Consulting-Specifying Engineer - November 2007 - (Page 45)

Exhaust design continues through the discharge stack. It is important that the hazardous vapors exhausted directly to the atmosphere for dilution not be recirculated back into the building Mathematical modeling, reduced-scale windtunnel studies or manual calculations are used to predict discharge stack behavior so recirculation cannot occur. Rain caps are inadvisable on exhaust stacks, because exhaust must discharge freely to the atmosphere. A minimum discharge velocity of 2,600 fpm will prevent rain from entering the stack when the fan is operating. Because rain has a terminal velocity of about 2,000 fpm a stack velocity of 3,000 fpm is recommended to resist downwash, increase effective stack height and allow a more optimum selection of the centrifugal exhaust fans to provide a stable operation point of the fan curve. High exhaust velocity is a poor substitute for stack height and effective stack design, unless one is specifying a high-plume exhaust fan that is designed for that application. With all these design parameters, don’t forget Undisturbed flow Roof recirculation region Roof wake boundry 1.5R 5:1 Uh Xc H hs Hc Lc 10:1 High turbulence region Building wake recirculation region L Lr Figure 2 Stack height design procedure. To manually predict stack exhaust behavior, determine the required stack height, represented by hs in the diagram. Source: ASHRAE that energy usage must be considered as well. As such, the guidelines provided in Chapter 44 of the 2007 ASHRAE Applications are an ideal source to manually predict stack exhaust behavior. A depiction of the design procedure for determining the required stack height, included in the ASHRAE Applications, is shown in Figure 2. At the end of the day, designing facilities with hazardous requirements can be challenging, but working out details through the design phase results in occupant safety and operations efficiency. Furthermore, involving the end user to understand the constraints of the control areas as they relate to the hazardous materials is vital in defining the requirements of the project as they relate to code regulations. Consulting-Specifying Engineer • NOVEMBER 2007 45

Table of Contents Feed for the Digital Edition of Consulting-Specifying Engineer - November 2007

Consulting-Specifying Engineer Contents Editor's Viewpoint Letters In The News M/E Roundtable Emergency Power for Healthcare 2007 Products of the Year Industrial-strength Lighting Hazardous HVAC Codes and Standards Management Report Jobs/Classifieds Specifier's Notebook On-Peak Performance Contents Best Battery Selector in Engine Starting Applications NEC 708: Practical Impact on Backup Power Systems Protecting Power: Specifying Outdoor Generator Enclosures Investing in Backup Power Systems Coes, Consultants, Manufacturers, and Standby Power Systems

Consulting-Specifying Engineer - November 2007

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