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Shell and tube heat exchangers are a vital part of the process in which these are installed and their condition can significantly affect the process economics in several ways. During the operation, deposition of salts t kes place on inside surface of the heat exchanger tubes which decreases its heat transfer. NFL carried out hydro-jet cleaning of tubes of final stage cooler of air compressor in Ammonia-! plant during March/April-015 shutdown. During hydro-jet cleaning, around 150 numbers of tubes were observed badly choked with hard scaling/ deposition. On detail inspection of choked tubes, it was observed that tubes were choked almost p to 2.5 meter length. The representatives of external agency expressed the~r inability as they do not have the drill of required length for de-choking. Options of chemical cleaning, replacement of tubes, plugging of choked tubes, etc., were explored. Mechanical department proposed to try the de-choking of the tubes mechanically by drilling method and in case it did not work, then tubes could be ;lugged. Since, time was available and with in-house effort, it was decided to proceed for de-choking of tube by drilling method. With efforts the tubes were de-choked successfully without any damage to tubes. Th article details the procedure ado ted for de-choking of tubes by drilling method.