Hand Sheet Making - Paper Testing

Hand sheet making

Hand sheets are created for a variety of uses; to determine the composition of stock/ pulp in a tank, to look at the cleanliness of papermaking stock, to test the strength properties of the pulp etc. There are a variety of ways to calculate the hand sheet weight and volume of stock needed to form a specific hand sheet.

Consistency method

Hand sheets are made to a grammage of 120gsm for testing to achieve this weight the stock volume needed is calculated. The hand sheet weight should be about 2.4g. (this depends on the diameter of the final hand sheet) Firstly the consistency of the stock sample is calculated. Once this is known a simple calculation can be made to work out the stock volume.

  

There will be a tolerance error using this calculation as the consistency of the stock sample can be inaccurate due to pouring the sample/ collecting the sample. A more accurate method can be used.

A good consistency measurement example can be found here; Technical Papermaking Consistency

Test sheet Method

Before making a hand sheet a test sheet is created to determine the amount of stock need to make a specific weight hand sheet. This method is ideal if the consistency is unknown.

First the stock needs to be diluted to around 0.3% consistency in a bucket. Using the same calculation above a volume of stock is calculated for the test sheet. The test sheet is formed and dried. Once the sheet has been dried the test sheet is weighed and recorded.

The weight is used in another calculation to determine how much extra or less stock is needed to achieve the desired weight. This step removes the errors made when creating the 0.3% solution.

 

This calculation will give you the new volume needed to make the hand sheet.

Forming Hand sheets for physical test of pulp

Using the test method as described by Tappi T205 hand sheets can be made from pulp ready for physical testing.

The stock samples are taken. The first step is to identify the amount of stock needed to make a standard 80GSM hand sheet. This can be achieved in two ways, either by conducting a consistency test on the pulp sample, which takes time and not always accurate.

The other method requires making a test sheet before the hand sheets.

Creating a hand sheet

With the hand sheet cylinder down (open) the hand valve is opened to allow water to pass through the wire, gently rubbing the surface allows any remaining fibres to be removed. The wire is now clean.

The cylinder is locked back in place. The valve is opened slightly to allow some water to fill up, next add 500ml of 0.3% stock solution. The water is filled up to 350mm from the wire to the inscribed line.

Insert the perforated stirrer 5 times for about 6 seconds trying not to spin the stirrer and remove the perforated disk from the liquid. Carefully remove the stirrer on the last up movement and wait for 5 seconds allowing entrapped air to leave and the fibres to settle. Fully open the drain and let the water drain under the vacuum from the water leg. The hand sheet will now form on the wire.

Blotting paper is placed on the formed hand sheet as well as a couch plate on top; the couch roll is applied with no extra force apart from the rolls weight. The couch roll is rolled five times across the plate. With the movement likened to opening a book, the disk and blotting paper is removed and placed onto the drying rings or hot plate.

Cleanliness

Creating hand sheet can be a good indication on the cleanliness of the stock; they can be used especially when troubleshooting quality problems on the machine. For example if a screen was passing/ the slots or holes were damaged you can see an increased level of contaminates in the hand sheet of the accepted stock line. It is a good idea to have a base line hand sheet of the process to be used as a comparison when trouble shooting appearance quality issues. 

Paper Machine Water Chemistry - Technical Papermaking

Paper Machine Water Chemistry

Process water is the name given to the back water on the paper machine, water that is reused within the papermaking process.

The backwater is a mixture of chemicals and fines that were not used the first time around (one pass retention). The back water can have detrimental effect ts to the process if the parameters of the water fall outside of specific ranges/ conditions. For example if the conductivity of the back water became too low or outside of normal operating conditions it would mean the retention on the wire is reduced.

Low conductivity (-18mV) shows there is a high volume of anionic trash within the system which when mixes with the cationic starch or polymer, hydrocol will bond with the trash rather than the fibres leading to poor formation on the wire, pick outs and deposits forming around the machinery and a reduction in run ability. 

pH of process water

pH can be described as the single most important aspect of wet end chemistry, this is because most/ all aspects of the chemistry relies on in some aspect the pH.

When the pH of the water increases (becomes more alkaline), the surface charge of the fibres also increases. This will affect the attraction of the retention aids and other cat-ionic substances to the fibres. The most undesired effect would be the substantial increase of bacteria in the system.

Another effect of high pH ius fiber swelling, fiber swelling is useful during refining because the higher surface area and increased flexability of the fiber leads to high de-fibrilation (versus cutting of the fiber). Caustic acid is added to create this effect.

fiber swelling is also used within de-inking plants. swelling of the fibers pre floatation allows the inks and binder to split and break off when the fiber swells this aids in the  bleaching and deinking process leading to high brightness of the finished pulp.

pH can affect quite strongly the dissolving ability of wood components and to changes in the dissolved substances. Increase in pH improves wood components dissolving ability in the water system and thus the amount of anionic particles dissolved and colloidal substances.

Because of the undesirable fractions within water, the water needs to be cleaned at some stage with different kind of methods like for example with disc-filtering and or chemicals. The wet-end of the paper machine contains the highest amount of water and its chemistry has to be controlled by a variety of chemicals like, retention chemicals, fixatives, de-foaming agents and biocides.

A decrease of the pH value leads to deposits on the machine as the precipitation of non-wood materials increases.  A fine balance has to be made; typically a pH of 7 is achieved within the water loop.

Machine Issues caused by change in pH
Increase in pH	Decrease in pH
Level of bacteria in water increases	Deposit precipitation increases – deposits on the machines
Higher amount of anionic trash in the system
Surface charge of fibres increase

Bugs/ Bacteria - Biocides

Paper machines run well when the operating conditions remain constant. Water plays a large part in paper production and any slight disturbances in the water can cause negative effects on the machine.

Bugs, bacteria thrive in water and damp conditions. Bacteria build up within the water system will cause a variety of issues. For example; Slime build up, lowering PH within the water (acidification) and upsetting the chemical balance within the water loops.

Biocides are used to control the bacterial problem. The main goal of the biocides is to limit the growth of sessile bacteria, i.e. those that are attached to surfaces. These are the bacteria that tend to build up, cause slime deposits and holes, hurt productivity, and hurt product quality.

Pulp Grinders - Mechanical Pulp Production

Types of grinders

Chain grinders

Chain grinders are the most common type of grinder, the chamber between the vertical chains houses the logs ready to be processed. The logs are continuously driven by the chains onto the surface of the revolving pulp stone. The chains apply a force on the log keeping them against the surface of the stone. Due to the surface profile fibers are torn out of the wooden compound. The logs are stacked horizontally due to the orientation of the fibers (minimizes fiber damage - increases fiber length)

Water showers are used to keep the stone clean and dilute the stock suspension. Water temp is usually higher than 80 deg. using a low water temperature can cause the stone surface to crack (due to temperature difference). Using hot water allows the stone to maintain the high temperatures. these high temperatures affect the lignin of the wood softening the material which in turn means the fibers can be separated with less damage

The basin collects the pulp washed off by the water showers. typically this produces a low consistency pulp, a thickening stage is added after to bring the consistency to a more manageable level. 

Pocket grinder

The pocket grinder is a technological advancement of the chain grinder above. processing the logs within a sealed "pocket" allows the sealed area to operate at a higher atmospheric pressure than the pocket grinder.

At higher pressures (approx: 5 bar) the boiling effect of water is affected. the higher temperature of the water will soften the lignin within the logs, longer fibers can be created with less damage done to the fibers. 

Revolving pulp stone is surrounded with a metal housing and 4 pockets, feeding of the pocket grinder was done manually. The debarked logs are pushed against the revolving surface of the pulp stone by hydraulically driven pistons. The pulp is collected underneath the pulp stone.  

 

The entire process of feeding the logs to the groundwood outlet is done under pressure. (up to 5 bar housing pressure, shower water temp up to app 120 deg) pressure affects the boiling temp of water. 


For more info on Mechanical ground wood follow this link
Stone Ground Wood - Mechanical Pulp Production

Stone Ground wood Pulp - Mechanical Pulp Production

Mechanical Wood Pulping

All mechanical wood pulping makes no attempt to remove an impurity known as lignin from the fibers. A product made from mechanical wood pulp will not be durable and will degrade rapidly especially in the sunlight.

Stone ground wood Pulp

This is the most commonly used method mainly due to the simplicity of the machine. one large grindstone which breaks up the descending log into individual fibers and fiber bundles.

Mechanical pulp is produced from fiber defibrillations from a stone grindstone. the temperature around of the pulp is around 80 to 125 deg, locally at the stone where the fibers are being broken down the temperatures can reach up to 170 deg.

this high temperature is an advantage because the lignin in the logs binding the fibers together starts to become more malleable allowing for less damage to the fibers, more intact fibers are produced. 

Thermo-ground wood pulp

The thermo-ground wood mechanical pulp is an advance on the stone groundwood process where the logs are treated with hot water. the logs sit above the stone grinder in a hot water bath. the hot water is used to make the lignin softer. when the lignin becomes soft the break down of the logs into fibers becomes easier leading to less energy being needed and higher quality pulp.
Temperature around 80 to 95 can be generated locally at the grindstone. The water temperature is usually around 80 deg C. 

Pressurized groundwood

This is another advance on the original stone groundwood pulp. this is mechanical pulp production where the grinding takes place under compresses air pressure usually about 1 bar above atmospheric pressure. the water pressure is there for greater than 95 deg C (due to high-pressure atmosphere raises the boiling point of water). this allows for higher grinding temperatures without steam flashing. the high temperature promotes the softening of lignin. this improves fiber separation and reduces the specific energy consumption of the pulp.
Grindstone temp is about 124 – 130 deg.

Basic principles – Stone groundwood process

The fibers are torn from the logs and washed from the stone by means of spray showers. The supply of fibers and fiber fragments go through a screening process to remove the large particles these are known as shives. The stock is then thickened.

At grinding the logs are heated up from the frictions of the grind stone caused by pressing the logs against the stone the wooden structure is softened, the lignin becomes more malleable when heated. The bonding between the fibers will be less. The shearing forces between the stone surface and the wood mean the fibers are torn out.

This process uses very little or no chemicals but is very energy intensive (1200 – 2100kw/ dry ton)

Under the pulp stone, a basin collects the groundwood pulp

The pulp is screened after this process to remove shives (over sized particles) and then thickened

Varieties of Stone groundwood pulp

Fine groundwood pulp

·         Low moisture content in the logs

·         Gentle pressing of the log in the grinding zone (low speed of the chain drives) leads to a lower load consumption

·         Grit size – small dia grit size

·         Less sharpening of the grindstone with small dia grit on the abrasive layer

·         Less sharpening of the grindstone

·         Higher consistency in the basin

·         High specific energy consumption
As a result, the shopper riegler value is high, low dewatering ability

Coarse groundwood pulp

·         Fresh wood high moisture content

·         Intense pressing of the log in the grinding zone – increased the load on grindstone

·         Grit size :usage of grind stone with large diameter grit

·         Sharp stone surface

·         Low consistency in the basin

·         Low specific energy consumption

·         Result in good dewatering ability – high shopper riegler

Log Storage - Wood Yard

Woodyard - Log storage

The main purpose of a wood yard is;

• To receive logs, wood residues and chips
• Debark logs and residues
• chip logs residues
• to store bark and chips

Raw materials for paper production

• small Diameter logs (approx.: 8cm to 40cm) from forest thinning's (coppicing)
• Sawmill residues sawdust, chips, unusable wood scraps

Traditionally used woods

• Softwoods typically spruce and pine (great fiber morphology)
• Hardwoods like birch, poplar beech, and eucalyptus

Wood handling

Separate processing or different species of wood, the mixing of wood species should be avoided.

Log storage

·         Orderly stacks

·         Piles of wood (need to be sprayed with water)

·         Storage in water (uniform moisture content)

·         Wood chips can be stored in silos or open-air storage piles

Logs for wood pulping

Usually,  cut down to 1m lengths and diameters of (10cm to 35cm) the moisture content ideally should be  > than 35% for optimal fiber removal

Quality control parameters

·         Amount (eg weight)

·         Moisture

·         Contamination

·         Shape (straight or bent)

·         Condition of the wood (Colour, fungal diseases, rotten wood)

The time taken between the tree being cut and converted into wood pulp should be no longer than 6 to 12 weeks to obtain optimum pulp characteristics. Also, logs should be moistened when in storage during dry weather.

Storage condition can affect the processability of the logs for example too dry logs can lead to fibers being ripped from the wood resulting in more shorter fibers and fines 

Storage conditions

·         Carefully piled up on a well-prepared log yard

·         Ensuring sufficient water drainage - prevents wood rotting

·         Ensure air circulation

To learn about the next stage of wood processing click here;
Stone Ground Wood - Mechanical Pulp Production
Pulp Grinders - Mechanical Pulp Production

Cat-ionic Starch - Technical Papermaking

Paper makers hold starch in high regard as one of the most important papermaking chemicals in producing high quality paper.

Cat-ionic starch is made up of modified maize starch that now has a slight cat-ionic charge (positive charge). Cat-ionic starch is added at the wet end of the machine. The slight cat-ionic demand of the starch allows it to bond with the anionic (negatively charged) fibers, fillers, fines, and other small unusable materials to improve dry strength in the sheet and even improve the retention of the sheet.

Cat-ionic starch is produced when a reactant chemical is treated with slurry of partially swollen starch granules.

Typical starch addition points in a approach flow system occur either before the machine chest in the thick stock system or after the machine chest at the start of the thin stock flow to the head box, Arguably both addition points allow for high strength, depends on the grade and the dosing quantity of starch. 

The problem with adding too much is that it will exceed the adsorption capacity of the surface, based on either the surface area or the limited extent of negative charge of the surfaces of fibers and other solid surfaces in the furnish. Excess starch beyond what adheres to the fibers in likely to cause foam, high biological oxygen demand (BOD) levels in the effluent, and poor retention and drainage. The performance of cationic starch as a strength agent sometimes can be improved by raising the pH; this will tend to make the fibers slightly more anionic and better capable of interacting with the starch.

When adding cat starch in a system with a high level of anionic trash the starch strength qualities can be improved with pretreating the stock with a highly charged cationic material to neutralize the anionic trash. 

Papermachine Cleaning Chemicals - Technical Papermaking

Wire conditioner

Wire conditioner is sprayed on to the wires of the fourdrinier machine. The job of the wire conditioner is to coat the wire and prevent stickie’s, glues, pitch and other tacky substances from binding with the wire plugging the holes in the mesh. Blocked parts on the wire will inhibit drainage leading to weight and moisture deviations as well as holes/ weak spots in the sheet. these will cause a lot of issues on the paper machine.

When problems occur on the paper machines wire like marks from stickie’s and hot melts a caustic chemical clean on the headbox can remove the hot melts. Caustic soda is used because it breaks down inorganic materials like chalk, ash and stickie’s.

Felt conditioner

Felt conditioner works in a similar way to the wire conditioner helps prevents stickie’s and pitch from attaching to the felt and blocking the felt. An excessive amount of pitch on the felts not only hinders dewatering but sticks off a small amount of fibers. After time this will build up and cause defects in the sheet, potentially leading to breaks.

Chemical felt cleaning and conditioning

Chemical precipitates found in the press section are inorganic as well as organic. The organic or hydrophobic types include rosin size, wet strength resin, and pitch and hydrocarbon oil. Inorganic precipitates include clay, calcium carbonate.  

Generally speaking, chemicals for cleaning felts are applied using either a continuous or a shutdown cleaning method. Continuous cleaning effectively keeps the felt open during its run rather than relying on the more difficult job of cleaning a plugged felt.

Chemical felt washes are done more infrequently on PM3/6 than on PM4, usually if the machine has been shut for a while a chemical wash is usually done but on crawl speed to allow better penetration of the chemicals in the felt. Using a lower speed allows the chemicals to bed in and reacts with the dirt and contaminates.

 Acid

Acid is used in two ways on the machines, continuously dosing and shock dosing. The acid is used continuously on all of the machines to help keep the felt open, rather than relying on the shock cleaning on a closed felt.

The Shock dosing is done at a high concentration of acid, it is important to ensure the water is running, neat acid will melt the felts.

Acid is used to clean the felts and to neutralise the caustic that has been previously dosed. The acid is dosed through the chemical sprays on the felts. Acid is dosed for ten mins with 5 mins flushing time after. The acid removes the broken down pitch, fines etc. in the felts. The acid is delivered in a higher concentration when shock dosed and more dilution water is added when the acid is used continuously to keep the felts cleaned. The caustic id a alkaline which is on the opposite spectrum to the acid.

Caustic Soda

Caustic is used to treat the felts. The chemical is shock dosed through the chemical sprays onto the felts to break up the pitch, ash, fines and dirt collected in the felts. The caustic is applied for 10 mins on each of the felts. The issue with caustic being used on the felts is the chemical can cause the felt to close up, reducing the efficient of water removal. After the chemical dosing the lines are flushed with fresh water to clean the pipe work and sprays.

 Anti-Scale Chemical

Anti-Scale chemical is important chemical because the papermaking process is water based. Scale is made up of minerals (mainly chalk and limestoneare composed mainly of calcium carbonate (CaCO₃), magnesium hydroxide (Mg(OH)₂), and calcium sulfate (CaSO₄)) within the water precipitating out and building up within water systems.

Industrial water systems using hard water can experience breakdowns as the scale builds up in pipes, boilers, water tanks etc. To maintain the level of scale within the water systems a chemical is doses at specific points to dissolve/ break down the chalky deposits preventing build up and ultimately costly breakdowns.

The effects of scale

Can restrict the flow through pipes as the internal diameter decreases as the scale builds up in the pipe.

Scale impairs the heat exchange between metals into the water. this reduces the cooling/ heating efficiency and can lead to the metal components over heating. This causes issues with the drying cylinders requiring more steam to dry the paper because not al heat can be transferred through the cylinder if a build up of scale occurs.

Below is a list of all the dosing points in the site. There are problematic areas that scale is likely to build up and can cause catastrophic breakdowns for example. If the dosing to the pump seal water failed the pumps can clog up with scale, pumping would be reduced and the mechanical could seize. 

Bentonite addition - Technical Papermaking

Bentonite (anionic smectite clay)

Bentonite is the name given to the anionic smectite clay material used to improve retention and drainage. The composition of the particles can be described as very thin plates caused by the salts in the clay. This gives a large surface area for the particle to bond with other particles and cationic polymer.

The bentonite has two functions, primarily when use either before or after a cationic polymer it serves as a drainage/ retention aid. When bentonite is used with cationic polymer it can be used to control the level of pitch, tacky materials (commonly referred to as Anionic trash).

Bentonite added down-stream to polymer improves dewatering on the wire. Best achieved when high mass cationic polymer is used has been added so that the stock furnish has a momentary net cationic charge. If the stock has a high level of Anionic trash it makes sense to firstly treat the stock with sufficient cationic polymer.

Bentonite works with the polymer to increase the dewatering capabilities of the paper web, the two chemicals work in unison to achieve this. Bentonite works as a drainage aid allowing the water to be removed more easily from the web. When adjusting the chemicals the set points of both the polymer and the hydrocol must be reduced or increased together, having a high polymer dosage and low hydrocol dosage or vice versa for example can decrease the drainage on the machine and cause poor formation.

Diluted Bentonite is added post screen and works by reforming the fiber flocculation’s that have been broken down  by shear forces going through the primary machine screen. Bentonite brings the smaller flocs together for better formation on the fourdrinier.

Wet end polymer addition - Technical Papermaking

Polymer (cationic polyacrylamides)           

Flocculation, retention, and drainage is affected by the quality of the backwater as well as the polymer characteristics, these characteristics affect the electrokinetic energy between the fibers namely the charge density, size of the particles, the weight of the molecules etc.

                                                                  

A balance has to be made between the flocculation of stock and the drainage of the stock. This balance results in a need for fiber flocculation to be limited but the flocculation of fine particles and retention additives should be maximized. As the fibers and fillers flock together to form bonds the drainage on the wire decreases. The molecular weight of the polymer affects the drainage and flocculation. With higher weight flocks are better formed. To improve the drainage the weight of the molecule has to be reduced. It is very important to choose the right polymer to keep the balance.

The total branching of the polymer affects the flocculation. Branching is the term used to describe the polymers ability to bond with multiple chemicals, fibers, fines etc. with industrial water (Backwater system/ dirty water) it was found that polymer with a high degree of branching and a higher weight had more resistance against shear forces and held overall better bonds. This made a positive effect on the retention and drainage of the stock. The dosing point of the polymer is before the primary screen where it will experience high shear forces. Using a polymer with both of these qualities will benefit the formation/ retention.

  Polymer used is a dry chemical mixed with dilution water and stored in a chemical tank before being dosed. The issue with the liquid polymer is that it can affect the chemical balance in the thick stock system (flocculation/ retention is reduced) a higher dosing rate is needed to achieve the same parameters as the dry polymer.

The Diluted polymer is added pre-Primary screen and used to create flocks of fibres by reducing the negative charge between the fibres. The polymer is made up of Nano particles and chains which bond together fillers and fines to create flocks. Polymer bonds between the individual fibres forming hydrogen bonds between them. The screen breaks these down into smaller flocks/ chains. This aids in the formation and will improve strength. 

Fourdrinier Retention Mechanisms - Technical Papermaking

Mechanisms of retention/ drainage

Retention aids traditionally based on Alum, Alum neutralizes the charge of the paper making furnishes and was seen a "fix all" regarding wet end chemistry problems. 

Modern Retention aids based on PEI single polymers used "bridging" as the dominant mechanism for retention. the first type of polymers were of high molecular weight which brougfht fibers and filler together and formed "bridges". Nowa days new micro particle systems follow a complex flocculation system to improve wire retention.

What is of interest are the flocculation properties of polymers (retention aids) because many components of the stock furnish (sludge, fillers, fines etc) are too small to be mechanically retained on the wire and need to be bound to the larger fibers through flocculation. The ideal scenario would be to restrict fiber to fiber flocculation and encourage the smaller particles and additives to flock to the fibers. This would give the best retention and dewatering of the sheet.

Bellow is a table that describes the elements of papermaking that will affect the retention on a fourdrinier machine, they can be catagorised into, Pulp conditions, Wire conditions and the additives added to the stock/ furnish

Stock Factors	Conditions of Wire	Additives
pH	Sheet grammage	Types and amounts of fillers
Consistency	Sheet formation	Shape and density of mineral particles
Temperature	Fabric characteristics	Types and amounts of other additives
Fiber characteristics	Type of dewatering elements	Order of addition
Degree of system closure	Machine speed	Ionic balance
	Shake (if used) – old technique used on high quality paper machines	Level of anionic trash

Improving retention on the wire has many benifits, primarly cost. Papermachines producing News print have a retention of about 50%, increasing this retention to 55% for example will reduce the amount of primary stock needed to an extra 45%. for example at a retention of 50% producing 25t/hr throughput the mass of stock through the headbox needs to be 50t/hr, by increasing the retention to 55% the mass through the head box is reduced to 45T/hr

Colloid Chemistry - Technical Papermaking

Colloid chemistry

The word "colloids" (used in science) describes materials that have at least one dimension that is smaller than 1 micro-meters. The word colloid does not give any indication on the chemical makeup of the particle.

Almost everything the papermakers deal with can be considered to be colloidal. Although fibers are larger than the classical definition, the fiber surface is highly porous, and micro fibrils of colloidal dimensions extend out into solution from the surface of a refined cellulose fiber.

Other colloidal particles common in papermaking furnish include fiber fines, filler particles, sizing emulsion particles, and retention aid molecules (for example; molecules so big that they no longer behave like regular molecules – high molecular weight polymers).

The average end-to-end distance of a retention aid polymer (500 nm) is much larger than the size of a typical colloidal particle (2 to 5 nm ).

When papermakers refer to colloids, they usually are most interested in the colloidal organic materials, including fatty acids, lignin by-products, and oxidized hemicellulose. These are often called "DSC" for "dissolved and colloidal materials," or "anionic trash."

Deposits form on papermaking equipment due to the “thermodynamic instability” of many materials suspended in water (oils, pitch, hot melts etc.). We can combat this by getting those materials to deposit/ bind onto fibers, thereby keeping their concentration low in the liquid, ergo less deposits on the machine.

Retention of colloidal materials is best achieved by a combination of coagulation (treatment to neutralize charges (conductivity), causing the particles to come out of the suspension) and flocculation (treatment with polyelectrolytes (high molecular polymers) so large that they can bridge between the surfaces).

Consistency Meters - Papermachine Automation

Consistency Meters

Rotor designed consistency meters

The most common consistency meter found, the rotor design CM (Consistency meter) sits just outside of the stock flow stream. A deflector rotor pulls stock into the recess where the measurement device rotates at a constant speed. The stock gets thicker the torque on the motor to maintain that speed increases. The consistency can then be measured against the motor torque. This is known as a strain gauge. These types of measurement devices require a certain flow to function correctly and can measure consistencies of 1% - 10%.

unlike fixed blade consistency meters the rotor design is not affected by the variations in stock flows because the device creates its own flow from the deflection rotor onto the measurment device.

Fixed blade Consistency meters

Fixed blade CM work in the same way as the rotor design except the paddle is placed within the fiber flow stream. The fixed paddles moves with the fiber flow. The consistency increases in the pip, this in turn increases the force against the blade. The force is measured by the meter and calculated to a consistency. This is another instance of a strain gauge. 

The limitation with this type of measurement device is the stationary aspect. As stock flows past the paddle, fiber and rejects can stick/ build up reducing the accuracy of the device.
Variable flow within the pipe will alter the consistency measurment. A higher flow will add a higher force onto the consistency meter resulting in a higher measured consistency.

The major advantage of this type of meaurment is cost - usually customers purchase a fixed blade/ dynamic blade consistency measurment will the intention of replaying it with a better model.

Microwave Consistency meters

Microwave transmitters work on the principle that sending microwaves through water the waves travel at a certain speed. When fiber is introduces the microwaves move faster through the stream. The consistency can therefore be measured depending on the speed of the microwaves being sent and received by the meter. 

Using the calculation;   Velocity = C / sqrt(e)   

Where; C = Speed of light in a vacuum

     E = Dielectric constant of liquid (water)

These devices are more accurate than the rotor/ blade design. The microwave Transmitter also has no moving parts for the pulp to affect/ build up on like the blade transmitter.

The CM is not affected by the flow rate, colour, and brightness, like traditional microwave ovens they are highly affected by metals. these consistency meters are used within very clean pulp systems like the aproach flow because the likleyhood of metals entering the stream are very low. The microwave transmitter needs to be the same size as the pipe being used. Due to the expensive nature of these devices microwave transmitters are typically used on smaller pipe work or substituted for cheaper models. 

For more Info on Instrumentation within paper-making check out my other blog posts!

Level transmitters;
https://www.papermakingbible.co.uk/2018/04/process-level-indicators-papermachine.html

PID controllers;
https://www.papermakingbible.co.uk/2018/02/pid-control-loop-parameters.htm

Consistency Measurement - Technical Papermaking

Consistency Measurement


Testing the consistency of a stock is the same as consistency of water solution. Consistency is the term given to the amount of solid matter within a body of liquid. The higher the consistency the more solid matter is present within the same volume of liquid. Consistency is measured in percentage; it’s a percentage/ ratio of solid matter to water in a certain volume.


To find the consistency we need to test the amount of dry content within a set volume of water. For example if we wanted the consistency of 100ml of water, and tested 25 gram of dry solids the consistency would be about 25%.

To capture the solid content within a solution a filter paper is used. Firstly the filter paper is weighed at 0% moisture (previously oven dried) and recorded. The stock sample is weighed (1g = 1ml) and recorded. The weighed stock sample is filtered through a vacuum and rinsed making sure all of the solids from the sample are on the sheet. The filtered solids and the filter paper go into the oven to remove all of the remaining moisture.

Once the sample leaves the oven it is weighed and recorded. This formula is used to calculate the consistency below.

Consistency=  (Mass of Solids)/(Volume of Liquid)

We have to calculate the mass of the solids captured on the filter paper, this is calculated below;

Mass of Solids=Total dry weight-Filter paper weight

Here are a list of results that I took from measuring the consistencies of a pulp preparation plant's stock flows through two fractionators and Long fiber screens.