Engineered Systems - March 2008 - (Page 44)

In which the authors take out the original unit designed specifically for K-12 and see how things have changed in the decades since it first went to school. Time and technology have solved some problems, yet an old-fashioned issue like ease of maintenance can still pose headaches (or backaches). All in all, this update about a well-known design option deserves a permanent spot in your three-ring reference binder. BY TOM DURKIN, P.E. AND BRIAN PARTINGTON, P.ENG. or 70 or 80 years, the classroom unit ventilator had been a widely accepted solution for school heating and cooling. But operational and comfort problems arose, and the unit ventilator (UV) fell from favor. It does still have a place in modern school design though, providing that the problems can be solved and all codes can be addressed. This article discusses the perceived UV issues and one designer’s successful solutions. A SHORT HISTORY LESSON Herman Nelson invented the UV in 1917. It consisted of little more than a hole in the wall, a fan, and a steam radiator. It was designed to address a recognized problem of that era, namely that there was insufficient fresh air being introduced into schools and that it was better to bring the air in through an engineered system rather than to allow infiltration, open windows, and drafts. It was the first and maybe only HVAC equipment designed specifically and solely for schools. In the years since 1917, cooling was added in many variations (chilled water, split DX, self-contained DX), and all possible heating sources (steam, hot water, resistance electric, and air and water source heat pump including geothermal) were available. The two big advantages of a UV school were (are) that they’re less expensive to build mechanically, and that they’re less expensive in the general construction since the building can be shorter as headroom for big ducts isn’t required. 44 En gi neer ed S y stem s March 2008 F By exact definition, a unit ventilator differs from a fancoil by virtue of the unit vent’s ability to introduce up to 100% outside air for economizer cooling. Most unit ventilator manufacturers offer multiple options and configurations, such that engineers have almost as much design flexibility with UVs as they do with air handlers. In addition to the traditional “under the window” cabinet-style unit vents (Figure 1), modern unit vents are vertical with ducted air distribution (Figure 2). THE PREREQUISITES Irrespective of whether one HVAC solution is less costly than another, in order for any solution to be truly viable, it must deliver consistent IAQ, occupant comfort, and humidity control. Only after these primary issues have been adequately and thoroughly addressed should secondary items such as construction cost, operating economy, longevity, and maintainability be considered. Another critical prerequisite is digital control. Many of the age-old UV problems can be solved by judicious application of modern DDC technology. THE ISSUES AND THE SOLUTIONS IAQ (consistent fresh air). There are several aspects to this, involving equipment design, building system design, and construction issues. Theoretically, setting an outside air damper to provide the appropriate outside air should be easy and very repeatable. However, the old

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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