Control engineering in the real world

Just to keep this sadly degraded newsgroups head a bit above the murky waters of solution manuals and questionable conferences.... No response
expected, just nice to tell my story to a few people, in addition to my work colleagues, that might appreciate it.
Working on a setup that controls an exothermic reactor, the temperature, amongst other things, is a bit critical. The reagents mix with water in a fairly typical CSTR, main handles being the addition rates. In order to help the temperature control, there are two water additions, hot and cold, from different parts of the plant. The system's been running, sort of, for years, the ops put parts of it in manual regularly when it goes unstable.
Typical for this sort of control configuration, which resembles spaghetti, it's not easy to see immediately where the problem is. But it looked more or less sensible, with some efforts to decouple the main interactions, and the main dependencies being dealt with by the right handles. The process guy finally sussed it - the 'hot' water is sometimes colder than the 'cold' water. There's been a suggestion to just switch the signals round when that happens, I'm digging my heels in.
:-/
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Bruce Varley wrote:

Why would you not be measuring and possibly controlling the water temperatures? Sounds like it is time to think again about the whole process and see if a better job could be made of it.
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Paul E. Bennett IEng MIET.....<email://Paul snipped-for-privacy@topmail.co.uk>
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Indeed, that's under consideration but there are issues in relation to other requirements. Not a simple problem unfortunately or it would be sorted by now.
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On Sat, 25 Jan 2014 15:47:24 +0800, Bruce Varley wrote:

I kind of agree with Paul and I kind of don't.
The part of me that agrees with him thinks that the control job will be much easier if the hot and cold water feeds are each held at a constant temperature -- that way the gain from how much water you add to the change in temperature will be a constant for each sort of water.
If the temperature of your two flavors of water isn't constant, then what's to keep a situation from cropping up where both are on the wrong side of where you need to be? What if you need water that's hotter or colder than either of your feeds?
Given a flexible enough controller I suppose you could measure the temperature of each water supply and dynamically adjust how much of each you're feeding. This would pretty much let you set the average temperature of the water inflow to be anything between the two feed water temperatures (and would kinda sorta automatically "switch the signals") -- but you still have the problem of needing water that's hotter or colder than what you can possibly get.
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Tim Wescott
Control system and signal processing consulting
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Tim Wescott wrote:

We do agree Tim. It was probably my brevity that led to confusion.
To be in control you have to measure the properties you are trying to control.
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