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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.

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))

To build this calculation in layers we are going to work in steps

  • Create the two most low level subtractions SUB(0,0) (Temp Hot Inlet - Temp Outlet Cold), (Temp Outlet Hot - Temp Inlet Cold)
  • Then add these two SUBS to the basket
  • Create a new SUB(0,0) and drag each previous subtractions calculations from the basket into the new Sub Calculation to get SUB(SUB(0,0),SUB(0,0))
  • Then add to the basket
  • Create a new LN(0) and Drag the SUB(SUB(0,0),SUB(0,0)) calculation from the basket into it
  • Then add to basket
  • Now its Time to create the final calculation. We have created the numerator and denominator of the calculation.
  • Numerator = SUB(SUB(0,0),SUB(0,0))
  • Denominator = LN(SUB(SUB(0,0),SUB(0,0)))
  • Create a DIV(0,0) and Add Both the Numerator and Denominator into calculation from the basket.
  • Now you have created the calculation


STEP 4 - USING YOUR CALCULATION

Once you have created your calculation and its displaying on the trend 

  • You need to change the name to make it easier to visualise - optional
  • Then add this calculation to the basket.
  • Once your calculation is in the basket you can drag and drop it anywhere you want it from the basket.

Save the Basket. Or copy the calculation into a KPI macro and save that so you don't loose your work. If working in a KPI make sure to save it