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``````Rates, Constraints, and Movement 381
Meeting the critical constraint requirement
change block. With these maximum rates, the Tank or Interchange will limit the rate of flow between the two residence blocks to a number less than infinite. This is shown in the Tank Constraint example discussed in “Tank and Interchange” on page 378.
Merge or Diverge blocks
If a Merge or Diverge block is between two residence blocks, the inflow and outflow branches may or may not require a critical constraint mechanism.
☞ For any Merge/Diverge mode, if a critical constraint has been placed on a Merge block’s out- flow branch, no critical constraints are required on its inflow branches. Likewise, a critical constraint on a Diverge block’s inflow branch means that no critical constraints are required on its outflow branches. If those constraints have not been placed, the critical constraint require- ment depends on the block’s mode.
The following table provides an overview of each mode’s requirements for critical constraints when neither the Merge block’s outflow branch nor the Diverge block’s inflow branch has a critical constraint. (In this table, the word “variable branch” means an inflow branch for the Merge block or an outflow branch for a Diverge block.)
Mode
Batch/Unbatch Distributional Neutral Priority Proportional Select
Sensing
Critical constraint requirements if there is no critical constraint on the non-variable branch
Only on one of the variable branches Each variable branch
Each variable branch
Each variable branch
Only on one of the variable branches (See Note, below) Each variable branch
Each variable branch
Note: For the Proportional mode, the variable branch with the critical constraint should not have a proportion <=0. Otherwise, that branch will be closed and the other variable branches will have potentially infinite effective rates. This is an error condition.
☞ Merge and Diverge blocks, including their modes, are described fully in the chapter “Merging, Diverging, and Routing Flow”.
The two examples that follow use the Minimum Valve model to illustrate some of the table’s concepts.
Discrete Rate

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