One example of controlling a temperature

[Pandiani]

Hello guys,
it's me again with (hopefully) interesting control loop issue. I have found an article in which is briefly described one steam's temperature control. As usual, I have questions and need your comments. Please read the attached file and comment.
Thank you very much.

 

[Peter Nachtwey]

Pandiani,have you changed locations again? What is CC?

What about the heater? Is the purpose of the steam to heat the heater or does the heater heat the steam? Is there much of a time delay between T1 and T2?

If there is a delay between T1 and T2 then it is best that the rate be determine at T1 where there is no or little dead time. I would simply use use dT1/dt for the derivative term instead of dT2/dt. If the steam is used to heat the load then keeping the difference between T1 and T2 constant would be handy. If you know the load or heat sink one could calculate what the difference between T1 and T2 should be.

This will be fun. Is this related to power generation?

 

[Pandiani]

Well, it had connection with power generation. It's old solution that I have found and was used in old regulation circuits before whole control system is replaced with modern DCS. I have found this scheme with poor description. I'll try to find as much informations as I can so we can analyze this together.

 

[10BaseT.]

Only one problem , you can't lower dry saturated steam temperature like that , where does the condensate generated go ? it plain and simply won't work , the only thing that should exit the heater is water . The only thing you can do is regulate the steam pressure , and thus it's temperature . The only way your model could possibly be made to work would be by using superheated steam , and the only control you would have is modulating the degree of superheat.

Having read Pandiani's second post , it seems that this is a desuperheater application .

 

[whish1]

I've worked in a powerplant and a moisture trap could be used to drain the condensate.We used to inject desuperheating demineralizd water into the steam imediately exiting the steam drum . No problem with setup

 

[Pandiani]

Only one problem , you can't lower dry saturated steam temperature like that , where does the condensate generated go ? it plain and simply won't work , the only thing that should exit the heater is water . The only thing you can do is regulate the steam pressure , and thus it's temperature . The only way your model could possibly be made to work would be by using superheated steam , and the only control you would have is modulating the degree of superheat.

Having read Pandiani's second post , it seems that this is a desuperheater application .

It's superheated stem, you're right. Pressure is well above 100 atm, and temperature is above 500°C. Like I said, I don't have much experience in practical control, and I have found this old solution (doesn't exist anymore), but I'd like to understand the real story behind it.

 

[10BaseT.]

You need to do a bit more research on steam first - for one , a conventional trap does NOT work with superheated steam - only when you finally achieve dry saturated steam do things become a bit more sensible .
You constantly refer to temperature , but in reality the most important thing to add to you model is pressure .
Remember how superheat is added - there can be no water while superheat is being added . Also , where does the exhaust steam go after T2 ? if it is to another process , then the backpressure of that process needs to be known before you can attempt to derive a model .
Importantly unlike the specific heat capacity of water, the specific heat capacity for superheated steam varies considerably with pressure and temperature and cannot be taken as a constant.
100 bar steam gives us a bit over 300C , so you are running around 200C superheat .
What does the heater do ? is it air heat ? and is the loading constant ?
Can you give some idea of the condition of the header steam , T1 temp and pressure and T2 temp and pressure ?
is there any possibility of T1 ever getting close to the dry saturated temperature for the system pressure ?