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Venturi Scrubber Calculator

Pressure drop equals 10 to the minus 6 times velocity squared times water gas ratio

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Venturi Scrubber Pressure Drop

The simplified Calvert equation relates the pressure drop across a venturi scrubber throat to the gas throat velocity and the liquid-to-gas volume ratio. Higher pressure drops yield better particulate collection efficiency but cost more energy.

P_drop = 10⁻⁶ × v² × L

How It Works

A venturi scrubber accelerates dirty gas through a narrow throat where scrubbing liquid is injected. The high-velocity gas atomizes the liquid into fine droplets that collide with and capture particulate matter. The pressure drop across the throat is the primary design parameter — higher pressure drops yield better collection efficiency but cost more energy. The simplified Calvert equation models this relationship as P_drop = 10⁻⁶ × v² × L, where pressure drop scales with the square of gas velocity.

Example Problem

A venturi scrubber operates with a gas throat velocity of 12,000 cm/s and a water/gas ratio of 1.3 L/m³. What is the pressure drop?

  1. Identify the known values: gas throat velocity v = 12,000 cm/s and liquid-to-gas ratio L = 1.3 L/m³.
  2. Determine what we are solving for: the pressure drop P_drop across the venturi throat.
  3. Write the Calvert equation: P_drop = 10⁻⁶ × v² × L.
  4. Square the velocity: v² = 12,000² = 144,000,000 (cm/s)².
  5. Multiply all terms: P_drop = 10⁻⁶ × 144,000,000 × 1.3 = 144 × 1.3.
  6. Compute the result: P_drop = 187.2 cm H₂O. This is a high-energy scrubber suitable for sub-micron particle capture.

When to Use Each Variable

  • Solve for Pressure Drop — when you know the gas throat velocity and liquid-to-gas ratio, e.g., estimating the energy cost of a proposed scrubber design.
  • Solve for Gas Velocity — when you know the allowable pressure drop and liquid-to-gas ratio, e.g., sizing the throat diameter for a target collection efficiency.
  • Solve for Water/Gas Ratio — when you know the pressure drop and gas velocity, e.g., determining how much scrubbing liquid is needed to achieve a required pressure drop.

Key Concepts

The simplified Calvert equation models pressure drop as proportional to the square of gas velocity and linear with the liquid-to-gas ratio. Higher throat velocities atomize the liquid into finer droplets, improving particle capture but consuming more fan energy. The trade-off between collection efficiency and operating cost drives every scrubber design decision.

Applications

  • Power plants: capturing fly ash and particulates from coal and biomass combustion flue gas
  • Steel mills: removing iron oxide fumes from electric arc furnace off-gases
  • Waste incineration: scrubbing acid gases and fine ash from flue gas simultaneously
  • Chemical manufacturing: controlling particulate and vapor emissions from reactors and dryers
  • Foundries: dust control from cupola furnaces and metal casting operations

Common Mistakes

  • Using throat velocity in m/s instead of cm/s — the Calvert equation expects cm/s, and mixing units yields results off by orders of magnitude
  • Designing only for clean-gas conditions — fouling and liquid maldistribution increase real pressure drops by 10–30% over calculated values
  • Neglecting downstream mist elimination — without a demister, entrained droplets carry captured particles back into the clean gas stream

Frequently Asked Questions

How does a Venturi scrubber remove particles from a gas stream?

The dirty gas accelerates through a narrow throat where scrubbing liquid is injected. The high velocity atomizes the liquid into tiny droplets, and inertial impaction causes particles to collide with and be captured by these droplets. A downstream cyclonic separator or mist eliminator then removes the particle-laden droplets from the clean gas.

What determines the collection efficiency of a Venturi scrubber?

Collection efficiency depends primarily on the pressure drop across the throat, which is controlled by gas velocity and liquid-to-gas ratio. Higher pressure drops create finer liquid droplets and more turbulent mixing, capturing smaller particles. The Calvert equation (P_drop = 10⁻⁶ × v² × L) quantifies this relationship. Particle size distribution, liquid viscosity, and gas temperature also play roles.

What is a typical pressure drop for a venturi scrubber?

Pressure drops typically range from 25 to 250 cm of water column. Low-energy scrubbers (25–50 cm) collect coarse particles above 5 μm, while high-energy units (150–250 cm) can capture sub-micron particles. Most industrial applications operate between 50 and 150 cm H₂O.

When should I use a venturi scrubber instead of a baghouse?

Venturi scrubbers excel when the gas stream is hot, humid, corrosive, or contains sticky particles that would blind a fabric filter. They also simultaneously remove some gaseous pollutants. Baghouses are better for dry, moderate-temperature gases with very high efficiency requirements.

What is the liquid-to-gas ratio in a venturi scrubber?

The liquid-to-gas ratio (L) is the volume of scrubbing liquid injected per unit volume of gas processed, typically expressed in liters per cubic meter (L/m³). Common values range from 0.4 to 2.7 L/m³. Higher ratios improve particle capture but increase water consumption and wastewater treatment costs.

How do you calculate venturi scrubber pressure drop?

Use the simplified Calvert equation: P_drop = 10⁻⁶ × v² × L, where v is the gas throat velocity in cm/s and L is the liquid-to-gas ratio in L/m³. The result is the pressure drop in cm of water column. This calculator handles unit conversions automatically.

What maintenance does a venturi scrubber require?

Key maintenance tasks include inspecting the throat for erosion and buildup, checking spray nozzles for clogging, monitoring pressure drop trends (increasing drop signals fouling), maintaining the recirculation pump and piping, and managing the scrubber blowdown water quality. Plan for throat replacement every 2–5 years depending on gas abrasiveness.

Venturi Scrubber Formula (Calvert Equation)

The simplified Calvert equation relates pressure drop to gas throat velocity and liquid-to-gas ratio:

Pdrop = 10−6 × v² × L

Where:

  • Pdrop — pressure drop across the throat, in cm of water column (cm H₂O)
  • v — gas throat velocity, in centimeters per second (cm/s)
  • L — liquid-to-gas volume ratio, in liters per cubic meter (L/m³)

The factor 10−6 is an empirical constant that makes the units consistent when v is in cm/s. Pressure drop is proportional to velocity squared, so doubling throat velocity quadruples the pressure drop and energy consumption.

Worked Examples

Power Plant

What pressure drop is needed to scrub fly ash from coal flue gas?

A coal-fired power plant routes flue gas at 15,000 cm/s through a venturi scrubber with a liquid-to-gas ratio of 1.0 L/m³.

  • Pdrop = 10−6 × 15,000² × 1.0
  • Pdrop = 10−6 × 225,000,000 × 1.0
  • Pdrop = 225 cm H₂O

This high pressure drop provides excellent sub-micron particulate capture but consumes significant fan energy. In practice, an ESP or baghouse may be more cost-effective for this application.

Foundry

How fast must gas travel to keep a foundry scrubber at 80 cm H₂O pressure drop?

A foundry cupola uses a venturi scrubber with L = 0.8 L/m³. The fan can deliver 80 cm H₂O. What throat velocity is achievable?

  • v = √(P / (10−6 × L))
  • v = √(80 / (10−6 × 0.8))
  • v = √(100,000,000)
  • v = 10,000 cm/s

At 10,000 cm/s (100 m/s), the scrubber efficiently captures particles above 1–2 μm. Finer particles require higher velocity or additional stages.

Chemical Plant

How much scrubbing liquid is needed for acid mist control?

A chemical reactor exhaust runs at 8,000 cm/s with an allowable pressure drop of 50 cm H₂O. What liquid-to-gas ratio is required?

  • L = P / (10−6 × v²)
  • L = 50 / (10−6 × 64,000,000)
  • L ≈ 0.781 L/m³

This moderate liquid rate is typical for acid mist scrubbers. Caustic solution rather than plain water is often used to simultaneously neutralize acid gases.

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