Problems in Stock Agitation

Introduction

Understanding stock agitation requires a basic knowledge of physics. Reading basic references, Oldshue and Yackel, helps too.  Insight into stock mixing also requires experience with stock behavior.  Since stock goes from liquid to solid suddenly, is opaque, and each stock varies in characteristics, art is required as well.  The following are some problems that demonstrate difficulties in real life, and an approach for analysis for each.

 

Retention Time Decrease, or Tonnage Increase

Most mills’ mission is to increase production on the equipment they have. So tonnage seem to double every 20 years.  This is good for profits, but tough on agitators. Since retention time decreases, the agitators’ ability to mix to uniform consistencies declines. Soon the next process suffers.  This is especially true with under the machine units, such as couch, press or dry end pits, because increase in storage volume would require moving machine foundations.

Increases in horsepower can compensate for lowered retention time, but stock must have enough time to makedown.  This varies with type and dryness. Couch broke allows more flexibility then press broke, and so on.  In processes involving broke, extraction plates can be added or changed to improve performance.  Pump out consistency can be raised to increase retention time, though this is somewhat contrary. In some cases, older types of mixers (some mills still use midfeather agitators) can be replaced.

Engineers should understand, as new machines are installed, that this is an inevitable process, and make some volume allowance.  Minimum makedown times should be considered along with maximum desirable pit dimensions for the present agitator.  Power is easy to add, especially in cross shaft agitators, but more volume will be unavailable in the future because of the decisions made now.

 

Shaft Length and Fluid Forces

Allowable shaft lengths in agitators depend on critical speed, allowable deflection and stress, in that order of importance.  Deflection and critical speed are related of course, so shaft stiffness is of primary concern.  Shaft stress can usually be handled by using better materials, and is a lesser problem.

For side insert agitators, there is really no substitute for shaft diameter, since deflection is critical to packing, and even more to mechanical seal performance.  Putting the bearings closer to the propeller usually impairs maintainability of the bearings and stuffing box.

In vertical and cross shaft agitators, pipe shafts are attractive because stiffness is increased.  This increased stiffness also allows a lot of flexibility in application, since more power can generally be added without affecting shaft stress.

Agitators with overhung shafts run up against the problem of fluid forces.  Generally agitators want to produce axial flow, or flow in line with the shaft axis since this provides the most mixing.  But no propeller is perfect at this, so radial forces are produced. Inflows impinge on or close structures interfere with flow. And it is possible that a blade may fail and be lost, imbalancing the load.  Or a propeller may be operated at the level of the stock. Fluid forces are produced at the propeller locations, perpendicular to the shaft. 

For vertical agitator reducers with overhung shafts, this condition is generally the limiting factor, since allowable overhung shaft moment is limited.  For side insert agitators, this is a problem since it adds load to the propeller weight.  The question is, how big a problem.  There is not a lot of data available.  A conservative approach is to use the formula

Fluid Force = HP x 126,000 / ( RPM x Prop Diameter inches)

This assumes the whole torque goes into fluid forces.  As a refinement, this number can be divided by the number of blades, since this imbalance would be that if one blade were lost.  But these are limits to what the forces may be, not what they actually are.

 

Critical Speed and Vertical Agitator Bridge Support Stiffness

Fluid forces create moments, seen as torque around bridges axes, which can drive additional deflection. These loads, seen as a couple on the 2 beams forming the bridge, produce additional deflection that in turn can produce wear on bottom bearings in tall vertical agitators, or imbalance on overhung units.

A stiffness analysis of the support bridge is required.  This can be very simple, assuming a simple beam and center mass, to calculate a critical speed.  If the bridge critical speed is near to the propeller blade rate, this may be a problem.  The blade rate can be the RPM, or it may be the RPM times the number of blades in a single group.  This would be the case if stock level in the tank passed through a blade group at any single elevation. This is one reason we use single blades spiraled up a long shaft if multiple impeller levels are needed.

The easiest cure is additional bridge stiffness.  Following the AISC code for unsupported beam lengths will get close to enough cross bracing to avoid this problem.

 
Power Draw, Older Agitators and Secondary Fiber

Most machines now operate on a furnish using some or all secondary fiber.  Since characteristics can vary a lot, it is difficult to choose appropriate stock factors. But we know that stock factors are lower then required for the 100% virgin fiber formerly composing furnish.

Further, older agitators were sized when power was cheaper, and less concern was given to motor size. Brinkley often added a motor size to agitator drives that were installed 25 years ago.  Not only are agitators over motored, but amperage draws show that motors are often not highly loaded, so that they are operating at lower motor efficiencies

Evaluation of actual mixing requirements can result in smaller motor sizes, and power savings.

How do you recoup these savings at the least cost?  Motors are often replaced with high efficiency motors as part of ongoing programs.  Amp draws on existing agitators should be taken at various operating conditions when this is done, and appropriate motor sizes installed.  Sometimes older agitator are not working at all, and the resulting poor agitation can also be remedied.

Vertical agitators with gear drives are most common.  Changing drive ratio is difficult and expensive.  New gears can be purchased, but most mechanics do not usually have sufficient practice working on gear drives to do a good job.  Increased rebuild frequency and expense is the result.

Vertical agitator power draw can be reduced by:

  • Removing a blade or blade group – but don’t imbalance the agitator, and don’t reduce agitation where you need it
     

  • Changing pitch on the blades – but this has limited effect
     

  • Trimming blades – but this may reduce propeller diameter to tank diameter ratio and result in stock dewatering and may reduce blade efficiency

 

Side Insert Agitator power draw can be reduced by:

  • Changing pitch on blades – again with limited effect
     

  • Changing RPM – easy to increase driver sheave size but the guard may be affected
     

  • Trimming blades – same as above

The manufacturer should be involved in this process, if they can be found.

 

Minimum Propeller Size

Since the primary problem in stock agitation is preventing dewatering, or settling out of the fiber, it is important to have a large enough agitator to influence the diameter to be agitated.  This is of course part of the blending, or pumping rate criteria, but practical rules of thumb are useful, since most agitation mistakes are made by having propellers that are ineffective in agitating the whole tank.

Vertical agitators are 2 or 3 times more efficient at producing flow then side insert agitators, since they have relatively larger propellers.  So horsepower can be reduced considerably using a vertical agitator, but the downside is initial cost.  A large reducer necessary for low RPM vertical agitators costs money, as does the bridge to support it.  But a vertical agitator doesn’t have a stuffing box seal, either.

In vertical agitators, our rules of thumb are that a propeller diameter (or swing) to tank effective diameter ratio of 0.2 to 0.25 is required for general mixing, and 0.33 or so for stock agitation.

For side insert agitators, since the process is to produce a momentum jet that carries across the chest, it is a little more difficult to quantify the effect.  Further, propeller size is limited by weight and shaft considerations. Prop diameter especially depends on the type of service.  However, we suggest the following as guidelines:

Tank DiameterProp Diameter
To 10 feet30''
10-14 feet36''
14-18 feet42''
18-24 feet48''
24 feet plus54''-60''

With current blade technology, larger diameter is inexpensive.  And larger blades do not increase a lot in weight.  So the inclination is to go to a larger diameter propeller.  

A word about propeller construction.  Manufacturers formerly cast all blades and hubs.  This produced a uniform material with excellent fatigue and shock resistance properties, and therefore a life that was measured in decades.  Current blades are often formed or fabricated from rolled steel, which is itself not a uniform product.  Stock service is not general mixing service, and blades designed for general service will fail.  Fabricated blades and hubs have to be well designed to stand stock service.

Another word about propeller construction.  The people who care most about propeller efficiency run ships.  These props are large diameter multi blade units with highly contoured variable pitch blades.  Not cheap to make, but efficient (and similar to the expensive cast blades that used to be used on agitators a lot).  Current high efficiency blades made out of formed plate are higher efficiency then flat plates.  “High Efficiency” propellers will save you motor size.  The check is in the mail.  We’ll do lunch sometime.

 

Odd Shaped Chests

For mixing, it is always helpful to have circular chests or square chests with height roughly matching diameter.  However, space or process considerations often make this impossible.

In most blending operations it is possible to deal with tanks with one side longer than another.  Stock is different, since minimum velocities must always be maintained, or it dewaters.  Stock cannot flow into or out of sharp corners, so tank bottoms should always have fillets. 

We use the concept of equivalent diameter to size vertical agitators, with the equivalent diameter equal to the diameter of a circle of equal area to the shape being agitated.  Definite practical limits exist, and vertical agitators mixing tanks with width in excess of length by a ratio of 1.25 is difficult.  For side inserts, the 1.25 ratio applies to the width of the wall on which the agitator is placed to the throw distance.  Multiple agitators can be used to get to longer tanks, and cross shaft agitators for couch or press pits. 

In a large diameter shallow tank, side insert agitators are not efficient, and a single vertical agitator propeller ½ its diameter off tank bottom is too far from the floor of the tank. So we suggest using 3 smaller vertical in a triangular planform.

Never install agitators with opposing flows toward each other as the dead spot is in the middle.

 

Baffling of Tanks to Produce Top to Bottom Agitation

Tank baffles are necessary in some vertical agitator applications.  Stock at consistencies above 3-3.5% is self baffling. Stock will swirl when consistency is less than 2.5-3%.  If swirling starts, mixing will be ineffective, and top to bottom motion that produces complete mixing will cease.  So baffles are required.

Standard baffling is (4) baffles of 1/12th tank diameter, or (2) of 1/8th  tank diameter.  For stock service, these should be set off wall by 1/3rd of their width.   In some area swirling is a good idea.  At the surface, it helps to draw in surface skim or additives, so keep the baffle top 2’ below the surface.  Keep the baffles a foot above floor level to aid in cleaning.

 

Low Stock Consistency

Stock agitators are designed to mix stock. This requires a lot of energy compared to typical blending operations. Often stock agitators are tested or run in water.  This results in a lot of splashing, and can result in excessive vibration and damage.  The fiber in stock, when introduced absorbs the mixing energy and results in smooth operation.  Care should be taken during startup, and if consistency varies widely in operation it may be desirable to lower mixer horsepower using dual speed motors or variable frequency drives.

 

Zone Agitation and Surface Motion

Is full chest motion necessary, or does it just sound like a good idea?  Most stock chests with side insert agitators will not show surface motion if stock depth is over ¾ of chest diameter.  Normally, this doesn’t matter, since stock is pumped out at the bottom, and that is the only place where a uniform consistency is important.

However, in Dump or Blow Tanks a floating layer is often created, which must be broken up to achieve uniformity.  In this case, surface motion is necessary, and generally cannot wait until stock level is drawn down sufficiently to mixing to occur.  Further, additives introduced on the surface need to be drawn in to be effective.  It is possible to introduce these behind the propeller on a side insert agitator.

The best solution for surface mixing is to use a vertical agitator with a blade within ½ blade diameter of the surface to mix in floats.  Of course, extensive operation of propellers at the stock surface may entrain air, or result in imbalance.

 
Direction of Primary Flow

Picturing flow in your mind allows qualitative solutions to problems. Most agitators are designed to pump in one direction, so getting the direction right is important.. 

Side Insert Agitators create a jet of flow out into the tank from the prop.  This acts like any other fluid jest and expands, entraining other stock. The idea is to have enough jet momentum when you get to the other side of the chest so that stock doesn’t dewater.  Fillets reduce this distance.  The flow returns to the back side of the prop along the chest sides, and above the top of the jet.  So places in a chest where flow out and back are low don’t get mixed. 

Vertical Agitators pump down from the prop, out to the tank sides, with the flow returning up the sides of the tank to the top of the tank.  You want the flow to sweep the tank floor so stock can’t deposit. If the flow doesn’t reach the tank sides with sufficient momentum, stock dewaters there. Prop diameter to tank diameter ratio is important. One the upper side, if the stock surface is more than 2 prop diameters above the upper propeller blade, little motion will be seen.

Cross Shaft Agitators are a special case.  The flow is along the shaft, in the direction of pumping.  The direction of pumping should be toward the extraction plate or pump suction.  This seems wrong, since this would short circuit.  However, pumping to the extraction point is correct, as sheet enters the flow at the pit surface, hopefully at least 1-2 feet above the top of the agitator to allow flow establishment and stop aeration.  The sheet then is pulled away from the pump out by the return flow, then comes down into the agitator at the opposite end of the pit and goes through the blade set to be broken up.