A fault with the dryer will lead to hydrolysis of the material in the barrel of the machine. This will cause lowered I.V. of the material. Lowered I.V. is the major cause of bad quality in preforms and bottles. More than 60% of all PET processing related faults can be traced back to the dryer.
Maintaining the dryer in optimum condition will allow the molding machine to perform at maximum efficiency and quality.
Airflow - The most important parameter, there should be nothing causing a restriction in the process and regeneration air flow.
Airflow - Most common problem is a blocked process filter, but also check blowers are operating correctly and the delivery hoses have not been squashed.
Temperature - Process Temperature should be in the region of 145~170ºC Depending on the resin supplier and the drying time. Also check the Regeneration Temperature which should be around 200~230ºC depending on the maker.
Temperature - Confirm process air temperature is correct on the display panel. The regeneration air temperature is not so easy to check since most regeneration controllers are hidden away in the electrical panel.
Time - Calculate (or measure) the time the material is in the hopper, this should be at least 3½-4 hours.
Time - If you know the capacity of the hopper in liters, you can calculate the PET quantity using a figure of 0.84kg/L as the bulk density. Compare this figure (kilograms of PET) against the current consumption of PET by the machine in kg/hr. If you are not sure, the safest method is to switch off the hopper loader and check the time necessary for the material to be consumed.
Dewpoint - Correct dewpoint may vary according to the manufacturer of the dryer, consult the maker's manual before assuming an error exists.
Dewpoint - Use a commercially available dewpoint meter to sample the air coming from the desiccant chamber. Typical vales may be anything between -20ºC and -50ºC. Check the manual for correct specification. In machines with more than one desiccant chamber, be sure to check all of them.
Check the correct function of all heaters on a regular basis using a clip-on ammeter. If one or more has broken, the others will be taking extra load.
Always check the calibration of instrumentation and thermocouples before assuming a problem exists
Showing posts with label d. Show all posts
Showing posts with label d. Show all posts
The dewpooint temperature
The dewpoint (or more correctly, the dewpooint temperature) is the temperature at which air can no longer support the existing quantity of water in vapour form.
Air always has a certain amount of moisture in the form of gas which is therefore normally invisible to the human eye. As the temperature of the air is lowered, the ability of the air to support this moisture is also lowered. At some temperature, a point will be reached where the excess moisture will change to liquid and will either become visible as a mist, or will be deposited on nearby surfaces as condensation. The temperature where this phenomena occurs is known as the dewpoint temperature.
Since the dewpoint temperature is not a real measure of the current air temperature, it is normal that the actual air temperature and the dewpoint temperature will be at different values. For example, in a PET resin dryer, the actual air temperature is typically 160ºC, while the dewpoint temperature is around -20ºC.
The dewpoint measurement is taken on the process air line just after the desiccant chamber(s). Many dryer makers fit an air sampling point with an outlet coming to the front panel of the dryer. When taking an air sample, allow adequate time for a stable result to be obtained and make sure to check all desiccant chambers. Each dewpoint meter may have different operating methods so please refer to the instruction manual. Although dewpoint is important for correct drying, the process airflow is by far the most critical. A dryer with good airflow and poor dewpoint is still partially effective, whereas a dryer with excellent dewpoint but poor airflow is useless.
Air always has a certain amount of moisture in the form of gas which is therefore normally invisible to the human eye. As the temperature of the air is lowered, the ability of the air to support this moisture is also lowered. At some temperature, a point will be reached where the excess moisture will change to liquid and will either become visible as a mist, or will be deposited on nearby surfaces as condensation. The temperature where this phenomena occurs is known as the dewpoint temperature.
Since the dewpoint temperature is not a real measure of the current air temperature, it is normal that the actual air temperature and the dewpoint temperature will be at different values. For example, in a PET resin dryer, the actual air temperature is typically 160ºC, while the dewpoint temperature is around -20ºC.
The dewpoint measurement is taken on the process air line just after the desiccant chamber(s). Many dryer makers fit an air sampling point with an outlet coming to the front panel of the dryer. When taking an air sample, allow adequate time for a stable result to be obtained and make sure to check all desiccant chambers. Each dewpoint meter may have different operating methods so please refer to the instruction manual. Although dewpoint is important for correct drying, the process airflow is by far the most critical. A dryer with good airflow and poor dewpoint is still partially effective, whereas a dryer with excellent dewpoint but poor airflow is useless.
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