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PID Issues

vooharmy
edited February 2016 in Accessories

Months ago I purchased all the parts to change my box to 4-20ma prop valve. Cant seem to get it to hold temp. Has lots of over/under shoot. I know a few people have this sort of setup and i am curious to know of its performance. I can tune it to hold reflux and the minute I set on a new temp its completely out. Its a pretty slow valve but I think its similar to @CothermanDistilling's IE Johnson valve. It occurs to me that the controller might be the issue its a rexc700 if a better controller might give me a better result I would definitely grab a replacement but if this is the expected performance I would choose to go back to a solenoid which I know to hold temp within .5c

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Comments

  • jacksonbrown

    Can you set it to manual?
    If so what is the flow rate at 100% and what is the max power going into the boiler?

  • vooharmy

    I can set it to manual but I have nothing to measure it with. I have been using mains water and a gate valve to adjust water flow. System is running around 7kw its a 5" still. When I am next in the shed I can grab a jug and stopwatch and work it out?

  • jacksonbrown

    @vooharmy said: I can set it to manual but I have nothing to measure it with.

    How about a drum, bathroom scales and a one minute timer.

  • grim

    Don't change out the PID, it won't make much difference.

    What valve do you have (specific model)? What flow rate range are you targeting? Where is the temp probe? What are your P, I, and D values?

  • jacksonbrown
    edited February 2016

    At 7kW I would suggest anything that gives over 4 or 5l/min is a bit big. How much over 5l/min suggest how much too big.

    To work out the bare minimum water required just use,

    60 * (kW-loses) / (SHw * (Tout-Tin)) = flow rate in l/min

    kW = power to the power.
    loses = heat lose for your system in kW. Mines about 0.300 kW
    SHw = Specific Heat value for water, 4.185
    Tout = Put the lowest vapour temp which would be about 75°
    Tin = What’s the highest feed water temp you might use, say 35°

    60 * (7-0.2) / (4.185 * (75-35)) = 2.4 litres a minute to get 100% reflux so the valve needs to adjust less than this to start taking product.
    This assumes a very efficient heat exchanger (deph) that will get the water close to the vapour temp.

    Loses will be will pretty consistent for your set up regardless of power settings. It's a function of the surface area and the DeltaT between air and still which is pretty consistent. You can measure it by lowering the power to the point where the take-off stops. At that point the power in is equal to the power out (loses).
    Mine doesn’t have a lot of surface area (4x4" plates on an insulated keg)

    If your bucket and stopwatch gains between 3 and 5 kg a minute of weight in a minute then you’ve got the perfect size valve.

    You can compensate a bit with the p value in the PID but if it’s way off you’re pushing shit up hill from the start.
    That’s why I think the flow rate is the first thing to check.

  • jacksonbrown

    @jacksonbrown said: kW = power to the power.

    That should read 'to the boiler'. And heat loss under it. Perhaps a mod can fix it. Sorry English is not my stong point.

  • CothermanDistilling
    edited February 2016

    OK, first you need to list all your devices by model number, and any settings they have, then where the temp probe is and what kind of condenser you are using, pressure and temp of water, etc...

    • PID model rexc700-xxx-xx*yy (what are the letters numbers after rexc700?)
    • P
    • I
    • D
    • min
    • Valve model
    • water pressure
    • water temp
    • condenser type and description
    • placement of probe

    Note - I am looking at the manual (PDF) I found on google and do not see a way to set the minimum valve opening, if you have the probe outside the condenser, and no minimum flow, you will never get it to work right.

  • vooharmy
    edited March 2016

    Sorry for the delayed response been a while since I have been in the garage and went down today to try another attempt. Measured out the flow and adjusted to 4L per minute with a gate valve. No improvement in the operation what I find is that the valve overshoots a great deal on the heating side of things. Stops up pretty well on the cooling side of things

    Here are the details of the setup:

    • PID model - rexc700-FK02-8*AN
    • P = 30
    • I = 800
    • D = 110
    • Min = ?? Not sure
    • Valve model = Johnson Controls VG1245AE+906GGA 1/2" two way ball valve with actuator
    • water pressure = Adjusted to 4L/Min (1Gallon/min)
    • water temp = 28.5°C / 77°F
    • condenser type and description: 5" Dephlem StillDragon
    • Probe position: 1/2" Socket into dephlem approximately 1/2way up on the opposite side to the supply.

    I have attempted a variety of other settings using the below method:

    1. Set all gains to zero.
    2. Increase the P gain until the response to a disturbance is steady oscillation.
    3. Increase the D gain until the the oscillations go away (i.e. it's critically damped).
    4. Repeat steps 2 and 3 until increasing the D gain does not stop the oscillations.
    5. Set P and D to the last stable values.
    6. Increase the I gain until it brings you to the setpoint with the number of oscillations desired (normally zero but a quicker response can be had if you don't mind a couple oscillations of overshoot)

    Not sure what to do next, put solenoid back into service while I nut out the issues.

  • CothermanDistilling

    OK, numbers look right... The only difference i see is that the sestos PID has a setting to have the current output set to a minimum value, I think I have mine at 77(7.7mA).. but since you have the probe in the condenser, that should not matter..

    rexc700-FK02-8*AN

    the F means reverse action, not sure about this, but should not be an issue, looks like it has more to do with fail-safe for spring return valves

    The K02 means K thermocouple with 0-400C range.. so P value of 30 is 120C 'proportional band' the Sestos is -50-1300C , so 30 is 405C (P is inverse, the smaller number means faster reaction)... my suggestion would be to increase the P number

    when you refer to the 'cooling side of things' and 'heating side of things', I am not quite understanding... I guess 'heating side of things' means you could be increasing power and cooling means decreasing power? On mine, if I go from 5500W to 11,000w to 16,500w and back, it takes a full minute or more to get the RV temp to the setpoint again, it may overshoot a degree or so and then comes to the setpoint... Maybe I should state that my goal is not to have the system control quickly for such a large change, but to stay at the setpoint over the course of the run, where the temp rises slowly... this also points to raising the P value and slowing the response.

  • vooharmy

    This has been an interesting journey for me and all the adjustments and the like that I have taken to improve the performance of my proportional valve have improved the performance of my solenoid. I think the biggest improvement has been in setting the flow rate of my water to something that makes sense. Currently with a solenoid valve and prop valve side by side I prefer the solenoid as its performance adjusts almost instantly to what ever is happening. Living in a house with three women means that someone is always messing with a tap or using water. Thank you to everyone who chimed in with advice on this one and believe me when I say I am thankful and appreciative to finally have a solution which makes me happy.

    -Voo

  • grim

    You were running a single solenoid on municipal water supply, correct?

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