DRYER CONTROL BY REGULATION OF HOT AIR SUPPLY RATE

Abstract

Method and apparatus for hot air drying granular polymeric resin material records heating air temperature at a heating air outlet, computes air flow rate through a hopper, maintains desired throughput of granular material being dried in the hopper until heating air outlet temperature is stable, and thereafter maintains air flow rate through the hopper and monitors that air flow rate to adjust the air flow rate to maintain a computed preferred air flow rate through the heating hopper.

Claims

1. A method for hot air drying granular material in a hopper having a granular material inlet and a heating air outlet, and a granular material outlet and a heating air inlet, comprising: a. introducing heating air into the hopper b. recording heating air temperature at the heating air outlet; c. computing heating air flow rate through the hopper; d. recording the stable heating air outlet temperature; e. incrementally reducing air flow rate through the hopper until heating air outlet temperature falls below the recorded stable heating air outlet temperature; f. incrementally increasing air flow rate until heating air outlet temperature returns to the recorded stable heating air outlet temperature; g. computing air flow rate through the hopper; h. continuously monitoring air flow rate through the hopper for variance from the computed air flow rate through the hopper and adjusting air flow rate through the hopper to return to the computed air flow rate; i. continuously monitoring heating air outlet temperature and if heating air outlet temperature is outside a predetermined range repeating step “h” until heating air outlet temperature is within a predetermined range of the recorded stable heating air outlet temperature.

2. The method of claim 1 further comprising maintaining the desired throughput of granular material being dried in the hopper while performing the steps of claim 1.

3. A dryer for granular material comprising: a. a heating hopper having a granular material inlet and a heating air outlet proximate the hopper top, and a granular material outlet and a heating air inlet proximate the hopper bottom; b. a blower; c. a conduit connecting the blower to the heating air inlet of the hopper; d. an air speed detector including a heating air outlet temperature sensor in the heating air outlet; e. a microprocessor connected to the blower, the temperature sensor, and the air speed detector, controlling blower speed in response to detected air speed to maintain heating air exit temperature at a selected level providing minimal energy usage by the heating air heater by periodically incrementally increasing and decreasing blower speed, measuring hearing air exit temperature and air flow rate when both are stable, and increasing or decreasing blower speed according to the measured stable heating air exit temperature being higher or lower than a preceding measured stable heating air exit temperature.

4. The dryer of claim 3 wherein the air speed detector further comprises: a. a second heating air outlet temperature sensor; and b. a heater positioned between the two outlet air temperature sensors.

5. The dryer of claim 3 further comprising a heater in the conduit.

6. The dryer of claim 3 further comprising a scale, connected to the microprocessor, for detecting weight of heated granular material exiting the hopper via the granular material outlet.

7. The dryer of claim 5 further comprising a transformer connected to the microprocessor and the heater, for furnishing electrical power to the heater.

8. A method for continuous hot air drying of granular material comprising: a. selecting a desired granular material throughput rate; b. selecting a desired inlet heating air temperature; c. using the selected granular material throughput rate, calculating the heating air flow rate required to deliver sufficient heat to the granular material to raise the temperature of exiting granular material to a level at which the granular material is adequately dry; d. monitoring temperature of heating air leaving the granular material; e. drying the granular material by blowing heating air therethrough at the calculated air flow rate until temperature of heating air leaving the granular material is steady within a preselected range; f. determining rate of heating air flow through the granular material from temperature of heating air leaving the granular material; g. adjusting heating air flow rate until actual heating air flow rate through the granular material equals the calculated heating air flow rate.

9. The method of claim 8 further comprising: a. continuously monitoring temperature of heating air leaving the granular material; and b. if temperature of heating air exiting the granular material is outside a preselected range incrementally adjusting heating air flow rate until temperature of heating air leaving the granular material is within the preselected range.

10. A method for hot air drying granular polymeric resin material, comprising: a. setting an input air temperature control to a desired temperature for the granular polymeric resin material to be dried; b. selecting a desired rate of throughput of granular polymeric resin material to be dried in pounds per hour; c. blowing heating air at the desired temperature and at a selected air flow rate into the bottom of a hopper in which the granular polymeric material is to be dried; d. monitoring temperature of air exiting the hopper; e. checking the granular polymeric material exiting the hopper for moisture and if found to be too moist, incrementally increasing the heating air flow rate into the bottom of the hopper until moisture lever in granular polymeric material exiting the hopper is at the desired level; f. recording heating air exit temperature (T1) and computing air flow rate (S1); g. maintaining the desired throughput of granular polymeric material being dried in the hopper until heating air exit temperature is stable; h. recording exit air temperature (T2); i. incrementally reducing air flow rate until heating air exit temperature of hopper falls below T2; j. incrementally increasing air flow rate until heating air exit temperature returns to T2; k. computing air flow rate (S2); l. continuously monitoring air flow rate for variance from S2 and adjusting air flow rate to return to S2; m. continuously monitoring heating air exit temperature and outside a predetermined range repeating step “1” until heating air exit temperature is within the predetermined range.

11. The method of claim 10 wherein the selected air flow rate in step “c” is 0.6 cubic feet per minute per pound per hour of granular polymeric material to be dried.

12. The method of claim 10 wherein granular polymeric material to be dried is introduced into the top of the hopper and the dried granular polymeric material is received from the bottom of the hopper.

13. The method of claim 10 wherein temperatures are sensed and air flow rates are controlled by a microprocessor.

14. The method of claim 10 further comprising maintaining desired throughput of granular polymeric material being dried in the hopper while performing steps “h” through “m”.

15. The method of claim 10 further comprising continuously introducing granular polymeric material into the hopper at the desired rate of throughput of granular polymeric material and releasing the heated granular polymeric material from the hopper at the desired throughput rate.

Description

DESCRIPTION OF THE DRAWING

[0034] FIG. 1 is a schematic representation of a granular material dryer manifesting aspects of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0035] In this description of the invention, the description is in terms of drying granular polymeric resin materials. However, the invention is not limited to drying granular polymeric resin material. Any granular material may be dried using the invention.

[0036] Referring to the drawing, a dryer 10 for drying granular polymeric resin material and embodying aspects of the invention is illustrated schematically in FIG. 1. In FIG. 1 hopper 12 is a part of dryer 10 for drying granular polymeric resin material. A hot air outlet conduit, connected to hopper 12 for hot air exiting hopper 12, is denoted 50. Within hot air outlet conduit 50 is an air speed detector 16 for detecting speed of air exiting hopper 12 via hot air outlet conduit 50.

[0037] A microprocessor 22 controls a blower 36 and a transformer 24. Microprocessor 22 connects to blower 36 via a blower control line 42. Microprocessor 22 connects to transformer 24 via a transformer control line 46. Microprocessor 22 further connects to a temperature sensor 32, which is resident within a hot air supply line 48, via connecting signal line 44. Microprocessor 22 further connects to a scale 34 via a connecting signal line 40. Scale 34 receives readings from a weight sensor denoted 20 in FIG. 1 via a connecting line signal 52.

[0038] Granular polymeric resin material to be dried enters hopper 12 via granular polymeric resin material inlet conduit 14 located at the top of hopper 12. Granular polymeric resin material that has been dried exits hopper 12 downwardly through granular polymeric resin material outlet conduit 18. Granular polymeric resin material supplied to dryer 10 and entering hopper 12 via granular polymeric resin material inlet conduit 14 will typically have come from a gravimetric blender in which virgin resin material, recycled resin material, colorant and other additives to facilitate molding or extrusion of the granular polymeric resin material, if the granular polymeric resin material is thermoplastic resin, have been added according to a pre-determined recipe and mixed. Granular polymeric resin material exiting hopper 12 downwardly through granular polymeric resin material outlet conduit 18 will typically be conveyed to a receiver which acts as a temporary storage receptacle for the granular polymeric resin material, where the granular polymeric resin material stays until it is needed for molding or extrusion by an injection molding press or an extruder.

[0039] In operation, granular polymeric resin material is fed into hopper 12 as indicated by the upper arrow G.sub.in at granular polymeric resin material inlet conduit 14. Heating air for drying the granular polymeric resin material is supplied via hot air supply line 48 with the air being driven by blower 36 and heated by heater 26, both illustrated schematically in FIG. 1. The heating hot air enters hopper 12 close to the lower extremity of hopper 12 as illustrated in FIG. 1 where hot air supply line 48 terminates at hopper 12. Positioned within hot air supply line 48 is a temperature sensor 32 which connects to microprocessor 22 via a connecting line 44. Inlet air temperature sensor 32 senses the temperature of heating air supplied to granular polymeric resin material within hopper 12 via hot air supply line 48 with heated air temperature sensor 32 supplying the sensed temperature to microprocessor 22 via a connecting line 44.

[0040] Microprocessor 22 controls operation of blower 36 and operation of transformer 24. Transformer 24 supplies electrical current at a desired voltage and amperage to heater 26 as illustrated in FIG. 1 with the amperage and voltage being output by transformer 24 being regulated by microprocessor 22.

[0041] An outlet airspeed detector for detecting airspeed within hot air outlet conduit 50 is designated generally 54 in the drawing and includes an upstream outlet air temperature sensor 28 located within hot air outlet conduit 50, a downstream outlet air temperature sensor 30 also located within hot air outlet conduit 50 and a heater 16 which forms a portion of outlet airspeed detector 54. Outlet air speed detector 54 determines rate of air flow independently of the diameter or cross-sectional area of hot air outlet conduit 50. This is effectuated by measuring air temperature at a first position along conduit 50 with such air temperature being measured by upstream outlet air temperature sensor 28. Air flowing through conduit 50 out of hopper 12 and past upstream outlet air temperature sensor 28 is heated by heater portion 16 of outlet airspeed detector 54. The temperature of the flowing air is then measured at a second, downstream position within hot air outlet conduit 50 by downstream outlet air temperature sensor 30. Air temperature measured by downstream outlet air temperature sensor 30 is subtracted from measured air temperature ascertained by upstream outlet air temperature sensor 28 to obtain a temperature difference. The power used by heater portion 16 to heat the air flowing through hot air outlet conduit 50 is divided by the product of the measured temperature difference and the specific heat of air to provide the air flow rate through hot air outlet conduit 50. This type of airspeed detector 54 and the operation thereof is disclosed and described in more detail in U.S. Pat. No. 8,141,270, the disclosure of which has been incorporated by reference.

[0042] During start up, calibration, and production operation of dryer 10, hot air supplied to the bottom of hopper 12 by line 48, with that air being pushed by blower 36 and heated by heater 26, has its temperature measured continuously by inlet heating air temperature sensor 32, with the measured temperature data being supplied to microprocessor 22 via connecting signal line 44. Microprocessor 22 works with inlet heating air temperature sensor 32, heater 26, blower 36, and hot air supply line 48 to assure that the temperature of the hot air supplied to the granular polymeric resin material, to be dried by dryer 10 within hopper 12 never exceeds a maximum temperature, which is typically furnished by the manufacturer of the polymeric resin. (Polymer manufacturers usually provide ideal drying temperatures and other data for their product).

[0043] Respecting representative particular polymers that may be dried using the apparatus and method of the invention, for acrylonitrile butadiene styrene (ABS) the preferred and maximum drying temperature is 190° F.; for polyamide (PA) the preferred and maximum drying temperature is 190° F.; for polybutylene terephthalate (PBT) the preferred and maximum drying temperature is 250° F.; for polycarbonate (PC) the preferred and maximum drying temperature is 250° F.; for polyether ether ketone (PEEK) the preferred and maximum drying temperature is 300° F.; for polyethylene imine (PEI) the preferred and maximum drying temperature is 300° F.; for polysulfone (PSU) the preferred and maximum drying temperature is 300° F.; for polyurethane (PUR) the preferred and maximum drying temperature is 280° F.; for polyethylene terephthalate (PET) the preferred and maximum drying temperature is 300° F.; for poly(p-phenylene oxide) (PPO) the preferred and maximum drying temperature is 250° F.; and for polyphenylene sulfide (PPS) the preferred and maximum drying temperature is 300° F.

[0044] In one practice of the method for hot air drying granular polymeric material according to the invention, an input air temperature control portion of microprocessor 22 is set to a desired maximum drying temperature for the particular granular polymeric resin material to be dried. Next, a desired rate of throughput of granular polymeric resin material to be dried is selected, with that throughput rate typically being in pounds per hour. Microprocessor 22 actuates blower 36, transformer 24 and heater 26 so that blower 36 proceeds to blow air at the desired drying temperature and at a selected air flow rate through hot air supply line 48 into the bottom of hopper 12, all as illustrated in FIG. 1.

[0045] In this practice of the invention, microprocessor 22 monitors temperature of air exiting hopper 12 with that temperature information being supplied to microprocessor 22 by connecting signal line 56. Signal line 56 receives temperature data from upstream outlet air temperature sensor 28 and provides that data to microprocessor 22. It is to be understood that the temperature of the outlet air leaving hopper 12 via hot air outlet conduit 50 could equally be provided to microprocessor 22 by downstream outlet air temperature sensor 30 with an appropriate adjustment being made for the difference between temperatures as sensed by upstream outlet temperature sensor 28 and downstream outlet temperature sensor 30 according to the amount of electrical power furnished to heater 16 of outlet air speed detector 54, or heater 16 could be turned “off” as needed, whereupon the temperatures sensed by sensors 28 and 30 would be the same

[0046] Heater 16 receives power from transformer 24; the electrical lines connecting heater 16 and transformer 24 have not been illustrated in FIG. 1 to aid the clarity of the drawing.

[0047] As the drying method proceeds, granular polymeric resin material exiting hopper 12 via granular polymeric resin material outlet conduit 18 at the bottom of hopper 12 is tested for moisture. If the granular polymeric resin material exiting hopper 12 via granular polymeric resin material outlet conduit 18 is found to be excessively moist, microprocessor 22, having been appropriately programmed, incrementally increases the hot air flow rate into the bottom of hopper 12, with such hot air being supplied via hot air supply line 48 until moisture level in the granular polymeric resin material exiting hopper 12 is at the desired level.

[0048] Exit air temperature, which may sometimes be designated T1 in the attached claims, is measured by either temperature sensor 28 or temperature sensor 30 and is recorded by microprocessor 22. At the same time, air flow rate out of hot air outlet conduit 50 is computed using the measured air speed provided by outlet air speed detector 54 and the known dimensions of hot air outlet conduit 50.

[0049] The desired throughput of granular polymeric resin material being dried in hopper 12 is maintained and temperature of air exiting hopper 12 via hot air outlet conduit 50 is continuously measured until the temperature of air exiting hopper 12 via hot air outlet conduit 50 is stable. This hot air exit temperature is preferably recorded and preferably noted as being the stable temperature by microprocessor 22 and may be designated T2.

[0050] Microprocessor 22 controlling blower 36, preferably incrementally reduces air flow rate into hopper 12 via hot air supply line 48 until the temperature of air exiting hopper 12 via hot air outlet conduit 50 falls below recorded stable exit air temperature T2. Microprocessor 22 then incrementally increases air flow rate by increasing the speed of blower 36, thereby forcing more hot air flowing through hot air supply line 48 and into hopper 12 to pass through the granular polymeric resin material therein and then to exit from hopper 12 via hot air outlet conduit 50, until the measured exit air temperature, as measured either by sensor 28 or sensor 30, returns to temperature T2 measured at the stable condition. At that time microprocessor 22 computes exiting air flow rate through hot air outlet conduit 50 using outlet air speed detector 54 and records that computed air flow rate at the stable temperature T2 condition as S2. Microprocessor 22 thereafter either intermittently or continuously monitors outlet air flow rate for variance from the computed air flow rate S2 and makes air flow rate return to S2 by adjusting the speed of blower 36. Microprocessor 22 also either intermittently or continuously monitors exit air temperature as measured by sensors 28 and/or 30 and if measured exit air temperature is outside a predetermined range, microprocessor 22 adjusts the air flow rate by varying the speed of blower 36 until exit heating air temperature is within the predetermined range.

[0051] When drying most granular polymeric resin materials, the selected air flow rate at the commencement of operation is usually computed as being 0.6 cubic feet per minute per pound per hour of granular polymeric resin material to be dried, where the weight in pounds per hour of granular polymeric material to be dried is selected as the desired throughput for dryer 10.

[0052] In FIG. 1 the letters G.sub.in and G.sub.out indicate that the granular polymeric material to be dried is introduced into the top of hopper 12 through granular polymeric resin material inlet conduit 14 and the dried granular polymeric material is discharged from the bottom of hopper 12 via granular polymeric resin material outlet conduit 18.

[0053] Granular polymeric resin material to be dried is preferably continuously introduced into hopper 12 at the desired rate of throughput of the granular polymeric resin material and is released from hopper 12 at the desired throughput rate. In other words, in the preferred operation of dryer 10, once the system has started and stable conditions have been attained for the heating air outlet temperature and the air flow through hopper 12, operation thereafter continues with the preferable continuous flow of granular polymeric resin material being heated during passage through hopper 12.

[0054] As noted above, the ratio of air flow rate in cubic feet per minute per pound per hour of granular polymeric material to be dried is typically set at 0.6 for initial operation of dryer 10. Heated air outlet temperature is monitored by microprocessor 22 receiving data from temperature sensor 28 or 30; heated air outlet temperature is preferably monitored constantly. If air outlet temperatures are, on average, higher than what experience has proven to be desirable for a given polymer, then both an operator and microprocessor 22 know that the 0.6 ratio of cubic feet per minute of air flow per pound per hour of granular polymeric resin material throughput is incorrect for that particular polymer and current environmental conditions.

[0055] Monitoring exit air temperature allows microprocessor 22 to adjust the 0.6 ratio in gradual increments so that on average the exit air temperature is held at the desired level for the granular polymeric resin material being dried. Using this approach of making small and gradual adjustments to the air flow rate/throughput ratio in software resident within and executed by microprocessor 22, and making small up and down adjustments in the speed of blower 36 based on feedback provided in the form of heating air outlet temperature and flow rate of heating air leaving hopper 12 via hot air outlet conduit 50, allows maintenance of a substantially uniform air flow rate during normal minute-by-minute and hour-by-hour variations in the process, which inevitably occur due to environmental and other factors.

[0056] By microprocessor 22 monitoring air flow rate of air exiting hopper 12 via hot air outlet conduit 50, if microprocessor 22 detects a change, microprocessor 22 adjusts the speed of blower 36 to return to the correct rate of air flow in cubic feet per minute exiting through hot air outlet conduit 50. Then, as microprocessor 22 is monitoring the exit air temperature while making the small adjustments noted above, over time microprocessor 22 learns and records whether the air flow goal amount of hot air through outlet conduit 50 in cubic feet per minute is correct for the particular granular polymeric resin material being dried and the current environmental conditions such as ambient temperature and relative humidity in the facility in which dryer 10 is located. If exit temperature is too high, this indicates energy usage is excessive.

[0057] In the course of detecting any excess heat over time, microprocessor 22 will gradually reduce the hot air requirement in cubic feet per minute by slowing blower 36 to a target cubic feet per minute air flow rate out of hot air conduit 50. The targeted cubic feet per minute air flow rate may be adjusted gradually, perhaps over several hours, to hold an average target air temperature for hot air exiting hopper 12 via hot air outlet conduit 50.

[0058] The ratio in cubic feet per minute of air flow to pound per hour of granular polymeric resin material throughput, in the software of microprocessor 22, is adjustable.

[0059] With this approach, microprocessor 22 uses the air flow rate information in cubic feet per minute out of hot air outlet conduit 50 to adjust speed of blower 36. Inlet temperature is held constant for a given polymer or other granular polymeric resin material being dried. Adjustments to the speed of blower 36 are made only after steady temperatures are maintained in heating air exiting hot air outlet conduit 50. Once substantially constant temperature at hot air outlet conduit 50 has been reached, air flow rate in cubic feet per minute is calculated for heating air exiting hopper 12 via hot air outlet conduit 50. Microprocessor 22 then alters speed of blower 36 in small increments, and may make corresponding small adjustments in power provided by transformer 24 to heater 26, in order to return heating air flow rate in cubic feet per minute out of hot air outlet conduit 50 back to the targeted air flow rate in cubic feet per minute. Over time, if the temperature of heating air exiting hot air outlet conduit 50 has reached a higher level than desired, or never reaches the minimum level required, the target cubic feet per minute is adjusted by microprocessor 22 altering the speed of blower 36, with such adjustments being made incrementally until temperature of heating air exiting hot air outlet conduit 50 falls into line with expectations.

[0060] Microprocessor 22, being connected to blower 36, inlet heating air temperature sensor 32, and outlet air speed detector 54 including temperature sensors 28 and 30, which are a part of outlet air speed detector 54, controls blower speed in response to measured air speed to maintain heating air exit temperature at a selected level, to provide minimal energy usage by heating air heater 26. Microprocessor 22 periodically incrementally increases and decreases speed of blower 36, and regulates transformer 24, as needed, measures heating air exit temperature, as detected by one of temperature sensors 28 and 30, and computes heating air outlet flow rate until both are stable, and increases or decreases speed of blower 36 according to the measured stable heating air exit temperature being higher or lower than a preceding stable heating air exit temperature. This procedure may be repeated over a substantial time period, namely days or even weeks, to be sure the ideal speed for blower 36 had been determined for a given granular polymeric material and for a given set of environmental conditions.

[0061] While the invention has been disclosed in detail for dryers in a configuration with granular polymeric resin material to be dried moving constantly through dryer 10, the inventive concepts are equally applicable to batch drying granular material, and dryer 10 may equally well be used for batch drying.

[0062] As discussed above and from the foregoing description of the exemplary embodiments of the invention, it will be readily apparent to those skilled in the art to which the invention pertains that the principles, structures and methods, disclosed herein can be used for applications other than those specifically mentioned. All such applications of the invention are intended to be covered by the appended claims unless expressly excluded therefrom.

[0063] The invention may be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. The disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive with the scope of the invention being indicated by the appended claims and equivalents, rather than by the foregoing description. All changes from the disclosed exemplary embodiments of the invention which come within the meaning and range of equivalents to the claims are therefore intended to be embraced therein. Specifically, embodiments and implementations performing substantially the same function in substantially the same way to achieve the same result are embraced by the claims.

[0064] As used in the claims herein, the term “comprising” means “including” while the term “consisting of” means “including so much and no more” and the term “consisting essentially of” means including the recited elements and those minor accessories appurtenant thereto which are known in the art to be required to facilitate the invention as claimed.