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Jet screen
CS chip screenChip optimizer rolls

Chip screening systems – the heart of the woodroom

Innovative designs for chip cleanliness and uniformity

The challenge

Forest product companies face the challenges of improving the efficiency and productivity of their wood processing operations to reduce the operating costs and to save raw material and energy. A key part of this is the chip screening process – the removal of pins and fines, and the re-direction of oversize/overthick chips for further processing.

The solution

ANDRITZ has a number of innovative approaches for highly efficient chip screening with a variety of wood species. The systems can be customized to fit the requirements and layout of a customer’s wood processing operation. The scope of supply can vary from complete plants to individual equipment delivery. ANDRITZ has a depth of experience in project management, ensuring reliable deliveries and excellent economic results for customers.

ANDRITZ has been working on alternative ideas in the chip screening process for a long time. This work has resulted in radical innovations. ANDRITZ chips screens are customized based upon:

  • wood species in the mill
  • final product of the mill
  • chipping capacity (if screened after chipping) or pulp production (if screened after storage)
  • infeed chip quality
  • amount and quality of purchased chips
  • climate

Chip quality is produced by the chipper, but can be improved by removing fines and pin fractions and by screening out overthick and oversize fractions, which can be re-processed.

ANDRITZ has two types of thickness screening systems, two types of chip gyratory screens, a disc screen, and several machines for processing overthick and oversize fractions:

Thickness screening

Gyratory screening

Processing oversize/overthick


  • Disc screen

JetScreen thickness screening

The JetScreen is a completely different screening method with superior flexibility and high capacity. The chip flow is distributed and discharged across the width of the screen at the top of the machine. The chips slide down the screen past a strong air impulse. The accept fraction is separated from the oversize, overthick, pins, fines, and any contaminants in the chip flow by the air impulse.

The pins and fines can be directed to the fines conveyor. Oversize and overthick chips are transported for further treatment. Sand, stones, and scrap material are separated for removal.

The pins, fines, and fine dust not suitable for pulping can be mixed with bark and other biomass for combustion in a power boiler.

The oversize and overthick chips can be sent to an HQ-sizer or chip optimizer for treatment. After treatment, the chips are returned to the accept flow. Accept chips passing through the HQ-sizer or chip optimizer remain undisturbed.

When the screening capacity is smaller than 1,000 m3 l/h, the two air impulse stages can be placed alongside each other, reducing the height of the JetScreen.

JetScreen features

  • Very clean chips without dust, sand, stones, or scrap
  • Very high screening efficiency at high capacity levels
  • Separation of chip fractions (size, thickness, and density) with air impulses
  • Easy to adjust and control the size of the fractions
  • No moving mechanical parts inside the screen
  • Very little maintenance required
  • Completely closed and dust-free screening


OPTI thickness screening

The chip flow is distributed and discharged to the whole width of the OPTI thickness screen on the top of the machine by the feeding conveyor.

In small-capacity screens, pocket rolls that screen out fines are mounted at the beginning of the thickness screen. The fines are transported by conveyor to be mixed with bark or other biomass.

Accepts from the OPTI screen fall down onto a conveyor, which transports them to chip storage or to a digester. Overthick and oversize chips are discharged to a chip optimizer or HQ-sizer for treatment. After processing, the accepts are mixed with the accepts from the OPTI screen.

In large-capacity screens, a pocket roll fines screen is mounted under the OPTI thickness screen. The fines and some of the pins are transported by conveyor to be mixed with bark or other biomass.

Two movable gates inside the screen adjust and control the amount of pins, fines, overthick, and oversize fractions separated.

OPTI screen features

  • Very high thickness screening efficiency at all capacity levels (separation is based on thickness of the chip fractions)
  • Superior separation of overthick and oversize chips
  • Excellent separation of the accept chips
  • Non-plugging, self-cleaning construction
  • Adjustable
  • Very little maintenance required
  • Dynamic loads minimized

Pocket roll fines screen features

  • Three-dimensional separation of fines
  • Accurate and high separation performance
  • Non-plugging, self-cleaning construction
  • Adjustable
  • ANDRITZ patent


CS gyratory chip screen

The CS chip screen is a gyratory type design suspended from four steel ropes. An electric drive with a single shaft and counterweights in the middle of the screen produces a freely vibrating and adjustable, horizontal circular motion by the screen.

Chips are fed through a wide infeed chute onto the upper screening deck. The oversized chips and slivers flow over the top deck to the rejects chute, which collects them for further treatment. Accepted chips drop to the middle and lower screening decks, where they are discharged to storage or the pulping process. Fines pass through all the decks and are collected at the bottom of the screen, which has a central discharge opening.

There are four different capacity levels for the CS screen. The standard screen has three screen decks. A fourth screen deck is available when very efficient pin chip recovery is required. The standard screening decks are made of mild steel. For abrasive conditions, the screen decks can be supplied in stainless steel.

The screen can be equipped with a protective dust covering. It consists of an infeed and discharge chute, dust cover, and a partial or complete cover for the top of the screen.

Servicing is accomplished by opening the screen rear panels, which expose the entire interior of the screen. The two drive shaft bearings are piped for automatic lubrication by a simple, central lubricating unit.

The frame construction of the CS gyratory screen is very rugged and simple, made of rectangular and square tubes. The screening decks are bolted to the frame.

CS gyratory screen features

  • Large capacity range: 3 or 4 screening decks
  • Stainless steel screening decks available for abrasive conditions
  • Rugged and simple construction
  • Effective overs  and fines separation
  • Self-cleaning due to the amplitude, frequency, and screening deck inclination
  • Easy maintenance


SCL gyratory chip screen

The sectional design of the SCL chip screen gives the screens their characteristic six- or eight-sided shape. Each screen plate is light in weight and easy to handle.

Chips are fed onto the center of the screen, which is suspended from steel wire ropes. The rotating motion of the screen, generated by a rotating counterweight, spreads the chips evenly over the distribution plates. The chips reach the screen plates at the point where the plates are widest and have the greatest capacity. As the requirement for capacity reduces, the screen plates become narrower. An optional third screen deck can be installed to reduce the load on the fines deck and improve selectivity. If the chip flow contains a large amount of oversize chips, the screen can be delivered with a coarse screen deck at the top of the screen to increase its capacity. The outlets for fines and oversize chips can be adjusted to fit the best plant layout.

SCL gyratory chip screen features

  • Screen for small capacities
  • Compact design
  • Layout-friendly
  • Easy maintenance



Knots in roundwood disturb the chipping process and create oversize fractions. A certain amount of the oversize fraction is also produced from log ends and breakage. In order to improve both chip quality and yield from the oversize fraction, ANDRITZ developed a “chip” chipper that employs a totally new technique for converting oversize chips to accept chips.

With its multiple anvil and special knife arrangement, the infeed spout of the HQ-sizer is able to align the chips to lie flat against the rotating disc in order to be cut according to their thickness. The nominal thickness of chips produced by the HQ-sizer can be adjusted. The HQ-sizer causes minimum harm to accept chips as they are well positioned before cutting and the proper size chips can pass by the cutting knife.

The HQ-sizer is capable of handling a chip flow that previously required two or more machines, such as chip crackers or chip slicers.

HQ-sizer features

  • Oversize and overthick fractions transformed into accept chips
  • Improved pulping yield and reduced rejects in pulp
  • Minimum fines and pins generation
  • Minimum mechanical damage to the fibers in chips treated
  • Follows gyratory or thickness screen
  • Low operating and maintenance costs


Chip optimizer

Unlike existing methods of oversize chip treatment, the chip optimizer employs a specially designed roll surface that bends the chips gently (reducing pins and fines as well as minimizing fiber losses). Since it has no wear components (e.g. slicer knives and anvils), the chip optimizer requires only basic maintenance without costly production shutdowns.

The design of the chip optimizer has been thoroughly researched and tested, and is patent-protected. Chips are not crushed, but bent when they pass between the rolls. Bending does not damage the wood fibers and keeps the wood fiber length at a maximum by producing internal cracks along the grain, which help increase cooking liquor penetration.

The chip optimizer consists of two horizontal, counter-rotating rolls. One roll is fixed and the other is movable. This allows adjustment of the nip between the rolls to different chip thicknesses and prevents damage to the equipment from metal or other foreign objects entering the machine. Both rolls are covered with bolted segments manufactured from a wear-resistant material. If needed, a special coating can be applied to the segments to increase corrosion resistance. Each roll is independently driven. This ensures smooth start-up, increased starting torque, and mechanical protection against overloading.

Chip optimizer features

  • Gentle bending action produces less fiber damage
  • Reduced pins and fines generation
  • Improved penetration of cooking chemicals
  • Improved pulping yield and reduced rejects in pulp
  • Minimum service requirements
  • Roll segments have long life and are easily replaced



The rechipper processes oversize chips into accepted chips with a minimum amount of fines generation. The performance of the rechipper is well-proven, and far superior to drum chippers or shredders.

The oversize fraction and slivers are gravity-fed to the rechipper via the infeed spout, where they come into contact with a rotating disc equipped with radially mounted knives. The knives cut the oversize chips to the required length. The clearance between the knives and the bed knife is easily adjustable and contributes to the high chip quality.

The excellent performance of the rechipper is aided by the V-shaped chute and its chipping geometry. This combination provides effective guidance and stabilizing of the oversize chips prior to and during chipping.

Rechipper features:

  • High chip quality compared to drum type rechippers and shredders
  • High capacity
  • Excellent operational reliability
  • Easy operation
  • Easily adjustable nominal chip length



ANDRITZ Oy, Finland

Phone: +358 (0)20 450 5555

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ANDRITZ Brasil Ltda., Brazil

Phone: +55 (11) 2168 0100

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Phone: +1 (770) 640 2500

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ANDRITZ Ltd./Ltée., Canada

Phone: +1 (514) 631 7700

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Related links


Bark processing


Chip screening

Chip storage equipment and reclaiming


Drum discharge – chipper infeed

Log handling


Wood receiving and conveying

Wood processing services

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