countercurrent cooling of blown film

Countercurrent Cooling of Blown Film

K. F. STRATER*

and J. M. DEALY

Department of Chemical Engineering McGill University

Montreal. Canada H3A 2A7

Presently used methods for the external cooling of blown film involve the use of an air ring located at the base of the bubble that blows air upward along the surface of the bubble. The air is heated as it rises, while the film is cooling and moving in the same direction. This is an example of cocurrent heat exchange, and the result is the accumulation of heated air around the upper portion of the bubble, which interferes with the cooling in this region. While rapid initial cooling is required to maintain bubble stability, we have explored the possibility of using countercurrent cooling for the upper region of the bubble. A standard air ring is mounted at the base of the bubble, and a circular shroud surrounds the bubble above this air ring. All the heated air is collected in an upper chamber surrounding the shroud and is exhausted by means of a secondary blower. The proposed scheme was found to increase heat transfer in the upper regions of the bubble and to permit the ducting of all heated air away from the film line.

INTRODUCTION n the film blowing process, the extrudate is- I sues, usually upwards, from an annular die

to produce a tube of molten plastic. This tube is closed at the top by the nip rolls, which also stretch the tube in the axial (machine) direction. The air pressure inside the bubble is main- tained at a level somewhat above the ambient pressure so that the tube is subjected to a hoop stress and bulges out at some point above the die to form a “bubble.” This phenomenon also produces stretching in the circumferential (transverse) direction.

To convert the tube of molten plastic to a film that can be wound onto a roll, it must be cooled from the die exit temperature to a level substan- tially below its melting point. This is usually done by blowing air at the external surface of the bubble by means of an air ring, which is an annular nozzle that directs air at the bubble around its circumference at a level just above the die.

The rate of the cooling process is the limiting factor in film production. If it is desired to pro-

* Present Address: Union Camp Corporation. Princeton. NJ 08540.

1380

duce a film of a given thickness and layflat width from a given resin, one can increase the production rate by increasing the extruder output only up to a certain point, beyond which it is not possible to apply sufficient cooling to permit a further increase.

Aerodynamics of the Film Cooling Process

The phenomenon that limits the amount of air that can be used to cool the bubble is its tendency to destabilize the bubble. The bubble is supported from above by the nip rolls, and its lateral position is stabilized to some extent by the collapsing frame and also, often, by a bubble sizing and stabilizing cage or by irises. How- ever, at its base, the bubble is held in place only by the tube of molten resin being extruded from the die. This allows the bubble to move and deform in response to rather moderate external forces. In particular, the aerodynamic forces arising from the interaction between the cooling air flow and the bubble surface can produce bubble motion and deformation. As the air flow rate increases, these forces increase, ultimately producing unacceptable nonuniformity in the film thickness and properties or even rupture

POLYMER ENGINEERING AND SCIENCE, MID-OCTOBER, 1987, Vol. 27, NO. 18


of the bubble. More recently, there has been some success in controlling these aerodynamic forces and even using them to regulate the shape of the bubble.

The general nature of aerodynamic forces can be understood by use of Bernoulli’s equation, which can be written as follows:

u2/2 + g z + P / p = constant. ( 1 ) This equation is rigorously valid only for the flow of an inviscid, incompressible fluid along a streamline. However, because of the low vis- cosity of air and the relatively small pressure changes occurring in cooling air streams, it is a good approximation to the actual case, except very ne r the film surface. I t does not account

bubble and provides a high level of disturbances that will exacerbate any tendency toward insta- bility.

E q u a t i o n 1 can be written in another form that involves changes in quantities:

for tur t ulence, which causes buffeting of the

Av2/2 + g A z + AP/p = 0. ( 2 ) Furthermore, in the case of cooling air flow, the gravity force is much smaller than the other terms so we can write:

Av2/2 + AP/p = 0. (3) Thus, an increase in velocity must be associated with a decrease in pressure and vice versa. Consider, for example, stagnation flow where the air flows toward a surface as shown in Fig. 1 . The velocity at the stagnation point is zero, and if the velocity and pressure far from the surface are u1 and P I , the pressure P2 at the stagnation point is related to these quantities by

A u ? / ~ + (Pi - Pz) /p = 0. (4)

Thus, the stagnation pressure, P2. is higher than PI and is given by

P2 = P1 + pu?/2.

7 “I ‘ P I

Another flow situation that is relevant to blown film cooling is venturi flow, in which the air flows through a contraction that causes an increase in velocity, as shown in Fig. 2. To infer something about the pressure, Pz . on the surface, we need to note that the pressure gradient normal to the streamlines is observed to be very small. Thus, we can use E q 2 to show that

P2 = PI - p(v$ - u 3 / 2 . (6) Since u2 > u l , we conclude that P2 < P I . I t is of interest in the present context to note that for a given average air velocity, heat transfer rates from the solid surface are much higher for stagnation flow than for parallel flow.

Air Ring Design Because of the importance of the cooling pro-

cess in film blowing and the complex interaction between the cooling air and the bubble, considerable attention has been given to cooling system improvement. In early air rings, the air impinged directly on the bubble surface generating a stagnation flow. This gave a high local heat transfer rate, but it also generated high positive pressure and severe buffeting. To avoid the destabilizing effects of this flow pattern, air flow rates were limited to rather low values. This led to the use of a deflector that diverted the air so that a “fan spray” flow pattern was produced. This permitted an increase in the air flow, but the flow passages were not streamlined and large scale turbulence was generated.

Although this type of air ring was widely used for several decades, efforts continued to develop improved units. Corbett ( 1 ) proposed the use of the venturi effect to create a smoother air flow and avoid stagnation flow forces. This improved bubble stability, but now the generation of a low pressure region tended to draw the melt out

Fig. 1 . Sketch showing stagnationflow.

“I ’ PI Fig. 2. Sketch showing venturiflow.

1381 POLYMER ENGINEERING AND SCIENCE, MID-OCTOBER, 1987, V d . 27, NO. 18

K. F. Strater and J . M. Dealy

toward the air ring, and this effect limited the air flow rate. Multiple, stacked air rings (2-6) and, more recently, the “dual lip air ring” (7-9) have been proposed to solve the problems of generating streamlined flow without generating large suction near the base of the bubble. The latter have been particularly effective for the processing of linear low-density polyethylenes, which are more sensitive to destabilizing forces than high pressure resins.

Cocurrent Vs. Countercurrent Cooling All of the cooling systems mentioned above

were designed to maximize the rate of cooling at the base of the bubble without destabilizing the bubble. All of them involve the upward flow of air, with all or most of the air being introduced near the bottom of the bubble. Thus, both the surface being cooled and the air are moving upward, and this is an example of cocurrent flow heat transfer. It is a situation that is gen- erally avoided in the design of industrial heat exchangers, because it does not give the maximum overall heat transfer rate for a given surface area and given inlet temperatures. In the case of the film-blowing process, it has the effect of allowing heated air to flow past the upper portion of the bubble at a rather low velocity. Thus, the cooling rate for this portion of the bubble is quite low, and high towers are necessary to permit sufficient cooling and proper collapsing of the film bubble. Collapsing a cylindrical bubble into a flat web should occur at a temperature low enough to prevent the film from blocking with itself and also to prevent the film in contact with the collapsing frame from stretching and thinning. These undesirable occurrences may lead to rolls that are un- windable or have nonuniform hardness and are thus unsatisfactory for further processing.

The use of countercurrent cooling would elim- inate these problems. The air would be introduced at the top of a shroud surrounding the bubble and withdrawn at the bottom. However, while increasing the overall heat transfer rate, countercurrent cooling would lead to a much reduced local heat flow near the base of the bubble. This in turn would lead to a higher frost line and to a less stable bubble. Thus, the ex- clusive use of countercurrent cooling does not appear to be feasible. However, cocurrent cooling at the base of the bubble might be combined with countercurrent cooling of the upper portion of the bubble to obtain the advantages of both schemes.

Proposed Combination Cooling System Our objective was to study the feasibility of

combined cocurrent and countercurrent cooling for the manufacture of blown film. Our wish to select the simplest configuration that would permit the evaluation of the concept led to a design in which a cylindrical shroud is mounted above a standard air ring. A toroidal chamber

surrounds the shroud near its top and commu- nicates with the shroud by means of a series of large holes. Air enters the space between the film and the top of the shroud under the influence of suction created by withdrawing air from the upper chamber. All of the air introduced by the air ring is also removed through this chamber. Thus, all the air used to cool the bubble is collected and prevented from accumulating in the neighborhood of the film line. In the sum- mer, this heated air can be ducted directly out- doors, while in the winter it can be used for space heating in the periphery of the plant building. Thus, there is also an energy conser- vation aspect of the proposed technology.

EXPERIMENTAL EQUIPMENT AND PROCEDURES

Resin The resin used in the experiments was a high-

pressure, branched film resin, DFDY 3312, manufactured by Union Carbide Canada Ltd. It contained antioxidant but no other additives. The melt index was 1.7 g/lO min, the melt flow ratio was 65, and the resin density was 923.5 kg/m3.

Extrusion Equipment and Cooling System A 1 -inch Wilmod extruder with a 24: 1 length-

to-diameter ratio was used to produce melt, which then passed through a screen pack and a 2.5-inch spider die. The extruder operating conditions are given in Table 1. All the experimental apparatus is described in detail else- where (10).

The cooling system is shown in Fig. 3. The single-lip, fan-spray air ring had a chamber that extended below rather than above the lip so that more of the bubble was exposed to view than with the standard Configuration (1 1). Three conduits carried cooling air from the primary blower to the air ring. For purposes of comparison, film was first produced using only the air ring so that only cocurrent cooling was involved (cases 1 and 8).

To make possible countercurrent cooling of the upper portion of the bubble, a shroud having an inside diameter of 26.4 cm and a length of 61 .O cm was mounted on the upper plate of the air ring. A toroidal air collecting chamber surrounded the shroud at a level 40.6 to 54.6 cm above the air ring. Air flowed from the shroud into this upper chamber by means of a series of holes in the shroud. Two ducts carried air from

Table 1. Extruder Operating Conditions.

Screw speed 50 rpm Production rate Extruder back pressure Melt temperature 205°C Blowup ratio Film gauge Nip roll speed

16.8 Ibm/h (7.7 kg/h) 3100 psi (12 MPa)

2.25 1.6 mils (0.025 mm) 25 ft/min (2.6 m/min)

m = mass

1382 POLYMER ENGINEERING AND SCIENCE, MID-OCTOBER, 1987, Vol. 27, NO. 18


COUNTERCURRENT COUNTERCURRENT AIR FROM AIR FROM ATMOSPHERE ATMOSPHERE

PLENUM

EXPELLED

SHROUD

AIR

COCURRENT

ATMOSPHERE

COCURRENT DIE

AIR FROM AIR FROM AT M 0s P H E RE Fig. 3. Sketch of cooling s y s t e m used in experimental s tudy .

the upper chamber to the secondary blower. The shroud was designed to extend some distance above the frost line and to accommodate blowup ratios to 3: 1 .

Five circular plates were mounted horizon- tally inside the shroud at several levels above the die. Each plate had a hole cut in its center with a diameter estimated to be about 1 inch greater than the bubble diameter at that level. These plates were intended to act as baffles, directing the cooling air flow near to the bubble surface.

The cooling air flow rates were determined by means of Pitot tube traverses at the outlets of the blowers. The air temperature was 27°C. and the flow rates used are given in Table 2. Two cocurrent and two countercurrent air flow rates were used. The greater of the two cocurrent air flow rates represents the maximum cocurrent cooling that was possible without destabilizing the bubble. The maximum countercurrent air flow rate was limited by blower capacity. The countercurrent flow rate was determined by subtracting the cocurrent flow rate produced by the primary blower from the flow rate in the secondary blower used to withdraw air from the upper chamber.

Measurement of Surface Temperature An Ircon SC34 infrared thermometer was

used to measure the surface temperature of the bubble during film production. This device is sensitive to the 3.4-pm spectral band, and in this region, LDPE has zero transmission. The reflectance was taken to be 0.04 and independ- ent of thickness, yielding an emissivity of 0.96.

To make possible the measurement of surface temperatures while the shroud was in position, a series of holes were cut in the shroud. These holes were taped over except when a temperature measurement was being made. No meas-

urements could be made within several inches of the die or in the area surrounded by the upper chamber.

Air Velocity Measurement Local air velocities in the vicinity of the bub-

ble were measured using a Disa constant temperature hot wire anemometer. The output was linearized on the basis of a calibration curve, and the sensor incorporated a thermistor for temperature compensation. Horizontal traverses were made at various levels. When the shroud was present, the anemometer probe was inserted through the holes mentioned above.

RESULTS To begin production, it was necessary to start

with only cocurrent air flow and establish the bubble in place before starting the second blower. All the bubbles were quite stable with the exception of that for case 11. In this case, the bubble expanded and contracted periodi- cally, suggesting the presence of a resonance phenomenon. This is reflected in large standard deviations in both layflat width and film thickness, as shown in Table 3.

When the countercurrent air flow rate was increased, however, (case 13) the oscillations disappeared and were replaced by a mild flut- tering that did not appear to affect bubble stability or film properties.

Surface Temperatures Figure 4 shows surface temperature as a

function of height above the die for cases 1 and 8, in which no countercurrent cooling was used and the shroud was not mounted above the air ring. The relationship between cooling rate and temperature distribution is obvious. Case 8, which had the highest air flow rate, also had

Table 2. Cooling Air Flow Rates.

Cocurrent Flow Rate Countercurrent Flow (m3/min) (m3/min) Case

1 2.92 0 (no shroud) 4 2.92 4.59 6 2.92 7.59 8 3.48 0 (no shroud)

1 1 3.48 4.59 13 3.48 7.59

Table 3. Film Dimensions.

Layflat Width Film Thickness

Case mean value standard mean value standard (cm) deviation (gm) deviation

1 22.9 0.020 39.4 0.032 4 22.8 0.041 39.6 0.041 6 22.5 0.047 39.9 0.042 8 22.5 0.035 40.1 0.055

1 1 22.4 0.21 40.4 0.33 13 22.1 0.040 40.9 0.062

1383 POLYMER ENGlNEERING AND SCIENCE, MID-OCTOBER, 1987, VoI. 27, No. 18

K . F. Strater and J . M . Dealy

I 1

i- a 150 a a 5 125

u

i-

I00

0 20 40 60 DISTANCE FROM DIE (cm)

Fig. 4. Bubble surface temperature us. distance above die, cases 1 and 8.

the most rapid temperature decrease. The frost line region is associated with the plateau, where crystallization is occurring. This is consistent with direct observation, from which the frost line height was estimated as 30 cm.

The influence of countercurrent air flow on the surface temperature is shown in Figs . 5 and 6. Data for the lower cocurrent flow rate and both countercurrent flow rates (cases 4 and 6) are shown in Fig. 5, along with data for cocurrent flow only (case 1) . For the lower of the two countercurrent flow rates (case 4), the introduction of the countercurrent flow actually reduces the cooling rate above the frost line compared with case 1 , while having little effect on the cooling rate below the frost line. The reason for this will become clear when the air velocity distributions are presented; the design of the shroud is far from optimal from the point of view of forced convection heat transfer. How- ever, at the higher countercurrent flow rate (case 6). there is a clear increase in the heat transfer rate above the frost line. Here, the high countercurrent flow rate more than compen- sates for the disadvantageous flow pattern.

This same phenomenon is also apparent in the case of experiments at the higher cocurrent flow rate, as shown in Fig. 6. For the lower countercurrent air flow (case 1 1 ) . the heat transfer above the frost line is not as good as for the base case (case 8), while for the higher countercurrent flow (case 1 3 ) the heat transfer is clearly improved.

Velocity Distributions The velocity distributions in the cooling air

flowing near the bubble are shown in Figs . 7 through 9. Velocities at levels below the upper chamber for cases 4, 6, 1 1 , and 1 3 were only slightly higher than in the corresponding base cases and are not shown. Upward velocity profiles for the base cases (1 and 8) are shown in

x C A S E I CASE 4

0 C A S E 6

0 20 40 60 80

Fig. 5. Bubble surface temperature us. distance above die, cases 1, 4, and 6.

DISTANCE ABOVE DIE (cm)

u 175 Y

x C A S E 8 A CASE I I

C A S E 13

0 20 4 0 60 80

Fig. 6. Bubble surface temperature us. distance above die, cases 8. 1 1, and 13.

DISTANCE ABOVE DIE (cm)

Figs. 7 and 8. These velocity distributions are of the general type associated with wall jets. It was not possible to make measurements within 5 mm of the surface, but the velocity at the film surface should be that of the film, about 0 . 1 3 m/s and in the upward direction.

Figure 9 shows velocity profiles measured at the top of the upper chamber for cases 4 , 6 , 1 1 , and 1 3 . The cocurrent air flow rate has no effect on the velocity distribution at this level for a given countercurrent flow rate. For cases 4 and 1 1 , the most important feature of these distributions is that the wall jet induced by the suction through the upper chamber has formed along the wall of the shroud rather than along the film surface. From the point of view of maximizing the heat flux at the film surface, this is a highly undesirable situation and ex- plains why the resulting temperatures are higher in this case than for the corresponding base cases. In retrospect, it is clear that the top of the shroud should have been designed to direct the downward induced air flow close to

1384 POLYMER ENGINEERING AND SCIENCE, MID-OCTOBER, 1987, Vol. 27, No. 78 ~~


h

E - 8 - u) \

10

- 8

E z = 19-6 cm > 6 c- * z = l0.g cm

0 z = 5-3 crn V 0

>

u) \

Y

i 4

CL 4 2

0 5 10 15 R A D I A L DISTANCE F R O M FILM BUBBLE (cm)

Fig. 7 . Air Velocity us. radial distance from film surface at various distances above the d ie for case 1 [z = 5.3, 10.9. 19.6. 30. 50.5 cml.

CASE 4 A CASE 6 0 CASE II

z.63-2cm 2.50.5 cm

o z = 36.3 crn . E

I Y

A zt19 .6crn

* z = 10.9 cm

0 5 10 15 RADIAL DISTANCE FROM FILM BUBBLE (cm)

Fig. 8. Air velocity us. radial distance from f i lm surface at various distances above the die for case 8 [z = 5.3. 10.9. 19.6, 30, 50.5 cm).

the film surface. For cases 6 and 13, the countercurrent flow rate is sufficient to enhance the heat transfer despite the unfavorable velocity distribution.

Film Properties A number of tests were carried out on the

finished film in each case. These included un- restrained shrinkage at 107OC, Elmendorf tear, birefringence, and sonic velocity. The results indicated that the countercurrent air flow rate had little effect on film properties for a given cocurrent flow rate.

CONCLUSIONS The results indicate that it is both feasible

and advantageous to apply cooling air in a countercurrent fashion to the bubble at levels near and above the frost line. Cooling rates are thus increased significantly in this region. This pre- vents the accumulation of hot air around the upper portion of the bubble and makes possible

4

I I 1 1 I I

0 I 2 3 4 5 6

Fig. 9. Downward air velocity us. radial distance from film surface at top of shroud [z = 58.7 cm) for cases 4. 6, 11, and 13.

RADIAL DISTANCE FILM SURFACE (cm)

the use of a shorter tower. In addition, building temperature control would be improved, and in the winter this would result in some energy savings. However, the use of combination cooling, as proposed here, requires the careful design of a shroud to surround the bubble from the die to a level somewhat above the frost line. I t must be possible to raise the shroud to permit the start-up of the film line, and the flow pas- sage for the countercurrent air must be opti- mized to provide velocity profiles consistent with high heat transfer rates.

ACKNOWLEDGMENTS This research was supported by grants from

the Natural Sciences and Engineering Research Council of Canada and Imperial Oil Ltd. Resins were supplied by Union Carbide Canada Ltd., and the experiments were carried out at that company's Montreal Technical Center. Mr. 2. Bakerdjian of Union Carbide provided useful technical advice.

REFERENCES 1. H. 0. Corbett, U.S. Patent 3,167.814 (1965). 2. D. R. Saint Eve and Ajit Kumar Bose, U.S. Patent

3,888,609 (1975). 3. F. J. Herrington, U.S. Patent 3,959,425 (1976). 4. F. J. Herrington, U.S. Patent 4,022,558 (1977). 5. F. J. Herrington, U.S. Patent 4.1 18,453 (1978). 6. D. N. Jones and S. J. Kurtz, U.S. Patent 4,330,501

(1982). 7. D. R. Hinrichs, U.S. Patent 3,548,042 (1970). 8. K. Masuda, K. Hasegawa and A. Okakmoto, U.S. Patent

3,568,252 (1971). 9. R. J. Cole, U.S. Patent 4,139,338 (1979).

McGill University (1 985).

(1974).

10. K. F. Strater. M. Eng. Thesis, Chemical Engineering,

11. R. Farber and J. M. Dealy. Polym. Eng. Sci., 14, 435

1385 POLYMER ENGINEERING AND SCIENCE, MID-OCTOBER, 7987, VOI. 27, NO. 18

countercurrent cooling of blown film

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