Metal Finishing Guide Book

2012-2013

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travelling to different areas of the building. The air make-up unit reduces fuel bills by eliminating drafts. Sizing The air make-up system should be sized according to the spray booth exhaust volume plus 10%. If the air make-up duct will be physically connected to the spray booth, then the 10% extra capacity can be disregarded; however, some means of volume adjustment must be allowed so that a proper input/exhaust volume balance can be obtained. This can be in the form of an adjustable drive on the air make-up and/or exhaust fan or volume dampers in the system. If the installation is new, then the manufacturer will know the needs of both the exhaust fan and the air make-up system. If the booth is older, the exhaust volume can be determined from the manufacturer's literature, computing from known booth velocity or from fan curves. Air make-up is most easily sized during initial booth purchase and installation. To determine if you require an air replacement unit, multiply your spray booth's exhaust fan rated capacity (cfm) by 20 (based on three changes per hour: 60 minutes/3 = 20). Using a 10 ft wide x 8 ft high spray booth rated at 125 fpm (with a total of 10,000 cfm exhausted) would be 20 times 10,000, or 200,000 ft3 of air. If your shop's cubic foot area is less than 200,000 ft3 of air, you should install an air replacement system. Types of Heaters An air make-up unit contains a heater to heat the air. The heater may be gas-fired (direct or indirect), steam or hot water, or electric units. Direct gas-fired heaters are the most economical choice. Indirect gas-fired heaters are only used when there are restrictions against the use of direct units. Steam or hot water heaters are the least efficient. They should only be used when there is an existing boiler that has additional capacity to handle the air make-up system. Electric units should only be used when alternative fuels are not available. The cost of this fuel is quite expensive. The formula for calculating costs is as follows: where cfm is the actual cubic feet of air delivered by the air make-up per minute, T is the temperature of the air leaving the unit (same as the space temperature), To is the average outside air temperature during heating season, 1.08 is the constant arrived by multiplying 0.075 (air density) by 0.24 (specific heat) by 60 min/hr, H is the total hours of operation from October through April inclusive, F is the BTU value of one unit of fuel (generally1,021 for natural gas per cubic foot), E is the efficiency of the unit (0.92 for a direct fired air make-up unit), and c is the cost of one unit of fuel (expressed in the same units as those used for F). The following example illustrates how the fuel cost formula works. A 10,000 cfm air make-up unit in a building in St. Louis operates 60 hr per week at 65��F space temperature. It is fueled by natural gas at $0.40/ft3. We find the annual operating hours by Remember, this represents the greatest cost to operate the air make-up unit. Actual cost could be less. Types of Air Make-Up Units There are four basic air make-up styles available. They are defined by their intake and discharge mechanisms and include horizontal intake/downdraft dis838

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