In short: A packed bed scrubber is sized from the gas flow first. The airflow and the chosen gas velocity set the tower diameter; the required removal efficiency sets the packing depth; and the liquid-to-gas (L/G) ratio sets the recirculation pump. EPA guidance notes that packed towers are typically designed to run at about 60 to 70% of the flooding velocity, and reports vendor L/G ratios of about 2 to 20 gallons per minute per 1,000 cubic feet per minute of gas. Final sizing depends on the pollutant, its concentration, the scrubbing liquid and the packing, so treat these as starting points. FIBROVX designs and builds FRP scrubbers to each project's airflow and chemistry.
How a packed bed scrubber works
Contaminated air enters near the bottom and flows up through a bed of packing. Scrubbing liquid is sprayed on top and trickles down over the packing, so gas and liquid meet over a large wetted surface. A mist eliminator at the top removes droplets before the clean air leaves. A sump at the bottom collects the liquid for recirculation.
Step 1: Define the airflow
Start with the actual gas flow at the scrubber inlet, at its real temperature and moisture. Include all hoods and openings, and allow for future connections. EPA's fact sheet gives typical packed-bed flows of about 500 to 75,000 scfm (0.25 to 35 standard m³/s).
Step 2: Choose gas velocity and diameter
As gas velocity rises, the upward drag eventually holds liquid in the packing. This is flooding, and the scrubber stops working properly. Designers choose a velocity below flooding, typically 60 to 70% of the flooding velocity per EPA guidance. The flooding point depends on the packing type and size and on the liquid and gas rates, so it comes from packing data.
Tower diameter then follows from:
Cross-sectional area = gas flow ÷ design velocity
Step 3: Set the packing depth
Packing depth depends on how much of the pollutant must be removed. Designers often express it as:
Depth = height of a transfer unit (HTU) × number of transfer units (NTU)
NTU rises with the removal efficiency. HTU depends on the packing, the liquid and the gas. Highly soluble or reactive gases, such as HCl scrubbed with water or caustic, need less depth than gases that absorb slowly.
Step 4: Set the L/G ratio and pump
| Parameter | Typical starting point |
|---|---|
| Design velocity | About 60–70% of flooding velocity |
| L/G ratio (packed towers) | About 2–20 gpm per 1,000 cfm |
| Inlet temperature for absorption | Usually 4–38 °C (40–100 °F) |
| Particulate in the inlet | Generally below 0.45 g/m³ to avoid plugging |
Values from EPA guidance; final design depends on the application.
The L/G ratio must keep the packing fully wet and provide enough capacity to absorb the pollutant. Too little liquid leaves dry zones; too much raises pressure drop and pump cost.
Minimum wetting rate
The L/G ratio needed to absorb the pollutant is sometimes lower than the liquid rate needed to wet the packing fully. When that happens, the minimum wetting rate for the packing sets the liquid flow, not the absorption duty. Packing suppliers publish minimum wetting rates. The EPA worked example below shows this case.
Step 5: Check pressure drop and the fan
Pressure drop across the packing, the mist eliminator and the ductwork sets the fan duty. Corrosive exhaust usually calls for an FRP fan and FRP ductwork.
Worked example: EPA's HCl absorber
The EPA Air Pollution Control Cost Manual includes a worked example for an HCl absorber using water. Its main inputs and results are summarized here, with metric values added.
| Item | EPA example value |
|---|---|
| Gas flow | 21,377 scfm (22,288 acfm, about 10.5 m³/s actual) |
| HCl at the inlet | 1,871 ppmv |
| Removal efficiency | 99% |
| Scrubbing liquid | Water |
| Packing | 2 in ceramic Raschig rings |
| Flooding factor | 0.7 (70% of flooding) |
| Cross-sectional area | 60 ft² (about 5.6 m²) |
| Tower diameter | 8.74 ft (about 2.66 m) |
| Number of transfer units | 4.61 |
| Height of a transfer unit | 2.24 ft (about 0.68 m) |
| Packing depth | 10.3 ft (about 3.1 m) |
| Pressure drop through the packing | 8.55 in of water (about 2.1 kPa) |
Three lessons come out of this example:
- The liquid rate was set by wetting, not absorption. The L/G ratio needed to absorb the HCl was far below the minimum wetting rate for the packing, so EPA recalculated the column with the minimum wetting rate. That raised the area to 60 ft².
- The packing depth follows from a simple formula. HCl is very soluble in water and no HCl enters with the liquid, so EPA uses NTU = ln(inlet ÷ outlet concentration). For 99% removal, that is ln(100) ≈ 4.61. Depth = NTU × HTU = 4.61 × 2.24 ft ≈ 10.3 ft.
- The actual gas velocity is moderate. 22,288 acfm through 60 ft² is about 371 ft/min, or about 6.2 ft/s (1.9 m/s).
The example uses ceramic Raschig rings, an older packing. Modern plastic packings have different flooding, pressure drop and transfer data, so the same duty can give a different diameter and depth. Use the data for the packing you will actually install.
Quick check of removal targets
Because NTU = ln(inlet ÷ outlet) for very soluble gases like this, the packing depth grows with each extra "nine" of removal:
| Removal efficiency | Inlet ÷ outlet | NTU = ln(ratio) |
|---|---|---|
| 90% | 10 | About 2.3 |
| 99% | 100 | About 4.6 |
| 99.9% | 1,000 | About 6.9 |
Each extra nine adds about 2.3 transfer units, so 99.9% removal needs about 50% more packing depth than 99%, at the same HTU. This simple form applies only when the gas is very soluble or reacts in the liquid, and the liquid enters clean. For other gases, use the full equation in the EPA manual.
Choosing the scrubbing liquid
| Gas | Common scrubbing liquid | Notes |
|---|---|---|
| Hydrogen chloride (HCl) | Water or dilute caustic | Very soluble; water alone often works |
| Hydrogen fluoride (HF) | Water or caustic | HF attacks glass; use a resin and veil suited to HF |
| Sulfur dioxide (SO₂) | Caustic (sodium hydroxide) | Needs an alkaline liquid and pH control |
| Chlorine (Cl₂) | Caustic | Reacts to form hypochlorite; control pH and temperature |
| Ammonia (NH₃) | Dilute acid, such as sulfuric | Very soluble; acid keeps it absorbed |
| Hydrogen sulfide (H₂S) | Caustic, sometimes with an oxidant | Common in odor control |
The liquid also sets the materials. FRP with a suitable vinyl ester resin is widely used for acid and caustic scrubbers, but confirm the resin and veil with the resin maker for your chemicals and temperature.
Pressure drop and fan
EPA notes that pressure drop in packed columns generally ranges from about 0.5 to 1 in of water per foot of packing. Add the mist eliminator, the inlet and outlet, the ductwork and any hood losses to get the total fan duty. In the EPA example, 10.3 ft of packing gave 8.55 in of water through the bed alone, so the fan must deliver more than that at the design flow.
Data to send for a quote
- airflow and temperature at the inlet;
- each pollutant and its inlet concentration;
- the required outlet concentration or removal efficiency;
- the scrubbing liquid you can use (water, caustic, acid) and wastewater limits;
- available space, power supply and indoor or outdoor location.
Customization
FIBROVX builds FRP acid fume scrubbers to order: tower diameter and height, packing type and depth, spray and mist eliminator design, sump size, inlet and outlet positions, ladders and platforms. Send your airflow and pollutant data for a sizing proposal.
Need a quote or a drawing check?
FIBROVX manufactures FRP products in our own factory in China and builds every item to order: sizes, resin system, color, molds and OEM branding. Send your application, dimensions or drawing, quantity and destination port.
Request a QuoteCommon mistakes
- Sizing on fan nameplate flow instead of actual flow.
- Running too close to flooding.
- Ignoring particulate, which can plug packing.
- Underestimating pressure drop, which leaves the fan short.
- No allowance for future hoods.
- Ignoring the minimum wetting rate, which can set the liquid flow and the diameter.
- Asking for "99.9%" without need. Each extra nine adds packing depth.
Key terms
- Flooding: When gas flow holds liquid in the packing and the scrubber stops working properly.
- L/G ratio: Liquid flow divided by gas flow.
- HTU / NTU: Height and number of transfer units, used to set packing depth.
- Mist eliminator: Device that removes droplets from the outlet air.
FAQ
What velocity is used for a packed bed scrubber?
A velocity below flooding, often 60 to 70% of the flooding velocity for the chosen packing.
What is a typical L/G ratio?
EPA reports vendor values of about 2 to 20 gpm per 1,000 cfm for packed towers.
How tall is the packing?
It depends on the pollutant and removal target. Depth is calculated from HTU and NTU.
Can one scrubber remove several acids?
Often yes, if one scrubbing liquid absorbs them all. Some mixtures need two stages.
Why did the EPA example tower get larger?
The L/G ratio needed for absorption was below the packing's minimum wetting rate, so the column was recalculated with the minimum wetting rate, which raised the area to 60 ft².
How much pressure drop does packing add?
EPA reports about 0.5 to 1 in of water per foot of packing for packed columns.
Can FIBROVX size a scrubber for my exhaust?
Yes. Send the airflow, temperature, pollutants and removal target. We will propose a tower diameter, packing depth and liquid rate, and build the scrubber in FRP to suit your site.
Related guides
- FRP Acid Fume Scrubbers Explained
- What Is a Mist Eliminator?
- Biofilter vs Chemical Scrubber
- FRP Tanks for Hydrochloric Acid
- Industries we serve
Sources
- US EPA, Air Pollution Control Cost Manual, Section 5.2 Chapter 1, Wet and Dry Scrubbers for Acid Gas Control: https://www.epa.gov/sites/default/files/2020-07/documents/cs5-2ch1.pdf
- US EPA, Air Pollution Control Technology Fact Sheet: Packed-Bed/Packed-Tower Wet Scrubber: https://frtr.gov/matrix/documents/Vapor-Treatment/Packed-Bed-Packed-Tower-Fact-Sheet.pdf