Apparatus and method for controlled drying of plastic pellets

A dryer for drying plastic pellets comprising:

• a hopper having a cylindrical body with a frusto-conical lower body portion, a spreader cone positioned within said hopper body, a heated air inlet at the bottom of said hopper and an air outlet at the top of said hopper;
• a pellet inlet port at the top of said hopper to admit pellets into said hopper, a pellet discharge port at the bottom of said hopper frusto-conical lower body portion to selectively discharge pellets from said hopper;
• a plurality of temperature sensors that provide an electrical temperature responsive signal positioned at vertical incremental distances from the bottom of said hopper to the top of said hopper whereby the temperature of pellets at the vertical level of each of said plurality of temperature sensors may be measured to provide an indication of the dryness of said pellets;
• electrical circuit means connected to said plurality of temperature sensors and to a micro-processor to provide temperature data from said hopper to said micro-processor;
• said micro-processor being programmed with the characteristics of said hopper and the location of said temperature sensors within said hopper, said micro-processor being programmable to receive information on the type of plastic pellets to be dried in said hopper;
• said micro-processor being arranged to process said temperature data from said temperature sensors to determine when pellets at a particular vertical level within said hopper have been at a target temperature for a sufficiently long residence time to be sufficiently dry to utilize in a manufacturing process.

A dryer for drying plastic pellets comprising:

• a hopper having a cylindrical body with a frusto-conical lower body portion, a spreader cone positioned within said hopper body, a heated air inlet at the bottom of said hopper and an air outlet at the top of said hopper;
• a pellet inlet port at the top of said hopper to admit pellets into said hopper, a valved pellet discharge port at the bottom of said hopper frusto-conical lower body portion to selectively discharge pellets from said hopper;
• a plurality of temperature sensors that provide an electrical temperature responsive signal positioned at varying vertical distances from the bottom of said frusto-conical lower body portion to the top of said hopper cylindrical body whereby the temperature of pellets at the vertical level of each of said plurality of temperature sensors may be measured;
• electrical circuit means connected to said plurality of temperature sensors and to a signal processing unit to provide temperature data from said hopper to said signal processing unit, said signal processing unit including a micro-processor, a multiplexor, an analog-to-digital convertor and a user interface;
• said micro-processor being programmed to include the number of said temperature sensors, the vertical position of each of said temperature sensors, the size of said hopper, and the inside diameter of said hopper;
• said micro-processor being programmable to receive the target temperature to which said pellets are to be heated, the residence time at which said pellets are to remain at said target temperature, the bulk density of said pellets, and the desired rate of through-put of pellets from said dryer;
• said signal processing unit being arranged to determine the weight of pellets within said hopper currently at target temperature, to determine whether said target temperature has been achieved for the required residence time, and to provide an indication of whether said hopper contains a sufficient amount of dry pellets to achieve said desired rate of through-put to a manufacturing process that said dryer is to supply with dry pellets.

The method of determining when a preselected amount of plastic pellets that are being dried by passing heated air upwardly through them within a dryer hopper are properly dry for use in a manufacturing process comprising the steps of:

• placing a plurality of temperature sensors within said hopper in contact with said pellets at varying vertical distances from the bottom to the top of said hopper;
• determining when each of said temperature sensors reaches a specified target temperature;
• measuring the residence time at which the pellets in contact with each of said temperature sensors remains at said target temperature;
• and
• calculating the amount of pellets in said hopper below the vertically highest positioned temperature sensor that has been maintained at said target temperature for said residence time to determine if said amount is equal to or greater than said preselected amount.