Universal Aerosol Can Material Rotary Style Water Bath 78 SPRAY July 2014 Hot Water Bath The “Clip” Style Universal Aerosol Can Water Bath Modern Concepts Another popular hot tank is the rotary or drum style. Aerosol cans are smoothly fed onto a large cylinder in rows of up to 24 or more. The cylinder then rotates to the next empty row. As it turns, cans are immersed in the hot water that covers almost half the wheel. Any leakers are typically daubed in a colored fluid by the operator so that when they are discharged onto the downstream conveyor belt, they can be quickly identified and discarded. Hot tanks are not without problems. Perhaps the most frequent is a lack of adequate surveillance. Operators are simply not utilized on some filling lines, especially in emerging countries. When they are present, they may be mentally or physically distracted. A gas bubble every three feet (0.92 m.) can equate to a propellant loss of roughly 14 grams per year, yet it will not be detected unless the operator searches the water with a concentrated effort that may be hard to maintain. A recent recommendation has been to change operators every 20 minutes. Essentially, all U.S. hot tanks are fitted with heavy mesh screens or sturdy clear plastic enclosures to protect personnel in the case of a very rare can explosion. Unless they are used in conjunction with an adequate ventilation system, the plastic may become coated with water condensate droplets and rivulets, frustrating any attempts at detection of leaks other than major ones, called gushers or geysers. A ventilation system is a good idea for several of reasons. If a number of cans were to discharge potentially dangerous amounts of flammable propellant (extremely rare), the system would safely remove it. Secondly, the refuse drum or box near the end of many hot tanks may contain various numbers of leaking dispensers, and these could build up a potentially explosive gas-air mixture. If the hot tank has an exhaust system, it is a simple procedure to install a 4" (100mm) diameter duct, with an “elephant trunk” and plastic standpipe that will go to the bottom of the waste container and remove any dangerous gas content. Over the years, several massive hot tank explosions have taken place, usually due to thermostat failure and serious overheating. It is not uncommon for the explosive or brisance effect from one can to cause dozens or even hundreds of surrounding cans to also explode. Hot water baths should not be shut down when filled with cans. On the same basis they should always be followed by a covered air-blast station to remove water from the valve cup and cans, and then by an accumulating table. The table should be kept fairly empty so that if a downstream problem were to occur, the cans in the hot tank could be smoothly discharged onto the accumulating table, thus preventing overheating. In addition to the thermostat, good engineering would suggest the installation of an excess temperature cut-off device. Some hot tanks are equipped with large dial or digital thermometers. If not, the water temperature can be statistically checked using the usual bi-metallic portable thermometers with 1.5" (38mm) diameter dials. Traces of residual water may remain in can/valve crevices, even though the dispensers are air-blasted and allowed a fair amount of time for evaporation. The rust discoloration that may follow on tinplate or steel surfaces can be avoided if the hot tank is kept at about 0.08% levels with corrosion inhibitors. Typically, five gallon (18.92 liters) quantities of either 40% sodium nitrite or 35% sodium benzoate solutions are added at intervals. The amount will depend on the rate at which fresh water is added—or waste water is discharged through standpipes. (Note: The sodium nitrite level can be quickly checked using a standardized potassium permanganate solution. It will be totally decolorized if an adequate amount of sodium nitrite is still present in the tank). Hot tanks that contain hard water and/or corrosion inhibitors can leave small water droplets on cans. When these evaporate, tiny white spots can be seen, especially on darker lithographed surfaces. By adding as little as 0.015% sodium hexametaphosphate to the water, a streaming drainage action develops that eliminates these droplets. Hot Water Bath Alternatives Critics of the hot water bath point to its very large size, water and energy consumption, safety issues, inadequacy to test leakage of heat-sensitive products, issues with manual leak detection and so forth as
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