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Rates, Constraints, and Movement
Blocks of interest
As discussed in the Introduction to this module, the movement of flow in a discrete rate model must take some time. It is a modeling error if the flow moves instantaneously throughout a model.
ExtendSim's discrete rate system attempts to move flow through the model as fast as possible. In the absence of any constraints, the effective rate of flow would approach infinity and the flow would move instantaneously throughout the model; this is never correct. For this reason, flow movement must be constrained by rates and conditions that are built into the model, and a lack of appropriate constraints is a modeling error that will stop the simulation run
In order to restrict flow rates:
• Discrete rate blocks are required to define their own sets of constraining flow rules
• Each area of a model must have one or more critical constraint mechanisms
Critical constraint flow rules, such as a block’s maximum rate, place an upper bound on the rate of flow, limiting it to a number less than infinite. The blocks’ aggregated set of flow rules ultimately defines how fast flow is permitted to move over time throughout the model.
This chapter discusses rates, the blocks that constrain flow, and how model conditions impact the rate of flow. It will cover:
• Rates,ratesections,andtheLParea
• Flowrulesfordefininghowablockpermitsflowtomovethroughit • The blocks that specify critical constraints
• How to meet the constraint requirement
• Acomprehensiveexampleofconstraintsandratesections
☞ Most of the models illustrated in this chapter are located in the folder \Examples\Discrete Rate\Rates and Constraints. The tutorial models mentioned are located at \Examples\Tutori- als\Discrete Rate.
Blocks of interest
The following blocks from the Rate library will be the main focus of this chapter.
Convey Flow
Delays the movement of flow from one point to another. Can accumulate flow to a maximum density, accumulate flow to fill empty sections, or act as a non-accumulat- ing conveyor.
Interchange
Used to mix flow with items, this block can also limit its maximum rate of inflow and outflow.
Tank
The block most frequently used to store flow can also limit its maximum rate of inflow and outflow.
Discrete Rate

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