Process question

Another question for you guys to mull over.

I have a process that I want to setup, Cascade control. I have two hot water tanks, one above the other, only one heating. I have a drive controlling a pump, that pumps the water from the bottom tank out through some piping and into the top tank where it eventually goes to the bottom tank again. There is heat tracing wired to a thermon controller and placed on the piping so temp. loss should be at a minimum.
I also have a turbine flow transmitter in line with the pipe so i can determine the flow.

This is what i was thinking: One flow loop, One temp loop of the water in the piping (RTD sensor).
The tank element is 240V. I need to control the water temp in the tank. regulate at "what ever temp." Is there a way that i could do this? One controller controlling the temp the other controlling the flow based on the required temp.
I was thinking i need something that could take 4-20ma input and vary 0-208V and handle max 20 amps.
Do such a thing exist? Any suggestions as to another way i could do this? One person that i've asked suggested to me that i could use a plc and have two timers setup so that i could adjust the amount of time my output is on. and use a PUC or SCR Relay to swith the 240V. Almost like Pulse width modulation idea. Not to sure about doing this. Any sugestions??

Thanks for the help.

Terry

Standard temperature controller

I may have missed something, but why not use a standalone temperature controller (like Omega 9222a)and a bank of solid state relays for the temperature control?

Thomas

We are going to be using a honeywell standalone controller for the temp.

A bank of Solid state relays?

Click to expand...

what do you mean? The controller is only going to put out 4-20 ma? Or maybe I can get 1-5v across a 250 ohm resistor? So you are suggesting on/off control for the temp loop?? Any suggestions at this point are welcome. I guess I don't require PID control for this loop. Would that still be part of a cascade loop then?

A pretty ambiguous question on the whole - what is the flow regulation for ? Is this a real process or just an exercise ?

You can certainly control the elements - I have just used a Stiebel Eltron burst fire controller with a 0-10V input , so that is easy .

I'm not sure why you want cascade control , if the water circulates (which it will unless you are very careless with the pipes ) then the there will be a normalization of temperature.
I can't see that this is cascaded control that you are trying to achieve , just two loops fighting with each other . How do you define what the flow setpoint will be ? the only way I would use flow is to prevent a temperature gradient in the top tank , using two measurements at either end of the tank .

Omega Emgineering and others have relays that convert a 4-20 mA signal to a variable pulse discrete output. You could use this to control the heater using solid state relays on the heater power.

If you go with a PLC you could just use the PLC discrete outputs to accomplish the same thing.

I don't understand your process completely. It doesn't seem that the pump and the flow meter enter directly into the temperature control loop, although they may be dictating how much heat is required. It seems you would be able to use a standard on/off contrl with a deadband, liek a thermostat, to control the temprature.

I'm with Tom. I'm not real sure what you are trying to accomplish based on you post.

You need a good cause/effect relationship between the inner and outer loop in order to do cascaded control. In addition the inner loop is usually much faster responding than the outer loop. An example that most people can relate to is the velocity control and current control loops in a drive. The velocity loop is the outer, slower-acting loop that requests a torque. The current loop is the inner, faster acting loop that makes sure the outer loop gets the torque it is requesting. The current loop actually drives the actuator.

OK, maybe I do understand what you are trying to do. I'm just not sure why the system is designed this way, if I'm right. You have one tank that will have a constant temperature liquid in it. You have another tank that whose temperature you are trying to control by introducing heated liquid from the other tank. Correct?

If this is the case, this really isn't cascaded control. You have two separate variables you are trying to control that are only coincidentally related.

You want to keep the heated water tenmperature setpoint as constant as you can. I mean, don't change the setpoint unless absolutely necessary. The temperature loop response for the non-heated tank will be non-linear enough without adding an additional twist. Also, unless energy loss is a big concern, keep the heated tank temperature as hot as you can get away with. This will give you better dynamic response as you change the setpoint on the non-heated tank.

If you do intend to change the heated tank temperature you will probably need to do some gain scheduling of the non-heated temperature loop as the system dynamics will change based on heated tank temperature. Also, try to model your energy losses from the non-heated tank and use that as a flow feed forward for you heated water flow. While not critical it can make the temp loop a bit easier to tune.

Lastly, don't get too anal about keeping the heated tank temperature absulutely on temperature. If it floats around a bit it probably won't make much difference.

If I'm wrong about what you are trying to do, sorry.

Keith

Sorry for any confusion. The process is only for demonstration purposes. A simulator. Small scale version of a system where bunker C is being pumped from a storage tank (suposedly located some distance away from boiler supply tank in bldg)to a boiler supply tank. The heat tracing is already in place and working. I have a drive hooked up to a pump for flow and a turbine flow TX. for feed back. The reason I'm using a hot water tank is to heat the water. I only have normal room temp. to cause a loss of heat in the piping. It would be nice to have a heat exchanger but I can't get one yet, maybe next year. I will post some pic's of the process as well. Right now i'm just thinking outloud if you will. I would like to have an example of an advance control system, Where I can decrease the flow rate (or increse) as the temp of the water changes. Flow Vs Temp. Key process variable is the temp of the water. This is not written in stone. any suggestions are welcome as to what kind of process i could setup. Although money is a limitation. I was hoping to use one of the advanced control setups ie. cascade, ratio, feed foward etc.

I was thinking that I could use 4-20 ma output from one controller (TiC)and a RTd for feedback. To control the temp of the water. This would give me a Temp. control loop. So I would have 1 Temp. loop and 1 flow loop with (FIC) output to the drive.
And I was thinking of some way to combine the two loops in a control scheme.

#1. I need to think of a control scheme.
#2. I need to find a device that will give me control over the heating element in tank.

Any suggestions. I would like to have as practical (real world) example as possible. I do have access to a PLC for this.(No PID) Simple PLC maybe a LOGO.
I have two maybe three controllers that I can use for TIC or FIC. I have two RTD's. I can let you know what controller i have in a few days time if that would help.

I'm not quite sure I understand the problem...is the heating element a digital device (on/off only)? If that's the case there are a couple of ways you can do it:

- Use a single loop. Leave the heating element on all the time and vary the flow rate to controll the temperature. The RTD is the PV and the pump drive is the CV. Use a temperature entry for the SP.

-Cascade the temperature loop to a flow loop. Again, leave the heating element on all the time and control the temp via the flow. The RTD is the temp looop PV and use a temperature entry for the SP. The output of the temp loop can them be scalled to the range of the flowmeter and used as the flow loop SP. The flowmeter is the flow loop PV and the pump drive is the CV.

I guess your tuning abilities and the stability of the inputs would determine which way is best suited.
Is this anywhere near what you are looking to do?

Yeah, That's sort of what i was thinking. Give me a few min. and i will try to post some Pic's. It may help to see the process as it is now. I will also see if I can find the P&ID for it.

Sorry, got bogged down with work again, I will try again tomorrow.