Applications Guide Tracer Graphical Programming - Trane

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Chapter 5 Cooling tower with variable-speed fan example
Writing the calculations module
According to the sequence of operation, the program must perform three
calculations:
• Change in water temperature across the cooling tower
• Ambient wet-bulb temperature
• Approach temperature
Calculating change in water temperature across the
cooling tower
Start with the simplest of the three calculations and assemble the blocks
needed to calculate the change in water temperature across the cooling
tower.
To calculate the change in water temperature across the cooling
tower:
1. Create a Wireless write block, name it ReturnTemp, and connect it to
the Return Temp input block.
2. Place a ReturnTemp and a SupplyTemp wireless read block in the
design space.
3. Place a Subtract block in the design space and connect it so that the
Supply Temp is subtracted from the Return Temp.
➤ Inputs to a Math block
ForsomeMathblocks,theinputportyouchooseforavalueisimportant.
For example, the Subtract block subtracts the bottom input-port
value from the top input-port value. Remember this relationship
when working with the Subtract and Divide blocks.
4. Place a Variable block in the design space and assign the analog variable,DeltaTemp,toit.Makesuretosettheblocksothatyoucan
writetothevariable.
5. Connect the Subtract block to the Delta Temp variable block
(Figure 87).
Figure 87: Change in temperature calculation
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