Process Flow Analysis
Process Details

We return to the Process worksheet to review details of the computations. The contents of rows 1 through 14 describe the process for product A. Columns B through L hold the data for the product and columns M through S hold results of Excel computations.






The adjusted time is usually a combination of the time required to set up a machine for production, , and the time required to actually perform the operation, . The net effect of these two components depends on the number of units processed for each setup, the lot size s. For purposes of estimating resource usage it is sufficient to allocate the setup time to the individual unit of product as

To illustrate consider operation A1 with processing time 3, setup time 120 and lot size 30. The adjusted operation time for these parameters is 7. It is shown in the Adjusted Time column (M) of the Excel display. Columns M through Q are interesting for systems with scrap and/or proportions. For this example with no scrap and all proportions equal to 1, the columns are shown but do not show any variation. The Operation Flow column (R) holds the flow through the operation expressed in minutes. Since the flow of finished goods is 60 per week and the number of minutes per week is 2400 minutes, the operation flow through each operation is 0.025 per minute. The WIP column shows the work-in-process at each operation. For processing operations the WIP is:

WIP = (adjusted time)*(flow rate)*(lot size)

For operation A1 this is: WIP(1) = (7)*(0.025)*(30) = 5.25 units

The total WIP for this process is the sum of the WIP values for the operations. This sum is computed in cell K3 as 43.5. The Throughput Time is computed from Little's Law as

Throughput Time = (System WIP)/(Flow Rate per min) = 43.5/(60/2400)) = 1740 minutes

This value is computed in cell K2. The computation of WIP and throughput time make the assumption that materials are processed and transferred in lots.

Similar results are computed for each product.


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Operations Management / Industrial Engineering
by Paul A. Jensen
Copyright 2004 - All rights reserved