CHAPTER 4: FLOW TIME ANALYSIS
4.3 Solutions to the Problem Set
Draw a process flow diagram.
[b]. The theoretical flow time is 36 minutes:
There are three paths through the system:
A: Take Order – Food – Deliver – Bill → 4 + 18 + 12 + 2 = 36 mins B: Take Order – Wine – Deliver – Bill → 4 + 4.8 + 12 + 2 = 22.8 mins C: Take Order – Cart – Deliver – Bill → 4 + 10 + 12 + 2 = 28 mins
Path A is critical so the TFT is 36 minutes
[c]. The flow time efficiency is 36/60 = 60%
The flow chart of the process is as shown in Figure TM-4.1.
Figure TM-4.1. Flow chart of the Kristen Cookies Process flow
Flow unit = 1 order of 1 dozen the theoretical flow time is 26 minutes. This is determined by adding the activity times from start to finish for 1 dozen cookies.
Flow unit = 1 order of 2 dozen. To determine the theoretical flow time for this flow unit, we first observe that certain activities can be performed in parallel. For example, while the oven is baking the first dozen, You can spoon the dough for the second dozen into another tray. Therefore, the flow time of such an order is not simply the sum of the activity times. A useful tool is a Gantt chart that shows the times during which different resources of interest are occupied for various activities. A Gantt chart for the three resources executing an order of 2 dozen cookies is shown in Figure TM-4.2. The dough for the 2 dozen cookies is mixed by You in 6 minutes and subsequently you spoon dough for 1 dozen in 2 minutes. Therefore in the 8th minute, the RM is ready to load the oven and set timer, which takes 1 minute. The oven starts baking the first dozen at the 9th minute and completes baking at the 18th minute. Meanwhile, You spoon the second dozen into another tray. At the 18th minute, the RM unloads the first tray from the oven and loads the second tray into the oven and sets the timer. So the second dozen starts baking at the 19th minute. While the second dozen bakes, the first dozen cookies cool and RM packs them into a bag, which takes a total of 7 minutes. At the 28th minute the second dozen finishes baking at which time the RM unloads the tray. After cooling for 5 minutes, the RM packs the second dozen in 2 minutes by the 35th minute. Finally, payment for the order and delivery to customer takes place in the 36th minute. Therefore the theoretical flow time for an order of 2 dozen cookies is 36 minutes.
An alternate way to compute the theoretical flow time is using the concept of bottleneck resources (discussed in Chapter 5). Oven is the bottleneck resource with 10 minutes of activity per dozen cookies. Observe that the first dozen goes into the oven at the end of 8 minutes. The second dozen will go into the oven 10 minutes after the first dozen. Therefore, the theoretical flow time of the second dozen = theoretical flow time of the first dozen + activity time at the bottleneck = 26 + 10 = 36 minutes. Similarly, theoretical flow time of 3 dozen = theoretical flow time of the first dozen + 2 * activity time at the bottleneck = 26 + 2*10 = 46 minutes.
Figure TM-4.2: Gantt Chart for Kristen’s Cookie; order size = 2 dozen.
With two ovens, the baking of the two dozen can be overlapped. So You can spoon the second dozen into another tray by the 10th minute. At this time, the RM is ready to load the second oven. The second oven will finish baking the second dozen by the 20th minute. The first dozen is out of the oven at the 18th minute after which it cools for 5 minutes and then the RM packs them in 2 minutes finishing at 25th minute. By this time, the second dozen has cooled and ready to be packed which takes another 2 minutes. Payment and delivery takes another 1 minute finishing the order in 28 minutes. 2.
With one big oven, both trays get loaded into the oven at the end of the10th minute. The timer is set only once; the baking starts at the 11th minute and finishes at minute 20. The two trays...
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