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Tutorial for Discrete Rate Systems
A basic discrete rate model
The key to discrete rate modeling is constructing a flow diagram using blocks from the Rate library to represent flows through the system. The Rate library is designed specifically for building discrete rate models. Blocks from other ExtendSim libraries, especially the Item, Plot- ter, and Value libraries, are often used with the Rate library to create discrete rate models.
The example in this chapter shows how to build a discrete rate model of a yogurt process; it will use many of the blocks from the Rate library. Starting with a simple model, then adding complexity and features, this chapter will show how to:
• Buildamodelofasimplerate-basedprocess
• Add a maximum flow rate that varies with the time of day
• Add a second supply of product that is occasionally shut down for maintenance
• Mix the two supplies according to a proportion
• Create a filling operation that puts the liquid yogurt into containers
• Add a conveyor to simulate a cooling process
• Package the containers into cartons
• Create a palletization area where the cartons are stored
• Add a second palletization area in parallel to the first
While this example model simulates a mixing, filling, and packaging process, the Rate library is useful for simulating many diverse concepts and processes.
This tutorial assumes you have completed the chapters in the main Tutorial module that starts on page 14 and that you have read the Discrete Rate Introduction that starts on page 324. It is also suggested that you complete the Discrete Event Tutorial that starts on page 108.
A basic discrete rate model
Rate-based models are mainly concerned with how quickly the flow moves in different sec- tions of the model, and what the yields will be, given the constraints and configurations of the model. A common rate-based model involves a flow of product moving from one holding area to another, with a valve that determines how quickly the flow moves.
The Yogurt Production model represents a process that takes a supply of liquids, converts it into plain yogurt, then mixes the yogurt with fruit. The fruited yogurt mixture is poured into individual yogurt containers and cooled and the containers are then packaged into cartons. The final step is to place the cartons on pallets for storage.
The assumptions for the final model are:
• The supply of liquid to make the yogurt comes from one location and the fruit comes from another location. Both locations have an infinite supply.
• Formostoftheday,theliquidisprocessedintoyogurtatarateof100gallons/minute.Since fewer workers are available during lunch, the processing rate decreases to 60 gallons/minute for that hour.
• After processing, the yogurt is routed to the mixing area.
• The fruit is processed and delivered to a mixing area at a rate of 8 gallons per minute when equipment is not undergoing maintenance, and a rate of 2 gallons per minute when it is.
• Each 10 gallons of mix is composed of 1 gallon of fruit and 9 gallons of plain yogurt.
Discrete Rate

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