Controlling Temperature

Discusses use of COCO, the process simulation and modelling software suite from AmsterCHEM, downloadable from http://www.cocosimulator.org

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Controlling Temperature

Postby Chani » 20 November 2020, 13:56

Hello,

I am trying to control the output temperature of a heat exchanger by applying a MeasureUnit, a Controller and a FlowConstraint. The output temperature should be 723K and according to the T the inlet cold stream flow should change.
The output information stream of the controller is invalid.
I think that as I am already giving the value of the outlet T in the heat exchanger, it fails to calculate. I don't really know how to configure the Heat Exchanger and make it work with the controller.
Thank you in advance!!
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Re: Controlling Temperature

Postby jasper » 21 November 2020, 08:28

To connect the input end of the information stream to the controller, it must be populated with a parameter. For that, the output end must be connected first. To do so, create a virtual information port on the flow constraint: double click, go to Ports, drop down the menu Port Operations, Add Virtual Information Port. Select either mole or mass flow, depending on which you want to control (does not matter much). Make sure it is an input port. Connect the information stream, it now contains a parameter corresponding to the controlled flow. Then connect the other end to the controller. Set the controller set point to 723 K. Set the initial guess on the controlled stream.

You should indeed also change the specification of the heat exchanger so that flow rate affects the outlet temperature.
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Re: Controlling Temperature

Postby Chani » 28 November 2020, 13:26

Thank you for your reply!

I changed the specification of the heat exchanger to "Max Heat Exchange" but I cant see an iteration going on (maybe its too fast) although the flow value of the controlled stream for the outlet temperature to be 723K is correct. Also, how can the vapor that leaves the exchanger has the same value as the hot stream that enters the exchanger? Maybe because the type is set to Max heat exchange?

I have tried all types of specifications for the exchanger but the message "maximum number of iterations exceeded" pops up.

How can I set the exchanger to only use an specific amount of heat till the outlet stream reaches the ideal temperature? Im confused...
Chani
 
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Re: Controlling Temperature

Postby jasper » 28 November 2020, 14:28

Max heat exchange makes that the outlet of one stream matches the inlet of the other stream, in counter current mode, for whichever stream causes the limiting amount of heat to be transferred.

If you want the outlet to be just at 723 K no matter how much heat is available, you can simply set outlet temperature as specification, but then of course you will not be able to control the inlet flow rate with this, as the product temperature does not depend on flow rate.

Other specifications that would work is heat transfer that is limited by some sort of efficiency, e.g. number of stages specified, or UA specified.

I cannot look at 'maximum number of iterations exceeded' without more info - it sounds like an infeasible specification. If you want me to look at it, please attach the case.
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Re: Controlling Temperature

Postby Chani » 28 November 2020, 17:35

Okay, as I want to control the outlet temperature of the heat exchanger with the flow of the inlet cold stream, the best specification would be "heat exchange" but again, I have no value to set it as initial guess. Any value I add, the error shows up "Maximum number of iterations exceeded".

I honestly think that in this particular case, I cant work with a controller because I have tried all possibilities and nothing works. I will have to set outlet temperature as specification and remove the measure unit, controller and flow constraint.

I am attaching the file. Thank you!!!
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Re: Controlling Temperature

Postby jasper » 30 November 2020, 12:18

Of course this does not work. You are always adding exactly 165895.81975 W to F2/ST, which is independent of the flow rate of F2/WC.

This kind of setup works if the heat that is exchanged depends on the flow rate, e.g. LMTD, Max. Heat Transfer or any of the Effectiveness specifications. But you cannot control the flowrate of a stream by a temperature set point if the temperature is independent of the flow rate, which is the case for a fixed amount of heat transfer, or a fixed temperature specification. It would of course work if you set a fixed temperature specification on F2/HV3, because in this case the amount of heat transferred depends on its flow rate.
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