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Lets calculate the Log mean Temperature difference on a heat exchanger. 

The calculation is this

LMTD = ((Temp Hot Inlet - Temp Outlet Cold)  -  (Temp Outlet Hot - Temp Inlet Cold))  / LN ((Temp Hot Inlet - Temp Outlet Cold)  -  (Temp Outlet Hot - Temp Inlet Cold))

#Previously when building calculations you would build them external from the system that the calculation is going to run in. 

NOTE - We are going to build this calculation using the Chart macro that is accessible in the Basket by selecting Trend. The method is exactly the same for building it in a KPI

STEP 1 - ADD TAGS TO BASKET

  • First step is to go to the search macro or a page that contains the relevant tags.

  • Right clicking or drag them into the basket.
  • Remember you have the option saving the basket.
  • Once you have added the tags you need in this calculation. We are ready to get started.

STEP 2 - TREND THE BASKET

  • Trend the basket
  • Were going to create out calculation on a new tag added to the trend window
  • Select Add tag under the trend settings
  • Enter "=" into the search to activate a calculation
  • Then open up the calculation builder

STEP 3 - BUILDING THE CALCULATION (2 Methods)

Two ways of building a multiple layer calculation. 

1.  Creating the Calculation Skeleton using the Eigen Calculation Builder then Dragging the relevant tags from the basket into the Calculation.

Step 1 - write out the calculation

We know the calculation we want to do is:

LMTD = ((Temp Hot Inlet - Temp Outlet Cold)  -  (Temp Outlet Hot - Temp Inlet Cold))  / LN ((Temp Hot Inlet - Temp Outlet Cold)  -  (Temp Outlet Hot - Temp Inlet Cold))

So we can start writing a few things in a calculation builder

  • A division is written like - DIV(0,0)
  • A subtraction is written like - SUB(0,0)
  • A subtraction with in a subtraction is SUB(SUB(0,0),SUB(0,0))
  • A LN statment is LN(0)

Now lets build the calculation

  • Numerator = SUB(SUB(0,0),SUB(0,0))
  • Denominator = LN(SUB(SUB(0,0),SUB(0,0)))
  • Wrap it in a Divide we get
  • DIV(SUB(SUB(0,0),SUB(0,0)),LN(SUB(SUB(0,0),SUB(0,0)))


Step 2 - Populate are calculation builder with the tags from the basket

  • Drag a tag from the basket into a spot.

NOTE - The trends at the end of each tag indicate it. If a tag is input wrongly this will not display. It will also indicate if your calculation is invalid.


2. Build the calculation in layers adding each layer into the basket.

DIV(SUB(SUB(0,0),SUB(0,0)),LN(SUB(SUB(0,0),SUB(0,0))))



STEP 4 - 












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