EXAMPLE 5.1: Bread Making
For the manager of a bakery a first priority is to understand the products that are made and the process steps required. Exhibit 5.4A is a simplified diagram of the bread-making process. There are two steps required to prepare the bread. The first is preparing the dough and baking the loaves, here referred to as bread making. The second is packaging the loaves. Due to the size of the mixers in the bakery, bread is made in batches of l0O loaves. Bread making completes a batch of 100 loaves every hour, which is the cycle time for the activity- Packaging needs only 0.75 hour to place the 100 loaves in bags.
From this we see that bread making is the bottleneck in the process. A bottleneck is the activity in a process that limits the overall capacity of the process. So if we assume that the bread-making and packaging activities both operate the same amount of time each day, then the bakery has a capacity of 100 loaves per hour. Notice that over the course of the day the packaging operation will be idle for quarter-hour periods in which the next batch of bread is still being made but packaging has already completed bagging the previous batch. One would expect that the packaging operation would be utilized only 75 percent of the time under this scenario.
Suppose that instead of having only one bread-making operation we now have two, as shown in Exhibit 5.48. The cycle time for each individual bread-making operation is still one hour per 100 loaves. The cycle time for the two bread-making lines operating together is half an hour- Because the packaging operation takes 0.75 hour to bag 100 loaves, the packaging operation now is the bottleneck. If both bread making and packaging were operated the same number of hours each day, it would be necessary to limit how much bread was made because we do not have the capacity to package it. However, if we operated the packaging operation for three eight-hour shifts and bread making for two shifts each day, then the daily capacity of each would be identical at 3,200 loaves a day (this assumes that the packaging operation starts up one hour after the bread-making operation). Doing this requires building up a shift's worth of inventory each day as work-in-process. Packaging would bag this during the third shift So what is the throughput time of our bakery?
In the original operation with just the single bread-making process, this is easy to calculate because inventory would not build between the bread-making and packaging processes- In this case the through- Put time would be 1.75 hours. In the case where we operate the packaging operation for three shifts, the average wait in work-in-Process inventory needs to be considered. If both bread-making operations start at the same time, then at the end of the first hour the first 100 loaves move immediately into packaging while the second 100 loaves wait. The waiting time for each l00Joaf batch increases until the baking is done at the end of the second shift.
This is a case where Little's Law can estimate the time that the bread is sitting in work-in process. To apply Little's Law we need to estimate the average work-in-process between bread making and packaging. During the first two shifts inventory builds from 0 to 1,200 loaves. We can estimate the average work-in-process over this 16-hour period to be 600 loaves (half the maximum). Over the last eight-hour shift inventory drops from the 1,200 loaf maximum down to 0. Again the average work-in-process is 600 loaves. Given this- the overall average over the 24-hour period is simply 600 loaves of bread. The packing process limits the cycle time tbr the process to 0.75 hour per t00 loaves (assume that the loaves are packaged in a batch). And this is equivalent to a throughput rate of 133.3 loaves4rour (100/0.75 = 133.3). Little's Law calculates that the average time that loaves are in work-in-process is...