Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 669 of 707

up water when working on small process tanks with high operating temperatures. In some operations, it is customary to replenish evaporative losses by rinsing parts over the tank. This practice increases the heat loading. Gallons of water each hour (drag-in or add), times 8.33 (lb/gal), times the temperature rise (water tempera- ture to tank operating temperature). gallons per hour 8.33 T rise Now determine total heating requirement by comparing initial heat-up requirements with the sum of the various losses. Assuming no additions or operating losses during the initial heatup, we can equate our heater size based on the initial heat-up requirement, plus the tank surface losses, plus the tank wall losses. This value must be compared with the operating requirements—tank surface losses, plus the tank wall losses, plus the rack (barrel) losses, plus the drag- in (make-up) losses. The larger value becomes the design basis for heater sizing. Heater sizing can proceed based on the heating method employed. Electric immersion heaters are sized based on 3.412 BTUH per watt-hour (3,412 BTUH per kilowatt-hour). Divide the design heating requirement by 3,412 to find kilo- watts of electric heat required. design heating requirements(BTUH)/3,412 The immersion heater sheath temperature will be higher than the solution tem- perature. Consult your immersion heater supplier for its recommendations where solutions have high temperature limits. Electric heaters have the potential of achieving sheath temperatures, particularly in air, and are capable of igniting flammable materials; therefore, it is essential that liquid level switches and high sheath temperature cutoffs be employed. Look for (or ask about) Underwriters Laboratory or other independent agency listing labels on electric heaters for assurance that the product meets a recognized standard. Verify and install the sheath ground to minimize personnel shock hazard and, as with all heaters, use a quality temperature controller for economical operation. Steam immersion heaters are sized based on steam pressure, overall transfer coefficients, area, and log mean temperature difference. The overall transfer coefficient is a value determined by several basic values: the ability of the heater material to conduct heat, the ability of the two fluid films that form on the inside and outside of the heater to conduct heat, and the resistance to the flow of heat caused by fouling or buildup. You can significantly alter the performance of immersion heaters by the choice of materials and the supply or the lack of supply of tank agitation. By selecting proper materials the fouling caused by corrosion is either reduced or eliminated. Clean quality steam will reduce internal fouling while properly placed agitation can enhance overall thermal performance. The precise calculation of the overall transfer coefficient is detailed and will not be covered here, but is available from your heater supplier. The following rule-of-thumb values can be used for estimating steam heater size. For metal coils, the range of values for the overall heat transfer coefficient is 100-200 BTU/hr/ft2 /O F. For plastic coils, the overall heat transfer coefficient ranges from 20-50. Use 150 for metal and 40 for Teflon. Now calculate the log mean temperature difference (LMTD) because the driving force for the heat exchange is a varying quantity that is expressed as this value. LMTD = (T1 - T2 668 )/[ln(T1 /T2 )]

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