Based on the power stats of this furnace and your heat pump (Heat Processing mod), I calculated that a single heat pump can power over 200 of these furnaces.

In practice, furnaces simply don't get hot enough unless very close to a furnace since heat pipes do not transfer heat effectively enough and the difference between the max. temperature of the heat pump (510) and required temp of the furnace (500) is so small.

I was thinking that this combination (possibly plus efficiency modules, not sure yet whether they apply here) could make for a very efficient coal-fired furnace station. Alas, much coal burned later, the design turns out highly impractical. (Was this intentional?)

After some experimentation, I find that a single heat pump can sufficiently heat a chain of 9 furnaces attached off one side. Since the only limitation is heat transfer, not heat generation, there should be no issue chaining more furnaces off a single pump in multiple directions (except that it will take a long time to get up to temperature). On the other hand there is little harm using extra heat pumps: they don't waste fuel.

Sorry I didn't get a chance to respond to you before. Yes, heat THROUGHPUT is the limiting factor for the heat processing mechanics I've been able to add to the game using the engine. My balancing is based on that concept. I agree that I can get about 9 on a single "heat chain". That was what I essentially meant to do. Although I am considering a major rebalance in the next release so let me know your thoughts.

After building a full nuclear-heat-powered direct-train-insertion smelter... meh, it was a neat concept but significantly more work than it's worth (partly because a significant amount of active logic was required to make train buffering work right). Again, throughput is the limiting factor — no matter the reactor size, those furnaces take a LONG time to heat up, but there's practically no limit to what one reactor can power. Probably this just can't be balanced right.