Guide to Venturi Scrubber Design: Calculations, Equations, Optimization, and Sizing

Author: Michael Klepik, Chief Executive Officer

Venturi scrubbers are high-intensity air cleaning devices, though they operate with high energy consumption. The gas velocity in the narrow section of the pipe ranges from 100 to 200 m/s (328 to 656 ft/s), and in some systems, it can reach up to 1200 m/s (3937 ft/s). At such speeds, the gas being purified breaks the liquid curtain, which is injected around the perimeter of the pipe, into fine droplets. This leads to intense collisions between aerosol particles and dispersed fluid, resulting in particle capture due to inertial forces.

Several critical parameters govern the performance and construction of a Venturi gas scrubber. These include:

• Gas flow rate (Qg) – Determines the size and velocity requirements of the throat.
• Liquid flow rate (Ql) – Directly impacts droplet formation and pollutant capture.
• Liquid-to-gas ratio (L/G) – A pivotal factor in system efficiency. The optimal l/g ratio in venturi scrubber varies by application but typically falls within the range of 0.5–2.5 gallons per 1,000 cubic feet of air.
• Pressure drop (ΔP) – A key engineering consideration that influences both removal efficiency and operating cost.

Governments and regulatory bodies set emission limits for pollutants.
1. The required efficiency for the venturi scrubber calculation is based on the concentration of the pollutant at the inlet and the maximum allowable concentration at the outlet:

Where:
C_out = required (or targeted) concentration at the outlet,
C_in = pollutant concentration at the unit inlet.

2. Specific energy consumption for air cleaning
The main energy parameter of a wet dust collector is the total contact energy, K_T, i.e., the energy consumption for processing a certain volume of gases with fluid per unit of time. The numerical value of this parameter is determined by the following expression:

Where:
K_T = specific energy consumption (kJ per 1,000 m³ of gas);
x = empirical coefficient based on apparatus configuration and operating conditions;
η = collection efficiency of the purifier (expressed as a decimal, e.g., 0.95 for 95%);
B = process-specific constant dependent on factors like unit geometry and gas properties;
ln = natural logarithm.

These Venturi scrubber design equations and formulas capture the nonlinear relationship between energy demand and pollutant removal efficiency. As the required efficiency approaches 100%, the term −ln(1−η) increases significantly, reflecting a sharp rise in energy consumption. The square root introduces a moderating effect, and the constants х and В allow for calibration based on experimental or engineering data.

3. The hydraulic resistance of the system is determined, including the pressure losses from the Venturi scrubber design equations and the cyclone droplet separator, and is expressed as:

Where:
m = specific water consumption for capturing fine particulate matter with central water injection (through a nozzle), typically taken as m=0.005–0.007 m³/m³. For coarse particulate matter in low-pressure tubes m=0.001–0.002 m³/m³;
P_w = pressure of the sprayed water, assumed to be in the range of 300–350 kPa.

4. The parameters of the cyclone droplet separator are calculated
a) The diameter of the cyclone separator, as part of the Venturi scrubber design dimensions, is calculated using the following formula:

Where:
L_c = gas flow rate, in m³/s;
V_c = effective air velocity in the cyclone, typically V_c=4.5−6 m/s.
b) Height of the cyclone droplet separator:

c) Hydraulic resistance of the cyclone droplet separator, Pa:

Where:
ξ_c = local resistance coefficient of the cyclone;
ρ = gas density, in kg/m³;
v_c = air velocity in the cyclone, in m/s.

5. The hydraulic resistance of the constriction tube is calculated, in Pascals:

Where:
∆P_vt = pressure drop in the tube;
∆P_p = total pressure drop across the apparatus;
∆P_c = pressure drop in the cyclone separator.

6. The gas velocity in the constriction tube under standard conditions is determined
(ρ=1.2 kg/m³; ρl=1000 kg/m³; ξg=0.15; ξl=0.63):

7. Geometrical dimensions of the Venturi Tube are calculated:
a) Throat Diameter:

L = the air flow rate, m³/h
b) Throat Length:

Where:
l_t​ = length of the throat;
D_t​ = diameter of the throat.
This formula is derived from the Venturi scrubber design calculation to ensure the proper throat length for efficient scrubbing and airstream management.
c) Inlet diameter of the converging nozzle:

Where:
D_in = inlet diameter, in meters;
V_in = air velocity at the inlet pipe, typically 15 – 20 m/s.

d) Length of the converging nozzle, in meters:

Where:
l_c = length of the converging section;
D_in = inlet diameter;
D_t = throat diameter;
α₁ = convergence angle (typically 25°–30°).

e) Outlet diameter of the diffuser, in meters:

Where:
D_out = outlet diameter of the diffuser;
L = gas flow rate, in m³/h;
V_out = air velocity at the outlet, typically 16–18 m/s.

f) Length of the diffuser, in meters:

Where:
l_d = length of the diffuser;
D_out = outlet diameter of the diffuser;
D_t = throat diameter;
α₂ = diffuser expansion angle, typically 6°–7°.

g) Diameter of the water injection nozzle, m:

G_w = water flow rate, in m³/s, calculated as G_w= L_c⋅m

The Venturi scrubber liquid-to-gas ratio is a critical parameter influencing both the collection effectiveness and the energy consumption of the system. This ratio defines the volume of liquid injected per unit volume of air and is typically expressed in gallons per 1,000 cubic feet or liters per cubic meter.

For most industrial applications, an optimal L/G ratio ranges between 0.5 and 2.5 gal/1,000 ft³. A properly adjusted ratio ensures sufficient droplet formation for effective pollutant capture while avoiding excessive solution use that could increase operational costs and cause downstream separation issues. Engineers must consider factors like particle size distribution, air temperature, and contaminant solubility when selecting the appropriate ratio for peak performance.

Several operating parameters significantly affect the configuration and performance of a Venturi, with pressure, air velocity, and temperature being among the most influential.

1. Gas Velocity at the throat directly impacts the particle collection performance and pressure drop.
Higher flow rates create more intense turbulence and finer particle formation, enhancing impaction and interception mechanisms. However, this also leads to greater energy consumption. Engineers must strike a balance between efficient particulate capture and acceptable power costs.

2. Operating Pressure
While most units operate at near-atmospheric pressure, in some cases, apparatuses may be pressurized. Increased pressure raises gas density, which in turn increases inertial forces and can improve droplet-particle interaction, enhancing collection performance. However, higher pressure also demands more robust construction and affects nozzle and pump specifications.

3. Air Temperature affects gas density, viscosity, and droplet evaporation rate.
Higher temperatures lower gas density, requiring adjustments to inlet design to maintain optimal velocity. Additionally, elevated temperatures can lead to partial evaporation of the washing fluid, reducing its effectiveness and potentially necessitating increased flow or pre-cooling before cleaning.

In summary, effective Venturi scrubber design must take into account the combined influence of velocity, pressure, and temperature to ensure the unit achieves the required performance without excessive energy consumption or mechanical strain.

Venturi scrubber sizing involves tailoring the physical dimensions of the system to meet specific flow and contaminant conditions. The dimensions of the converging and diverging sections, as well as the throat, must be adjusted according to the process flow rate and target removal performance.

Determining appropriate venturi scrubber dimensions requires careful analysis of air characteristics, contaminant profiles, and physical constraints. Undersizing leads to insufficient removal, while oversizing increases capital and operational expenses.

Engineers often consult venturi scrubber sizing design charts and software tools to find the optimal configuration. The venturi scrubber size must match the airflow throughput and pollutant load to achieve desired outcomes.

Evaluating the performance starts with the ability to calculate the collection efficiency of a venturi scrubber. The collection efficiency primarily depends on particle size distribution, droplet size, relative velocity, and system geometry. Performance increases with higher speeds and finer fluid particles, which improve particle capture through inertial impaction and diffusion.

Venturi scrubber calculation and optimization also involve balancing pressure drop and liquid usage to ensure economic feasibility while maintaining environmental compliance.

The TORNADO FB Venturi Packed Bed Scrubber by Torch-Air is a two-stage air purification system designed for high-efficiency removal of dust, smoke, chemicals, and vapors from gas streams. It combines a Venturi pressure tube with a fluidized packed bed column filled with random packing materials, such as Raschig rings and Pall rings, to enhance mass transfer and pollutant capture. This configuration allows for cleaning efficiencies of up to 99.9%, depending on the level of pollution. The system is engineered to handle gas flow rates ranging from 600 to 30,000 cfm (1,000 to 51,000 m³/h) and is known for its stability under fluctuating loads and temperature variations. With a 5-year guarantee and a service life exceeding 10 years, the TORNADO FB offers a reliable and eco-friendly solution for industrial air filtration needs.

The venturi scrubber centrifugal force plays a secondary but important role in enhancing particulate removal, especially for larger or denser particles. As the gas-liquid mixture passes through the throat, high velocities generate turbulent flow and sharp directional changes. These dynamics cause heavier particles to experience centrifugal effects that force them toward the case walls, where they collide with fluid sprays and are captured more efficiently. While inertial impaction and droplet-particle interactions remain the dominant mechanisms, the added influence of centrifugal force contributes to improved overall collection performance, particularly in apparatuses designed for high-velocity operation or handling coarse particulate matter.

Improving performance without significantly increasing pressure drop requires careful design and smart operational decisions. Strategies include:

• Optimizing the L/G ratio: Using only the necessary amount of fluid to ensure proper droplet formation and effective pollutant capture, without overloading the apparatus.
• Enhancing spray distribution: Using fine atomizing nozzles and maintaining high relative velocities to maximize the phase contact surface area.
• Minimizing unnecessary restrictions: Designing smooth transitions and streamlined inlet/outlet passages reduces turbulence-induced losses.
• Using hybrid or multistage systems: Where extremely high efficiencies are needed, secondary mist separators or cyclonic aids can be added downstream without increasing Venturi inlet resistance.
• Ultimately, achieving high performance at low power cost requires iterative optimization—balancing geometry, flow dynamics, and empirical adjustments informed by real-world operation.