Process Flow Models - Batches
Some operations may be performed in batches. Here when a batch size other than 1 is specified, we assume that all items in the batch are processed simultaneously. A common example is a heating process where several units are placed in a furnace and treated for a fixed amount of time.

 To include batches in the analysis, click the Batch button on the dialog. Batches are often associated with lots, so we select the Setup time and Lot Size button. We include Resources and Time Functions. The example has five operations arranged in series. When batches are present the resource amount column contains a formula that is: Resource amount = 1/batch size Two of the operations have batch sizes other than 1. The first operation, Op1, has a processing time of three hours, but the batch size of three indicates that three units are processed at the same time. The resource use per unit is 1/3, and the net effect on the use of Machine 1 is one hour per unit. Op1 contributes three hours to the Throughput Time and three units to the WIP. The transfer lot size of 1 means the units enter and leave machine 1 one unit at a time. The Furnace operation is different because it processes batches of five. The lot of five units enters the furnace altogether and they complete process in five hours. The setup lot, transfer lot and batch size are all the same. The operation contributes five hours to the throughput time and 25 units to the WIP.

We have included lot change operations before and after the furnace because the transfer lot changes from 1 to 5 for the furnace operation and then back to 1 for Op4. The time computed (2) in the Time Function column (O) is the average time required for accumulating items before the furnace and dispensing the items after the furnace. The results clearly show the impact of batch processes on throughput time and WIP.

We modify the example by changing the lot and batch size for the furnace operation to 10 units. We have also included a setup time of three hours. Clearly the batched operation has a great impact on the WIP.

The batch size must always be positive. A number greater than one implies that items share a resource. A number less than one means a single item consumes more one resource hour per unit of processing time. Zero or negative batch sizes are not allowed.

Operations Management / Industrial Engineering
Internet
by Paul A. Jensen