Heat Exchanger: the Double-Pipe Residual Exchanger (Bell, 1923)

A heat exchanger built from stock pipe: runs of one pipe inside another, joined by return bends. Hot residuum from the stills flows one way in the outer pipe while cold crude flows the other way in the inner one, so the crude reaches the stills already warm. Variable: 2 in within 3 in, or 3 in within 4 in, and the number of runs (Bell's Fig. 73 has 16). Expect a transfer rate of 14 to 18 B.t.u. Source: H. S. Bell, American Petroleum Refining (1923), pp. 156-162. Unknown: run length, packing. Picture: Fig. 73 of the book.