GB2122384A - Plastics material drying cycle - Google Patents

Abstract

A plastics material drying cycle consisting of a process air blower 2 for moving process air through a closed cycle sequentially consisting of a desiccant means 6, a process air heater 10 for heating the process air to a selected temperature, a body of plastics material 18 to be heated, and to the return or inlet side of the blower 2 in which the volume of the process air is controllably bypassed from the discharge side to the inlet side of the blower 2 in response to the discharge temperature of the process air from the body of material 18 being dried. Preferably the rate at which such discharge temperature varies is sensed to control the rate of change in bypass of the process air. <IMAGE>

Description

SPECIFICATION Plastics material drying cycle In the plastics processing industry it is well known that a precise method of preconditioning the plastics material to be processed with respect to moisture content and temperature is required in order to subsequently obtain satisfactory production of plastics articles. One well know preconditioning cycle is a closed air cycle having a process air blower for discharging pressurized process air sequentially through a desiccant means to control the moisture content of the process air, an air heater to heat the process air, heating a body of plastics material in a drying hopper with the dry heated process air to within a selected temperature range so that the heated material has a temperature and moisture content to permit the material to be subsequently processed into products having the quality and appearance as is desired commercially.After passing through the drying hopper, the process air flows to the return or inlet side of the blower to permit the process air discharged from the drying hopper to be recycled. Thus, the process air flows through an essentially closed cycle which is isolated from ambient air. As is known, the body of plastic material may consist of one of numerous moisture permeable pellets or pellets and regrind material as are commonly used in the plastics processing industry. Further description of this well known drying cycle is not required to one skilled in the relevant prior art and in view of the prior publications on such drying cycles such as the "Engineering Data No. 9300 and 9309" bulletins by Conair, Inc., and US Patent 3,487,603.

As also known, some plastics material require heating to temperatures above those normally used for plastics drying and "high performance" dryers having the above described cycle are used for such purposes.

Such "high performance" dryers also include a heat exchanger, such as a water-cooled heat exchanger, in the process air path between the drying hopper and the inlet side of the blower to lower the temperature of the process air returning from the dryer hopper for efficient moisture removal by the desiccant means. Again, further description of such a cycle is not believed to be necessary in view of the prior publications thereon such as Conair bulletin "Engineering Data No. 9309" and the above identified patent. All of the publications and US patent identified herein are incorporated herein for amore complete understanding of the cycle of this invention.

According to the present invention there is provided a plastics material drying cycle in which a blower circulates process air through a closed flow path having at least a process air heater and a drying hopper sequentially located in such flow path on the discharge side of such blower and with the process air discharged from such drying hopper being returned to the inlet side of such blower, the volume of the process air flow through said air heater and said drying hopper being varied without directly discharging process air from said flow path.

In the improved drying cycle a controlled volume of process air is bypassed from the discharge side to the inlet side of the blower in response to selected temperatures of the process air being discharged from the drying hopper. All bypassed process air is bypassed prior to the process air entering the desiccant means and the air heater so that the energy supplied by the air heater can be reduced. As is known, the air heater is energized in response to the inlet temperature of the drying hopper. Normally the inlet temperature of the process air to the drying hopper is maintained within a selected temperature range with the energy output of the heater being varied in response to the fluctuations of the drying hopper inlet temperature.In this invention the temperature characteristics of the process air as discharged from the drying hopper are sensed to control the flow of bypass process air such that when the hopper air discharge temperature increases to or exceeds a preselected value, the volume of process air supplied to the drying hopper is decreased by opening a modulating or variable volume flow valve in the bypass line and, when the hopper air discharge temperature decreases to a preselected value or a lower value, the flow of process air through the modulation valve is decreased or discontinued and the volume of process air to the drying hopper is increased.

Preferably the rate at which such discharge temperature increases is utilized to control the flow of process air.

Two embodiments of the invention will now be described, by way examples, with reference to the accompanying drawing in which: Figure 1 is a schematic representation of a drying cycle constructed in accordance with the principles of this invention; and Figure 2 is a partial schematic representation of the drying cycle as shown in Fig. 1 in which a heat exchanger is used.

The closed plastics material drying cycle as modified by this invention is well known and consists of a suitable process air blower 2 having a high pressure discharge line 4 connected to the inlet of a suitable desiccant means 6, a discharge line 8 from the desiccant means 6 to the inlet side of a suitable air heater 10, a discharge line 1 2 from the heater 10 to the inlet side of a suitable drying hopper 14, and a discharge line 1 6 from the drying hopper 1 4 to the inlet side of the blower 2.In such well known drying cycle a preselected quantity or body of plastics material 18 is retained in the hopper 14 and, after being heated and dried therein, portions of the material are sequentially discharged from the drying hopper 14 with the quantity of each discharge being dependent upon the material demand of a subsequent material processing operation such as the material requirements of a moulding machine 20 connected via a conduit 22 to receive the material discharged from the hopper 1 4. The drying hopper 14 is also sequentially resupplied with unheated plastics material in response to the availability of space within the hopper 14 via a hopper loading 24 and a supply conduit 26 in communication with a suitable source 28 of unheated plastics material as is well known.Hopper 14 is dimensioned to provide a preselected amount of heated or dried material over a unit of time as is required on a continuous basis by the subsequent processing machine 20, for example in pounds per hour. The incoming process air to the drying hopper 1 4 must have a sufficient heat content or drying capability to heat and dry the required pounds of plastics material to permit supplying of dried plastics material on a continuous basis for subsequent processing. As is known, the maximum or continuous supply of dried plastics material is not always required and consequently with a lower material demand the heat content of the incoming process air to the hopper 14 can be reduced.Heretofore, with electric resistance heater means 30 the temperature of the incoming heated air to the hopper 1 4 is maintained within a selected inlet temperature range. For such purposes a temperature sensor 32 is provided in line 1 2 immediately prior to the hopper 1 4 having an output signal directly proportional to the temperature of the process air as it enters the hopper 14. The output signal of the temperature sensor 32 activates a suitable control means 34 connected in the supply line 36 to the electrical heater means 30 to suitably control the electrical energization of the heater means 30. By varying the on-off time of the heater means 30 the heat content of the process air of the hopper 14 will also vary.

Heated process air enters the hopper 14 and travels or permeates through the body of plastics material 1 8 in heat exchange relationship so that the material 1 8 is heated to within a selected temperature range which is satisfactory for subsequent processing. By selecting a hopper 14 of suitable size with reference to the material to be dried, the condition of the material to be dried and the demand for dried material, the material being dried in the hopper 14 is retained in the hopper 14 for a sufficient period of time, known as the dwell time, to achieve the desired drying of such material. In transversing the hopper 14 the process air heats the material 18 while simultaneously absorbing moisture from the material 18.Such heat exchange causes the temperature of the process air in hopper 1 4 to be reduced relative to the inlet temperature of the process air to the hopper 14. The cooled, moisture carrying, process air is discharged from the hopper 14 via line 1 6 to the inlet side of the blower 2.

Blower 2 increases the pressure of the process air with the pressurized process air passing via line 4 to the desiccant means 6 wherein the moisture content of the process air is reduced to a satisfactory level to permit the process air to subsequently absorb moisture in the hopper 14. Process air flows from the desiccant means 6 via line 8 to the air heater 10 wherein the process air is heated as heretofore described. In such a cycle the entire volume of process air discharged from the hopper 14 is dehumifidified in the desiccant means 6 and heated in the air heater 10. In this regard ambient air will enter the hopper 14 during the periods when material 1 8 is being discharged from hopper 14 and the periods when material to be heated is being supplied to the hopper 14.Inasmuch as the cycle and the components thereof as heretofore described are well known, further description thereof is not necessary for an understanding of this invention to one skilled in the relevant art.

In the improved drying cycle of this invention a process air modulating valve 40 is connected via a line 42 to the lines 4 and 1 6 to selectively bypass process air around the blower 2 prior to the process air being dehumidified and heated as previously described. Inasmuch as the inlet of valve 40 is in flow communication with the high pressure side of blower 2 process air will only flow from line 4 to line 1 7 through valve 40 during the drying cycle process; that is, when the blower 2 is operating. A suitable temperature sensor 44 is connected to line 1 6 at a suitable location therein the sense the temperature of the process air as the process air is being discharged from the hopper 14. Sensor 44 has an output signal directly proportional to such discharge temperature. In the basic cycle of this invention the sensor 44 is connected to a suitable control 46 for controlling the electrical energization of a suitable actuator 48 so that the gate of the valve 40 is moved towards or into a fully open position when the discharge temperature of the process air from the hopper 14 is at or exceeds a preselected upper temperature and the gate of the valve 40 is moved towards or into a closed position when the discharge temperature of the process air from the hopper 14 is at or below a preselected lower temperature.

Thus, in instances when the valve 40 is closed and the discharge temperature of the process air from the hopper 14 in line 1 6 achieves or exceeds such preselected upper temperature the valve 40 is opened via the action of sensor 44, control 46 and actuator 48 to bypass a portion of the process air around the blower 2 via line 42 and a valve 40. Upon opening the valve 40 the nonbypassed portion of the process air flows through the desiccant means 6, heater 10 and hopper 14 at a reduced volume as comparted to the volume of process air flow therethrough when valve 40 is closed. With the reduced volume of process air flow through the air heater 10 the heat output of the heater means 30 will raise the temperature of the reduced volume of process air in the air heater 10 at a faster rate than occurs when all the process air flows through the air heater 10.Sensor 32 senses when the heated reduced volume of process air reaches or exceeds a preselected upper temperature and will via control 34 cause the heater means 30 to be de-energized. Thereafter the heater means 30 need only supply sufficient heat to the reduced volume of process air to maintain the reduced volume of process air supplied to the drying hopper 14 at a temperature as controlled by sensor 32. Sensor 32 senses when the heated reduced volume of process air reaches or falls below the lower limit of a preselected temperature range causing the heater means to be energized via control 34.

As long as the temperature of the process air in line 1 6 is below the preselected upper temperature at which sensor 44 actuates the control 46 the sensor 32 via control 34 controls the energization of heater means 30 to maintain the incoming process air to the dryer 14 within the desired temperature range. In the event the process air temperature in line 1 6 again reaches or exceeds the preselected temperature at which sensor 44 actuatates control 46, the output signal of sensor 44 will via control 46 actuate the actuator 48 to further open the valve 40 and increase flow of bypass process air. As the flow of bypass process air increases the flow of process air increases the flow of process air through the drying cycle decreases up to the point that valve 40 is in its fully open position.Valve 40 is sized to provide sufficient bypass process air flow as would be required in an operating drying cycle in which the valve 40 is intended to be used. Control 46 is provided with a control means (not shown) to energize an alarm in the event the valve 40 is in its full open position and the process air temperature in line 1 6 exceeds the temperature range through which the sensor 44 via control 46 controls actuator 48.

In the event the process air temperature in line 1 6 reaches or falls below the lower temperature of the temperature range desired in line 16, the output signal from the sensor 44, will, dependent upon the temperature in line 16, via control 46 move the valve 40 towards or into the closed position to reduce the bypass air flow and increase the process air flow through the drying cycle. As the process air flow through the drying cycle increases the sensor 32, via control 34, controls the energization of heater means 30 to maintain the temperature of the incoming process air to the hopper 14 within the desired range.In the event that valve 40 has been partially closed and the temperature in line 1 6 remains at below the lower temperature of the temperature range desired within line 16, the output signal from the sensor 44 will, via control 46, move the valve 40 to a more closed position to further reduce the bypass air flow. Thus, the drying cycle of the invention constantly monitors the temperature of the process air in line 1 6 to provide a heat balance between the heat required by the flow of material through the hopper 14 and the heat provided by the heater means 30. Thus, during periods of low heat demand for drying material 1 8 in hopper 14 the electrical energy consumption by the heater means 30 is reduced.Inasmuch as any suitable known modulating valve 40 and actuator 48 can be used for this invention further description thereof is not necessary for the understanding of this invention. Also the sensors and controls described herein can be of any suitable type to control the opening and closing of valve 40 as described.

With improved cycle heretofore described it will be realized that a heat balance is established in which the variable demand for heat by the material 1 8 is balanced with the heat content of a variable volume of process air.

Within the limits as set by the preselected actuating temperature signals to the control 46 the volume of process air is reduced for low (on a relative basis) heat demand periods and increased for high (on a relative basis) heat demand periods. Various factors such as the incoming material temperature, the moisture content of the material, the type of material to be dried and the through put requirements for the material will cause variations in the heat required to dry the material; thus, the temperature selected for opening and closing the valve 40 will vary and, accordingly, specific operating temperatures have not been set forther herein.Control 46 is preferably provided with logic means to detect the rate at which the discharge temperature of the process air is increasing so that the gate of the valve 40 can be opened to bypass a selected volume of process air prior to temperature of the process air being discharged from the hopper 14 actually reaching the temperature required for actuating the actuator 48 as heretofore described. It is also to be realized that the control 46 is selected to minimize movement of the valve 40 in the event of intermittent temperature spikes in line 1 6.

Inasmuch as any suitable control for actuating the actuator 48 as described can be employed, further description of such control is not necessary for the understanding of this invention.

Fig. 2 illustrates a modification of the drying cycle heretofore described in which a heat exchanger 55 as is known is inserted in line 1 6 prior to the connection of line 42 with line 16. Heat exchanger 55 is required in certain high temperature drying cycles in which the discharge temperature of the process air from the hopper 14 will be higher than the cycle previously described. Other than providing a control 46 to operate with higher temperature conditions, the operation of the cycle with the heat exchanger 55 is the same as the cycle previously described.

Claims (11)

1. A plastics material drying cycle in which a blower circulates process air through a closed flow path having at least a process air heater and a drying hopper sequentially located in such flow path on the discharge side of such blower and with the process air discharged from such drying hopper being returned to the inlet side of such blower, the volume of the process air flow through said air heater and said drying hopper being varied without directly discharging process air from said flow path.

2. A plastics material drying cycle as claimed in claim 1, in which said varying is controlled by the temperature of the process air discharged from said drying hopper.

3. A plastics material drying cycle as claimed in claim 1 or claim 2, in which said varying is by selectively bypassing process air directly around said blower.

4. A plastics material drying cycle as claimed in claim 3 wherein said varying is by bypassing a variable volume of process air.

5. A plastic material drying cycle as claimed in any preceding claim, in which the moisture content of the process air is reduced prior to flowing through said air heater.

6. A plastics material drying cycle in which a process air blower sequentially produces a process air flow through at least a process air heater and a drying hopper with the process air discharged from the drying hopper being returned to the inlet side of the air blower the improvement comprising, an actuable modulating valve for controlling the flow of air therethrough, means for connecting said valve to the discharge and inlet sides of said blower to provide a bypass flow path for process air around said blower, sensor means connected with respect to the process air being discharged from said drying hopper to produce output signals at preselected temperatures of said discharged process air and control means responsive to selective ones of said output signals to open and close said modulating valve.

7. A plastics material drying cycle as claimed in claim 6, in which said control means is responsive to the rate at which said discharge temperature varies.

8. A plastics material drying cycle as claimed in claim 6 or claim 7, in which the process air flows through a desiccant means prior to flowing through said air heater.

9. A plastics material drying cycle as claimed in any one of claims 6 to 8, in which means co-operate with said process air flow to control the heat output of said air heater.

10. A plastics material drying circle as claimed in any one of claims 6 to 9, in which means co-operate with said process air flow to reduce the heat output of said air heater when said modulating valve is at least partially open.

11. A plastics material drying cycle substantially as hereinbefore described with reference to and as illustrated in Fig. 1 or Fig. 2 of the accompanying drawings.