General Process Considerations In The Selection Of Agitators For Simple Applications And Impeller Design
The first thing to keep in mind is proper positioning of the agitator with respect to the vessel geometry. If not, it is like placing your General Manager in Gang Man’s position. Performance is liable to fail.
The general thumb-rule in any mixing problem is to determine the amount of energy or power required to perform the operation; then selecting the most efficient method of applying this energy in terms of low a> first cost, b> operating cost & c> maintenance cost. The major steps in selecting a mixer from the process standpoint are as follows.
- Definition of the problem.
- Description of materials to be mixed.
- The mixing cycle.
- Classification towards purpose.
Definition of the problem.
A clear picture what is to be attained. E.g. to make up and hold in uniform suspension 10% slurry of hydrated lime or to dissolve 30 Kgs. of sugar in 50 liters. water in 5 minutes time.
Description of materials to be mixed.
Complete physical properties of each ingredient should be known, including sp.gr., initial and final viscosities, easiness of miscibility, etc.
The mixing cycle.
A continuous operation with constant flow rate and retention time is always preferred. If it is batch operation, to obtain the followings.
- Batch size,
- Time allowed or reqd. for operation,
- Sequence of adding materials,
- Changes of product characteristics during agitation,
- Whether the mixer is to be operative during filling or emptying the tank.
Classification towards purpose.
- Blending of miscible liquids.
- Dispersion (liquid- liquid, solid-liquid).
- Heat exchange.
- Solids suspension.
- Chemical reaction.
- Extraction including washing and leaching.
- Gas dispersion, absorption, and stripping.
Blending of miscible liquids.
Easiest of agitating applications and normally axial turbine or hydrofoil impellers are preferred. Jet mixing by recirculation with an ordinary pump is equally good at lower cost.
In dissolving, we want provision of high flow rate and low shear past the solid surface. Unless the solid is polymeric or sticky or viscous in nature, it is an easy operation. Data solicited are solid percentage, physical characteristics with interim changes, temp, solubility and permissible dissolving time.
Dispersion (liquid- liquid, solid-liquid).
Dispersion refers to mixing of non-miscible liquids or of solids in liquids, into somewhat homogeneous mass whose stability is measured by its life before reasonable separation occurs. Power input varies greatly depending on purpose and impellers generally used are pitched blade turbine, or saw tooth cutter. This is most critical of mixing problems and unless properly understood, the design is liable to fail.
Dispersion (liquid- liquid, solid-liquid).
Used to speed up heat transfer by forced convection. Apparently simple but for critical applications, following data helps.
- Tank dimensions and details of jacket and coils, preferably G.A.drawing.
- Heat transfer co-efficients.
- Specific heats and thermal conductivities.
- Temp. of batch at start and end of cycle.
- Temp. – Viscosity curve of components.
- Whether solid suspension is included.
Imparting a high power to break the molecular chains and to form the oil-water emulsion with a non-ionic surfactant. Other applications are mainly in paints & lubricants sector. Some emulsion may be steady for years and some break within minutes. Static mixers are preferred than agitators for high instantaneous power resulting better emulsion but washing pump & pipeline is problematic.
It is simple physical (like mixing) operation but power consumption varies greatly on purpose e.g. a> complete motion of solids, b> complete suspension of solids, c> complete uniformity. Power req. is in ratio of 1: 2: 5 for said operations.
It can be considered as combination of blending, dissolving, heat transfer, extraction, gas dispersion, and solid suspension etc. Usually an easy task from agitator designer’s point but to be sure, pilot plant study is always recommended. A haphazard selection is vulnerable and over design (like peripheral impeller tip speed) has various detrimental effects on the final product.
Extraction including washing and leaching.
This is normally a continuous counter- current (fluidized bed) process like solid suspension involving water to be well mixed up with other ingredients and the ingredients separates out by gravity separation. Usually of interest for mining people.
Gas dispersion, absorption, and stripping.
Gas is impregnated from bottom as small bubbles and intimately distributed throughout the liquid usually resulting a chemical reaction. Generally curved vane impeller or multiple turbines are preferred with high speed. Fully baffled tanks should be tall and narrow in construction. Pressurized chamber accelerates the process. A better way is by static mixer employing a liquid pump and a pressurized semi- permeable solid wall to impregnate gas under pressure.
It is opposite of dissolving and is accomplished by cooling a saturated solution or by heating to drive out the solvent. The heat transfer requires a good flow. Satisfactory handling of crystals is of prime importance. Pilot plant data are desired. Generally crystals deposit at the bottom but if process deserves to be uniformly suspended, much study on the crystal structure & sensitivity is to be made for speed selection. Fluid-foil or aerofoil impellers with high flow and low shear are suggested.
Now let us try to understand basic impeller design.
As like fan (for air handling) or hydel power turbines, much research has been conducted towards agitator impeller design and it is a very wide subject with much more further scope to research and improve. Impeller is designed mainly keeping in mind the purpose or application and sometimes custom built.
The basic designs are as follows.
- Marine Propeller– Looks like a table fan blade. Suitable for high flow and low shear e.g. blending, dissolving, heat transfer, etc. Unsuitable for solid suspension, dispersion, extraction, gas dispersion, etc. flow efficiency- 50-60%. Getting outdated but manufacturers with their old proven designs are still sticking to that.
- Pitched Blade Turbine – Consisting 4 nos. 45 degree vertically inclined flat blades (like ceiling fans), has reasonable flow and shear compromising both. Mainly used for medium viscous liquids and solid suspension. Most versatile in its mode of operation.
- Hydrofoil is supposed to be a foil in water and looks like pitched blade turbine with an angled cut on the lower periphery. Its flow efficiency is better than marine propeller, replacing the later in most applications and is most desired for mixing purposes but unsuitable for high shear application.
- Aerofoil is having a classic stand fan blade type look and its properties are almost like hydrofoil impellers. Of interest for mining people for mineral washing and beneficiation for energy efficiency.
- Curved vane impeller looks like straight Pelton wheel designed to hold moving particles for some time e.g. for air absorption.
- Stator rotor– Old outdated design supposed to bring highest shearing action and usually specified for oil-water emulsion.
- Saw Tooth Cutter is reasonably good for handling medium to high viscous liquids. It is most economic for emulsification. Bad design for blending simple solutions as well as handling semi solids (grease, honey etc.). Suggested for oil-water emulsion, lubricants, paints, etc.
- Anchor (with its namely look) is used for heat transfer from bottom or scraping viscous liquids.
- Helical Ribbon looking like screwed ladder or double helix model of our gene. With its diameter close to tank diameter, is used for viscosity above 80 poise.
Another very effective design for handling viscous fluid is to use pitched blade turbine placed at bottom of draft tube (equaling 70% of tank dia.) at medium speed.
I am very sorry for my weakness even in rough schematic drawing that could have helped in better understanding.
Till date we very often encounter motor power selection much on higher side with no necessity. Again lower speed was thought to be superior in terms of application.
Now the international trend is that if the process permits (unless molecular chains break up or organic growth is hampered in biological processes) lower speeds are only negotiated with motor power reduction without affecting the application parameters.
Basic power calculation for turbulent flow is :-
P=K*NS* S.G*N3*D5/ 160000
P= BREAK HORSE POWER (H.P.)
K= A CONSTANT VARIES GREATLY FROM 0.3 UPTO 8 AND IS DEPENDENT MAINLY ON BLADE GEOMETRY, TANK DIMENSIONS, BAFFLES, PHYSICAL FLUID CHARACTERISTICS, ETC. TERMED AS THE IMPELLER NUMBER.
NS= NO. OF IMPELLERS.
S.G. =SP.GR. OF THE LIQUID.
N= ROTATIONAL SPEED RPM.
D= IMPELLER DIAMETER IN METER.
Considering gear efficiency and motor margin, motor power is determined.
Power calculation in viscous fluid is cumbersome and then also is unreliable. We shamefully take motor margins as 50% minimum. For basics, BHP varies with 1st. power of rpm and 3rd. power of impeller diameter for viscous impellers. In fact for thumb rule at low speed, wattage equals the weight of total mass in grams transferred except in thixotropic conditions.
For both turbulent and viscous motion flow rate is determined by
Q= FLOW RATE IN CUBIC METER PER MINUTE.
K= PUMPING NUMBER MAX. 0.7 (FOR HIGH FLOW & LOW SHEAR) AND MIN. 0.01(FOR LOW FLOW & HIGH SHEAR).
D= IMP. DIA IN MTRS.
The agitator price is somewhat very little in comparison to what a process would gain and deserves from a reasonably good design. Pilot plants do come and go but the commercial plant determines the profit percentage. It is always suggestable to go for much in-depth of pilot plant studies before arriving at the big risk time.
Furthermore, in case of any doubt, checks and cross checks from different agencies are likely to be obtained and it is available at reasonable cost. Designer should have adequate experience in the proper application you are trying to achieve.
In case you have any quarries/ suggestions, I would be glad to share our knowledge.
Data Usually Required For Proper Design.
Purpose of agitation.
Materials to be mixed with individual physical characters and their quantities.
Tank dimensions preferably with sketch.
Electrical: 1 ph. Or 3 ph., flp or not etc.
There is no perfect design and I strive to provide you low cost agitator as per best of my knowledge to suit your purpose and to fetch gold both for you and me. With the above notes, please do not get agitated if I ask for your valued order with little knowledge and no experience of my own as on 07-01-2014.