PID or not?

I have a cooling water system tied to a PLC5 that, until recently was controlled manually. I am not experienced with PID and was wondering if I could implement it for my task. I have an open loop cooling water system comprised of cooling towers with fans, a holding tank with temperature probe, heat exchangers and pumps. I am tasked to control the cooling tower fans in "Auto" mode. Currently they are operated manually as needed. I should also mention the fans are capable of hi and low speed.
I have started to write ladder logic and I keep questioning if PID in some form could work here. I have three modes of operation, summer, winter and economizer. The desired temperature during any season, at this point, is 70 degrees farenheit. I will also have access to outside air temperature for sure and am trying to get outside air humidity available as well. My logic needs to understand the relationships of outside air temperature and humidity, the use of the fans in hi or low speed and mode of operation (summer, winter or economizer).
I've read through numerous posts and am impressed with the depth of knowledge in this area on this site. Your feedback is kindly appreciated.

DublinStout

Yes PID could work. But I think the first question is do you really need that precise control? What is your desired water temperature and what is the tolerance on that? If you have been doing fine let us say with a desired temp of 90 plus or minus 5 or maybe 10 you may not need PID (??)

90 +- 1 maybe PID,
90 +- 0.1 more than likely PID,
90 +- 0.01 definitely

Dan Bentler

You didn't say what you are controlling but when we design a controller to control room conditions inside a manufacturing facility with equipment that uses a chiller in conjunction with outside air: Think about Enthalpy.

We use enthalpy calculations to decide whether outside air will work or we need the chiller with outside air or just the chiller.

DublinStout said:

I will also have access to outside air temperature for sure and am trying to get outside air humidity available as well. My logic needs to understand the relationships of outside air temperature and humidity, the use of the fans in hi or low speed and mode of operation (summer, winter or economizer).
DublinStout

Click to expand...

This is what prompted me to mention enthalpy. I've done a few Summer-Winter-Auto applications for room conditions.

PID control is mainly for continuous output to a valve, motor speed and whatever. You have off, low and high, so some other logic might be better.

In a simplistic way you could use
way above: high
slightly above: low
at or below: off

This will of course need refining dependent on whatever other factors you're using.

Edited to add:
If the operators are able to keep a stable temperature manually, ask them how they go about it. May give some useful insights automating their thought process.

It seems as though you'd be better off not using a PID if you only have 2 speeds available.

While it would work it wouldn't particularly help.

I'd shoot for 80 or 85 degrees not 70. 70 might be a little tough in the summer. You might want to have a motorized valve between the tower feed and return line to blend water. Why not use a PID? You'll probably have to add some additional parts but they would pay for themselves with the energy and chemical savings. I'd also think about rotating pump operation so sludge and fall-out is minimized in the idle pumps. As long as you're at it, why not monitor system TDS, make up water TDS, make up water totalizer, and chemical/biocide treatment as well?

Agree 100% with todster

Dublin

Thinking on this some more
1. If you have lots of bucks and a month downtime then tear out all of existing system. Rebuild existing pumps for reinstall. Get chillers to cool water and use heat from them for heating office space in winter (and maybe summer air conditioning) and cold city water. Retain cooling towers for summer use.

2. OK #1 just wont work. Take a hard look at how existing system works and is put together. My guess is it is a bunch of add ons.
Install PLC and have it control fan speed on cooling towers via PID and VFD. Same for circ pumps. You should easily be able to adapt existing water chemistry controls to PLC and maybe PID possibly with VFD for injection pumps.
CAUTION you can run pumps TOO slow for two reasons
A fan in motor is not able to cool motor
B pump will not pump this far down on pump curve - if pump is not pumping it is not cooling the seals / shaft packing. CHECK PUMP CURVES

Once you have the PLC in and running then you can start REALLY evaluating system taking into account enthalpy max desired temp max delta T, and what pieces of equipment have these values. When you have these numbers you can select control sensors and actuators and install them. The big advantage of doing it a bit at a time is multi
1. You can learn as you go and from your mistakes
2. You can evaluate a component for reliabilty and decide to use it on other portions of system.
3. If you design and install to gain best efficiency you can use money saved to help fund the next upgrade.

Dan Bentler

I don't think he's going to be able to run the pumps on VFD like that. The water flow for a cooling tower would be from the basin in tower to pump, then pump to heat exchangers, then back to nozzles in tower or blended back to pumps. The heat exchangers most likey have a temp. compensating valve on them. If the flow is too low and heat in exchangers exceed 120 deg. then fall out and scale may occur and possibly polymerization. It would be difficult to control the pump to provide the correct flow to every heat exchanger. The tower would need the flow to be at a certain GPM and higher to efficiently distribute it on the fill.
What I would do first is buy a few authoritive books on cooling towers and approach the subject knowingly. If you enjoy viewing PDF's, you could go for the Google degree.

Welcome the forum DublinStout.

I've done a number of cooling towers and I've done them with PID and without. If your tower fans aren't on a VFD then there isn't much to be gained by using PID.

I use a staging algorithm for the cooling tower fans. Tower pumps are usually run at constant full flow, todster touched on just a few of the reasons for doing so. You can stage up and stage down the tower fans using a series of temperature switch points and timers. Make sure each switch point has some hysteresis built in and incorporate some minimum on time and minimum off time for each fan motor so that you aren't constantly bumping a motor on/off if the temperature is hovering right at a switch point. Make sure that you have a freeze-stat set at some value way above where you might start making ice in the winter. Even a tiny small chunk of ice will wreak havoc with a pump. I shut the fans down at 68F and shut off the feed pumps at 65F, just to make sure we don't get anywhere near making any kind of frost in the tower on even the coldest winter days.

Your PLC program does not need to understand the relationship between outside air temperature and humidity. But you do. The ultimate tower temperature you can cool to is determined by the wet bulb conditions at the tower loacation. Calculating the current wet bulb temperature in your PLC from temperature and humidity is very difficult and requires the use of a psychrometric look up charts and also a barometer in addition to temperature and humidity. In the end it does not provide any information that will improve your control if you designed the system properly, so IMO you should just drop that from the project. The best cooling towers can reliably cool water down to within seven degrees F of the wet bulb temperature. There are tables available that give the wet bulb temperature history for your area - your tower vendor probably can provide them. If your tower is properly sized for the load then find the highest wet bulb temperature in the table and add seven degrees. That is the best temperature setpoint that you can reliably reach year round. In winter you can do a bit better. For example, here in the Salt Lake City area we live in a high altitude desert so we have a low wet bulb temperature at 64F. That means that I can reliably make 71F water year round with a cooling tower. Your location indicates you are in Massachusetts, but not where. Checking the wet bulb for Boston, I see that it is 75. That means that you can reliably make 82F water year round with a cooling tower in Boston. You can easily hit your 70F set point in the winter, but you are not going to get there in the summer time, no matter how large you make the tower and no matter how many fans you run, if you are in or around Boston that is. If you are somewhere else then find out what the highest summer time wet bulb is there.

I suggest something like this
Stage = 0 - all fans and feed pumps are off.
Stage = 1 - tower feed pump is on.
Stage = 2 - tower feed pump + 1 fan.
Stage = 3 - tower feed pump + 2 fans
Stage = 4 - tower feed pump + all fans.

You can also use an alternator to rotate which fans come on first so that all get even usage.
Do a forum search using key words such as wet bulb and cooling tower and fan or pump alternator. You will find many threads where we have discussed the topic.