Drop hammer

There is provided a drop hammer rapping device used for cleaning outer surface of a heat exchanger tube by beating and impacting it. The device comprises a longitudinal extended rotatable shaft (6), which is held by at least two supports, a unit (7) provided for rotation of the shaft (6), a plurality of radial arms (9) which are rigidly connected with a shaft (6) at a distance from each other. Each arm (9) is rigidly connected to said shaft (6) with a orderly increasing circumference angle relative its previous neighboring arm in the perpendicular perpendicular direction of said shaft (6). A drop hammer (10) has a hammer head (11) and a hammer handle (12) and is fixed at each radial arm (9) at its outer end via a hammer joint (13). The hammer comprises of an anvil or striking handle (4) which are mounted on or contact with collecting pipes of a tubular heat exchanger unit (1), which exposed ends are rapped by a drop hammer (10) when a shaft (6) rotates. In order to reduce or remove a repulsive force occuring