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324 Introduction
What this chapter covers
Discrete rate modeling is based on rates of flow that change when events occur. In a discrete rate system, quantities of “flow” (material, product, data, etc.) are located in one or more parts of the model. During the simulation run, the flow moves from one location to another at a cer- tain speed, called the effective rate. The movement between blocks that hold or route the flow follows paths, rules, and constraints that are set in the model.
As discussed in “Modeling methodologies” on page 45, the primary modeling approaches are continuous, discrete event, and discrete rate. In some situations (listed later in this chapter), simulating a system using discrete rate modeling is a more natural fit compared to using con- tinuous or discrete event modeling. Processes that are event driven, rather than time driven, do not lend themselves to continuous simulation. Systems where there is no “item” that can be identified, or when there are so many items that identification is meaningless, can be more nat- urally represented using a rate-based approach rather than discrete event modeling. Further- more, rate-based models run faster than discrete event models and are applicable to thinking in terms of flows, tanks, rates, and so forth.
☞ Any system or process that involves a quantity of something that is stored at one place, then moves to another place at a rate per time unit, can be simulated using discrete rate modeling.
Like continuous and discrete event modeling, rate-based modeling can help you perfect pro- cesses and products. It is useful for planning resource capacity by determining the rate at which products are being processed or sold. It is helpful for testing various schedules to maxi- mize process efficiency. And it can be used to analyze the effect of processes on the internal and external environment.
☞ For information about discrete rate modeling in general, including how it differs from continu- ous and discrete event modeling, see “Modeling methodologies” on page 45.
What this chapter covers
• Discrete rate application areas
• Simulating discrete rate systems
• Blocks for doing rate-based modeling
• An introduction to some important discrete rate concepts:
• LPtechnology
• Layout of a discrete rate model
• The Executive block
• Connectorsandconnections
• Flowunitsandunitgroups
• Flow rates: constraining, effective, infinite, and potential
• How the Discrete Rate module is organized
Discrete rate application areas
Discrete rate simulation is used in two diverse areas:
• To model commodities that would normally be considered “stuff” rather than “things”, for example powders or liquids, gases and other fluids in the following areas:
• Petrochemical • Manufacturing
Discrete Rate

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