wants to be caught short, so everyone is biased toward bigger units.” In his article, “Sizing and Selecting Equipment for Proper Humidity Control” in AHRI Magazine (Spring 2003; available at www. acca.org), Hourahan rejects rules of thumb, such as 500 to 600 sq ft/ton or 350 to 450 cfm/ton. “Those ideas may have worked satisfactorily 30 years ago because buildings were not that tight and not that energy-efficient,” he said. Today, he said, engineers need to do load calculations for energy, IAQ, and humidity. “The programs available from ACCA and from all of the manufacturers deal with the basic issues. How well is the building constructed? What leak- Web Extras In the online version of this article, there are two extra sections. One is a section on installation, commissioning, and recommissioning. The other provides a hyperlinked list of recommended resources, some of which were called out in the article. See more at WWW.CSEMAG.COM 46 Consulting-Specifying Engineer • MARCH 2009 http://www.acca.org http://www.acca.org http://WWW.CSEMAG.COM
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