Page 375 - ExtendSim User Guide
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Creating Flow 349
Creating flow
The Convey Flow, Interchange, and Tank blocks are residence-type blocks; they hold flow and can serve as sources of flow for the model. There are two ways flow can be introduced into a discrete rate model:
1) BysettinganinitialcontentsforaConveyFlow,Interchange,orTankblock.Thesethree residence blocks can be preloaded with initial amounts of flow, either a finite amount or, in the case of the Tank and Interchange blocks, an infinite amount. Depending on the block and options that are selected, the initial amount is:
• Created by entering a constant number, or selecting “infinite”, in the block’s dialog • Entered in a table for each segment (Convey Flow block)
• The attribute values for any number of initial layers (Tank block)
• The value of an item attribute (Interchange block)
• Getting the value from a ICO valuein connector (Interchange block)
2) Onceaninitialcontentshasbeenset,undercertainconditionsthecontentsoftheTankor Interchange block can be changed during the simulation run. An example is the Inter- change block when it is set to Tank only exists while item is in it. In this case, you can choose to have each arriving item reestablish the initial contents.
Empty and not-empty
If a Tank or Interchange block’s initial contents is finite, its status can alternate between the empty and not-empty states. This change of state has an impact on the effective rate calcula- tions:
• IftheTankorInterchangeblockisnotempty,itsflowlevelcanfall.Consequently,theeffec- tive outflow rate could be higher than the effective inflow rate.
• If the block is empty, it cannot provide more flow than what it concurrently receives. In this case, the effective outflow rate has to be less than or equal to the effective inflow rate.
When a Tank or Interchange block changes state between empty and not empty, ExtendSim will calculate a new set of effective rates. For more information about rates, see “Rates, rate sections, and the LP area” on page 371.
☞ The Convey Flow block has a different mechanism for calculating a change of state between empty and not-empty. For more information, see the “Delaying Flow” chapter.
Tank initialization
A Tank’s initial contents can be: • Entered in the dialog as:
• 0(thedefault) • Anumber
• Infinite
Options for initial contents
• Specifiedbyflowattributevaluesandbeeitherfiniteorinfinite.(Flowattributesaredis- cussed starting on page 353.)
To simply cause the Tank to have an infinite amount of initial contents, select the Initial con- tents option, then check the (infinite) check box. For example, in the Yogurt Production model from the Discrete Rate Tutorial, the Tank blocks that represented Liquid Supply and
Discrete Rate

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