Water temperature control/Hot Cold MIX

MJC · Mar 13, 2013

Gents, I have a small project that is in an early stage and I need some suggestions.

We have two ¾” pipes that carry water one cold one hot. Customer wants to be able to control the outlet of water temperature thru an entry at HMI.
My idea is to have a motorized control valve on the cold water side to modulate the set point of the outlet temperature using a PID instruction. Not sure of what the rest of controls will be.
This is the first time I doing something like this and I’m open for suggestions, I think I will be looking at a micrologix 1100 but have to see if it can handle HSC flow meter.
The hot water is coming from a regular house hold type boiler and cold is just city water so I will have to look into what the temperatures are.
Thank You all

CalG · Mar 14, 2013

Too bad your pipe size is that large.

I have a sweet little "mixer" with a programmable digital display that was made for photographic dark room chemistry.

Makes water temp a breeze...if only the pipe size were a match ;-)

Peter Nachtwey · Mar 14, 2013

How do you expect to use the flow meter?
The way it is right now the outflow temperature will be somewhere between half way the not and cold temperature and the hot temperature. You can modify that somewhat with what you call valve. I assume valve is manually operated. Do you know how the modulating valve affects flow as a function of the control signal?

bce123 · Mar 14, 2013

have you thought of using an Insta Hot heater. http://www.insinkerator.com/en-us/Household-Products/Water-Products/Pages/Model-Hot1.aspx
you could chance up the controls a bit.

g.mccormick · Mar 14, 2013

Why not use a 3 way mixing valve to mix the hot and cold?

CalG · Mar 14, 2013

A flow meter could be very useful in modulation Amplitude determinations

Peter Nachtwey said:
How do you expect to use the flow meter?
The way it is right now the outflow temperature will be somewhere between half way the not and cold temperature and the hot temperature. You can modify that somewhat with what you call valve. I assume valve is manually operated. Do you know how the modulating valve affects flow as a function of the control signal?

Nothing worse than a control that swings to the limits because the sampling is remote from the control.

For this example, low flows (or stopped flow) could easily send the modulation valve full open trying to warm up the temp probe. all the while, no hot water is making it to the probe.

The control then would be structured 'If the flow is small. then the max valve change is small" regardless of temp.
A pre-emptive assist for any PID. function.

EHIRL · Mar 14, 2013

I was planning to do something similar a couple of years ago, I was thinking of having a valve on both lines to keep the flow constant but might not be needed for your application.

Peter Nachtwey · Mar 14, 2013

CalG said:
Nothing worse than a control that swings to the limits because the sampling is remote from the control.

Agreed the flow must be known and the distance between the control valves and the temperature sensor is dead time. This project hasn't been thought out.

MJC · Mar 16, 2013

Thank you all for your kind and helpful response.
Peter you are absolutely correct about how the setup in previously attached image would function.
The flow/totalizer would act as a totalizer for requested amount of water per operators input, that why I mentioned a HSC for the PLC
I don’t know how the valve signal will affect flow here.
Here is a basic rundown of what the process involves.
Operator is to enter a total in gallons to request up to 30 gallons of water. Operator also must enter the water temperature. This is for a mixing batch of marinate.
I found a proportional mixing valve that might just work.I understand that valve ports are too small for this application but something
similar
may just work
The temperature probe will be placed no further than 6” away from the mixing valve.
https://customer.honeywell.com/resources/techlit/TechLitDocuments/95-0000s/95-2505.pdf

MJC · Mar 17, 2013

Looks like I found what I was looking for 2 in 1 its a nice unit. Hassmfg.com

Water temperature control/Hot Cold MIX

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