Page 409 - ExtendSim User Guide

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Comprehensive example
The following example illustrates many of the concepts from this chapter. The top line of the Tank Constraint model, shown on the right, has two rate sections, two critical constraints, and one relational constraint.
☞ The sections that follow use the abbreviation FPT to indicate “flow units per time unit”.
Rate sections
Rates, Constraints, and Movement 383
Comprehensive example
Tank Constraint model
Rate sections are determined internally by a communication between Rate library blocks and the Executive (Item library). The boundaries between rate sections are established at the begin- ning of the simulation run; they do not change during the run even if the effective rates change.
At the beginning of the simulation run:
• TheFillingvalvehasamaximumrateof10,getsitsinflowfromaninfiniteSource,and sends its outflow to an empty Storage tank that has a capacity for 100 flow units. The system will thus calculate an effective inflow and outflow rate of 10 FPT for the Filling valve at the start of the simulation run. (This will change once the Tank fills.)
• TheStoragetankhasaacapacityfor100flowunits,getsitsinflowfromavalvewithamax- imum rate of 10 FPT and sends its outflow to a valve with a maximum rate of 5 FPT. At the beginning of the simulation run, its effective inflow rate will thus be 10 FPT and its effective outflow rate will be 5 FPT.
• The Emptying valve has a maximum rate of 5 FPT and sends its outflow to an infinite Sink. Its effective inflow and outflow rate is 5 FPT.
At the start of the simulation run, the Storage tank’s effective inflow rate is different from its effective outflow rate. Thus the first rate section for the Tank Constraint model starts at the Source block’s outflow connector and ends at the inflow connector on the Storage tank. The second rate section starts at the Storage tank’s outflow connector and ends at the Sink’s inflow connector.
Critical constraints
There are two critical constraints in the top line of the Tank Constraint model. The first critical constraint is the 10 FPT entered in the Filling valve’s maximum rate field. The second is the 5 FPT entered in the Emptying valve’s maximum rate field.
Relational constraint
Relational constraints define the way the effective rates of different sections are related to each other. At the beginning of the simulation run there are no relational constraints – the effective inflow rate is independent of the effective outflow rate. When the Tank (which has a finite capacity of 100 flow units) becomes full, it applies one critical constraint: inflow rate must be less than or equal to outflow rate.
Simulation’s impact on the effective rates
Since it is empty at the start of the run, the Storage tank's initial set of flow rules will not include placing any restrictions on its inflow rate. Consequently, the initial effective rate of flow through Rate Section 1 is limited only by the Filling valve's critical constraint of 10 FPT.
Discrete Rate
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