Controlling temperature of water

Hi,

This is a control problem. I am currently planning to control temperature of water by mixing hot and cold water through a three way valve. The three way valve will be actuated by a motor which is controlled from a (0-10 v) signal. I am planning to use PID to control the temperature output by mixing hot/cold water.

Is there a better way to control temperature of water or has someone did this sort of project before.

Thanks for your time.

Raj

raj57 said:

Hi,

Is there a better way to control temperature of water or has someone did this sort of project before.

Thanks for your time.

Raj

Click to expand...

I think we would have to know more about the process itself to answer that question.

Why can't you just control the temperature of the hot water?

What is generating the hot water?


Think a three way mixing valve would work just fine as long as the hot water side is always greater than the set-point and the cold water side is always less then the set-point ( but that goes without saying I suppose)

In addition to what Mickey said, is the hot or cold water at a constant temperature or does it vary a lot.

I would try a temperature sensor on the outlet with a PID loop. Potentially there are safety issues if this is feeding say a tap or a sink so you may need to start with cold and then add hot which will take time to settle at the required temperature

You can also use a thermostatic mixing valve.

yes two ways to do this.
first is as you did with a simple three way valve ad steady as possible supply.
another is forward by measuring the temp and the flow

I would think the reason to use a three way valve with hot and cold water is to reduce the time it takes to get the desired temperature as well as giving you the ability to decrease the temperature as quickly as you can increase it.
Having said that, raj57, we aren’t doing your homework are we?

Hi everyone,

Thanks for your replies.

Hot and cold water are set at constant temperatures. I have actually built the mechanical system ( a rotary actuator on the three way valve) and was wondering whether there would be simpler ways to control temperature with these inputs

The system takes two inputs [ a hot and cold water line supplied by a factory , these are set at constant temperature]. The user sets a temperature that is limited by Coldwater- limit < user temp < hot-water limit

Thanks for the quick feedback everyone

shooter said:

yes two ways to do this.
first is as you did with a simple three way valve ad steady as possible supply.
another is forward by measuring the temp and the flow

Click to expand...

Shooter can you elaborate on your second method.

Thank you

nonuke said:

You can also use a thermostatic mixing valve.

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Mhm, i thought thermostatic mixing valves can only output a constant temperature or can they be controlled. How can they be controlled (will it be controlled by a motor?) I am not sure i am a newbie haha.

Thanks for your time and quick feedback.

cjd1965 said:

In addition to what Mickey said, is the hot or cold water at a constant temperature or does it vary a lot.

I would try a temperature sensor on the outlet with a PID loop. Potentially there are safety issues if this is feeding say a tap or a sink so you may need to start with cold and then add hot which will take time to settle at the required temperature

Click to expand...

The temperature of hot and cold water are constant and i am currently using a temperature sensor at the output of a three way valve. I see that makes sense.

Thanks for the quick feedback

Firejo said:

I would think the reason to use a three way valve with hot and cold water is to reduce the time it takes to get the desired temperature as well as giving you the ability to decrease the temperature as quickly as you can increase it.
Having said that, raj57, we aren’t doing your homework are we?

Click to expand...

Nope not at all, this is just a question for curiosity and asking experts for their opinions.

Thanks for the quick feedback though!

raj57 said:

Mhm, i thought thermostatic mixing valves can only output a constant temperature or can they be controlled. How can they be controlled (will it be controlled by a motor?) I am not sure i am a newbie haha.

Thanks for your time and quick feedback.

Click to expand...

you are correct. You can only change the Set Point manually.

I think Shooter's feed forward suggestion is based on calculating the heat energy change needed to increase or decrease the main fluid volume temperature. It is based on:

Change in energy = mass x heat capacity x change intemperature

You can use this equation combined with the temperature of both fluids to calculate exactly the mass (volume = mass / density) of cold water to add to the main tank to drop its temperature a specific amount.

Note that for either the feed forward or feed back PID control to work it is essential to have the tank well mixed to minimize temperature gradients.

I agree with Tom.
I have done a few projects like this. First I send x amount of cold water (using a meter), and from the above formula, I calculate the amount of hot hater and just send the correct amount. Works all the time

I think you will have difficulties if you vary the flow rate of both the hot and cold water inlets, although this is made easier if, as you have said, they are constant temperatures,

It is much easier to bring the outlet temperature to the setpoint by "blending" - e.g. adding hot water to cold water to bring its temperature up, or by adding cold water to hot water to bring its temperature down. Usually one or the other inlet water feeds will be "full-flow" and its flowrate is not usually controlled, you control the opposite water to "blend" with it to achieve the desired temperature, which is the way most domestic thermostatic shower mixers work.

What you choose to control will be dependent on the application, you can blend hot into cold if the target temperature is nearest the cold temperature, or you can blend cold into hot if the target temperature is nearest the hot temperature.

Of course, if the target temperature range extends from the cold inlet to the hot inlet, your 3-way valve arrangement can be justified, but controlling it will be more difficult.