Abstract: A hand-holdable, portable welding apparatus for smoothly joining the lumens of sections of thermoplastic tubing systems, including those that may be located in a cramped work site. The integrity of the lumens is maintained when the tubes are being welded by use of a selectively rigidizable, non-elastomeric mandrel of PTFE or PFA having a predetermined maximum size dimensioned to the ID of the lumens being joined. The welding unit employs a resiliently mounted coaxial heater block in conjunction with arcuate radiator inserts that are centered about the tubes to be welded regardless of the diameter of the tubes. Spring-loaded dogs grab the periphery of the tubes as they are inserted into the welding unit and prevent them from moving apart when heat is applied.