Build Advance Calculations using the Basket

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. 

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.

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.