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Water cooling control circuit

Discussion in 'General Electronics Discussion' started by Roland King, Aug 30, 2015.

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  1. Roland King

    Roland King

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    Aug 30, 2015
    As a complete electronics ignoramus, I’m hoping someone here can give me some assistance, please… I am attempting to automate the water cooling circuit on a distillation column, and to date have come up with an arrangement where a PID, sensing the coolant water temperature through a type K thermocouple, operates a solenoid valve that turns the water supply on and off. The problem with the latter is that it does not control the water supply to an acceptable level of accuracy – when the PID is set to a nominal temperature of 53 deg. C, the coolant water fluctuates from between 40 and 70 degrees. (The water flow rate involved is low at around 0.5 litres/minute). As I see the problem, I am using a 1/2" solenoid gate valve which the control circuit determines is either on or off, and this arrangement cannot achieve the required accuracy. Could my problem perhaps be solved by using a different type of valve (e.g. a globe valve which is designed for producing a throttled flow) and a more sophisticated electronic control that will keep the valve partly open by varying amounts and thereby avoiding the wild temperature fluctuations I’m getting at the moment - or maybe there is a completely different approach to take? Any help and advice on this matter will be much appreciated, as will also some guidance on relevant component selection and sourcing.
     
  2. Alec_t

    Alec_t

    2,950
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    Jul 7, 2015
    Welcome to EP!
    The obvious question: have you explored different tuning values for the PID parameters?
    I wonder if the problem is not so much the solenoid valve but more the coolant system? A very small thermal mass may be more difficult to hold at a constant temp than a larger mass, for example. Also, a low coolant flow-rate may be inadequate for a large distillate flow rate. What is the total volume of the coolant system, and what amount of distillate are you trying to cool?
     
  3. Roland King

    Roland King

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    Aug 30, 2015
    Thank you for your prompt reply. I did say I am an ignoremus in these matters so being asked whether I have 'explored different PID tuning values' leaves me feeling like turning up in the examination room without having done any of the course work! If it helps, my set up was originally tested by placing the thermocouple probe in cups of hot water at various temperatures - the response of the circuit was very quick and accurate (the valve opening and closing within 1 degree C of the value set on the PID). Secondly, the cooling on this column (it's a proprietary item) is intended to be carried by means of a manual valve and a digital thermometer, and under that arrangement I have been able to hold the temperature to within an acceptable 2 or 3 degrees of nominal. I'm trying to automate the distillation process which takes around 2 hours so that I can go off and do something other than stare at a thermometer for that length of time! As regards your other questions, the total volume of the coolant system will be small - I'd guess no more than 150 - 200 ml, and the amount of distillate collected is around 5 litres over the 2 hour period.
     
  4. Alec_t

    Alec_t

    2,950
    803
    Jul 7, 2015
    That would involve quite rapid temperture changes, whereas the distillation temperature changes may well be much slower. So the optimum PID parameters would not be the same in both cases. Have you tried tweaking the PID values under actual cooling system working conditions? Perhaps boosting the 'D' setting would help speed things up?
     
  5. Roland King

    Roland King

    3
    0
    Aug 30, 2015
    Thanks, Alec_t - I'll give it a try.
     
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