f50 There are three equal minimal values : processing time of 10 hours for job 5 on machine A and for jobs 2 and 3 on machine B. According to rules, job 5 is scheduled next to job 4 in the first schedule or next to job I in the second schedule. Job 2 then is scheduled next to job 7 in the first schedule or next to job 4 in the second schedule. The optimal sequences are shown below. 14 5 3 2 7 6 1 5 3 2 4 76 Now we can calculate the elapsed time corresponding to either of the optimal sequences, using the individual processing times given in the problem. The elapsed time and idle times for machines A and B will be same for either sequence. The details corresponding to the first schedule are shown in table 5.5. TABLE 5.5 Machine A Machine B Idle for Job Time in Time out Time in Time out machine B 1 0 3 3 11 3 4 3 9 11 17 0 5 19 19 31 2 3 19 34 34 2 46 56 2 7 55 56 59 0 6 55 66 66 67 7 Thus the minimum elapsed time is 67 hours. Idle time for machine A is 1 hour (66th-67th hour) and for machine B is 17 hours. 5 9 44 3 34 46 46 EXAMPLE 5.4-3 A refrigeration company has six plants located in different parts of a city. Every year it is necessary for each plant to be completely overhauled. The overhauling is carried out in two stages A and B, and each stage requires a crew of workmen with completely different skills. The work on stage B can start only when stage A has been completed. The plant has to be closed for the entire period of the overhauling. The company, at present, is following the schedule of the overhaul of the six plants as given below: Time required by the Crew (days) Plant: P P2 P; PA PS Po Crew A: 12 10 Crew B: 10 9 14 6 8 (1) Determine the optimal sequence. (ii) If downtime of any of the six plants costs 5,000 per day, idle time for crew A costs *1,500 per day and idle time for crew B costs 2.500 per day, which of the two schedules, the present one or the one determined in part (i), will be more economical? What are their respective costs ? 9 10 8 10 7