View Source

Table of Contents

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.

Help Pages > Build Advance Calculations using the Basket > Screen Shot 2018-10-26 at 10.38.33.png

Help Pages > Build Advance Calculations using the Basket > Screen Shot 2018-10-26 at 10.39.03.png

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

Help Pages > Build Advance Calculations using the Basket > Screen Shot 2018-10-26 at 12.07.50.png

Help Pages > Build Advance Calculations using the Basket > Screen Shot 2018-10-26 at 12.09.55.png

Help Pages > Build Advance Calculations using the Basket > Screen Shot 2018-10-26 at 12.09.34.png

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.

Help Pages > Build Advance Calculations using the Basket > Screen Shot 2018-10-26 at 12.10.50.png

Help Pages > Build Advance Calculations using the Basket > Screen Shot 2018-10-26 at 12.12.13.png

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))))

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

Create the two most low level subtractions SUB(tag,tag)
THen add these two SUBS to the basket
Create a new SUB(0,0) and drag each previos sub into the new Sub Calculation to get SUB(SUB(0,0),SUB(0,0))
Then add to the basket
Create a new new calcualtion with LN(0) then add the SUB(SUB(0,0),SUB(0,0)) into the calculation
Add to basket

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

Help Pages > Build Advance Calculations using the Basket > Screen Shot 2018-10-26 at 14.45.49.png

Help Pages > Build Advance Calculations using the Basket > Screen Shot 2018-10-26 at 14.46.41.png

Help Pages > Build Advance Calculations using the Basket > Screen Shot 2018-10-26 at 14.47.10.png