Design of Blend Chest Agitators; Guidelines, Data Required and Agitator Sixing

Steven F. Drury, Director of Applications at Sharpe Mixers.

Sharpe Mixers Director of Applications Engineering Steve Drury, PE presented a paper Design of Blend Chest Agitators; Guidelines, Data Required and Agitator Sixing at the recent TAPPI sponsored PEERS conference in Tacoma Washington.

Steve’s paper covered what a mixer vendor needs to know from the user to be able to design a cost effective agitator for any dilute stock application:

Mixing and blending are critical to a successful pulp/paper mill operation. The mass concentration of the pulp suspensions (aka consistency) in chests must be uniform at all locations. This is typically done with open face high efficiency impellers (hydrofoils). Along with a properly sized agitator the proper chest design and feed/pump suction locations are also critical to uniform consistency. One method used to provide uniformity is the momentum method which this presentation is based on. The mathematical model for the momentum method will be presented along with guidelines for chest designs and agitator locations.

A successful agitator design relies on good data and information from the mill engineer such as geometry of the chest and the process information outlining what is expected of the agitator.

This presentation will describe what an agitator supplier must know to design the equipment and describe the “why” so the plant engineer can understand not only the importance of chest geometry, feed and pump suction locations, but the process information as well: stock characterization, throughput and temperature.  A good understanding of the relative importance of these design factors will help the plant engineer in making decisions for optimization, retrofits and upgrades.

A typical blend chest design will be used to illustrate the design steps.

Mixing of dilute stock (up to 6.0% OD consistency) is a very important unit operation in the pulp and paper industry; however very little published information is available for the plant engineer’s use to size a mixer. Standard mixer design books such as Handbook of Industrial Mixing (1) and Fluid Mixing Technology (2) have sections on pulp mixing but detailed procedures or design data is not included. To the best of this speaker’s knowledge the only detailed book on pulp and paper agitation is TAPPI’s Pulp and Paper Agitation (3) The procedures outlined in this book are based on the concept of momentum flux (4) also known as QV (Q for flow and V for velocity). Using this concept along with mixer power and flow equations a relationship of mixer impeller diameter and rotational speed can be developed.

In order to size a mixer (power, speed and geometry) knowledge of the application (process specification) is critical. Knowledge of the mechanical requirements (mechanical specification) will allow for geometric and strength design.

Once the process parameters and chest geometry are set the mixer can be sized for power, speed and impeller geometry. A sample problem will illustrate this procedure.

As chest geometry will have a significant effect on mixer sizing. A series of figures showing recommended geometries will be presented. 

Attention to mechanical design is important. Reliability, seal selection, bearing life, maintainece access etc. all contribute to capital and operational cost. A short description of critical speed, shaft design, service life and bearing life will be presented in order to understand the impact of these features for cost and reliability.

Evaluating competing designs can be challenging, as each vendor will more than likely have different selections for power and impeller speed. Some guidelines for comparison of operating costs vs. capital costs are shown along with process results (power, torque and QV)

Information on mixer design, evaluation, standards and science can be hard to find. A few published resources are presented for reference.

PEERS, an acronym for Pulping, Engineering, Environmental, Recycling, Sustainability, PEERS represents the most dynamic, forward leaning topics in the forest products arena today.  PEERS offers an expert-led program that has been proven to provide real-world results in each of these critical areas of the mill.  Planned and led by working professions, every session has been designed to address real challenges attendees face on the job.

Presenters and attendees are pulp and paper experts/equipment vendors/operators/consulting engineers and plant engineers directly involved with mill operations.  These are the people who "make it work".