Comparison of two different cooling methods for extrusion

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This paper will compare the total power consumption of two different means of heating/cooling systems: air and water. For a single 90mm extruder, the total power consumption, output rate, and thermal …

This paper will compare the total power consumption of two different means of heating/cooling systems: air and water. For a single 90mm extruder, the total power consumption, output rate, and thermal control will be used to compare the two cooling means. Four different resins will be used.

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  • 1. COMPARISON OF TWO DIFFERENT COOLING METHODS FOR EXTRUSION PROCESSES Timothy W. Womer Walter S. Smith Richard P. Wheeler Xaloy Corporation, New Castle, PA Abstract This paper will compare the total power consumption of two different means of heating/cooling systems: air and water. For a single 90mm extruder, the total power consumption, output rate, and thermal control will be used to compare the two cooling means. Four different resins will be used. Introduction Heat can be added or removed from the extruder barrel with air or water cooling. Air cooling is ideal for processes that do not require high energy removal. It is less expensive for the hardware, easier to maintain, has lower operating costs, and requires less space compared to fluid cooling. Air cooling provides for slower changes in temperature compared to water cooling. Water cooling is best suited for processes that require high energy removal. Compared to air cooling, the equipment is more expensive, requires higher maintenance to prevent fouling, and requires more space and a water pump. Thermal instability can also occur if the cooling water flashes to steam. Large thermal gradients produced by water cooling can also contribute to excessive thermal strain and stress in the extruder. shown in Figure 2. A continuously running water pump is shown in Figure 3. This pump is a 1000 Watt. Figure 4 shows one zone of the air cooled setup. Each of the five zones contains a set of 3000 Watt heaters (6000 Watts per zone). Each heater is cast aluminum with cooling fins. The 205 Watt blower is to the right in Figure 4. Each blower is activated by the zone controllers. Each blower is rated at 7.5 cmm (265 cfm). Each zone is isolated by baffles. Figure 5 shows an overview of the air cooled system with all heaters, baffles and blowers installed. The top cover has been removed in Figure 5. The heated air exits in an air gap just under the top cover shown in Figure 6. Also shown in Figure 6 is the 711mm (28”) Flex-lip Sheet die and the Dynisco Screen Changer. The die was set to 2.5mm (.100”). The Screen Changer was loaded with a breaker plate and a 20/40/60/20 screen pack. A melt probe was inserted in the melt stream between the screen changer and die. A low shear barrier mixing screw was used for all testing. It was specifically designed for polypropylene with a long feed section. A Fluke Data Acquisition System was used to acquire data from the process. It will be referred to as NetDAQ. Resins Equipment The extruder used for this study was a 90mm (3.5”) x 24:1 NRM Extruder with five temperature zone controllers. It is equipped with a 112 kW (150 Hp) DC motor. Max screw speed is 129 rpm. Figure 1 shows the extruder. Four resins were used for this study. • ExxonMobil LDPE LD100BW, MFR of 2.0 g/10 min • Novachemicals Novapol HD-2007-H HDPE, MFR of 8.5 g/10 min • ExxonMobil PP 9852EI, MFR of 2.1 g/10 min • Eastar EB062 PETG, IV of .75 dl/g The water cooled system consisted of five zones. It is a closed loop system. Each zone has a set of 3000 Watt heaters (6000 Watts per zone). Cooling of each zone is controlled by a solenoid that opens and closes a valve. Heat is pulled from the system through a heat exchanger and discarded. The solenoids and heat exchanger are both Experimental Procedure Each of the four resins was extruded with water cooling and then with air cooling for a total of eight one-hour tests.
  • 2. For each test, the barrel and screw were completely cleaned. The die was pre-heated two hours prior to each one hour test, and the barrel was pre-heated for one hour before the testing started. Steady thermal conditions were then assumed to prevail throughout each hour long test. The four resins were run with the water-cooled system first. Once the water-cooled trials were completed the extruder was retrofitted for air-cooling. The same controllers used for water-cooling were used with the air cooling. Between switching of the systems the heater amperage and voltage were checked on each zone. For each one hour test, the extruder was started and set to a speed of 75 rpm. The thermocouple temperatures, the amount of time the heaters were on, motor amps, screw speed, melt probe temperature, and the amount of time the blowers ran (air cooling) were all monitored and recorded every .02 seconds a NetDAQ. Melt temperature was measured every ten minutes with an IR gun and a handheld melt probe. Output rates were measured and recorded every twenty minutes. The data were then extracted from the NetDAQ and compiled with a spreadsheet program. The amount of time the heaters and blowers were on was used in conjunction with the heater amperage and voltage to calculate the energy (kilowatt-hours) consumed by each heater and blower during the hour long test. The same was done for the drive motor energy. The energy added to the polymer was calculated from the difference between the polymer product melt temperature and the feed temperature. Presentation of Data and Results The water-cooled system used slightly more energy than the air-cooled system for all four polymers as shown in figure 7. There was little difference between the HDPE runs shown in Figure 8. Power consumption for the drive was almost equal. The main difference was power used between the cooling systems. The water cooled used about 22% more energy compared to the air cooled. The same patterns are seen with the other tests. Please reference Figures 9 and 10. LDPE tests had similar values between the systems with the water using 7% more energy. The PP runs had the lowest total power consumptions with comparable values. The air cooled used 20% less energy than the water cooled. Figure 11 shows the highest power required for all the runs. This came during the PET trials. The major difference was the power usage for heating/cooling. The water cooled used 80% more energy than the air cooled. Output rates were higher for the water-cooled system on 2 out of the 4 resins. Please see Figure 12. Temperature control varied according to resin. With respect to only the heating/cooling system LDPE had the highest power consumption for all resins mainly because of power needed in zone 3. This zone was cooler during the whole trial for both systems. Please see Figure 13. HDPE exhibited a similar pattern of a cooler zone 3 for both systems as well. Please see Figure 14. PP had no apparent differences between the two systems. This is confirmed in Figure 15. PET was the only resin that required extensive cooling in Zone 1during the trials. The air system couldn’t maintain the actual temperature to the set point. This is illustrated in Figure 16. Discussion of Data and Results One of the major differences between the water and air systems was the continuous running of the water pump. This consumed 1kWhr for all water cooled tests. Since water cooling is an abrupt mean of heat extraction energy is removed quickly and many times resulting in excess energy removal. So energy must be added back into the system to keep the barrel at temperature. Air cooling is more gradual and doesn’t over cool a barrel section as easily as water cooling. So unless extensive cooling is needed then water cooling can be avoided. Air cooling should be a sufficient system for most properly designed extruders. Water cooling would be useful when many different polymers are to be processed by the same extruder. With a given screw design, some polymers may require extensive cooling or heating to produce the desired product temperature. This may require the added heat capacity the water provides. But it is versatility at the cost of thermal stability and excessive energy consumption. This can be seen by the high energy consumption values for HDPE and LDPE. Zone 3 actual temperature values were low during the whole test. This zone required constant power for both resins and both cooling systems. A different properly designed screw would alleviate this problem. The screw was specifically designed for PP. More cooling was required to run the PET resin on Zone 1. The air cooling system could not control this zone. However the water cooling could control this zone, but naturally used more energy to do so. The PET output rates were 5% higher for the air cooled. The water lowered the temperature of the first zone which lowered the solids conveying to reduce the output. So, output rate can also be affected by the cooling means, especially as it affects solids conveying.
  • 3. Conclusions 1. Cooling of the extruder barrel should be minimized. Excessive cooling will require more motor power. 2. Heating of the extruder barrel should be minimized. Excessive heating will produce large thermal gradients in the melt and non-uniform product melt temperature distribution. 3. Air cooling is recommended for an extruder dedicated to a given product. However, the screw must be properly designed to not require excessive cooling or heating to maintain product temperature. 4. Water cooling finds uses when a given extruder is used to process multiple polymers and rates with the same screw. Water cooling can provide great energy transfer so that product temperature can be maintained in spite of a screw that is not optimized for a given polymer at a desired rate. Figure 1-90mm x 24:1 NRM Extruder with water cooled system Solenoids References 1. 2. 3. C. Rauwendaal, Polymer Extrusion, Hanser Publishers, NY, 1986 E. Steward; W. A. Kramer, Air vs. Water Cooled Single Screw Extruders, ANTEC 2003 J. Wortberg; T. Schroer, Novel Barrel Heating with Natural Gas, ANTEC 2003 Manifold Heat Exchanger Figure 2-Water cooled system Heat Exchanger and Solenoids
  • 4. Flow Meters Water Pump Figure 3-Water cooled system-Water Pump ` Figure 5-Air cooled system-Overview Baffle Air Gap Air Cooled Heater Blower Screen Changer Die Figure 4-Air cooled system-Single zone Figure 6-Die, Screen Changer and Air Gap
  • 5. Total Energy Consumed for Each System Comparison of Total Kilowatt-hours for Processing PP 100 87.75 45 86.54 41.87 39.64 40 80 70.49 69.33 70 60 52.2 50 Water Air 50.05 41.87 40 39.64 30 20 Power Consumption (KWh) Energy Consumed (KWh) 90 35 30.27 Total Drive Heat/Cooling 25 20 15 11.6 9.36 10 5 10 0 0 HDPE LDPE PP PET Water Resin Processed Figure 10-Power Consumption for PP for both systems Comparison of Total Kilowatt-hours for Processing HDPE Comparison of Total Kilowatt-hours for Processing PET 80 100 70.49 69.33 70 87.75 50 Total Drive Heat/Cooling 40 30 20 11.14 8.73 10 Power Consumption (KWh) 90 60.6 59.35 60 Air Heating/Cooling System Figure 7-Total Energy Consumption for the 8 tests Power Assumption (KWh) 30.28 30 Water 85.85 70 60 Total Drive Heat/Cooling 50 40 30 20 10 0 86.54 84.07 80 3.68 0.69 0 Air Water Heating/Cooling System Air Heating/Cooling System Figure 8-Power Consumption for HDPE for both systems Figure 11-Power Consumption for PET for both systems Throughput Rate for Each System Comparison of Total Kilowatt-hours for Processing LDPE 350 307 60 Total Drive Heat/Cooling 40 34.21 33.25 30 20 17.99 Throughput Rate (kg/hr) 50.05 50 Power Consumption (KWh) 292 300 52.2 250 200 166 164 169 Water kg Air kg 171 150 106 97 100 16.8 50 10 0 HDPE 0 Water Heating/Cooling System Air Figure 9-Power Consumption for LDPE for both systems LDPE PP PET Resin Type Figure 12-Output Rates for all 8 Tests
  • 6. Temperature Control for LDPE of both systems versus setpoint Temperature Control for PET of both systems versus setpoint 260 260 250 250 240 240 Temperature C Water Air Setpoint 220 200 200 190 190 180 180 Zone 1 Zone 2 Zone 3 Zone 4 Zone 5 Zone 1 Figure 13-Temperature Control of LDPE for both cooling systems 260 250 240 230 Water Air Setpoint 220 210 200 190 180 Zone 1 Zone 2 Zone 3 Zone 4 Zone 5 Figure 14-Temperature Control of HDPE for both cooling systems Temperature Control for PP of both systems versus setpoint 260 250 240 230 Water Air Setpoint 220 210 200 190 180 Zone 1 Zone 2 Zone 3 Zone 4 Zone 5 Figure 15-Temperature Control of PP for both cooling systems Zone 2 Zone 3 Zone 4 Zone 5 Figure 16-Temperature Control of PET for both cooling systems Temperature Control for HDPE of both systems versus setpoint Temperature C Water Air Setpoint 220 210 210 Temperature C Temperature C 230 230