COMPARISON OF TWO DIFFERENT COOLING METHODS FOR EXTRUSIONPROCESSES

Timothy W. WomerWalter S. Smith

Richard P. WheelerXaloy Corporation, New Castle, PA

Abstract

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

Introduction

Heat can be added or removed from the extruder barrelwith air or water cooling. Air cooling is ideal forprocesses that do not require high energy removal. It isless expensive for the hardware, easier to maintain, haslower operating costs, and requires less space comparedto fluid cooling. Air cooling provides for slower changesin temperature compared to water cooling.

Water cooling is best suited for processes that requirehigh energy removal. Compared to air cooling, theequipment is more expensive, requires highermaintenance to prevent fouling, and requires more spaceand a water pump. Thermal instability can also occur ifthe cooling water flashes to steam. Large thermalgradients produced by water cooling can also contributeto excessive thermal strain and stress in the extruder.

Equipment

The extruder used for this study was a 90mm (3.5”) x24:1 NRM Extruder with five temperature zonecontrollers. It is equipped with a 112 kW (150 Hp) DCmotor. Max screw speed is 129 rpm. Figure 1 shows theextruder.

The water cooled system consisted of five zones. It is aclosed loop system. Each zone has a set of 3000 Wattheaters (6000 Watts per zone). Cooling of each zone iscontrolled by a solenoid that opens and closes a valve.Heat is pulled from the system through a heat exchangerand discarded. The solenoids and heat exchanger are both

shown in Figure 2. A continuously running water pump isshown in Figure 3. This pump is a 1000 Watt.Figure 4 shows one zone of the air cooled setup. Each ofthe five zones contains a set of 3000 Watt heaters (6000Watts per zone). Each heater is cast aluminum with coolingfins. The 205 Watt blower is to the right in Figure 4. Eachblower is activated by the zone controllers. Each blower israted at 7.5 cmm (265 cfm). Each zone is isolated bybaffles. Figure 5 shows an overview of the air cooledsystem with all heaters, baffles and blowers installed. Thetop cover has been removed in Figure 5. The heated airexits in an air gap just under the top cover shown in Figure6.

Also shown in Figure 6 is the 711mm (28”) Flex-lip Sheetdie and the Dynisco Screen Changer. The die was set to2.5mm (.100”). The Screen Changer was loaded with abreaker plate and a 20/40/60/20 screen pack. A melt probewas inserted in the melt stream between the screen changerand die.

A low shear barrier mixing screw was used for all testing. Itwas specifically designed for polypropylene with a longfeed section.

A Fluke Data Acquisition System was used to acquire datafrom the process. It will be referred to as NetDAQ.

Resins

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

Experimental Procedure

Each of the four resins was extruded with water cooling andthen with air cooling for a total of eight one-hour tests.

For each test, the barrel and screw were completelycleaned. The die was pre-heated two hours prior to eachone hour test, and the barrel was pre-heated for one hourbefore the testing started. Steady thermal conditions werethen assumed to prevail throughout each hour long test.

The four resins were run with the water-cooled systemfirst. Once the water-cooled trials were completed theextruder was retrofitted for air-cooling. The samecontrollers used for water-cooling were used with the aircooling. Between switching of the systems the heateramperage and voltage were checked on each zone.

For each one hour test, the extruder was started and set toa speed of 75 rpm. The thermocouple temperatures, theamount of time the heaters were on, motor amps, screwspeed, melt probe temperature, and the amount of time theblowers ran (air cooling) were all monitored and recordedevery .02 seconds a NetDAQ. Melt temperature wasmeasured every ten minutes with an IR gun and a hand-held melt probe. Output rates were measured andrecorded every twenty minutes.

The data were then extracted from the NetDAQ andcompiled with a spreadsheet program. The amount oftime the heaters and blowers were on was used inconjunction with the heater amperage and voltage tocalculate the energy (kilowatt-hours) consumed by eachheater and blower during the hour long test. The samewas done for the drive motor energy. The energy addedto the polymer was calculated from the differencebetween the polymer product melt temperature and thefeed temperature.

Presentation of Data and Results

The water-cooled system used slightly more energy thanthe air-cooled system for all four polymers as shown infigure 7.

There was little difference between the HDPE runs shownin Figure 8. Power consumption for the drive was almostequal. The main difference was power used between thecooling systems. The water cooled used about 22% moreenergy compared to the air cooled.

The same patterns are seen with the other tests. Pleasereference Figures 9 and 10. LDPE tests had similarvalues between the systems with the water using 7% moreenergy. The PP runs had the lowest total powerconsumptions with comparable values. The air cooledused 20% less energy than the water cooled.

Figure 11 shows the highest power required for all theruns. This came during the PET trials. The majordifference was the power usage for heating/cooling. Thewater cooled used 80% more energy than the air cooled.

Output rates were higher for the water-cooled system on 2out of the 4 resins. Please see Figure 12.

Temperature control varied according to resin. With respectto only the heating/cooling system LDPE had the highestpower consumption for all resins mainly because of powerneeded in zone 3. This zone was cooler during the wholetrial for both systems. Please see Figure 13. HDPEexhibited a similar pattern of a cooler zone 3 for bothsystems as well. Please see Figure 14. PP had no apparentdifferences between the two systems. This is confirmed inFigure 15. PET was the only resin that required extensivecooling in Zone 1during the trials. The air system couldn’tmaintain the actual temperature to the set point. This isillustrated in Figure 16.

Discussion of Data and Results

One of the major differences between the water and airsystems was the continuous running of the water pump.This consumed 1kWhr for all water cooled tests. Sincewater cooling is an abrupt mean of heat extraction energy isremoved quickly and many times resulting in excess energyremoval. So energy must be added back into the system tokeep the barrel at temperature. Air cooling is more gradualand doesn’t over cool a barrel section as easily as watercooling. So unless extensive cooling is needed then watercooling can be avoided. Air cooling should be a sufficientsystem for most properly designed extruders.

Water cooling would be useful when many differentpolymers are to be processed by the same extruder. With agiven screw design, some polymers may require extensivecooling or heating to produce the desired producttemperature. This may require the added heat capacity thewater provides. But it is versatility at the cost of thermalstability and excessive energy consumption.

This can be seen by the high energy consumption values forHDPE and LDPE. Zone 3 actual temperature values werelow during the whole test. This zone required constantpower for both resins and both cooling systems. A differentproperly designed screw would alleviate this problem. Thescrew 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. Howeverthe water cooling could control this zone, but naturally usedmore energy to do so. The PET output rates were 5% higherfor the air cooled. The water lowered the temperature of thefirst zone which lowered the solids conveying to reduce theoutput. So, output rate can also be affected by the coolingmeans, especially as it affects solids conveying.

Conclusions

1. Cooling of the extruder barrel should beminimized. Excessive cooling will require moremotor power.

2. Heating of the extruder barrel should beminimized. Excessive heating will produce largethermal gradients in the melt and non-uniformproduct melt temperature distribution.

3. Air cooling is recommended for an extruderdedicated to a given product. However, thescrew must be properly designed to not requireexcessive cooling or heating to maintain producttemperature.

4. Water cooling finds uses when a given extruderis used to process multiple polymers and rateswith the same screw. Water cooling can providegreat energy transfer so that product temperaturecan be maintained in spite of a screw that is notoptimized for a given polymer at a desired rate.

References

1. C. Rauwendaal, Polymer Extrusion, HanserPublishers, NY, 1986

2. E. Steward; W. A. Kramer, Air vs. Water CooledSingle Screw Extruders, ANTEC 2003

3. J. Wortberg; T. Schroer, Novel Barrel Heatingwith Natural Gas, ANTEC 2003

Figure 2-Water cooled systemHeat Exchanger and Solenoids

Figure 1-90mm x 24:1 NRM Extruderwith water cooled system

Heat Exchanger

Manifold

Solenoids

`Figure 3-Water cooled system-WaterPump

Figure 4-Air cooled system-Single zone

Figure 5-Air cooled system-Overview

Water Pump

Flow Meters

Air Cooled HeaterBlower

Baffle

Die

Screen ChangerAir Gap

Figure 6-Die, Screen Changer and Air Gap

Total Energy Consumed for Each System

70.49

52.2

41.87

87.75

69.33

50.05

39.64

86.54

0

10

20

30

40

50

60

70

80

90

100

HDPE LDPE PP PET

Resin Processed

Ene

rgy

Con

sum

ed(K

Wh

)

WaterAir

Comparison of Total Kilowatt-hoursfor Processing HDPE

70.49 69.33

59.35 60.6

11.148.73

0

10

20

30

40

50

60

70

80

Water Air

Heating/Cooling System

Po

wer

Ass

umpt

ion

(KW

h)

TotalDriveHeat/Cooling

52.250.05

34.21 33.25

17.9916.8

0

10

20

30

40

50

60

Water AirHeating/Cooling System

Po

wer

Co

nsu

mp

tio

n(K

Wh

) TotalDriveHeat/Cooling

Comparison of Total Kilowatt-hoursfor Processing LDPE

Comparison of Total Kilowatt-hoursfor Processing PP

41.8739.64

30.27 30.28

11.69.36

0

5

10

15

20

25

30

35

40

45

Water Air

Heating/Cooling System

Po

wer

Co

nsu

mpt

ion

(KW

h)

TotalDriveHeat/Cooling

Comparison of Total Kilowatt-hoursfor Processing PET

87.75 86.5484.07 85.85

3.680.69

0

10

20

30

40

50

60

70

80

90

100

Water Air

Heating/Cooling System

Po

wer

Co

nsu

mpt

ion

(KW

h)

TotalDriveHeat/Cooling

Throughput Rate for Each System

166 169

106

292

164171

97

307

0

50

100

150

200

250

300

350

HDPE LDPE PP PET

Resin Type

Thr

oug

hpu

tRat

e(k

g/h

r)

Water kgAir kg

Figure 7-Total EnergyConsumption for the 8 tests

Figure 8-Power Consumption for HDPEfor both systems

Figure 9-Power Consumption for LDPEfor both systems

Figure 10-Power Consumption for PP forboth systems

Figure 11-Power Consumption for PETfor both systems

Figure 12-Output Rates for all 8 Tests

Temperature Control for LDPE of both systems versussetpoint

180

190

200

210

220

230

240

250

260

Zone 1 Zone 2 Zone 3 Zone 4 Zone 5

Tem

per

atu

reC

Water

Air

Setpoint

Temperature Control for HDPE of both systems versussetpoint

180

190

200

210

220

230

240

250

260

Zone 1 Zone 2 Zone 3 Zone 4 Zone 5

Tem

per

atu

reC

Water

Air

Setpoint

Temperature Control for PP of both systems versus setpoint

180

190

200

210

220

230

240

250

260

Zone 1 Zone 2 Zone 3 Zone 4 Zone 5

Tem

per

atu

reC

Water

Air

Setpoint

Temperature Control for PET of both systems versus setpoint

180

190

200

210

220

230

240

250

260

Zone 1 Zone 2 Zone 3 Zone 4 Zone 5

Tem

per

atu

reC

Water

Air

Setpoint

Figure 13-Temperature Control of LDPEfor both cooling systems

Figure 14-Temperature Control of HDPEfor both cooling systems

Figure 15-Temperature Control of PP forboth cooling systems

Figure 16-Temperature Control of PETfor both cooling systems