Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Temperature (o Solution F) 100 120 140 160 180 Table I. Heat losses from Liquid Surfaces ambient temperature from the desired operating temperature (if the shop tem- perature is kept very cool during winter months, it might be wise to use this tem- perature as the average ambient temperature). Temperature rise = T operating minus T ambient [To - Ta = T rise] Determine an adequate heat-up time to suit your production requirements. The traditional 2-hour heatup may prove costly and unnecessary since using this val- ue usually provides a heater more than twice the size necessary for heat mainte- nance. A 4- to 6-hour heatup more closely approximates the heat maintenance val- ue but may impose production constraints deemed impractical. Long heat-up times can be overcome through the use of 24-hr timers; however, unattended heat- starts carry the responsibility of tank liquid level monitoring and approved overtemperature safety shutoffs. With this data, the initial tank heating requirements can be determined. A BTU is the amount of heat required to raise one pound of water one degree Fahrenheit. A BTUH is that amount per hour. Initial BTUH(Q) = Weight Trise s.h./Heat-up time where s.h. is specific heat. This should be the actual value from the process sup- plier (water is 1.0). Calculate the approximate heat loss from the tank surface and tank walls. (Use the data shown in Tables I and II.) The losses from the tank surface can represent the most significant loss affecting heater sizing. The addition of even a partial or loose-fitting cover will reduce these loss- es. The tank surface area is simply the width in feet times the length in feet. You can use inches instead of feet, but then must divide the results by 144 to obtain square feet. If you install partial covers, such as removable covers extending from the tank edge to the anode busing, use the remaining "open" dimensions. The covered Temperature (o Solution F) 100 120 140 160 180 Metal Tank or Thin Plastic (BTU/hr/ft2 ) 170 340 615 900 1,590 Table I I. Heat losses from Tank Walls and Bottoms 666 Insulated Tank or Heavy Plastic (BTU/hr/ft2 ) 290 560 995 1,600 2,750 Nonventilated Losses (BTU/hr/ft2 170 340 615 900 1,590 ) Ventilated Losses (BTU/hr/ft2 290 560 995 1,600 2,750 )

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