Engineered Systems - October 2008 - (Page 52)

October gets us to the cusp of winter in a lot of the country. The cold-weather threats to good performance are outnumbered only by the possible strategies to combat them. Here, the authors look at humidity, economizers, pipe and coil insulation and placement, and more as they assess the design opportunities and pitfalls once the temperature takes a dive. BY RICHARD D. MILES, LEED® AP, AND STEVEN P. FURGESON, P.E., CEM, CHFM, LEED® AP he design of effective HVAC systems in cold climates requires careful attention to several key factors. These include outdoor drybulb/wetbulb (db/wb) temperatures, indoor temperature and humidity requirements, the quality of the building envelope, insulation of pipes and coils, airside economizers, and building pressurization, among others. Proper attention to these issues will result in thermally comfortable indoor environments with a minimum of humidity and moisture issues. This discussion focuses on Maine, where many of the buildings we have designed are located. Maine is generally indicative of HVAC design parameters in the northern tier states of the U.S. OUTDOOR DESIGN TEMPERATURES The table for climatic conditions listed in the 2005 ASHRAE Handbook — Fundamentals, indicate, for example, that in Portland, ME, the 99.6% design outdoor db temperature is -1.6°F. This means that based on accumulated historical climate data, the outside air temperature should only drop below the -1.6°, 0.4% of the time. Practical experience however, has proven that this occurs quite a bit more often. If one takes a look at values for the 99.6% and 99% db temperatures in the 2001 ASHRAE Handbook — Fundamentals vs. the 2005 edition, you would see, for about 20 stations located mostly in cold climates, the winter design temperature values are quite a bit lower. This is due to an algorithmic change, explained in ASHRAE (2004a), in which previous heating db design conditions were calculated from the db/dewpoint joint frequency matrix. 52 En gi neer ed S y stem s October 2008 T INDOOR TEMPERATURES AND HUMIDITY In order to determine what the design T is (temperature difference between the outdoor design temperature and the indoor temperature) both must be defined prior to beginning load calculations. We have already given an example of what might be used for wintertime outside air temperatures. To determine the desired indoor air temperatures, discussions must take place between the building’s owner/occupant and system designer. There are several factors that must be considered when determining a thermally comfortable environment. These are metabolic rates, clothing levels, air temperature, air speed, and humidity. Generally speaking, winter indoor temperatures typically range between 68° to 72°. For systems where humidity control is not designed into the HVAC system, humidity levels will generally follow outside ambient humidity levels. ASHRAE Standard 55, “Thermal Environmental Conditions for Human Occupancy” states that there are no lower limits for humidity within a building. However, when humidity levels drop below 25%, occupants tend to experience non-thermal comfort issues, such as dry skin, irritation of mucus membranes, and static electricity generation. BUILDING ENVELOPE CONDENSATION One of the most critical design decisions made is in regards to a building’s envelope. There is very little point in designing a state-of-the-art HVAC system if the envelope designed to contain it is less than optimum. ASHRAE 90.1 Energy Standard provides specific criteria for the minimum construction requirements of a building’s envelope. The potential for condensation on exterior envelope elements is

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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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