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  1. #1
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    tuning hvac loops

    Hellloo

    I'm looking for some good websites or books about tuning pid loops especially for hvac control.

    Thank you

  2. #2
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    Like a noob introduction?

    Or a veteran service tech's advanced theory?

  3. #3
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    Do you like math or twiddling knobs?

  4. #4
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    deltafrank, you've got mail.

  5. #5
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  6. #6
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    You have more mail

  7. #7
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    Quote Originally Posted by miller_elex View Post
    Like a noob introduction?

    Or a veteran service tech's advanced theory?
    No big-bang theories. No endless calculation. Looking for specific loop application and the major points regarding it.

    Example :

    The modulation output for a vapor boiler should be set in a Proportional mode only. Let's say you have a 5 psi band and your setpoint is 13 psi and you have a 15 psi security relief valve so you don't want to overshoot 13 psi. Make sure the output signal is 0 as soon as you reach 13 psi.

    This is simple yet, but let's say you always tune loops with a large proportional band and make the integral do all the work, in this case, you'll probably get your face burn before the integral will begin to decrease the signal.

    Example 2 :

    What is the best way to control 3 water/water thermopumps, who are divided in 6 stages (2 stages each), and you have to take in consideration the minimum off delay on each of the compressors?

    These are the kind of informations I'm looking for.

  8. #8
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    The control tuning book suggested is a good place to start.

    I come from a process control background rather than HVAC but it all ends up being the same pile when it comes to tuning. What comes out of the controller is no different whether you have a heat exchanger or a chemical mixer being controlled by it.

    I generally used to tune using the seat of your pants method. You bump the process and watch the change in the input to the controller and the output of the controller. The shape of your temperature to time graph will tell you a lot of your process and how it is tuned. Understanding it will help you decide what PID parameters you want to increase or decrease.

    It all comes down to gain. How much you are adding to the system and how close you are to unity gain. How you add it and the RC time constants of your process will be your limiting factor of what you can get away with. You trade off speed for stability, sometimes you can get what you want sometimes not.

    Tomorrow I'll look through my files and see which might be suitable, a bit late tonight.

  9. #9
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    Quote Originally Posted by deltafrank View Post
    No big-bang theories. No endless calculation. Looking for specific loop application and the major points regarding it.

    Example :

    The modulation output for a vapor boiler should be set in a Proportional mode only. Let's say you have a 5 psi band and your setpoint is 13 psi and you have a 15 psi security relief valve so you don't want to overshoot 13 psi. Make sure the output signal is 0 as soon as you reach 13 psi.

    This is simple yet, but let's say you always tune loops with a large proportional band and make the integral do all the work, in this case, you'll probably get your face burn before the integral will begin to decrease the signal.
    Simple enough, you can not use Integral as the main controlling function. You look at what you want to achieve with your control and you pick the best method there. There is a reason you would use a ton of Proportional in this case. You have a hard limit and you do not want to go over it.

    Did the same kind of thing in a heat treat furnace that needed a tight band but was not allowed to overshoot by ten degrees. Had a high limit controller set at the top of the band to knock off the power when the temperature hit the limit.

    Just used enough Integral to keep long term stability but it was a fraction of the Proportional term. As my boss says 'It is what it is.' You can only use the equipment you have, and in the way it is designed to operate. As you already know using mainly Integral here is not a good idea.

    Example 2 :

    What is the best way to control 3 water/water thermopumps, who are divided in 6 stages (2 stages each), and you have to take in consideration the minimum off delay on each of the compressors?

    These are the kind of informations I'm looking for.
    I would use a PLC on this one. Again you would have to know the operating characteristics of the equipment and build up a program.

    There are control strategies for the more complex applications as long as they are relatively standard. The ISA publishes Standards that control people follow, I am guessing this is what you are looking for.

    As an example look at BOILER CONTROL SYSTEMS

    http://www.wbdg.org/ccb/DOD/UFC/ufc_3_430_11.pdf

    Look at the FIGURES towards the end. There are a number of loops shown.

    The ISA sells these standards as a recognized method of doing control. You still have to know what it is you want and how to set them up. Good luck finding specific rules on parameters. It all depends on the equipment.

    As an example we have a operating room where the doctors can adjust the temperature and humidity in the theater. As far as I am concerned the engineer who designed the system was not quite on the ball. The temperature and the humidity sensors are in the return duct.

    The doctors want tight control of the temperature and the humidity but with the sensors as far from the occupied space as you can get there is a big lag between when the control action happens and the result is sensed and the controller has time to adjust.

    Basically there is a big lag in the system. Just like your first example by the time the controller knows something is wrong things are already out of control. Forget about tight control.

    Now place the two sensors closer to the action, you have less lag, and you can jack up the gain.

    Same equipment but one different detail and you can use different PID parameters and get a different result. No cookie cutter rule will give you the right values to cover both examples. That is where education comes in.

    Not sure how much you know but the following link is not too bad a start.

    http://www.controlguru.com/wp/p71.html

    The book referred before is still a good idea. If you have specific applications that you basically redo all the time it would be a good idea to look up a Standard if one is available for it. Also manufacturer's technical support should be able to help you. It is their equipment after all.

    There are courses to get you up to speed if you are so inclined.

    http://www.isa.org/Template.cfm?Sect...ontentID=80170

    http://www.idc-online.com/pdf/traini...umentation.pdf

    If you have a specific question there might be someone here to help out but a blanket question asking for information to cover many circumstances may not get too much of a response. I myself would not mind seeing a book on rules to start off on.

  10. #10
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    Thank you for the answer and the links.

    I'll take a look at it.

  11. #11
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    Quote Originally Posted by deltafrank View Post
    Thank you for the answer and the links.

    I'll take a look at it.
    Should have posted this link instead.

    http://www.controlguru.com/pages/table.html

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