Metal Finishing Guide Book


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the mixing room and through a first-stage filter to collect large dirt and dust particles. The prefiltered air then moves through the ceiling fan for continuous air exchanges. Air then moves through the ceiling plenum filter to further purify room air of contaminants for a clean air mixing environment. AIR MAKEUP An air makeup unit can lower heating Fig. 19. Roof-mounted horizontal and cooling intake blast. costs. When air make-up is added, the building exhaust system works more efficiently. The information in this section will help to determine when an air make-up system is needed. Air make-up is the air required to maintain safe and effective building operation by replacing exhausted air. When an exhaust fan is installed in a building, exhausted air must be replaced from outside. This is done either through the cracks and openings in a building or with an air make-up, or Fig. 20. Inside ceiling air replacement, unit, which introduces outside mount vertical intake. air into the building. This air is usually filtered, cooled, or heated. Installing an exhaust system without an air make-up unit is a good example of heating ventilation air by accident rather than by design. Air always flows from a higher pressure area to a lower pressure area. Installing an exhaust fan in a building creates negative pressure within the interior space. Air Fig. 21. Inside ceiling mount horizontal intake. will flow from the higher pressure outside the building to the lower pressure inside. Because most buildings are closed in, the flow is restricted, but not completely. Cracks around doors and windows and in the masonry and vent stacks allow air to flow into the building. This air creates drafts and cold spots until it can mix sufficiently with space air to reach room temperature. The normal heating system must work longer and at higher temperature to heat the air seeping from the outside. In addition to the increased heating cost, the negative pressure keeps the exhaust fan from doing 785

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