It has been well documented that a Kraft wood pulp digester fed with a uniform chip size and a controlled volume of pins and fines provides a measurable improvement to the pulping process and pulp quality. Chips less than 7 mm thick pulp more completely than thicker chips, giving a uniform raw material for easily controlled pulping and bleaching. Additionally, removing the `pins’ and `fines’ from the digester feed improves operating performance.

Chip thickness screening (CTS) is a wood chip segregation and conditioning process that removes the overthick chips from the chip furnish and processes them either by slicing or crushing so that they can be uniformly cooked along with the screen accepts.

Most mills recognized the benefits of CTS in the early 1990’s and installed CTS systems; however, there were significant technological advances in the late 1990’s that many mills missed and still have not upgraded their equipment to the latest, most efficient technologies.

Although the actual pulping process improvements resulting from the application of CTS may vary from mill to mill, generally improvements can be found in the following areas:

  • Improved wood utilization.
  • Increased digester maximum production.
  • Increased yield.
  • Decreased digester alkali application.
  • Recovery boiler solids reduction.
  • Increased chip screens system availability.
  • Less mechanical damage due to foreign materials such as rocks.

This article provides a brief description of the CTS process and summarizes the potential benefits. For excellent reference papers on chip thickness screening and the benefits, see the references included at the end of this article.


Chip thickness screening is a purely physical mechanical process and since its initial application, has been continually improving in efficiency of operation and utilization of equipment.

A typical CTS system consists of a scalping screen for gross-overs removal, primary thickness screen(s) to remove the overthick chips, secondary screen(s) to separate the accepts chips from the pins and fines and tertiary screen(s) to separate the pins from the fines. Some mills utilize a 4th (quaternary) level of screens for further refining the fines fraction. In addition, an air density separator and either a chip slicer or a chip crusher (conditioner / cracker) is utilized to further process the overthick chips.

Unscreened wood chips are reclaimed from the chip storage area and are conveyed to the scalping disc screen where the `gross oversized’ pieces such as sticks, frozen lumps and large stones are removed. The acceptable material passing through the scalping screen is directed to a primary screen that segregates acceptable chips (accepts <8-9 mm) by thickness from overthick chips.

The overthick fraction passes over the end of the primary screen and is conveyed to an air density separator (ADS) where rocks, knots and other `heavies’ are separated from the lighter overthick chips. The ADS discharges the heavies into a reject bunker and conveys the acceptable material to the overthick treatment equipment where the recovered overthick chips are sliced or crushed to produce an acceptable chip for cooking, before being diverted to the accepts conveyor to the pulpmill.

The `fines-rich’ fraction passing through the primary screen, is diverted to the secondary screen(s) for further pins / fines processing, and the `clean’ portion of the accepts fraction from the primary screen is directed to the accepts conveyor to the pulpmill.

The secondary screen(s) separate the remaining accepts chips from the entrapped `pins’ and `fines’ and directs the cleaned accepts to the accepts conveyor to the pulpmill. Pins and fines are diverted to the tertiary screen(s), which separate the fines from the pin chips.

Dirt and grit are concentrated in the fines fraction rejected from the tertiary screen, which is usually diverted to the wastewood hog fuel flow to the biomass boiler.

Refer to the attached typical process flow diagram (PFD).CTS PFD-001

The percentage of pins in the digester furnish that a Kamyr digester can accept and remain operating satisfactorily, typically varies from 5 to 10%. So, depending upon the amount of pins in the chip flow and the digester’s ability to handle pins, a portion of the pin chips (0 – 100%) is diverted from the tertiary screen to the accepts flow to the pulpmill, while the balance is rejected with the fines. It becomes a fine balance between rejecting too much fibre and maintaining an efficient digester operation.

Typical guarantees for single and dual-line chip thickness screening systems are:

  • 100 units per hour per line (1.0 unit = 200 ft3 or 5.6m3)
  • 95% overthick removal efficiency (ORE)
  • 90% fines removal efficiency (FRE)


2.1      Chip Quality and Digesting Tests

Information regarding the incoming chip supply to the screening system is of primary importance for process design. However, information regarding the current chip quality data is often very limited. In order to accurately specify and design a chip thickness screening system, it is recommended that a comprehensive chip sampling and testing program and a corresponding cooking test program be instituted to clearly define the present operating circumstances and determine the optimum chip size and pins and fines content that best suits the mill’s particular pulping process.

After the installation of a CTS system, the chip sampling program should continue to be utilized to monitor the quality of the incoming wood chips and the performance of the screening system.

For chip sampling information, refer to the article titled “Requirements for Biomass Sampling”, included elsewhere on this website.

2.2      Typical Chip Supply to the Screening System

The following table summarizes a typical chip supply to the screening system.

Classification Definition / Test Screen Size Average Bad Case
Gross Over-sized Chips > 45 mm RH 1.8% 5.3%
Overthick Chips <45 mm RH; >8 mm slot 9.4% 19.5%
Accept Chips <8 mm slot; >7 mm RH 81.9% 57.0%
Pin Chips < 7 mm RH; > 3 mm RH 4.9% 12.2%
Fines / Pan < 3 mm RH 2% 6%

The definitions for the various classifications differ from mill to mill. For instance, some mills consider accepts as that material passing through a <7 mm slot and pins as that material passing through a combination of >3 mm RH and <2 mm slot.

Comprehensive chip data is necessary in order to produce a reliable result. It is recommended that prudence be exercised when selecting the final numbers as the worst scenario will vary considerably.

At the primary screen, the design should strive for the highest possible overthick removal efficiency (ORE), while limiting accepts carry over (ACO) to a minimum. The primary screen must be capable of handling all types of variations in chip geometry, as the chip sources and quality will change over time.

The secondary screen separates the accepts from the pins and fines. Fines removal efficiency (FRE) and accepts fibre loss (AFL) are the key factors at the secondary screen. AFL and fines carry over (FCO) are the key factors at the tertiary screen.

2.3      CTS System Sizing and Capacity

The size of a CTS system should be based on the maximum sustainable peak digester production rates, as this will determine the maximum chip flow through the CTS system. Conversely, the potential savings attributable to CTS should be based on the annual average production rate.

Factors to consider when sizing a CTS system include:

  • Steady flow through a Kamyr digester.
  • Surge flows through batch digesters.
  • Different species have different moisture contents, bulk densities, particle size distributions (PSD) and fibre yields.
  • Screening losses and efficiencies with different types of screens.

2.4      Anticipated CTS System Performance

CTS systems provide a marked improvement in chip quality over screening systems using only gyratory or vibrating screens. It is anticipated that a well designed and operated CTS system will be able to reduce overthick chips in the furnish to the digester to less than 2% and fines to approximately 0.5%.

The table below summarizes the anticipated chip quality from a modern CTS system.

Overthick to Digester Target <2%
Pins to Digester (see note) Target <7%
Fines to Digester Target <0.5%

Note: In order not to reject too much fibre, the pins content in the feed to the digester should be the maximum that the digester can tolerate.

When doing a study assessing the viability of a CTS system, it is necessary to prepare a detailed mass balance, which shows the anticipated system performance based on the average and bad case chip sampling data shown above. See the attached mass balance sample. CTS BAL2a


Industry experience has identified several direct benefits attributable to the use of CTS in the Kraft pulping process. These benefits are well-documented and include the following:

  • A more uniform cook following a reduction in rejects by a factor ranging from 1.5 – 3.
  • An increase in yield through lower reject rates. Most Kraft mills with CTS systems experience lower rejects and higher yield. A 2% increase in yield and a 50% reduction in rejects are typical values for North American mills that convert to CTS screening.
  • A direct cost benefit from increased yield at a higher kappa number.
  • Reduced load for recovery operations because of reduced alkali usage per ton of pulp, resulting in either better chemical and energy efficiency or increased production.

These benefits are achieved primarily because of the supply of more uniformly-sized chips to the digester that translates into a more-uniform cook, cleaner pulp delivered to the bleach plant and lower chemical usage for bleaching to a given brightness.

Benefits can be derived from:

  1. Treatment of overthick chips.
  2. Reduced small particle generation (SPG).
  3. Removing fines from the digester furnish.
  4. Recovery of pins and rejected accepts.
  5. Recovery of acceptable overthick fibre.
  6. Removal of rocks, grit and dirt.
  7. Reduced Liquor Usage.

3.1      Uniform Cooking of Overthick Chips

Overthick chips do not fully pulp during the Kraft pulping process and are discharged as knots. When these under-cooked `knots’ are re-cooked, they produce a pulp with much lower yield and strength than pulp from properly sized chips.

The amount of knots rejected in a pulping line gives an indication of the proportion of overthick chips in the digester feed. For a mill without a CTS system, which uses predominantly sawmill residual chips, the amount of knotter rejects normally amounts to 2% to 3% of pulp production. With CTS and oversize chip treatment, knotter rejects are typically reduced to less than 0.5%.

Recirculating under-cooked ‘knots’ in a pulping system occupies digester space. Removal of the overthick chips allows a greater quantity of accepts chips to fill the volume otherwise occupied by recirculated knots on their first or subsequent pass through the digester, and allows production to be increased in a digester-limited mill.

3.2      Fines Removal

Fines occupy the interstitial spaces between larger chips, so removing all of the fines will reduce the digester throughput. Also, removing all of the fines represents a considerable fibre loss. However, excess fines can cause considerable operating difficulties in a Kamyr digester, such as level control problems in the chip chute and pluggage in the top separator or extraction screens, resulting in lost digester production. Removal of fines and reduction in the overall percentage of undersized chips in the digester feed improves the circulation rate and increases yield by reducing overcooking of undersized chips.

Fines absorb pulping chemicals and are removed when the chemicals are processed in the recovery boiler. An excess amount of fines places additional load on the recovery boiler and results in lost production in a recovery-limited mill.

Gyratory screening systems are not effective at removing fines from the digester furnish during wet or freezing weather conditions. The newer styles of secondary and tertiary screens are able to remove fines much more efficiently, allowing digester production to be maintained throughout adverse weather conditions.

3.3      Reduced Fines Generation

The first CTS systems utilized rechippers or chip slicers. Rechippers indiscriminately chop-up over-sized wood pieces into random bits, generating large quantities of small particles. Chip slicers were an attempt to control the thickness of the re-processed overthick chips by re-slicing them to the desired thickness. However, slicers make one good chip and one thin chip out of every overthick chip and generate a lot of pins and fines.

A typical rechipper or a poorly maintained chip slicer will degrade 20%-40% of the processed material into fines. Assuming that 11%-12% of the incoming chip supply is comprised of over-sized and overthick chips, the amount of degraded material coming from a rechipper or slicer is substantial. This degraded material is either added to the digester furnish, increasing the fines content or is returned to the chip screens, where it is removed and lost. This lost fibre must be replaced by the purchase of incremental chips. One mill calculated the value of fibre lost to fines to be approximately $1.5M per year.

Additionally, rechippers or chip slicers require considerable maintenance to remain in effective operating condition and if not maintained or operated properly will result in even greater amounts of small particle generation (SPG).

Modern CTS systems utilize chip crushers, which crush the overthick chips, creating fissures into which the digesting chemicals can penetrate, thereby permitting the chips to digest at nearly the same rate as accept chips. CTS systems that utilize chip crushers normally operate for many months without appreciable maintenance and SPG is limited to ~1.5%.

3.4      Recovery of Pins and Rejected Accepts

Old style screening systems (no CTS) often reject acceptable chip fibre with the screened-out chip fines. In the quest to remove all the chip fines, gyratory screens will often have 5/16” – 3/8” openings with the result that acceptable mini-chips and pin chips in the 3-7 mm thickness range are often rejected. Accepts rejection rates of 2%-3.5% are not uncommon, based on experience at coastal mills.

More than 90% of these lost accepts would be recovered by modern secondary / tertiary screens and could be reintroduced into the digester furnish in metered amounts.

3.5      Recovery of Rejected Overthick Fibre

In an old style screening system, oversized / overthick but good fibre is rejected along with rocks to landfill. In a modern CTS system, this lost fibre would be efficiently separated and recovered at the ADS system and reprocessed in the overthick crusher.

3.6      Removal of Rocks, Grit and Dirt

CTS screening systems remove some of the rocks, sand and grit. Rocks larger than the disc interface opening (IFO) of the scalping screen will be removed with the gross-oversized chips (typically >2.5”).

Rocks larger than the primary screen openings (>8mm) will be diverted to the ADS system with the overthick chips and will be discharged from the ADS with other `heavies’ into a rejects bunker.

Sand, grit and dirt smaller than the tertiary screen openings (<3mm) will be removed from the chip flow with the rejected chip fines.

Stones smaller than the primary screen openings and larger than the tertiary screen openings will remain in the chip flow to the digester (i.e. <8mm, >3mm). These rocks could be removed by processing the accepts chip flow from the secondary and tertiary screens through a 2nd ADS system designed for the service.

Similarly, the small stones rejected with the fines from the tertiary screen could be removed from the fines rejects flow by processing the fines rejects through a 3rd ADS system designed for the service.

Removing the rocks, sand and grit from the pulpmill furnish has the benefit of reducing blow line plugging and mechanical damage to the knotters, blow tank pumps, digester feeders and pulp screens, resulting in increased digester operating time, hence higher production rates.

One mill has estimated damage to mechanical equipment from rocks in the chips, to be about $500,000 per year.

3.7      Reduced Pulping Liquor Usage

Because the amount of overthick chips and fines are reduced where chip thickness screening is utilized, the white liquor application to the digester can be reduced, which reduces the loading in the recausticizing and kiln areas. This in turn, reduces direct operating costs per tonne of pulp produced.

Mill data indicates a 5% decrease in alkali for a 2% increase in yield. A reduction in alkali is expected for two reasons – less wood substance is dissolved and a lower residual is required to produce clean pulp.

A reduced alkali requirement of 0.6% AA/BDt wood has been observed in the recausticizing / kiln area resulting from the installation of a modern CTS. This level of reduced alkali usage has been observed at several B.C. mills with CTS installations.


4.1      Yield Improvement

With the installation of a modern CTS system, pulp yield is expected to rise by 1.5% – 2%. This has been observed in several BC mills that have installed CTS systems. This is roughly equal to a reduction in fibre usage of 0.035 BDU per ADt of pulp produced.

4.2      Reduction in Bleaching Chemicals

In one mill, improved uniformity and reduced shive content in the pulp was conservatively estimated to reduce bleaching costs by $1 per ADt .

4.3      Offloading the Recovery Boiler

For mills that are recovery limited, a CTS system would help to offload the recovery boiler, enabling a production increase.

4.4      Manning Reduction

Modern CTS systems operate continuously with no dedicated operator in attendance, and with no rechipper or chip slicer, maintenance costs are much lower.

4.5      Other Potential Benefits

Other savings associated with CTS utilization, include the following:

  • Reduced steam usage.
  • Improved washing.
  • Improvements in pulp quality.
  • Defoamer reductions.

Such allowances are difficult to quantify and the net effect of such allowances is small compared to the main benefits identified above.

4.6      Total Benefits

If all of the above mentioned benefits are realized, the replacement of an old style, non-CTS screening system with a modern CTS system can result in annual savings of $5.0M to $6.0M CAD.


Upgrading an old-style CTS system with a modern CTS system will, of course, not see the full savings, as most of the savings were realized when the original CTS system was installed.

Primary Screen Upgrades

Old style CTS systems utilize disc type of thickness screens and chip slicers. Disc screens experience high wear and do not maintain their accuracy for long. Maintenance costs are high and progressively thicker chips are accepted as the discs wear, often moving outside the chip thickness `sweet spot’. Modern `bar’ type thickness screens do not wear quickly and maintain their accuracy for long periods of time.

Overthick Processing Upgrades

As previously mentioned, chip slicers degrade the material being processed, generate high volumes of fines and require considerable weekly maintenance. Modern chip `conditioners’ or chip `crackers’ generate minimal fines and run from year-to-year without requiring continuous maintenance.

Secondary / Tertiary Screen Upgrades

Flexible mat screens are very effective in separating accepts chips from pins and fines in wet and freezing climates.

Roll screens are very effective in separating accepts from pins and fines, and in separating pins from fines in warm climates.

Also refer to the article titled “Screens for Woody Biomass”, which is included elsewhere on this website.


The following companies provide the best chip thickness screening and fines screening equipment, which are used in most pulpmills in North America.

Rader (Terrasource) –

Acrowood –




AA                                                                                   active alkali

ADS                                                                                 air density separation

ADt                                                                                  air dry tonne

ACO                                                                                accepts carry over [efficiency]

AFL                                                                                 accepts fibre loss

BC                                                                                   British Columbia

BDt                                                                                  bone-dry tonne

BDU                                                                                 bone-dry unit = 2,400 BDlbs

CAD                                                                                 Canadian dollar

CTS                                                                                 chip thickness screening

FCO                                                                                fines carry over [efficiency]

FRE                                                                                 fines removal efficiency

GO                                                                                  gross oversized

ft3                                                                                    cubic feet

IFO                                                                                  interface opening

lbs                                                                                   pounds

$1.0M                                                                              one million dollars

MC                                                                                  moisture content (wet basis)

mm                                                                                  millimeter

m3                                                                                   cubic meters

OT                                                                                  overthick

ORE                                                                                overthick removal efficiency

PRE                                                                                pins removal efficiency

PSD                                                                                particle size distribution

RH                                                                                  round hole

SPG                                                                                small particle generation

Unit                                                                                 200 ft²

uph                                                                                 units per hour

Copywrite © 14 February 2016


Bertil Fagerland, Rader Companies Inc., circa 1980, “Chip Thickness, Its effect on Kraft Pulping and the Economic Impact”

Brian Briscoe, Westar Timber, CPPA 1985 Spring Conference, “Chip Thickness Screening, Post Evaluation at Celgar Pulp”

Doug Christie, Lutra Enterprises, circa 1987, “Status of Thickness Screening of Wood Chips”

K. Kreft & S. Javid, Acrowood Corporation, 1990 Tappi, “A New Roll Screening Technology”

Paul Thimons, Mead Corporation, 1991 Tappi Journal, “Chip Thickness Screening with an Oscillating Bar Screen”

John T. Henry, P.H. Glatfelter Co., 1993 Pulping Conference, “Conditioning of Over-thick Chips, An Alternative to Chip Slicing”

George Strakes, Rader Companies, 1997 Pulp & Paper Magazine, “Overthick Chip Conditioners Improve Mill Pulping Efficiency”

Brent J. Grendys, Weyerhaeuser Canada Ltd., 1998, “The Benefits of Screening and Conditioning Overthick Wood Chips”

Desmond Smith, Acrowood Corporation, Paper Industry October 2005, “Pulping System Sensitivity to Chip Quality”


About the Author

Paul Janzé has more than 30 years experience in engineering design, project management, equipment manufacturing and maintenance, primarily in the forest products and energy industries. His industrial material handling experience includes: biomass handling and processing including forest residuals, logs, lumber, chips, pellets, woodwaste, corn stover, straw and poultry litter, deinked pulp, sludge and biosolids; municipal solid waste (MSW); limestone, coal, ash handling and petroleum coke.

He has a keen interest in technologies which recover and utilize waste materials and convert them into products such as wood pellets. Paul’s specialties are fibre flow analysis and mass balances, process optimization and designing novel solutions to complex processing and handling problems.

Paul can be reached at: Advanced Biomass Consulting Inc., tel: 604-505-5857, email:

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