PID phillosophy

I need to clarify this a bit.

If you read about industrial heat exchangers you can find information on flow rate vs thermal exchange. In large tube a shell heat exchangers an increase in flow can result in more turbulence and better thermal exchange plus a few other dynamics as well. However, there are many types of thermal exchangers. You can prove the concept I mentioned above with a small piece of copper tubing and a pan of hot water, which I just did to reconfirm to myself. Very low flow from the faucet resulted in warm discharge water and higher flow resulted in cold discharge water. Cold discharge water means very little thermal exchange.

Sorry for the confusion.
 
I'm not sure it would happen in this situation with a fan blowing air on a a hot work piece. In some heat exchangers the coolant flow can be high enough so it doesn't have time to transfer enough heat. Yes it is cooling but the other part of the heat exchanger is being heated so it runs away. You can measure the outlet temperature of the coolant and see that once the flow is high enough it starts decreasing in temperature because its not in the exchanger long enough to carry the heat away. Once again it depends on the process and it would most like be known by the process engineers.

The process does indeed reverse because in normal operation a rising PID output results in a cooler hot side process and a hotter coolant output. Once a certain flow is achieved an increase in output results in a hotter hot side process and a cooler coolant output temperature.
I think you may be confusing what's going on in your example. Assuming you're using this HX to cool something, if the coolant flow is higher, then the coolant outlet temp will be lower because you're putting a set amount of energy (from the hotter product) into a greater mass of liquid (the coolant) than before. Think about putting a cup of water on the stove for 1 minute vs. a gallon. Which would be hotter?

At the same time, the higher flow of coolant will remove heat quicker from the the product side of the HX in your example. The product will exit the HX at a lower temp with a greater coolant flow. Reversal of control direction wouldn't happen.
 
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I think you may be confusing what's going on in your example. Assuming you're using this HX to cool something, if the coolant flow is higher, then the coolant outlet temp will be lower because you're putting a set amount of energy (from the hotter product) into a greater mass of liquid (the coolant) than before. Think about putting a cup of water on the stove for 1 minute vs. a gallon. Which would be hotter?

At the same time, the higher flow of coolant will remove heat quicker from the the product side of the HX in your example. The product will exit the HX at a lower temp with a greater coolant flow. Reversal of control direction wouldn't happen.

It's easy to get into an apples vs oranges comparison here. Plate chillers/heaters are different than tube and shell/hairpin heat exchangers. Sometimes you just want to heat or cool the liquid in one side of the exchanger and not affect the other side. Other times you want to cool the out going finished product and preheat the incoming feedstock. Other times you may want to use an existing heat source to preheat a feedstock to a fraction of the temperature of the heat source using a ratio/blending controller . I think what you are saying maybe correct but there are many types of heat exchangers.
 
It's easy to get into an apples vs oranges comparison here. Plate chillers/heaters are different than tube and shell/hairpin heat exchangers. Sometimes you just want to heat or cool the liquid in one side of the exchanger and not affect the other side. Other times you want to cool the out going finished product and preheat the incoming feedstock. Other times you may want to use an existing heat source to preheat a feedstock to a fraction of the temperature of the heat source using a ratio/blending controller . I think what you are saying maybe correct but there are many types of heat exchangers.
Yeah, in a single pass setup if you raise the flow rate of one side of the HX, the temperature change will will be less on that side even though energy transfer is greater. But if your PV is outlet temp of one side, and your CV is the flow rate of one side or the other, the control direction will still always be in the same. I don't see how a reversal could ever happen.
 
Well, this has turned into a very interesting thread actually.
I appreciate everyone input. Lets talk some more about this cascaded control loop idea.

Is it needed?
If so what applications is it good for?

What are some of the added complications about it.
 
gain scheduling for non linear systems (Siemens)

Hi guys and gals



I have a severely non linear system that i am trying to develop a controller for.

The non linear curve can be broken down into 3 linear-ized sections.

With a single fine tune in the s7-1200 i can get close but when the controller set point travels from one linear region to another we experience large overshoots and large errors.

Research tells me that gain scheduling may be an idea. a set table of gives each used for a different range of the process variable.

Has anyone implemented this before?
Recommendations on how to implement this?



Thanks
 
Hi guys and gals



I have a severely non linear system that i am trying to develop a controller for.
Why keep this a secret? I hate wasting time asking questions.

Has anyone implemented this before?
Recommendations on how to implement this?
Yes, a few of the people on this forum have been to our advanced training.
The video is long and unrefined. Watch till you get the idea.
http://deltamotion.com/peter/Videos/Delta Computer Systems Advanced Training.mp4
Basically we use a cubic spline or what many would call a cam table to calculate gains for each angle so the gains are continuously changing.
The user can tell the swing arm to move in degrees. We use cubic splines to convert the degrees to linear motion and a set of gains for each angle.
 
I was pretty sure i created a new post for this

Not sure how it ended up in my old post

Can this be moved to a new thread without violating the forum rules?

Also, occasionally i am under contract of non disclosure so i must be limited with details.
 

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