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Gravimetric dosing of additives at high temperatures? Nothing to sweat about
Gravimetric systems for additive dosing are usually meant for work with materials at regular temperatures, i.e. 50C to 450 C. In some cases it's necessary to dose materials at higher temperatures, reaching 1200 C and sometimes up to 2000 C. Working at this high temperature range is very challenging and regular gravimetric feeders can't take it – they malfunction and are ruined.
Note that in this case we are not referring to a situation in which the main raw material is at a high temperature as a result of the use of a dryer, and the additive should be at a low temperature, as is usually the case when manufacturing products out of PET. In such cases, the additive is liable to melt as a result of the intense heat retained by the dried raw material, which goes from the neckpiece through the body of the feeder to the additive itself. Therefore a neckpiece cooled by cold water is used, preventing the transfer of heat to the feeder. This time the issue at hand is a case in which the additive itself is at a high temperature.
What's the challenge of working with an additive at a high temperature?
A high-temperature additive is liable to cause two critical components in the feeder not to work properly or even to be ruined.
The first component is the load cell, a component that is very temperature-sensitive. The maximum temperature for load cells differs from one type to another and among different manufacturers, but for the most part it's between 400C and 700C. Above that temperature, the load cell may continue to function, but the manufacturer doesn't guarantee it, and when the temperature rises above a certain level it will be totally ruined.
The second component that is sensitive to high temperatures is the engine. Feeder engines are usually designed to work at temperatures up to 500C, and sometimes up to 800C. A temperature that's too high is liable to significantly shorten the life of the engine.
Naturally, the high heat of the additive reaches the load cell and the engine through the body of the feeder, so the challenge is to keep these components at low temperatures by preventing the heat from reaching them, thus ensuring the proper functioning of the feeder.
How can you protect the critical components in the feeder from the intense heat that comes from the additive?
At LIAD Weighing and Control Systems we've developed a special feeder that is capable of working at high temperatures. The development was based on the use of a number of mechanisms that protect the critical components from the high heat of the additive. The first mechanism uses cold water to cool the load cell. Similarly to the cooling of the neckpiece during the manufacture of PET products, the cold water system used for cooling the mold is utilized here as well. The water enters the load cell's cooling system and leaves it, circulating constantly.
The second mechanism cools the engine in the same fashion. Circulation of cold water in and out of the engine's cooling system keeps it at a consistently low temperature.
In addition, there are two mechanisms that maintain a low temperature in the body of the feeder. One makes use of the factory's air pressure system, constantly releasing cold, compressed air inside the body of the feeder. The other is a fan installed at the back of the feeder that draws out the hot air.
Alongside the active cooling of the body of the feeder, steps were taken to reduce the transfer of heat from the additive to the load cell. The connection between the weighing hopper and the load cell was switched from stainless steel to an insulated material with low elasticity. Special insulated material was also put into use at the back of the feeder's weighing hopper, reducing the transfer of heat from the weighing hopper to the body of the feeder.
This technology ensures the longevity of the feeder and maintains proper and optimal functioning of both the load cell and the engine at extreme temperatures of up to 2000C. This development moves us one more step forward in the manufacture of complex products and in the achievement of a technological advantage in a competitive market.