Design of Water Jet Ejector Vacuum system

Hello Everyone ...!!

Back after a gap to post something interesting which is very much referred topic in our daily work-life especially when we work on vacuum distillations. 

This post is dedicated to one of our beloved reader Mr. Ritesh Pandey.

Before that i would like to welcome you all back to PharmaCalculations.com! Today, let’s deep dive into one of the simplest yet powerful technologies used in pharmaceutical manufacturing — Ejector Vacuum Systems. Whether you are running a distillation, drying, or crystallization unit, creating vacuum efficiently can be a real game-changer.

Let’s look at a realistic design case for a water jet ejector system used for methanol distillation, complete with line sizing and numerical calculations.

Prior jumping into the topic lets have some basic questions which could help us to understand the things better.

What is an ejector vacuum system?

An ejector vacuum system uses a high-velocity fluid (called motive fluid) to entrain gases or vapours from a process and discharge them at a higher pressure. It creates vacuum by converting motive fluid pressure into velocity through a converging-diverging nozzle, entraining vapours via suction.

What is a motive fluid in an ejector system?

A motive fluid is the pressurized fluid used to generate vacuum. It is typically water, steam, or compressed air, depending on the application. The fluid accelerates through a nozzle, creating a low-pressure zone to entrain vapour or gas from the suction side.

How do I select the motive fluid for my ejector?

Selection depends on:

• Availability & cost (e.g., cooling water vs. steam)

• Vacuum level required (steam offers deeper vacuum)

• Contamination concerns (water preferred if product contamination must be avoided)

• Temperature sensitivity of product
  For pharmaceutical applications, water jet ejectors are preferred due to cleanliness and cost-effectiveness.

What is the difference between single-stage and multi-stage ejectors?

• Single-stage ejector: Used for moderate vacuum applications (≥ 200 mmHg abs).

• Multi-stage ejectors: Achieve deeper vacuum (< 100 mmHg) by using intermediate condensers and multiple stages, especially in steam ejectors.

Why is a barometric leg used in water jet ejector systems?

A barometric leg (vertical discharge pipe >10.3 m) prevents backflow of water into the system and provides enough head to discharge without pump assistance. It's essential when discharging into an open drain or tank below vacuum.

How does the suction capacity of an ejector change with motive water pressure?

Higher motive pressure increases velocity at the nozzle, improving vacuum and suction capacity. But beyond optimum point, efficiency drops. Typical water pressures for good operation range 2.5–4 kg/cm²g.

Can an ejector handle condensable vapours like methanol or acetone?

Yes, but condensers are often used downstream to reduce vapour load and minimize backpressure. For large-scale distillations, a direct contact or surface condenser helps improve ejector efficiency.

What are typical design margins to consider?

• Vacuum margin: Design for 10–20 mmHg below target

• Flow margin: Design for 10–20% more vapour load

• Line sizes: Include ~10–15% extra area to reduce pressure drop and future fouling

What are common reasons for ejector underperformance?

• Insufficient motive fluid pressure or flow

• Blockage or scaling in nozzle/suction line

• Incorrect backpressure due to high discharge resistance

• Condensation of vapour inside suction line (reduces entrainment)

Can ejectors be used continuously?

Yes. Ejectors have no moving parts, so they are ideal for continuous operation in processes like:

• Distillation

• Vacuum drying

• Crystallization

• Solvent recovery

How do I determine the capacity of an ejector?

Capacity is usually specified in Nm³/hr or kg/hr of vapour it can entrain. You need to:

• Estimate vapour generation rate from distillation/drying load

• Choose vacuum level (suction pressure)

• Match with motive fluid flow and pressure

What is the thumb rule for water jet ejector sizing?

• For every 1 Nm³/hr of vapour load, require 3–5 m³/hr of water at 2.5–4 kg/cm²g.

• Suction line velocity: 15–25 m/s

• Water line velocity: 2–3 m/s

• Discharge line: Sized based on two-phase flow

Can I use the same ejector for different solvents?

Not necessarily. Design is based on molecular weight and vapour load. Each solvent may need tuning.

Why prefer water jet ejector over mechanical vacuum pumps?

Simplicity, no moving parts, cost-effectiveness. But consumes more water and less precise vacuum control.

How do I ensure the ejector is working properly?

Check the vacuum gauge, water pressure, and discharge flow. Install isolation and drain points for maintenance.

CASE STUDY

Now, its time to jump into the case. Here it is:

We want to distil out 4 KL (4000 L) of methanol in a 5 KL SS316 reactor under vacuum, with pressure not less than (NLT) 600 mmHg vacuum, i.e., 200 mmHg absolute pressure. The ejector will use water as the motive fluid, and we aim to complete the distillation in 6 hours. Let’s design the ejector system and suction line.

Now lets solve this case step wise:

Step 1: Process Understanding & Design Basis

Parameter	Value
Volume of methanol to distil	4000 L
Boiling point of methanol at 200 mmHg	~45–48°C
Vacuum required	600 mmHg vacuum (absolute pressure = 200 mmHg)
Distillation time	6 hours
Reactor volume	5 KL (SS316)
Cooling water available at	25°C
Water pressure at ejector	~3 kg/cm²g (4 bar abs)
Allowable line velocity	15–25 m/s (for vapour)

Step 2: Vapour Load Estimation

Methanol Mass

$$\text{Mass} = 4000\ \text{L} \times 0.791\ \text{kg/L} = 3164\ \text{kg}$$

Flowrate Over 6 Hours

$$\dot{m} = \frac{3164}{6} = 527.3\ \text{kg/hr}$$

Molar Flowrate

$$\text{MW}_{\text{methanol}} = 32.04\ \text{g/mol} = 0.03204\ \text{kg/mol}$$
$$\dot{n} = \frac{527.3}{0.03204} = 16457.7\ \text{mol/hr}$$

Molar Volume at 200 mmHg, 318 K

$$V_m = \frac{RT}{P} = \frac{8.314 \cdot 318}{26660} = 0.0991\ \text{m³/mol}$$
$$\text{Volumetric flowrate} = 16457.7 \cdot 0.0991 = \boxed{1631\ \text{Nm³/hr}}$$

So, methanol vapour load = 1631 Nm³/hr, assuming instantaneous vaporization

Step 3: Motive Water Flowrate Estimation

Practical Assumption:

Not all methanol vaporizes at once — assume only 1–2% flashes per minute.
Let’s conservatively assume a realistic average vapour load = 15 Nm³/hr

Use water-to-vapour ratio = 4:1 (standard for ejectors at 200 mmHg)

$$\text{Motive water} = 15 \times 4 = \boxed{60\ \text{m³/hr}}$$

Step 4: Suction Line Sizing

Inputs:

• Vapour flowrate = 15 Nm³/hr = 0.00417 m³/s

• Vapour density @ 200 mmHg & 318 K = ~1.1 kg/m³

• Target velocity = 20 m/s

$$A = \frac{Q}{v} = \frac{0.00417}{20} = 2.085 \times 10^{-4}\ \text{m}² \Rightarrow D = \sqrt{\frac{4A}{\pi}} = \sqrt{\frac{4 \cdot 2.085 \times 10^{-4}}{3.1416}} = \boxed{0.0163\ \text{m}} = \boxed{1.6\ \text{inch}}$$

Select 2" NB pipe for vapour suction line.

Step 5: Motive Water Line Sizing

Flowrate = 60 m³/hr = 0.0167 m³/s
Target water velocity = 2.5 m/s

$$A = \frac{0.0167}{2.5} = 6.68 \times 10^{-3}\ \text{m}² \Rightarrow D = \sqrt{\frac{4 \cdot 6.68 \times 10^{-3}}{3.1416}} = \boxed{0.092\ \text{m}} = \boxed{3.6\ \text{inch}}$$

Select 4" NB pipe for motive water line.

Step 6: Discharge Line Sizing

Total discharge = vapour + water ≈ 15 + 60 = 75 m³/hr = 0.0208 m³/s
Target velocity = 2.5 m/s

$$A = \frac{0.0208}{2.5} = 8.32 \times 10^{-3}\ \text{m}² \Rightarrow D = \sqrt{\frac{4 \cdot 8.32 \times 10^{-3}}{3.1416}} = \boxed{0.103\ \text{m}} = \boxed{4\ \text{inch}}$$

Select 4" NB pipe for discharge line (multiphase flow margin)

Hope you understand the design of Water jet ejector.

For any queries, please feel free to comment or contact us at pharmacalc823@gmail.com

Comments are most appreciated .......!!!

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About The Author

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Hi! I am Ajay Kumar Kalva, Currently serving as the CEO of this site, a tech geek by passion, and a chemical process engineer by profession, i'm interested in writing articles regarding technology, hacking and pharma technology.
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