Agitator and Impeller Types: How to Specify a Tank Agitator

An agitator is a motor and gearbox, a shaft, a seal and one or more impellers, and the impeller decides what it can do. Impellers come in three families. Axial flow impellers, the hydrofoil and the pitched blade turbine, pump liquid along the shaft and suit blending, solids suspension and heat transfer in thin to medium liquids. Radial turbines, such as the six blade Rushton, push liquid out towards the wall and are used mainly to disperse gas. Close clearance anchors and helical ribbons sweep almost the full vessel diameter and handle very viscous products [1, 2, 3].

The impeller also sets the power. In one published test a Rushton turbine's power number was 5.55 against 0.36 for a hydrofoil [3]. On those figures, at the same speed and diameter, the Rushton draws about 15 times the power. A motor rating means little until the bidder states the impeller, diameter, speed and power number.

Why this matters now

An enquiry that says only "mixing tank with agitator" invites bids that cannot be compared: a hydrofoil from one vendor, a pitched blade turbine with a bigger motor from another, a Rushton from a third. A datasheet that states the duty, fluids and vessel, and asks every bidder for the same figures, lets you settle the choice during technical bid evaluation, before the nozzle and baffles are fixed.

Match the impeller to the duty and viscosity

Viscosity enters through the impeller Reynolds number, Re = ρND²/μ: density times speed in turns per second times impeller diameter squared, divided by viscosity. AIChE's CEP magazine treats flow as fully laminar below an Re of 10 and turbulent above 10,000. It sets out the conventional guide: hydrofoils for thin liquids in turbulent flow, pitched blade turbines for the transitional range, and close clearance designs, the helical ribbon for example, for viscous products in laminar flow. The same article argues that hydrofoils also work well in laminar duty [2].

For blending and solids, axial impellers circulate the whole batch; in the university test, at equal power input, down pumping axial impellers gave shorter mixing times than radial turbines [3]. Tell bidders whether solids must just leave the bottom or be uniform, as these are different targets [6].

For gas, EKATO notes the Rushton's relatively low flooding limit, the gas rate at which the impeller is overwhelmed, and a large fall in power once gas flows [1]. Chemineer states its parabolic blade BT 6 has nearly six times the gas dispersing capability of a Rushton [4]. As gassed power is lower, check the motor also covers ungassed power.

Dispersing one liquid in another needs both shear and pumping. Chemineer offers its P 4 axial impeller for such immiscible blending, and an adjustable high shear impeller, ChemShear, for very fine dispersions [4]. Jacket choice for heat transfer is covered in our guide to jacketed vessels and glass lined reactors.

Baffles and shaft seals

Baffles are vertical plates on the vessel wall. Without them, thin liquids swirl round with the impeller and form a surface vortex [5, 7]. The usual arrangement is four flat plates, each one twelfth of the vessel diameter wide, at 90° spacing. Where fouling matters they can stand off the wall by a third of their width, and by half at higher viscosity [7].

Baffles also change the power. In one study, removing the baffles around an off centre hydrofoil in water cut measured power by about 40% at the same speed [7]. So the vendor's power figure must assume the baffles you will actually fit.

The seal is where the shaft enters the vessel. Lip seals suit low pressure. Packing in a stuffing box costs less but relies on a constant leak of lubricant along the shaft, unwelcome where contents are toxic or volatile. Mechanical seals are the usual choice for closed or pressure rated vessels and volatile products; one mixer maker quotes them from vacuum tight to 25 bar, and names shaft deflection among the main causes of seal failure [8]. For toxic or flammable contents, ask for a double mechanical seal: it has the lowest leakage, any leak goes into the vessel rather than to atmosphere, and it often needs a lubricating liquid supply [5].

Where the area is hazardous, put the zone on the datasheet. In Great Britain the Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations 2016 cover mechanical as well as electrical equipment [9], so ask about the gearbox and seal, not only the motor. This is general information, not legal advice.

Power number, power and tip speed

Shaft power is P = Np × ρ × N³ × D⁵, with P in watts, ρ in kg/m³, N in turns per second and D in metres. In a baffled tank the power number Np is constant above an Re of about 10,000 [2] and depends on impeller and tank geometry. One university rig with D/T of one third measured 0.36, 1.19 and 5.55 for a hydrofoil, a down pumping pitched blade turbine and a Rushton, with power measured at the motor, so possibly including drive losses [3]. Chemineer's figures, in a 2008 presentation hosted by ISPE Boston, are 0.28, 1.3 and 5.0 for its HE 3, P 4 pitched blade and D 6 Rushton [5]. Below the turbulent range Np changes with Re, so ask for the vendor's power curve.

A 10% larger impeller at the same speed draws 61% more power, and 10% more speed draws 33% more. In turbulent flow power is proportional to density, so a factory run on water understates the power a denser product will need.

Two derived figures help compare bids: power per unit volume, in kW/m³, and tip speed, π × N × D. At equal power a low power number hydrofoil turns faster than a Rushton of the same diameter, so ask each bidder which figure they designed to.

What to put on the agitator datasheet

Based on Dynamix's application data sheet [6], give every bidder:

• the duty and how success is measured: blend time, solids off the bottom or uniform, gas rate or heat duty
• vessel diameter, height, heads, open or closed, and minimum, normal and maximum liquid levels
• baffles, coils and other internals
• each liquid's proportion, density and viscosity, and the final mixture's
• solids proportion, density, particle size, settling rate, and whether abrasive or sticky
• foaming, batch or continuous, and pressure, vacuum and temperature ranges
• wetted materials, top or side entry, nozzle size and preferred seal
• electrical supply, variable speed and hazardous area zone

Ask each bidder to return impeller type, diameter and speed, the power number used, absorbed power at the highest density (and ungassed), motor rating, tip speed and seal arrangement.

Practical example

Take an illustrative vessel 2.4 m in diameter, filled to 2.4 m, holding about 10.9 m³ of an aqueous product at 1,050 kg/m³ and 5 mPa s. Three bidders each propose a 0.8 m impeller (one third of the vessel diameter) at 84 rpm, or 1.4 turns per second. Power numbers are the university rig's [3], for illustration only.

• Re = 1,050 × 1.4 × 0.8² ÷ 0.005 = about 188,000, so flow is turbulent.
• ρ × N³ × D⁵ = 1,050 × 2.744 × 0.3277 = 944 W, multiplied by each power number.

Suppose the pitched blade bid has a 1.5 kW motor and a 95% efficient gearbox, both illustrative. At 1,050 kg/m³ the motor delivers 1.12 ÷ 0.95 = 1.18 kW, 79% of rating. If the batch can reach 1,300 kg/m³ and stays turbulent, shaft power rises to 1.39 kW and motor load to 1.46 kW, over 97% of rating. A water test at 1,000 kg/m³ shows only 1.07 kW. A rival bid with the same impeller at 0.88 m, 10% larger, draws about 1.81 kW if the power number holds, or 1.90 kW at the motor, beyond a 1.5 kW rating. So check every bid at the highest density.

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