Depends on the metallurgy of your heat exchanger. Copper, and most copper alloys, no problem. But any contact with ferrous metals, HCL creates ferric chloride which can pit the base metal fairly quickly. Like "as you watch" quickly.
All I can get out of the chemical rep (Nalco) is that HCL is typically inhibited with an organic cation.
So if you pull your exchanger bundle and you are sure that its metallurgy, (tubes, tube sheets and brazing filler) are compatible, sure go ahead. But if you are flushing while in place, I'd reconsider. High risk? Probably not. But low reward. For a few bucks more, I'd want an inhibited acid cleaner.
Barnacle buster, which I believe is Phosphoric acid, is probably safer anyways.
I'm not a chemical guy what so ever. But I have seen operations groups, trying to save money or time, due considerable damage to vessels, boilers, and heat exchangers while chemically cleaning them. I'm the maintenance guy who gets to repair the mistakes...