---
title: "ID fan / FD fan / PA fan"
description: "Industrial boilers use three principal fans to manage gas and air movement:"
canonical_url: "https://sylio.co/glossary/id-fan"
last_updated: "2026-06-28T02:29:33.730Z"
---

Industrial boilers use three principal fans to manage gas and air movement:

<table>
<thead>
  <tr>
    <th>
      Fan
    </th>
    
    <th>
      Function
    </th>
    
    <th>
      Location
    </th>
  </tr>
</thead>

<tbody>
  <tr>
    <td>
      <strong>
        ID (Induced Draft)
      </strong>
    </td>
    
    <td>
      Pulls flue gas through the <a href="/glossary/convective-pass-backpass">
        convective pass
      </a>
    </td>
    
    <td>
      Downstream of the <a href="/glossary/electrostatic-precipitator">
        ESP
      </a>
      
       or <a href="/glossary/baghouse">
        baghouse
      </a>
      
      , before the stack
    </td>
  </tr>
  
  <tr>
    <td>
      <strong>
        FD (Forced Draft)
      </strong>
    </td>
    
    <td>
      Pushes combustion air into the burners
    </td>
    
    <td>
      Ahead of the air heater air-side inlet
    </td>
  </tr>
  
  <tr>
    <td>
      <strong>
        PA (Primary Air)
      </strong>
    </td>
    
    <td>
      Conveys pulverised coal from mills to burners
    </td>
    
    <td>
      Between the coal mills and the burner deck
    </td>
  </tr>
</tbody>
</table>

## Why fans foul

- **Fly-ash deposition on ID fan blades** unbalances the impeller, causing vibration and bearing wear
- **PA fan blade build-up** from sticky coal fines
- **FD fan inlet vane fouling** from atmospheric dust accumulating on the air-intake filter or vane assembly

ID fans on coal-fired and [biomass](/glossary/waste-to-energy) plants are particularly prone to blade fouling; a trip-causing imbalance is a regular outage risk.

## Sonic horns on fan housings

[Sonic horns](/glossary/sonic-horn) installed on the upstream ducting and at the fan inlet keep the blades clean by preventing the dust from settling onto them in the first place. Cement preheater ID fans are a particularly common installation.

## Related terms

- [Boiler](/glossary/boiler)
- [Convective pass / backpass](/glossary/convective-pass-backpass)
- [Sonic horn](/glossary/sonic-horn)