Possibly they would be okay as a source of heat for smelting small quantities of metal, making glass etc. But thermodynamics suggests it's always going to be less efficient to feed fuel to a creature and then have that creature produce heat than it is to create the heat directly. Unless you have multiple dragons you'll never get 100% uptime on the dragonfire because they'll need time to eat and so forth. Overall it doesn't sound well suited to large industrial-scale operations.
Your best bet might be that dragonfire has magical properties and is required for certain alchemical processes, or for smelting small amounts of metal that requires to be infused with magic (magic rings, amulets, swords etc.) It's slavery on a small scale and only the wealthiest people can afford to have a dragonforge, because the feeding and security requirements are so tough to meet. Perhaps it's even required that the dragon be required to cooperate in the process, infusing the right magic into an item being created. This would set up an interesting tension: the dragons are intelligent and hate being enslaved, so what hold do the people have over them that is so strong that they co-operate despite the slavery? Does emergent knowledge of that hold then become a lever that could be used by other players in the story?
(Edit, to answer the questioner's query about what the hold might be since they've already been subjugated and chained).
Well, my proposed use for them was that they are an essential part of the process of making magic items and have to co-operate in order for the magic item to work properly, by imbuing the correct magic. So you perhaps need a stronger hold than mere chains to ensure that these intelligent, once-proud, extremely resentful creatures don't sabotage your magic ring. (A dragon might perhaps, instead of a true +6 ring of dodging, make a ring that appears to be a +6 ring of dodging but which only works when the wearer's pulse is below 110. Thus, in the heat of battle, it stops working and the bearer becomes much easier to kill.)
What that hold is, is up to you: it's a plot detail for you as the author to answer, not a world building question. But it's one that allows for some interesting possibilities.
If on the other hand you just want dragons as a means of turning meat into fuel, that still seems inefficient when you could just burn wood or coal (growing plant based fuel is much more resource efficient than growing plants to feed to animals to feed to a dragon which in turn will make heat). Yes you might be able to get a hotter flame, but how hot do you need? Ancient people were able to make furnaces well above a thousand degrees to make glass, fire pottery etc. and your world has a 17-18th century level of technology. Short of something a bit more special and unique such as my example above, I don't see why your people would bother keeping them as slaves at all. Except maybe as high status dangerous pets, a bit like Jabba the Hutt's Rancor Monster in Return of the Jedi.
