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Rates, Constraints, and Movement 379
Defining a critical constraint
The Tank Constraint example shows two flow streams with identical behavior. In the bottom flow stream, Tank 2 uses the options Maximum inflow rate and Maximum outflow rate to replace the filling and emptying valves found in the upper flow stream.
Tank Constraint model. Top stream with two Valves to constrain flow; bottom stream with maximum rates defined in Tank 2.
☞ Instead of using a Valve block to constrain flow, setting maximum inflow and/or a maximum outflow rates in a Tank or Interchange block can be used to satisfy the model’s requirements for a constraint.
Convey Flow
The Convey Flow block calculates critical constraints for its inflow and outflow connectors separately. The critical constraints are derived from model conditions and settings in the dia- log.
• The critical constraint for the Convey Flow block’s inflow is calculated by multiplying the block’s effective speed by its maximum density entry.
☞ The effective speed can be less than or equal to the speed set in the dialog. If the block is non- accumulating, or if it is accumulating but cannot accumulate more, and the block’s ability to deliver flow exceeds downstream demand, the effective speed will be lower than the entered speed.
• Thecriticalconstraintfortheblock’soutflowistheresultofthemultiplicationoftheblock’s speed setting by the density of flow present at the outflow end of the block.
☞ Setting the initial contents or capacity for a Convey Flow block is discussed in the chapter “Storage and Units”. The “Delaying Flow” chapter shows how to use the Convey Flow block to delay the movement of flow in a model.
Merge and Diverge
The critical constraint for one or more of a Merge or Diverge block’s branches can be implic- itly specified under certain conditions. Most often, the result would be a rate of 0 (zero).
Discrete Rate

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