---
title: "Plenum (clean side / dirty side)"
description: "A plenum is a gas-flow chamber inside a baghouse. Every baghouse has at least two:"
canonical_url: "https://sylio.co/glossary/plenum-clean-side-dirty-side"
last_updated: "2026-06-28T02:29:25.022Z"
---

A **plenum** is a gas-flow chamber inside a [baghouse](/glossary/baghouse). Every baghouse has at least two:

- **Dirty plenum** — below the [tubesheet](/glossary/tubesheet); accepts the incoming flue-gas flow; surrounds the outside of the [filter bags](/glossary/filter-bag); contains the bag-and-cage assemblies
- **Clean plenum** — above the tubesheet; collects filtered gas leaving the inside of each bag; routes to the outlet duct and [ID fan](/glossary/id-fan)

The pressure difference between the two plenums is the baghouse [differential pressure](/glossary/differential-pressure-baghouse), the headline operational KPI.

## In a pulse-jet baghouse

A [pulse-jet baghouse](/glossary/pulse-jet-baghouse) adds a small inlet plenum above the clean plenum that houses the air-receiver tank and the manifold of pulse valves. Each valve fires downward through a venturi into the open top of a bag, momentarily reversing flow.

## In a reverse-air baghouse

A [reverse-air baghouse](/glossary/reverse-air-baghouse) compartment alternates between *filtration mode* (gas flows from dirty plenum, through the bag wall, into the clean plenum) and *cleaning mode* (compartment isolated; reverse-air fan flows clean gas back through the bags into the dirty plenum).

## Related terms

- [Baghouse](/glossary/baghouse)
- [Tubesheet](/glossary/tubesheet)
- [Filter bag](/glossary/filter-bag)
- [Differential pressure (baghouse)](/glossary/differential-pressure-baghouse)