> "countercurrent heat exchangers" Very interesting! The concept makes a lot of sense, and I was able to find some companies online that produce such a thing,

> "countercurrent heat exchangers" Very interesting! The concept makes a lot of sense, and I was able to find some companies online that produce such a thing, often under the phrase "counterflow heat exchanger". Frustratingly there's little details online for actual pricing, but you can find the bare exchange units on ebay for $200-300.