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High Temperature Exhaust Fan
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Exhaust fans can be grouped into two general categories: emergency smoke evacuation and process ventilation. Generally speaking, emergency smoke evacuation fans may never be used, but they must be installed and be capable of exhausting high temperature air and smoke in the event of a fire. In contrast, high temperature process ventilation requires continuous duty exhaust of high temperature air, fumes or particulate. Both application types are uniquely different requiring special construction and system design considerations. In this article, we will examine both types of exhaust applications, looking first at the emergency smoke exhaust category. Since emergency smoke exhaust deals with lifesafety issues, there are governing bodies in place that identify and regulate specific design and performance standards. The administration and organization of the various governing bodies is subject to modification based on the needs of the industry. Currently, four such agencies are Industrial Risk Insurers (IRI), Southern Building Code Congress International, Inc (SBCCI), the National Fire Protection Association (NFPA), and Underwriters Laboratories, Inc. (UL). IRI insures properties all over the world based on an informational manual, which details the construction requirements that belt drive emergency heat and smoke exhausters must meet in order to be covered by IRI.
SBCCI is a not-for-profit organization of government officials from the United States and several foreign governments, which serves a strong leadership role in the delivery of model building codes. The purpose of the NFPA can be summarized into three main categories. First, NFPA promotes the science and improves the methods of fire protection and prevention, electrical safety, and other related safety goals. Secondly, it obtains and circulates information on these subjects. And thirdly, it secures the cooperation of its members and the public in establishing proper safeguards against loss of life and property. The fourth organization is Underwriters Laboratories, Inc.. UL is a non-profit, independent organization that maintains and operates laboratories for examination and testing of devices, systems and materials to determine their relation to life, casualty hazards and crime prevention. UL has three safety standards that apply to emergency smoke exhaust products. UL705 is concerned with the mechanical and electrical construction to insure safe operation. All electrical components (motor, wiring, switches, enclosures, etc.) must be ULlisted. UL793 is concerned with the lifting mechanism for the butterfly dampers and the fusible link. In order for a product to be listed in the UL Directory under ÒPower Ventilators for Smoke Control Systems,Ó it must meet the requirements of both UL705 and UL793. Additionally, UL must witness a full-scale test of a fan operating for the required time at the specified elevated and temperature.
So what makes one fan more capable of sustaining higher temperatures than another fan? Each model has a recommended maximum operating temperature based on the construction materials, drive components, and airflow characteristics. The limiting temperature is determined to be the highest temperature that any component of the fan assembly will reach during any operating cycle. Similarly, the maximum operating temperature is typically determined to be the lowest temperature that begins to exceed the capacity of any one component. For example, in some cases the bearings may be the limiting component, while in other cases the fanÕs impeller construction material may be the limiting component. The construction material is perhaps the most obvious element of the fan to consider when dealing with a high-temperature application. In general, aluminum withstands maximum temperatures up to 250 F, standard carbon steel up to 750 F, and 316 stainless steel up to 1000 F. Critical components are many times constructed of ferrous materials to withstand the higher temperatures. If temperatures were to exceed 300 F, for example, aluminum would be eliminated as a construction material option. Other construction considerations include bearing type, drive component selections, means of ventilation and cooling of the drive components, and insulation options.
The most common way of simplifying construction and component specifications to accommodate high temperature applications is to maintain separate categories based on the specified temperature range and time requirements.
Northern Fan Co. has four Heat Option Packages for high temperature operation. Heat Option I construction is designed for continuous operation between 200 F and 500 F. Heat Option II construction meets specifications requiring the fan to exhaust 500 F air for a minimum of four hours in an emergency smoke removal situation per IRI requirements. Heat Option III construction meets the specifications requiring the fan to exhaust 1000 F air for a minimum of 15 minutes in an emergency smoke removal situation per SBCCI. This construction also surpasses the IRI requirements for 500 F for a minimum of four hours. Heat Option IV construction meets specifications for UL Listed ÒPower Ventilators for Smoke Control Systems. This includes the IRI requirement of 500 F for a minimum of four hours, the SBCCI requirements of 1000 F for a minimum of 15 minutes, and the Snow Load Test for butterfly dampers in UL-793.
While it may be tempting to choose a higher heat option than necessary "just to be safe", doing so can add considerable and unnecessary cost to the job. For example, selecting HT Option III when HT Option II is adequate adds insulation and high temperature bearings. These items would be considered "overkill" and add unnecessary extra costs.
For additional information please refer to http://www.northernindustrialsupplycompany.com/products/industrialfan_blowers.html
Industrial Process Engineer
Northern Fan Co.
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