Consulting-Specifying Engineer - November 2007 - (Page 44)

space effectively, then recirculated air may be acceptable. Air dilution calculations will help determine if the amount of ventilation and recirculated air will dilute the environment to safe levels. If there is a risk that vapors may not be effectively captured or if hazardous materials are used openly in the space that will equate to 25% of the lower explosive limit, one will need to increase the ventilation requirements. One effective approach in such a scenario is a combination of low exhaust pickup points around the perimeter of the room and uniform supply air from the ceiling. Exploring exhaust systems Exhaust systems falls under the International Mechanical Code, Section 510. The engineer should consider the density of each hazardous material in the area. For flammable solvents heavier than air, low-point exhaust systems pick up hazardous vapors that collect at floor level. Collection points are then required to be a minimum of 12 in. above the finished floor elevation. Similarly, exhaust duct velocities should be high enough to collect and convey hazardous fumes. For solvent fumes, duct velocities range from 1,000 fpm to 2,000 fpm. Heavier molecules and particles require even higher velocities, up to more than 4,500 fpm. When selecting the exhaust diffuser, the effective open area of the capture grille must be calculated to ensure a high capture velocity at the grille face. Unlike commercial applications, exhaust noise is a much lesser consideration than face velocity. Separate, independent exhaust systems also are required for each material, and they must exhaust directly to the outdoors. One’s building department may make exceptions if the engineers show that different solvents are compatible with and can safely share an exhaust system. According to the International Mechanical Code, a hazardous exhaust system is required for normal operating conditions under the following circumstances: • To limit flammable vapor, gas, fume, mist or dust to concentrations below 25% of the lower flammability limit • If a NFPA health-hazard rating of 4 is present at any time • If a NFPA health-hazard rating of 1, 2 or 3 exceeds 1% of the median lethal concentration for acute inhalation toxicity. The International Mechanical Code also mandates that fire suppression be installed in hazardous exhaust ductwork larger than 10 in. diameter. Of course, the intent of this requirement is to protect against hazardous materials such as paints and other flammables that can deposit residue on the duct interior and cause duct fires. Solvents and some other materials actually clean the inside of the duct and are evaporated readily. With no potential for buildup of flammable substances inside the ductwork, one may be able to persuade the building department to waive the requirement for sprinkler protection. Wi t h CI TY M U re o sph e atmo LTI systems an ibilities flows freely. f poss CITY MULTI building comfort solutions are transforming the HVAC world, permitting new design possibilities and providing unparalleled comfort and control. Proven CITY MULTI systems are flexible enough to satisfy even complex building design applications and deliver unsurpassed dependable performance with no surprises. They’re also backed by our full support today and tomorrow. CITY MULTI will transform the way you think about HVAC. See a new world of possibilities at transforminghvac.com Input #229 at csemag.com/quickResponse 44 Consulting-Specifying Engineer • NOVEMBER 2007 http://transforminghvac.com http://transforminghvac.com http://csemag.com/quickResponse

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

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.