---
title: "Convective pass / backpass"
description: "The convective pass (also backpass) is the downstream section of a boiler where heat transfer is by conduction across tube banks rather than radiation from a flame. The convective pass contains, in order of decreasing gas temperature: the finishing superheater, the reheater, the primary superheater, the economiser, and finally the air heater."
canonical_url: "https://sylio.co/glossary/convective-pass-backpass"
last_updated: "2026-06-28T02:29:25.783Z"
---

The **convective pass** (also **backpass**) is the downstream section of a boiler where heat transfer is by conduction across tube banks rather than radiation from a flame. The convective pass contains, in order of decreasing gas temperature: the finishing [superheater](/glossary/superheater), the [reheater](/glossary/reheater), the primary superheater, the [economiser](/glossary/economiser), and finally the [air heater](/glossary/air-heater).

## Why the convective pass is the prime sonic-horn zone

Three reasons:

1. **Deposits are dry**, not molten. Ash arriving at convective surfaces has cooled below its sticking temperature; it deposits as a friable layer that acoustic energy can lift.
2. **Surfaces are extensive** and partly inaccessible to retract sootblowers — perfect for non-contact cleaning.
3. **Heat-rate sensitivity** is high. Every degree of approach temperature loss in the economiser or air heater translates directly into fuel cost.

A typical large utility boiler benefits from **8–20 sonic horns** distributed across the convective pass, complementing existing steam [sootblowers](/glossary/steam-sootblower).

## Sequencing

Horns are fired in a programmed sequence that respects [compressed-air](/glossary/compressed-air) supply, avoids overlapping firing on adjacent fields, and times their action between sootblower cycles to maintain continuous low-level dust release.

## Related terms

- [Boiler](/glossary/boiler)
- [Economiser](/glossary/economiser)
- [Superheater](/glossary/superheater)
- [Reheater](/glossary/reheater)
- [Air heater](/glossary/air-heater)
- [Sonic horn](/glossary/sonic-horn)