# Wet Scrubber for Local Exhaust Ventilation

How a rooftop wet scrubber cleans LEV exhaust: gas rises through a spray tower, droplets capture particles and gases, and a mesh pad removes mist before the clean\-side fan\.

By Omkar Marathe

Topic: Machines

[Open the interactive Jig](https://ask.jig.com/j/mRwjFCtOhC4Q)

## Transcript

### 1. Induced\-draft spray tower on an LEV system

This rooftop unit scrubs the exhaust from a local exhaust ventilation \(LEV\) system before discharge\. Air rises from the hoods through a Ø700 mm riser, is scrubbed in a Ø1800 mm welded polypropylene spray tower and leaves through a centrifugal fan and stack\. The fan is on the clean side, so the tower, ducts and hoods all run under suction\.

Layout follows the supplied general\-arrangement sketch\. Airflow \(18,000 m³/h\) and unlabelled dimensions are illustrative design values\.

### 2. Riser, reducer and inlet

The Ø700 mm riser from the 13th floor expands through a concentric reducer to Ø800 mm, turns through a long\-radius elbow and enters the shell with its invert 1250 mm above the terrace\. At 18,000 m³/h that is about 13 m/s in Ø700 and 10 m/s in Ø800\. The amber tracers mark the contaminated gas\.

Size ducts to stay above the contaminant's minimum transport velocity for the actual process \(HSG258 / ACGIH Industrial Ventilation\)\.

### 3. Lower chamber and sump

Gas enters above the liquor surface, turns upward and spreads across the full 1800 mm section; the sudden expansion drops coarse particles straight into the sump\. At 18,000 m³/h the superficial velocity in the shell is about 2\.0 m/s, within the 1–3 m/s typical of spray towers\. The sump holds the recirculating liquor and collects captured solids as sludge\.

The section is cut toward your viewpoint\. Drag to turn the view and inspect any side\.

### 4. Countercurrent spray header

At 3600 mm a header of nine downward full\-cone nozzles drives droplets against the rising gas\. Particles are captured mainly by inertial impaction on droplets, and soluble gases are absorbed into the liquor\. As a low\-energy scrubber, a spray tower runs below about 5 in\. w\.c\. \(1\.2 kPa\) and is efficient for particles above roughly 5–10 µm; fine PM would need L/G above 3 L/m³\.

Switch the pump off and the dust passes through untreated\. Sources: EPA\-452/F\-03\-016; EPA wet\-scrubber monitoring guidance\.

### 5. Mesh\-pad mist eliminator

Between 3850 and 4100 mm, just above the header, a knitted mesh pad strips entrained droplets before the gas reaches the Ø857 mm outlet\. Without it, carryover wets the fan and stack and discharges captured contaminant with the exhaust\. Log its pressure drop separately from the tower differential: a rising value indicates fouling or flooding\.

Captured droplets coalesce on the wires and drain back against the gas flow\.

### 6. Clean\-side fan and stack

The outlet gooseneck drops to a centrifugal fan on an isolated skid\. Placing the fan downstream keeps every joint upstream under negative pressure, so leaks draw air in rather than releasing contaminant\. The fan handles saturated air, so its casing needs a low\-point drain, and the stack discharges vertically above the parapet\.

Under COSHH, LEV must be thoroughly examined and tested at least every 14 months \(HSG258\)\.

### 7. Liquor circuit and controls

The recirculation pump draws from the sump and returns liquor to the header; make\-up water replaces evaporation and blowdown, and sludge is drained from the bottom\. Interlock liquor level, pump flow and tower differential pressure with the fan at the local panel, and dose and monitor pH on acid\-gas duties\. Treat the liquor against bacterial growth, including Legionella\.

Choose a layer to isolate the gas path or the liquid circuit\.