Run No. 53, from September 8 to 10, 1952, was an N-T-U crude recycle cracking operation using the reaction chamber. Estimated maximum permissible operating conditions were employed for the purpose of collecting data to establish a practical limit of severity. The operating conditions for the one test period conducted were 950 degrees F. heater outlet temperature and 300 p.s.i.g. recreation chamber pressure. These conditions were too severe for prolonged operation and the run was terminated after 48 hours of operation because of coking of the pressure control valve in the reaction chamber to flash chamber transfer line.