water mixing 'on the fly'

[tjagiello]

Hi guys,

I was wondering if anyone has any experience with water mixing 'on the fly', which is basically changing the output water temperature quickly by adding cold water to a normally hot water line. I'm looking at a range of 88-94*C, flow rate of around 180ml-300ml when the whole process takes roughly 30s. The hot water line is driven by a pump which has a variable speed drive connected to it and allows to change the flow rate 'on the fly' as well.

Initial idea was to add a PID and let it control the mixing valve, but perhaps someone has better suggestions on how to solve this?

Regards,
Tom

 

[jmw]

Depends on vatious factors such as the accuracy you need and the stability of the temperature in the two streams.
I have designed systems for hot and cold fluid mixing using a control valve, temperature sensor and PID controller. If there is a lot of variability in the stream temperatures then you may have problems if the valve tries to control outside of is optimum control band.
Simplest system would be to use something like a waxstat or a shower mixing tap... there are industrial versions avaiable.

JMW

http://www.ViscoAnalyser.com

 

[tjagiello]

Hi JMW,

thanks for your reply.

Can you recommend any manufacturers for valves that can be used for mixing? would a simple needle valve on the cold water supply line work, as in adding cold water to cool down the hot stream?

The hot water will be coming from a PID controlled boiler, so it will be rather stable, the cold water supply line might not be so great though, but still stable throughout longer periods of time (cold tap water supply). I'd be aiming for around 0.5*C accuracy.

The thing I'm worried about when using the 'cool down' approach (adding cold water to the hot water) is that it won't mix properly and partially the stream is going to be cold and the rest will be hot.

Regards,
Tom

 

[roydm]

You would get much better control by measuring the 2 flows and ratio controlling cold to hot using a Temperature / Flow cascade loop.
Primary Temperature loop setting the flow ratio Cold to Hot.
You might also want to measure the hot water and somehow put that into the calculation as feed forward.
That's my 2c worth

Roy

 

[jmw]

The hot and cold water not mixing is a maybe. It depends on how much turbulence you generate when you mix the flows. If in doubt, install some kind of static mixer.
It is a good point because there is an example quoted of water from two different natural sources still being sperately identifiable in a pipeline some hundred meters or so downstream.
In myc application I was dealing with ho and cold oil at 25l/min.

JMW

http://www.ViscoAnalyser.com

 

[ilan7]

Use Static Mixer. Consult with the process / piping guys about it - has to be at the right size and the right construction material to perform as it should.