I have a chiller application where in i need to maintain the Chilled water outlet temp at constant 1 deg C. even if there is a change in the Chilled water inlet temp by 0.5 or 1 deg still my system should deliver constant 1 deg C output. The final control element is a steam control valve which controls the steam flow to the system. I will be using a 3-wire PT100 sensor at inlet & outlet of chilled water for the same.
PLease guide me how i will be able to achieve this requirement.

Thanks..

 

The usual problem with heat exchangers is response time which may make it difficult to achieve the degree of control you require.

A technique suggested by Mark Shelley (then of Fluor) and successfully employed for heating is to split the flow stream prior to the chiller and chill one stream and then recombine with the bypass stream in a static mixer.
A temperature sensor downstream of the static mixer will control the flow proportion going through the chiller.
A second temperature sensor in the output from the chiller will control the refrigerant.

In this way part of the stream is chilled below the target temperature but when recombined with the unchilled flor the temperature can be very accurately controlled by the flow proportion as this provides very fast response to temperature changes while any trends in temperature are compensated for by the control of the refirgerant which is necessarily slower.

This was used to control hydrocarbon heating to obtain a temperature stable to less than 1degF despite process stream temperature variables.

 

Since you have a measure of both inlet and outlet, you should use this to your advantage. Use a simple feedforward control scheme. Set it up as follows:

* PID Loop, with PV from outlet temperature
* Feedforward signal from inlet temperature

This way, the chiller will react to any changes in inlet temperature immediately. You will not have to wait for the outlet temperature to start to drift.

To see one way to do this, visit:
http://www.expertune.com/WhitePaperFeedforward.html

-George

 

a related approach to both of the above is employed for humidity control in some environmental test chambers

http://www.humiditycontrol.com/howitworks.pdf