High Performing Buildings - Winter 2008 - (Page 12)

that saves 66.6% of the Standard 90.1 budget building requirement. The cooler water also increases the period of time when the heat exchanger is efﬁcient to run. The normal selection is for a three-degree approach, but the heat exchanger was selected to provide 100 tons of capacity when operating at a onedegree approach to entering condenser water. With the combination of the cooling tower and the heat exchanger, chilled water leaving the heat exchanger can be cooled to within four degrees of ambient wet bulb temperature whereas the normal selection parameters would only approach within eight degrees of LESSONS LEARNED Several things have become apparent during construction and operation. Air Leaks It is more difficult to seal a building against air leaks than it is to agree to a leakage target with a contractor. Much effort was put into sealing the building, but actual leakage significantly exceeds the target. The target was an ambitious 0.1 air changes per hour (ACH). The ventilation system was set up to deliver enough air to compensate for 0.5 ACH. Trend logs of building pressure show that actual leakage exceeds 0.5 ACH. Larger Tank A larger rainwater tank would improve the water performance of the building. While the installed tank provides 528,000 gallons per year to the gray-water system, the precipitation that falls on the roof averages 631,000 gallons per year. Unexpected Dust Roofs are dusty places when the rain begins to fall, and it is necessary to service the filters and strainers on the gray-water system more frequently than on other hydronic systems. Control Sequences Very complex control sequences require additional designer time to ensure that the end performance matches expectations. ambient wet bulb. An example of how this helps: If the return water from the building is at 66°F and the ambient wet bulb temperature is at 60°F, then the heat exchanger can reduce the 66°F return chilled water to 64°F and reduce the chiller load by a little more than 8%. The standard selection would require that the ambient wet bulb temperature be 54°F to contribute the same 8%. And, at a 54°F ambient wet bulb, the actual performance of the selected cooling tower and heat exchanger would be 33% with water sent to the chiller at 58°F. This also means that the heat exchanger can assume the entire building load at a higher ambient wet bulb temperature which results in many more hours of available cooling operation without having to operate the chiller. Chilled Water Distribution System Because HVAC energy consumption is largely the movement of heat from an area where it is not desired to an area where it is unobjectionable, considerable attention was given to the energy necessary to transport heat. For this project, the UFAD system offered the opportunity to further reduce the amount of chilled water that had to be pumped. The UFAD system uses 62°F supply air, a temperature that can be produced with relatively warm chilled water. If all dehumidiﬁcation requirements are met by the dedicated ventilation units, then 54°F water will efﬁciently meet the sensible cooling requirements of the space and cool the air to 62°F. The amount of ventilation air varies with the amount of air required Winter 2008 to properly pressurize the building and to comply with ANSI/ASHRAE Standard 62.1-2001, Ventilation for Acceptable Indoor Air Quality. To ensure adequate dehumidiﬁcation, the minimum pressurization airﬂow was used to calculate the required dew point of the ventilation air. That dew point, 47°F, led to 42°F as the chilled water temperature necessary to achieve the required dehumidiﬁcation. The ventilation unit coils were selected to provide 47°F leaving air with chilled water rising from 42°F to 54°F. The UFAD units were selected to provide 62°F leaving air with chilled water rising from 54°F to 66°F. If the loads are equally divided between ventilation units and UFAD units, then the overall rise is from 42°F to 66°F or 24 degrees. This only requires half the water ﬂow of a conventional 12 degree rise system, yielding a design ﬂow of one gpm of water per ton of refrigeration and reducing pumping energy. As discussed previously, a variable Natural convection pulls cooled air off the water’s surface and up through the glass-wrapped stairs. 12 HIGH PERFORMING BUILDINGS Photo © Timothy Hursley

Table of Contents Feed for the Digital Edition of High Performing Buildings - Winter 2008

High Performance Buildings - Winter 2008 Passing On the Gift: Heifer International Headquarters Head of the Class: University of Florida’s Rinker Hall How Far Can You Go? Pearl River Tower The Proof Is Performance: How Does 4 Times Square Measure Up? Lighting the Way: Two Guilford County Schools Montreal’s Retail Example: Mountain Equipment Co-op® (MEC)

High Performing Buildings - Winter 2008

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