Beyond the impeller: understanding agitator types, design & performance-

"Welcome, curious minds!"

          Selecting the right agitator is critical for achieving efficient mixing, maintaining product quality, ensuring reliable heat and mass transfer, and operating a process safely. The selection of an agitator depends on several factors, including the reaction mass, viscosity, density, operating volume, required mixing intensity, solid suspension requirements, gas dispersion, and heat-transfer requirements.

          In this blog, we will discuss the basic concepts of agitation and mixing, different types of industrial agitators, their applications, advantages and disadvantages, important dimensionless numbers, agitator design parameters, power and pumping calculations, and the Chem Scale for agitation intensity.

         To make the concepts easier to understand, a worked example is also included at the end of the blog.

Agitation vs. Mixing: what is the difference?

        If you ask three different engineers to explain the difference between agitation and mixing, you may get three different answers. Although the terms are often used interchangeably, there is a fundamental difference between them.

Agitation	Mixing
1. Agitation is the induced motion of a fluid in a specific circulation pattern within a vessel.	1. Mixing is the process of distributing two or more separate phases or components throughout one another.
2. It can occur within a single homogeneous phase, such as water.	2. It generally involves two or more distinct phases or components, such as solid-liquid or gas-liquid systems.
3. It creates fluid flow, velocity, and circulation using a mechanical device such as an impeller.	3. It aims to achieve uniform blending or distribution of different materials.
4. It involves bulk movement of the fluid without necessarily changing its composition.	4. It involves the physical intermingling of different components to reduce or eliminate concentration gradients.

   

          In simple terms, agitation creates movement within the vessel, while mixing uses that movement to achieve a desired level of uniformity between different components.

        There are several types of industrial agitators, and each type is designed for a particular process requirement. The selection depends primarily on fluid viscosity, density difference, required shear, solids suspension, gas dispersion, heat transfer, and desired circulation pattern.

Some key dimensionless numbers and formulas-

Reynolds number (Re)-If flow profile is laminar (Re < 10), transitional ( 10 < Re < 10⁴) and turbulent (Re > 10⁴)

Re= (Da² x n x ρ)/µ

Tip speed of agitator (Ts)- it determines maximum shear rate at the outer edge of the impeller blades.

Ts = π x Da x n

No. of impellers on an agitator shaft (N)-

N =( Hl x Sg)/ D

Spacing in impellers of agitator (S)–

S = Hl /( N-0.5)

Agitator shaft length (Ls) is generally calculated as-

For vertical cylindrical tank with tori spherical dishend,

Ls = L + 0.25D

For rectangular tanks or cylindrical tank with flat dish end,

Ls =L

Power required for agitation- it is calculated using the dimensionless power number (Np)

P = Np x n³ x ρ x Da⁵

 If 2 impellers on same shaft then power requirement would be,

P = 2 x power requirement of single impeller

If 3 impellers on the same shaft then power requirement would be,

P = 2 x power requirement of single impeller + 0.75 x power requirement of single impeller

If 4 impellers on the same shaft then power requirement would be,

P = 2 x power requirement of single impeller + 0.75 x power requirement of single impeller + 0.5 x power requirement of single impeller

Pumping rate(Q)- Volumetric flow rate generated by the impeller using the flow number (Nq).

Q = Nq x n x Da³        

Bulk velocity (Vc)- Average velocity of the fluid moving across the tank cross-section A.

Vc = Q/A

Scale of agitation-

R = Vc (ft/min)/ 6

Where,

Da- Agitator diameter (mtr or ft)

D- Vessel diameter (mtr or ft)

Hl- Height of liquid in tank (mtr or ft)

L- Length of tank (mtr or ft)

Ls – Length of agitator shaft (mtr or ft)

n- speed of agitator (rpm)

ρ- Density of fluid (kg/m3 or lb/ft3)

µ- Dynamic Viscosity of fluid (pas- sec or lb/ft sec)

Np- Power number

Nq- Flow number

A-C/s of agitated tank (m2 or ft2)

Different Industrial agitators design specifications

Table below provides typical operating speeds, power numbers, pumping factors, and standard impeller-to-vessel diameter ratios for industrial impellers operating in the approximate range of 100–300 RPM.

These values should be considered typical reference values rather than universal design constants, because actual values depend on impeller geometry, Reynolds number, baffles, vessel geometry, and operating conditions.

Agitator	Type	Agitator speed (rpm)	Power Number (Np)	Pumping factor (Nq)	Impeller diameter
Axial flow turbine	3- Bladed	100-300	1.35	0.6	Da= 0.3 x D
	4-Bladed		1.4	0.69
	5-Bladed		1.45	0.78
	6-Bladed		1.6	0.87
Marine Propeller	3- Bladed	100-300	0.3	0.33	Da= 0.3 x D
	4-Bladed		0.33	0.34
Turbo Propeller	3- Bladed	100-300	1.35	0.6	Da= 0.3 x D
	4-Bladed		1.4	0.69
	5-Bladed		1.45	0.78
	6-Bladed		1.6	0.87
Disc blade turbine	Std	100-300	5.0	0.7-0.8	Da= 0.3 x D
Flat blade turbine	Std	100-300	5.0	0.7-0.85	Da= 0.3 x D
Backward blade turbine	Std	100-300	5.0	0.65-0.85	Da= 0.3 x D
Paddle	Low speed	50	5.0	0.03	Da= 0.8 x D
Anchor	Proximity	50	5.0	0.07	Da= 0.8 x D
Gate	High solidity	50	5.0	1	Da= 0.8 x D

 

SCALE OF AGITATION (Chem Scale 1-10)

Chem Scale is used to quantify the mixing intensity needed for a given process as well as provide a frame of reference for mixing scale-up. This simple 1-to-10 scale has been used as a valuable tool for quickly describing mixing intensity.

Type	Description	Chem Scale	Bulk fluid velocity(ft/min)	Typical application
Mild	Little surface motion, no splash, mild ripple	1	6	Most storage applications. low viscosity blending of liquids with minimal difference in density & viscosity. Viscosity in the range of 100 cps and below. light solids moving on tank bottom, no suspension.
		2	12
		3	18
Moderate	Slow rolling liquid surface. occasional wave with little splash	4	24	Low to medium viscosity blending with specific gravity difference less than 0.6 with viscosities generally below 5000 cP. Solids moving off bottom but not suspended anymore than 20% throughout tank. Easy heat transfer.
		5	30
		6	36
Vigorous	Some splash with boiling action at surface, occasional vortex	7	42	Medium viscosity liquid/liquid blending. Specific gravity difference less than 0.85 with viscosity less than 10,000 cP. Moderately suspended solids throughout 75% on tank. good heat transfer.
		8	48
Violent	Splashing and large wave formations, erratic vortex formation.	9	54	Liquids with viscosities less than 25,000 cps with specific gravity difference of 1.0. Solids suspended to complete uniformity
		10	60

 

 To further understand above concept, let us solve below mentioned example,

Reaction mass viscosity (µ): 60 cP

Density (ρ): 900 kg/m³

Liquid Level: Maximum and minimum height.

Rotational speed (n): 200 RPM

Impeller type: Pitch blade

Reactor capacity: 5 kL (5 m³)

Working volume: 80% reactor occupancy,  calculate power required and scale of agitation.

Let us solve this problem,

         Standard aspect ratio for reactor is (L / D) kept around 1.3-1.4. Consider 1.4 for calculation.

 Volume of reactor is calculated as,

   V = (π / 4) x D² x (1.4 D) + 2 x (84.66 x 10^-3 x D³)

Reactor diameter (D) is calculated as,

5 = 1.26882 x D³ ---> D = 1.579 m (approx. 1570 mm).

Reactor shell height (L) would be,

L = 1.4 x 1.579 ---> L = 2.2 m (approx. 2200 mm).

Reactor occupancy is 80%, then reactor working volume would be, 0.8 x 5 = 4 KL

Liquid height (Hl) would be,

4 = (π / 4) x 1.57² x HI + 2 x (84.66 x 10^-3 x 1.57³) = 1.728 mtr ---> consider 1.8 mtr

      No. of Impellers required on agitator shaft are

      (N) = Liquid Height (Hl) x Specific Gravity(Sg) / Reactor dia(D)

 (1.8 x 0.9) / 1.57 = 1.03 ---> Rounded up to 2 units.

Impeller Diameter (Da) would be,

 Da = 1/3 x D = 1570 / 3 = 523 mm ---> consider 550 mm.

Spacing in impellers of agitator (S)–

S = Liquid Height (Hl)/ (No. of impellers on agitator shaft (N)- 0.5)

 = 1.8/(2-0.5) = 1.2 mtr

Agitator shaft length  (Ls) is calculated as,

  Ls  = Reactor shell length (L) + 0.25 x reactor dia (D)

        = 2.2 + 0.25 x 1.57 = 2.57 mtr  ---> approx. 2.6 mtr

Reynolds Number will be, Re = (Da² x n x ρ) / µ

 Re = [ (0.55)² x (200 / 60) x 900 ] / (60 x 10^-3) = 15,125

       Flow is fully Turbulent because Re is greater than 10,000.

Power Number (Np) = 1.3 for pitched blade (taken from Re Vs Np graph)

Theoretical Power requirement is calculated as, P = Np x ρ x n³ x Da⁵

P = ( 1.3 x 900 x (200 / 60)³ x 0.55⁵ ) / 1000 = 2.18 kW

Power per Impeller is, 2.18 kW / 0.746 = 2.923 HP /impeller.

As we have 2 impellers on same agitator shaft so power required for 2 impellers on shaft would be,

       P = 2 x power requirement of single impeller

       P= 2x 2.18 = 4.36 KW

As final motor power selection, transmission losses are add on.

Consider Gland Losses: Add 10% overhead ---> 4.36 + 0.436 = 4.796 KW.

Transmission Losses: Add 10% gearbox losses ---> 4.8 x 1.1 = 5.28 KW.

Consider std size 5.75 KW /7.5 HP motor.

Pumping Factor for pitch blade is Nq = 0.8

Pumping capacity of agitator is calculated as, Q = Np x n x Da³

= 0.8 x (200/60) x 0.55³ =0.4436 m3/sec ---> 1597 m3/hr

Bulk velocity (Vc) of fluid will be,

= 0.4436/ ((π/4) x 1.57² =0.23 m/sec ---> 45.27 ft/min

Scale of agitation is calculated as, R = Bulk velocity (Vc)/6

 R = 45.27/6 =7.54  ---> 8

Mixing scale is vigorous.

 

I hope this article helps you understand the fundamentals of industrial agitators and provides a useful starting point for agitator selection and preliminary design.

Happy Reading, and keep learning!

Your comments are highly appreciated.