Abstract: A modular in bed tube unit including a hatch cover bolted over a hatch-opening in the front wall of a fluidized bed boiler. The modular unit also includes a feed fluid manifold, a steam manifold, and a plurality of in-bed tubes connected at one end to the feed fluid manifold and that are connected to and extend through the hatch cover in the fluidized bed of the combustion chamber and extend from the bed back through the hatch cover to the steam manifold. Bolted connecting flanges, between the feed fluid pipe of the boiler system and between the steam manifold and the steam pipe of the boiler system can be quickly disconnected. The entire unit can be quickly removed and replaced when the in-bed tubes become deteriorated. For water wall boilers, the tubes of the front water wall are bent inwards at the hatch opening to allow positioning of the in-bed tubes. The modular unit can be adapted to multiple boiler units.