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ptc-66830521-Visitor
1-Visitor
October 8, 2014
Solved

Given/Find Solve Block "Variable Undefined"

  • October 8, 2014
  • 1 reply
  • 1926 views

I'm working on a heat exchanger analysis problem and was going to use a Given/Find Solve Block to solve a system of equations. There are two heat exchangers in series. Near the top of my mathcad sheet I input the given values for the hot inlet fluid temperature and cold inlet fluid temperature, the physical properties, and mass flow rates. I then list my initial guesses and create the Given/Find block but get an error when using the Find function. Any hints/tips/tricks/concerns?

Thanks!

Best answer by Werner_E

There were some errors in your sheet:

1) a typo when you set the inital guess values (ratings.2 instead of rating.2)

2) You must put all variables you are seacrhing in one Find command.

3) You use a variable T.ci twice which is never assigned a value.

4) This is not a failure but helps to get a solution. You don't have to search for the two ratings variables, you can assign them after the solve block.

5) Sometimes a solve block simply doesn't work as excpected because of the use of units and you get a solution when you get rid of them temporarily. Always worth a try.

See attached. We get a solution but it definitely is not a correct solution.

1 reply

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Werner_E25-Diamond IAnswer
25-Diamond I
October 8, 2014

There were some errors in your sheet:

1) a typo when you set the inital guess values (ratings.2 instead of rating.2)

2) You must put all variables you are seacrhing in one Find command.

3) You use a variable T.ci twice which is never assigned a value.

4) This is not a failure but helps to get a solution. You don't have to search for the two ratings variables, you can assign them after the solve block.

5) Sometimes a solve block simply doesn't work as excpected because of the use of units and you get a solution when you get rid of them temporarily. Always worth a try.

See attached. We get a solution but it definitely is not a correct solution.

P
ptc-66830521-VisitorAuthor
1-Visitor
October 8, 2014

Thank! With your reworked Solve Block I was able to get the correct answer. I forgot to paste over the T.ci values from the original worksheet. The reason why I didn't post the original was becuase it was a very large worksheet with other problems already worked. The reason why you got an "incorrect answer" was because you accidentally set rating.1 = rating.2 when you were double checking your answers. Thanks for the help!

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