Proper selection of chipper design and operating parameters can improve both woodyard and kraft pulp mill performance

Improved Chipper Design Yields Better Chips For Chemical Pulping

The increasing cost of wood has further increased interest in chip quality at its production source. High processing losses in chip screening and pulping inefficiency associated with fines and pin chips have been quantified.11 A chipper designed to produce fewer fines and pin chips would provide considerable savings, both in raw material screening retention of good fiber, and by reducing black liquor solids loading rates as a by-product of improved kraft cooking.² A chipper which produced fewer overthick chips, while achieving a more consistent thickness-to-length ratio, would significantly improve pulping uniformity and pulp consistency, even in mechanical pulps.

Chipper parameters critical to the production of quality chips include spout angle, knife edge angle, Lambda angle, knife speed, and chip length setting (Figure 1).

Figure 1: Chips form at the compression face of the advancing knife.

The uniformity of chip size and the percentages of each chip size fraction produced by a chiper are keys to pulping performance. Pulp production, pulp strength, and pulp quality are directly tied to chip size and chip quality.

How does a chipper make chips? Chips are formed during chipping due to a compression force of the leading knife face against the wood (Figure 1).⁹ When the chipper knife enters the log, compression stress occurs against the cut log end. This compression force, and the wood's resistance to longitudinal splitting into individual chips, determines the chip thickness to chip length ratio produced, as well as pin chip and fines generation.⁷

The compression force is produced by the chipper's Lambda angle ^(l). Lambda angle is defined as the mathematical result of subtracting the combined spout, pull-in, and knife edge angles from 90°. It represents the extent to which the compressive face of the knife forces through the wood: small l angles slice through like a knife while large l angles pound through the log like a dull chisel.

The combination of chipper geometry and wood quality produce a range of chip lengths and thicknesses which may be more or less uniform (Figure 2).

Figure 2: The goal of chip production is greater uniformity.

The chip thickness to chip length ratio typically varies from 1:4 to 1:10 (Figure 3).⁵ Within a given wood species, the combined effects of chip length setting, spout angle, knife edge angle, pull-in angle, l angle and knife velocity are primary parameters that can be modified to optimize the relationship between chip length and thickness. Secondary factors are the knife to anvil clearance, the counter knife face angle, and the distance from the tip of the knife to the counter knife.

Figure 3: Consistency of chip thickness across a range of chip lengths produced more uniform chips.

Chip length setting affects the distribution of chip thicknesses by shifting the entire range of chip size up or down. Increasing chip length at a given set of knife package angles will increase the average thickness of the chips produced (Figure 4). The chip length setting is calculated as the knife edge projection distance above the face plate (or wear plate) divided by the sine of the spout angle.

Figure 4: Changing chip length set-up shifts the chip size distribution. Longer Chips (B) are thicker than shorter chips (A).

Pilot plant studies. A 66-in. 4-knife chipper was used to evaluate the effect of changing spout angles and knife speeds on Southern yellow pine and oak. Three different spout angles were studied: 30°, similar to some Scandinavian chippers; 35°, suspected to be close to an optimum setting; and 38°, commonly used in past chipper designs. Knife speeds were varied from approximately 4,400 ft/min to 6,700 ft/min. The chips produced were caught in a cyclonic chip collector in a gentle fashion so that chip damage did not occur. Samples were classified using a ChipClass classifier with standard tray sizes (Table 1).

Pin chip generation. Knife velocity had a significant effect on pin chip production. In tests on Southern yellow pine, increasing knife velocity resulted in more pin chips at all spout angles. Slightly fewer pin chips were produced at the two highest spout angles in pine.

There was little effect of spout angle on the generation of pins in oak. Pin chip levels increased with higher knife speeds, but there was a minimal affect of spout angle. The level of pin chip generation was about half the level observed in pine chipping.

Overthick chip generation versus pin chip production. Slower knife velocities produced higher levels of overthick chips. This effect was most pronounced at the 38° spout angle, where small changes in speed can result in large changes in the quantity of overthick chips produced. The intermediate and low spout angle settings had less overthick production at all speeds and were less sensitive to knife velocity across the range measured.

These results demonstrate the trade off between generating pin chips and overthick chips. A major chipper performance challenge is to find an optimum combination of chipping speed and hardware angles to produce the preferred compromise between pin chip generation and overthick generation. We concluded that the proper compromise meant chipping in the range of 5,000 ft/min to 6,500 ft/min.

It should be noted that optimum chips were produced without the use of post processing devices such as card breakers. To achieve this, the research chipper was configured so that once the chips were produced they passed as freely as possible through the chipper disc slots and then gently exited and decelerated into a cyclonic sample collection device. It would be possible, for instance, to use a slow knife velocity which would produce low amounts of pins but high levels of overthick chips, and physically reduce these latter in size through the use of card breakers. This would generate pins and fines in the post-chipping process, thereby offsetting the positive effects of slow speed.

Large accept chips versus small accept chips. Since greater uniformity of chip thickness improves the cooking process, minimizing the production of overthick chips is not only beneficial (with low pin chip generation), but in the acceptable chip size range, a narrower thickness range with more chips between 4 mm and 6 mm is also preferable. This is another performance challenge for chippers.

Using pine, a 35° spout angle gave the best chip quality, including a higher percentage of large accept chips within the acceptable range with minimal overthick and pin chip generation. In tests on oak, the 35º spout angle also produced more larger accept chips, although the effect on performance was not as significant as in the pine tests. A 38° angle on oak significantly increased the generation of overthick chips.

Fines and oversize chip generation. Selection of an optimum spout angle and knife velocity should be based on pin chip and overthick chip generation, as well as on the distribution of large to small accepts. Fines or oversize chip generation are not major considerations for several reasons.

Across the range of knife velocities tested, fines generation fluctuated only 0.2%, showing a minimal effect of knife velocity on fines generation. Spout angles also only varied by 0.2% fines, indicating an insignificant effect. In all cases, the fines were less than 0.5% of the total chips generated.

The test results also show that the variation in oversize chip fraction quantity was more due to wood species than to either knife velocity or spout angle. In the oak tests, the oversize fraction varied between 1.1% to 1.5%. In pine, oversize results ranged between 0.1% and 1.1%. There was no apparent relationship between changes in spout angle or knife velocity and the generation of oversize. The presence of knot wood, compression wood, or log growth patterns are believed to have been the major cause of oversize, rather than chipper set up.

NEW CHIPPER FFEATURES AND PERFORMANCE. Based on these test results, as well as other tests not covered in this paper, Acrowood redesigned its whole log chipper spout. From the pilot plant results it was concluded that chipping at slower speeds (between 5,000 ft/min to 6,000 ft/min) was beneficial, with better performances achieved near 5,500 ft/min. It was also concluded that chipping at slower speeds (4,500 ft/min to 5,000 ft/min) produces an unnecessarily high percentage of overthick material requiring "post processing" methods such as card breakers.

The new-design horizontal feed whole log chipper parameters include: a 35º spout angle, slower speed chipping, and l angle and a counter knife configuration specifically selected to optimize chip quality.

The disc hardware was designed to optimize chip quality while minimizing maintenance costs. Features include: helicoidal face plates; adjustable angle counter knives; and generously tapered chip slots with bolt-in, replaceable chip slot liners.

At a chip plant in Mississippi, recent field results have been obtained on chipping Southern hardwood. The chipper easily provided greater than 90% acceptable chips (Table 2). The performance of the field unit confirms the pilot plant chipper results, through low production of pin chips and low production of overthick chips.

Future considerations. As mills evaluate the benefits of optimizing chipper performance, the following is a suggested list of potential considerations.

• New chippers should produce at least 90% acceptable chips. Minimizing pin chip generation is critical. The tradeoffs of producing overthick chips versus pin chips need to be considered.
  
• Minimizing overthick chip levels and providing greater uniformity in chip thickness has been an ongoing endeavor of the pulp and paper industry since the 1960s. Newly configured chippers produce less overthick chips and provide more uniform chip thickness. In addition to superior thickness control, mills should consider producing longer, thinner chips. Longer chips provide greater fiber length and pulping strength. Chippers set for longer chips achieve higher chipper production rates. In the past, chipping at a longer chip length setting was undesirable due to the generation of overthick.
  
• In considering new chipper installations, the pros and cons of chipping capacity versus chipper performance should be carefully weighed. Similarly, chipping at higher speeds to provide higher production has also been determined to be a major source of higher pin chip generation.10 Future chipper selections should be based on the system's optimum performance target, rather than on higher production capacities.
  

Desmond Smith is sales manager and technical director and Shannon R. Javid is president and general manager, both with Acrowood Corp.