INNOVATION May-June 2012

of entrapment in elevators if the pressure difference across the elevator doors is too high. Quite literally, elevator doors can fail to open under these conditions. However, this is somewhat dependant on maintenance practices. Exfiltration losses in elevator rooms can be reduced or eliminated by using mechanical cooling. Depending on the design, the need for large holes through the building envelope for cooling could be eliminated completely. Sealing the elevator penthouse to prevent air leakage will reduce exfiltration rates through the elevator shafts. For a hypothetical system with variable frequency drive controlled elevators in 20+ story multi-unit residential building, the sensible cooling load for a single elevator (including the heat gains caused by the microprocessor controls, the variable frequency drives, the drive motor and mechanical devices) would be approximately 2.6 kW including a duty cycle factor. If we add in 20 W per square meter for miscellaneous base build loads and 2.6 kW for a second elevator we would have total sensible load in the elevator penthouse of just under 5.4 kW. The installed cost of a split system air conditioning unit capable of meeting this load (approximate capacity of 7.2 kW) would be close to $15,000. If we assume that we can reduce exfiltration losses significantly by controlling the losses from the elevator penthouse then the simple payback for an air conditioning system may be less than four years and perhaps lower if we offset the installation costs against the savings from not installing an exhaust fan and dampers. Based on this information, we strongly encourage designers to carefully review cooling strategies employed in elevator machine rooms in high-rise towers. There are both energy and life safety considerations that should be factored into the design. In combination with this effort, ventilation strategies should also be reviewed in all buildings. According to CMHC, the corridor ventilation system “represents a reasonable target for energy conservation efforts due to their impact on the building energy use.” v Jeff Besant PEng is a principal with Besant and Associates Engineers Ltd, a firm providing electrical and mechanical consulting services in BC, Alberta and Saskatchewan. John Lovatt PEng MSc is an instructor at BCIT in the Architectural Science Program. He has published award winning research papers on Stack Effect in High-Rise buildings.

However, even with dampers there can be high leakage rates through the building openings. Dampers may not provide enough of an air barrier to prevent considerable exfiltration losses. The problem is worse if there are no dampers. ASHRAE 90.1 specifies maximum air leakage rates for dampers (21 L/s per square meter at 250 Pa). However, not all dampers perform the same. John Knapp (“Damper Leakage Rates – More Important Than Ever,” 2007) notes there can be 10-to-1 performance difference in the leakage rate between dampers supplied with proper seals versus a damper supplied with no seals at all. The air leakage path through the elevator system also includes the elevators and the top of the elevator frame. Gregory Cahanin (“Change is in the Air,” 2005) suggests that elevator doors have gap leakage areas of 315 cm 2 to 668 cm 2 . If there are two elevators, then the total leakage area per floor for the elevators doors can be 1,336 cm 2 . We found with a tape measure in sample buildings that the typical gap leakage areas at the top of elevator shafts are between 1,500 cm 2 and 3,000 cm 2 . (Note: This is at the cable holes and support frame for the elevator overhead gear traction motors. The air must travel through these restrictions—the sum of which is the overall pressure developed by stack effect and the corridor system). The other possible air leakage path for ventilation air is through the exit stairs. The air leakage area for the exit stairs depends on the construction of the exit doors and the construction of the exit stairwell. Alex McGowan (“Heat Transfer Through Roll-up Doors, Revolving Doors and Opaque Non-residential Swinging, Sliding and Rolling Doors, 2006)” found that at 75 Pa pressure difference (the standard criterion for NFRC 400), the air leakage for an emergency exit door was measured at a range of 3.95–10.6 L/s per square meter of projected area (0.78–2.08 cfm per square foot), depending on the weatherstripping condition. Using these data for effective gap areas and air leakage rates, we can conclude that most of the air that doesn’t reach the suites exhausts through the elevator shafts and out of the elevator penthouse. In their 2008 article “On Smoke Control by Pressurization in Stairwells and Elevator Shafts,” Richard Miller and Don Beasley stated that the elevator doors have an effective leakage of over four times that of exit doors. With elevator systems, exfiltration losses are a concern in terms of energy wasted and sustainability; however, there are also life safety considerations. In high-rise towers, there is a risk

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