Engineered Systems - October 2008 - (Page 54)

Cold Climate HVAC Challenges the perimeter can prevent difficult to remediate drafts, which can overcool surfaces. It is these cracks that lead most often to frost or ice formation. Better controls or zoning on the humidification system may also be necessary to prevent over dispersion of moisture. On one project, a separate control zone for the exterior envelope was designed. This zone supplied non-humidified, independent temperature-controlled air to the window headers to ensure that condensation on older windows would not occur. Although the original intent of this system was to provide dryer, warmer air to the most likely point of condensation in the building (the windows), it was difficult to ignore the resemblance of this zone to a conventional automobile defrost system. FIGURE 2. Typical VAV system with central relief fan. (Schematic courtesy of Trane Engineering.) COLD AIR INTRUSION There are several methods to eliminating cold air intrusion. Obviously, a tight, wellinsulated envelope that does not allow the conduction of heat to the exterior is optimal. This helps to prevent the surface temperature from falling below the dewpoint of the air. Lack of proper insulation between window sills and the exterior are common locations for this problem. Eliminating cracks along www.protechinfo.com • 800.766.3473 • sales@protechinfo.com WHEN DOES AL29-4C stainless steel • precision TIG-welded seems • patented built-in locking bands • triple-lipped directional gaskets • standard adjustable vent lengths BECOME A SUPERIOR GAS VENT & CONDENSATE MANAGEMENT SYSTEM? When they are engineered into advanced systems that contain quality components with welded seams & silicone free joints ® Now Available in 3” - 24” Vent Diameters Input 153 at www.esmagazine.com/instantproductinfo PIPE/COIL LOCATION ISSUES The location of piping system components is critical in cold climates. Obviously, any pipe that conveys water without glycol should be run inside the building’s complete insulation barrier. While this fact seems intuitive, floor plan considerations, constructability considerations, and the general quality of workmanship sometimes test this well-known requirement. We have seen many instances of cracked sprinkler and plumbing lines that were located inside an incomplete insulation barrier. The locations of heating and cooling coils are also an item of concern. These coils in AHUs have the potential to be in the ‘line of fire’ of cold, outdoor air. Unfortunately, it only takes a small current of freezing, stratified air to pose a risk to coils. Some common resolutions to this problem include sufficient freeze-stats, draining coils in the winter, adding glycol to the water, constant flow circulators on a dedicated coil loop, and backup control schemes. Although freeze-stats are a first line of defense, it is not uncommon for these devices to be installed improperly, limiting their effectiveness. Facility departments draining the coils on a spring/fall cycle can resolve this. Glycol can increase overall pumping energy but provides a relatively secure method of preventing frozen coils. Another timetested solution for selected heating coils is the installation of a dedicated pump to keep the water in the coil moving. This method works well but can increase installation and operation costs. Some control schemes continuously monitor the inside temperature of the air handler. If the temperature drops too low, a sequence of steps may commence. These steps can include de-energizing the unit, increasing hot water flow through the coil either through ESM03084Protech.indd 1 54 En gi neer ed S y stem s October 2008 2/19/08 1:55:05 PM http://www.protechinfo.com http://www.protechinfo.com http://www.esmagazine.com/instantproductinfo

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Engineered Systems - October 2008 Contents Editor’s Note Letters Back2Basics HVAC Challenge Case in Point Commissioning Building Automation HydroTech HVACR Designer Tips Special Insert: Engineer's Technical Library Fall 2008 Living Wild! Chilled Beams in Chicago Phoenix Convention Center: On The Rise Cold-Climate HVAC Challenges Humidification Strategies For Data Centers & Network Rooms Special Advertising Supplement: Tabb Talk Computers & Software Issues & Events Products Literature & Catalogs Glossary Classifieds Advertiser Index Tomorrow’s Environment

Engineered Systems - October 2008

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