Can anybody suggest to me what the above might mean in practice?

 

It sounds like some pretty specialized vocabulary. Can you give us any more context?

 

The sentence is part of the description of a boiler. Which is a 3-pass wet back design where the first pass is a furnace, and the other are small bore tubes.

 

Three pass means that the flue gases (the products of combustion of a heater or boiler) are passed back through the heat exchanger three times in order to maximize the heat reclaimed, and minimize the heat lost when those gases are released.