fire-tube boiler

[ fahyuh r-toob, -tyoob ]
/ ˈfaɪərˌtub, -ˌtyub /

noun

any boiler for generating steam by passing hot gases and other combustible products through tubes (fire tubes) immersed in water to a chimney or uptake.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019