Temperature Control

Sounds like you need to go to a simple PLC - with maybe 2 analog inputs and 1 analog output. That way, you can program your 10% open for the set time period using the timer functions in the PLC.

Just depends on how much you can/want to spend.

Yes - that is what I have done - the "micro processor" is essentially a PLC, just limited I/O no add on I/O cards.

Would something like this be of interest?

The software is free.

Yeah - we have used that here on a small cabinet cooler.

I did a 1/8th DIN controller and a MicroLogix controller for about the same price for this application. The controller is a couple steps to program for a PID loop control. The MicroLogix will basically be just - digital input to this address causes that output on tht address. Pretty simple programming. My programmer preferred that route. The Horner would have worked the same way as my combo does.

I checked the Ogden model and it does appear to do what I want it to. Am waiting for a quote to compare cost with my 2 piece arrangement. (temp control and micro processor)

A setpoint programmer doesn't necessarily do what you described, i.e. holding a fixed output for a period of time.

Now it's entirely possible that a ramped and/or setpoint held in soak (dwell) mode will achieve the a good enough end result for you, but you should be aware that the heat exchanger valve will see the output from the controller's PID for whatever however it's tuned, it will not see a fixed % output, like you could set in a manual mode.

For now, I am stil going with the two piece system. A micro processor that will output a set 5.6ma for a timed period, based on a digital input from a switch, then after time out, pass the analog signal through from the PID temp controller, or, after a second digital input do the same. This allows a selector switch that can either be used to start the system and walk away, and in a set time period, the system will go to PID control, or select the 5.6ma output for a few minutes less than the set time period by selecting the PID output via the switch. (user flexibiblity) Switch will be a three postion (OFF/START/RUN). Either of the last 2 options will run the system. You just selct START and walk away, or if in a hurry, select START then RUN to get to PID quicker. This assumes some responsibility on the operators part, not to switch too fast.

Hello Phys,

I surmise that the earlier warm up with 1.6 m.a. added to the TIC's 4 m.a. out (0% signal) to crack open your TCV, main steam valve, worked, but not without operating or maintenance difficulties. Why not add a smaller "preheat" on-off control valve with full flow, manually set for steam demand, in parallel with the existing TCV? The operator can follow the temperature rise on the TIC above. You will not be wearing out the trim of the larger TCV by running it so close to shut 0ff. This is a permanent solution, not a patch that will be understood by both operations and maintenance personnel. The "preheat" steam flow can be manually set if needed for seasonal operating differences.

Regards,

Luther M

Good point - wear and tear on the TCV. The whole idea of this control scheme is to avoid wear and tear on the much more expensive heat exchanger from thermal shock. We now need to weigh cost of maintenance on a control valve versus replacement costs of a heat exchanger. We have always just allowed the TIC to run the TCV on past projects with no regards for thermal shock to the heat exchanger, but this customer is concerned about thermal shock. I think I should mention possible valve wear before the quote becomes a job. A small automated gate valve around the TCV for a small leak by to pre-heat is a good idea. Thank you.