At the Hannover coal mine in Bochum (Germany), a former Krupp plant and now a site of the Westphalian Museum of Industry, one of the few pit ventilation systems of the Ruhr mining industry from the time before the World War II is preserved. Loads and damages from the operating time as well as an improper repair 30 years ago are responsible for a number of heavy damages, which must now be repaired if this important technical monument is to survive. Description of the technical monument: The pit fan system at shaft 1 of the Hannover mine was built in the 1920s (presumably 1929). The manufacturer is indicated by a sign "Westfalia Dinnendahl Bochum, Essen plant". The plant consists of branched underground ducts connecting the shaft with the ventilation system, four shut-off slides, a pair of fan wheels with helical sheet metal hoods and two concrete diffusers. The wheels were designed to be driven by steam engines. In the central machine house, however, only one fan was driven by a steam engine, the other by an electric motor. The axles of the large fan wheels are mounted on pedestal bearings (cast iron) on a concrete foundation. The steel construction of the hoods consists of riveted and screwed angle profiles in different sizes with sheet steel fillings. Parts of the fan impeller may be made of cast iron. In the literature, information on this type of fan can be found in C.H. Fritzsche: Lehrbuch der Bergbaukunde, Volume I, 9th edition, Berlin 1955, p. 651 ff. The type is described as "centrifugal fan". The lecture deals with the procedure for survey and documentation, damage analysis, method development and feasibility studies. The online portal [www.indumap.de](http://www.indumap.de) served as toolkit. The results can also be used as a guideline for the treatment of similar structures of large industrial monuments.