Multi-Stage Water Bath Control System

P

Thread Starter

Peter

I have a temperature control system where constantly moving fluid must be heated via heat exchanger to keep a process chamber at constant temperature. Find a simple diagram of the system here:

https://www.dropbox.com/s/5f784pexc1z1w08/Tank Chamber Temp control.png?dl=0

1. Fluid enters from reservoir at ambient temperature (20-26 deg C).

2. Hose passes through a peristaltic pump that can operate from 50 to 400 mL per min.

3. The fluid passes through a heat-exchanger in a heated water bath ("Tank"). The tank is about 750 mL and heat-exchanger is about 1.5m in length. The Tank temperature is controlled by a "Heater Output" signal (0-100% PWM) and has a "Tank Temp" sensor

4. Out of the Tank heat exchanger, the fluid flows through a section of about 20" of hose, outfitted with an "Infrared Temp" sensor to the "Chamber".

5. The Chamber is about 400 mL in volume, insulated, and where the experiment is taking place. "Chamber Temp" is the ultimate process variable we are trying to control. It should stay at the "Chamber SP", typically 37 deg C +/- 0.5 deg C, with a critical limit of never going above 37.5 deg C. This set point of 37 can be set anywhere between 30 and 37 deg C.

The "experiments" are expected to have a warm-up time, then can run anywhere from 30-90 minutes, keeping Chamber Temp at the Chamber SP.

Right now we have a PID-based control process running. There are two stages:

(1) Warm Up - Here, we use the Tank Temp as the process variable and warm up the tank to somewhere near the Chamber SP. Through experimentation, we have found the dead time (time to see any change in PV, when MV is changed) is about 150 seconds. And the time constant is much greater - it takes about 800 seconds to stabilize from 34 deg C to 37 deg C. This stage is relatively straight forward and we can tune a PID loop to keep the tank at a certain SP when pump is off and everything is static...

(2) Chamber Control - Here, is where things get more complicated. When the pump starts running, fluid passes through the water bath, through the Infrared Temp sensor, then in to the Chamber. The goal is to control the Chamber Temp by controlling the Heater Output of the Tank. It has been a challenge to tune the PID loop, as the pump may be at different rates, the tank may be warmer or cooler than the Chamber, and it takes soo long for a change in Heater Output to affect the Chamber Temp. due to the mass of the Tank, time lag, and mass of the Chamber.

My question is around the Chamber Control stage described above:

- Is PID appropriate here? It is VERY slow, long delays, and hard to tune.

- Is it best to have one loop using Chamber Temp to manipulate Heater Output? Or to have two "stacked" loops where one can observe the Chamber Temp to set a desired Infrared Temp, then a second to use the Heater Output to control Infrared Temp? Or even three:
PV-MV
1. Chamber Temp - Infrared Temp
2. Infrared Temp - Tank Temp
3. Tank Temp - Heater Output

- Is there a way I can characterize the system more quantifiably to help tune the loop or give you guys more information?

Thanks,
Peter
 
C

Charles Moeller

Peter,

The best instrumentation and controller often can not compensate for poor system design:

1. The temperature sensor for heater control should sense the working fluid sent to the chamber instead of the heater tank liquid. (The large thermal mass of the tank prevents timely response.)

2. Minimize heater tank volume to reduce thermal mass, or provide a more-closely-coupled heater.

3. If the working fluid only provides thermal management for the chamber and does not otherwise change, then you may close the loop by returning the chamber effluent (fluid out) to the pump input (fluid in). Pump speed will then have much less effect on temperature. You may need a standpipe or surge tank with makeup from the reservoir for the closed loop design.

4. If the working fluid is changed during its passage through the chamber, have the fluid out preheat the fluid in a separate heat exchanger.
 
Temperature baths work best with low thermal mass and low heat flows. Hence they are used with capillary viscometers. Yes there will be a warm up time. For a capillary viscometer the thermal mass of the capillary is low and the heat flow into the system from the fluid is low at around 50ml/min.

With heat exchangers the problem is different.
They can provide a lot of heat very rapidly even with high flow rates but the problem is response time.

This problem is solved here:
http://www.viscoanalyser.com/Clipboard 5.png

The heat exchange fluid flow control is under a slower response PID and which compensates for trends.

The mixer control valve is on fast respopnse PID control and compensates for temperature variation in the mix. Note that a static mixer is used and the pipework is well insulated.
 
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