# Agitator Power and Impeller Reynolds Number Calculator

> Source: https://netray.co/tools/agitator-power-and-reynolds-number-calculator
> Publisher: Netray (netray.co), AI and integration engineering for ERP and manufacturing systems.
> Section: Free Tools

## Short answer

Calculate agitator power from power number, speed and diameter, with Reynolds number and kW per cubic metre.

## Category

manufacturing

## Industry

Chemical Manufacturing

## Audience

- Process engineer
- Mechanical engineer
- Production manager

## Tool

**Kind:** calculator

**Inputs:**

#### Impeller type (power number Np)

**Key:** np

**Type:** select

**Default:** 5

**Options:**

##### Rushton turbine (about 5.0)

**Value:** 5

##### Pitched blade turbine (about 1.3)

**Value:** 1.3

##### Marine propeller (about 0.35)

**Value:** 0.35

##### Hydrofoil (about 0.3)

**Value:** 0.3

**Help:** Fully turbulent, baffled tank values. Confirm with the vendor.

#### Impeller speed

**Key:** rpm

**Type:** slider

**Default:** 120

**Min:** 20

**Max:** 400

**Step:** 5

**Unit:** rpm

#### Impeller diameter

**Key:** d

**Type:** number

**Default:** 0.6

**Min:** 0.05

**Step:** 0.05

**Unit:** m

#### Liquid density

**Key:** rho

**Type:** number

**Default:** 1000

**Min:** 500

**Step:** 10

**Unit:** kg/m3

#### Liquid viscosity

**Key:** mu

**Type:** number

**Default:** 1

**Min:** 0.1

**Step:** 0.5

**Unit:** cP

**Help:** Water is 1 cP. Heavy oils and polymer solutions can be thousands.

#### Batch volume

**Key:** vol

**Type:** number

**Default:** 5

**Min:** 0.1

**Step:** 0.5

**Unit:** m3

**Outputs:**

#### Speed

**Key:** n

**Expr:** rpm / 60

**Format:** decimal

**Description:** Revolutions per second.

#### Power drawn

**Key:** p_w

**Expr:** np * rho * pow(n, 3) * pow(d, 5)

**Format:** number

**Description:** Watts, P = Np x rho x N^3 x D^5.

#### Power drawn (kW)

**Key:** p_kw

**Expr:** p_w / 1000

**Format:** decimal

**Primary:** true

#### Power per unit volume

**Key:** pv

**Expr:** p_kw / vol

**Format:** decimal

**Primary:** true

**Description:** kW/m3.

#### Impeller Reynolds number

**Key:** re

**Expr:** rho * n * pow(d, 2) / (mu / 1000)

**Format:** integer

**Description:** Above about 10,000 is fully turbulent.

**Chart:**

#### Power drawn vs impeller speed

**Type:** line

**X:** rpm

**From:** 20

**To:** 300

**Steps:** 28

**Series:**

- p_kw

**Disclaimer:** Power number is only constant in the turbulent regime (Re above roughly 10,000). In viscous or transitional flow Np rises, so use vendor data. Add allowance for seal, bearing and gearbox losses when sizing the motor.

## Content

**Intro:** How much power does your agitator really draw? Enter impeller type, speed, diameter and liquid properties to get shaft power, power per cubic metre and the Reynolds number that tells you whether the formula applies.

**Sections:**

#### How to use it

**Body:** Select the impeller type, which sets a typical power number, then enter speed in rpm, impeller diameter in metres, density and viscosity. Add the batch volume to see power per unit volume, a common scale-up criterion. The sweep shows that power rises with the cube of speed, so a small speed increase costs far more energy than expected.

#### The relationships

**Body:** In the turbulent regime power is P = Np x rho x N^3 x D^5, with N in revolutions per second. The impeller Reynolds number is Re = rho x N x D^2 / mu, with viscosity in Pa.s. Because power scales with the fifth power of diameter, a modest change in impeller size has a very large effect on load.

**Bullets:**

- Double the speed, power rises eightfold
- Increase diameter by 10 percent, power rises about 61 percent
- Typical blending duty is a few tenths of kW/m3
- Gas dispersion and solids suspension need more

#### Common mistakes

**Body:** Using the turbulent power number at low Reynolds numbers underestimates power for viscous liquids. Another mistake is ignoring baffles: an unbaffled tank forms a vortex and draws less power. Finally, a variable liquid level during filling changes the effective power drawn, so check the worst case, usually the full vessel with the densest charge.

**Bullets:**

- Confirm tip speed limits for shear sensitive products
- Check motor size with margin of 10 to 20 percent
- Gassed power is lower than ungassed power
- Record agitator speed in your batch record

**Cta:** If agitator speeds and mixing times are part of your batch records in an ERP or MES, Netray can help review how they are captured. Ask us.

**Faq:**

#### What is a power number?

**Answer:** It is a dimensionless constant that relates power draw to density, speed and diameter for a given impeller and tank geometry. It is roughly constant in turbulent flow and measured by impeller manufacturers.

#### What Reynolds number means turbulent?

**Answer:** In a baffled stirred tank flow is considered fully turbulent above about 10,000. Between 10 and 10,000 is transitional and below 10 is laminar, where power number depends strongly on Reynolds number.

#### Is power per volume a good scale-up rule?

**Answer:** It is widely used for blending and mass transfer, but not always. Tip speed, mixing time and Reynolds number may matter more for shear sensitive, suspension or reaction-limited processes. Choose the criterion by the controlling mechanism.

#### Does the motor need to match calculated power?

**Answer:** No, it needs margin. Add allowances for gearbox and seal losses and for variation in fluid properties. Also check start-up torque, especially when solids have settled.

## Search terms

agitator power calculator, mixer power calculation, impeller power number, Reynolds number stirred tank, power per unit volume mixing, Rushton turbine power, agitator motor sizing

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