Engineered Systems - March 2009 - (Page 30)

In this overview of a three-school hydronics project in Texas, the author demonstrates how control is critical and why it’s worth putting some thought into alternate design scenarios along the way. He also offers a couple of tips for retrofits in constantly occupied spaces. BY ROLANDO LEGARRETA, P.E. n 2005, the El Paso Independent School District, in EL Paso, TX commissioned Alegro Engineering, L.P. to implement IAQ improvements to two elementary schools and one middle school. The schools’ existing (and original) A/C systems, installed in the ’70s, included central evaporative coolers, hot water reheat systems, and constant flow pumping systems. With this system set-up for use during the cooling season, there was no way to provide individual classroom control. One of the goals of the project was to provide individual room control during the heating and cooling seasons, as well as to maintain relative humidity (rh) at no more than 60% year round. CONTROL IS KEY In order to achieve the owner’s require30 En g i neer ed Sy stem s I ments during the schematic design phase, Alegro Engineering designed a system with central AHUs and variable flow terminal units with hot water reheat coils to each space and classroom. Not only had this system been used with success in other school projects, but individual room control could be achieved through this design. The hydronic portion of the systems included air cooled chillers and constant flow primary pumping systems. On the heating side, the system consisted of water boilers, a primary pumping system with constant flow, and a secondary system with variable flow pumps. All AHUs were provided with pre-heat coils, and most terminal units were provided with re-heat coils. All secondary pumping systems were provided with two-way valves throughout except for one AHU, and both the chilled- water coil and the hot-water coil were designed for constant water flow. We used a three-way valve to avoid a pump working on a dead-head situation. Each selected coil would require 10% to 15% of the maximum flow. When the opinion of probable construction cost for each project was prepared, we found out that there was not enough money to provide each school with the required upgrade. In a team effort, and working with the owner, areas that could remain with evaporative cooling systems for all three schools were designed and planned as alternate design solutions. DESIGN CONSIDERATIONS Based on the sequences of operation planned for the project, the AHU preheat coils were selected to increase the temperature from March 2009

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Engineered Systems - March 2009 Contents Editor’s Note Back2Basics Case In Point Commissioning Building Automation HVACR Designer Tips Application Checklist Teaching An Old School New Tricks Basics For Absorption Chillers A New Haven For Sustainable Schools Glossary Classifieds Advertiser Index Tomorrow’s Environment

Engineered Systems - March 2009

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