I assume you are referring to an Induced Draft fan.
Fired heaters can be designed to draft naturally with the exhaust gases rising up the stack by virtue of the temperature difference. This would tend to drop the pressure in the firebox and draw in combustion air. If this is not sufficient to produce the required heat input, you can add a forced draft fan to force in additional combustion air. But, this could result in positive pressure in the firebox which can be a problem. You can add an induced draft fan to draw out the exhaust gas and force it up the stack. Some heaters have neither, some have one but not the other and some have both.
The induced draft fan can be an enclosed impeller much like a pump impeller. It can be a squirrel cage fan. It has to be designed to run at the temperature of the exhaust gas.
I apologize if I misunderstood your question and you have some other meaning for "ID Fan".
