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Mixing Flow and Items
Using the Interchange block to mix items with flow
area where the Max & Min block (Value library) is used to translate the cycling item's location into a stage number. Since the cycling item remains in the Stage 1 queue, the model is now in a Stage 1 holding pattern. Consequently, Valve 1 is opened (so the Reception tank starts receiv- ing flow) and Valves 2 and 3 are closed.
Stage 2: Open Valve 2 and release flow to Processing
Once the Reception Tank reaches the full state, Gate 1 is opened and Gate 2 is closed. This allows the item to move on to the Stage 2 queue. The result is that Valve 2 opens, Valves 1 and 3 close, and flow starts moving from the Reception tank into Recycling and Processing. Gate 2 remains closed while the Reception tank empties.
Stage 3: Process the flow
Once the Reception tank is completely empty, Gate 2 is opened, Gates 1 and 3 are closed, and the item enters an Activity block labeled “Stage 3”. While the item remains in Stage 3, all three Valve blocks remain closed. The Activity block, which has a delay of 2 minutes, is used to keep the flow in the Processing tank for some period of time so it can be processed.
Stage 4: Open Valve 3 and release the flow
Once processing is completed, the item leaves Stage 3 and moves into the Stage 4 queue. Since Gate 3 is currently closed (because the Processing tank is not empty), the model is now in a Stage 4 holding pattern and Valve 3 opens. Once the Processing tank is empty, Gate 3 opens and the item cycles back to the State 1 queue where the whole processes starts all over again.
☞ While this model is useful for demonstrating how mixed mode models can control item and flow movement, the same behavior can be created without items using the Valve block’s Hys- teresis option. For more information, see “Setting hysteresis in a Valve” on page 411.
Using the Interchange block to mix items with flow
The Rate library’s Interchange block is unique in that it allows items and flow to interface directly with each other. Flow can enter the Interchange block not only through its inflow con- nector but also through the arrival of an item. Conversely, flow can exit the block through its outflow connector or through the exiting of an item.
The use of the Interchange block was introduced on page 342 of the Discrete Rate Tutorial, and the block’s capacity to hold flow is discussed starting on page 362 of the Flow Sources, Storage, and Units chapter. This chapter will describe how the Interchange block interfaces flow with items.
☞ The Interchange block is where an item can be filled with flow or emptied of flow.
There are a number of occasions where it can be useful to provide items with the ability to store, transport, and empty flow as they move from one section of a discrete rate model to another. For example, the attribute capabilities used to distinguish one item from another can also be used to distinguish one block of flow carried by one item from another block of flow carried by a different item. This can be an especially useful modeling construct since flow units by themselves are indistinguishable from each other.
Behavioral rules
The Interchange block has two very different modes (“Tank only exists while item is in it” and “Tank is separate from item”) that affect how the block behaves. (These modes were intro- duced on page 362 and will be discussed more fully on page 426.) Even so, the Interchange block always follows a fundamental set of rules:
Discrete Rate

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