PID? - adding heat to create constant temp water flow

[flatfootskier]

My sister lives on a dairy farm. They want to heat milk in a plate heat exchanger using hor water at a steady 96 deg C. This needs to be +/- 0.25 deg C.
It seems your average supplier expects a supply of steam to heat a water circuit, injecting either on a pulse or proportional basis. It certainly is an easy way of delivering huge amounts of heat.

I am keen that they utilise their solar PV to heat a bulk tank to say 85 deg C nominal and then use this preheated water with a comparatively small inline immersion.
Overall heat requirement is about 40kW, but the preheating should only require a few kW of top-up.

Does anyone have any feel for the important factors of the design, particularly with regard to the controller & sensor type / positioning?
I think I know enough to make a start, but without more learning I'll doubtless learn a few things the hard way...

Many thanks.

 

[Reloadron]

using hor water at a steady 96 deg C. This needs to be +/- 0.25 deg C.

That is a pretty tight tolerance, not impossible but not inexpensive. I would be starting with the sensor and likely looking towards a good PRT rather than thermocouple or thermistor solutions. If water is to be the exchange medium I would be using distilled water and I am assuming the heat exchange to be a food grade stainless steel. As to the controller I would likely choose a quality unit from Omega Engineering or similar manufacturer. I also like Newport Process Control Systems. This is where a phone call or email to one of their process and applications engineers is a good step towards a solution. Lastly as to the heaters I like Watlow Heaters. They manufacture a wide range of heaters including immersion types and again have very good applications engineers. I would think about heating in a tank (having an expansion tank) and then final heating inline with an inline immersion heater.

When I did something similar I used a 3 way proportional valve to mix hot and cold water to get my final water temperature, this worked for my application but there is likely several ways to get where you needs to be including possibly some flow control of either the water of milk.

Ron

 

[flatfootskier]

Thanks - some very useful snippets there and I've been looking at datasheets. I must say I hadn't thought about actually speaking to someone - but it makes perfect sense.
You assumed correct - pre-heating in bulk tank with a smallish inline top-up
Why distilled water ? Intrigued by that!

 

[Reloadron]

Why distilled water ? Intrigued by that!

While I don't know your location or more important the quality of the "tap" water one side effect of using water in a heat exchange system is things like calcium building up within the heat exchange unit. Frequently filters are placed upstream of the process and distilled water used to prevent things like calcium buildup in the exchange itself. I also mentioned stainless steel as here in the US or similar countries the FDA and various government regulations figure into food processing, especially with a product like milk. While I never worked the food industry I have worked quite a bit with heat exchange and snn chemical build up in the exchange units after the fact.

Ron

 

[flatfootskier]

Nominally, the bulk water will be considered 'dirty.' Whilst it'll flow through stainless plate heat exchangers it will be stored in a mild steel (boiler grade though) heat store, so it'll have corrosion inhibitor. Milk inside a plate heat exchanger has to sit at a higher pressure than any heating fluid in case of gasket failure. I live in a soft water area & don't really think about it, but this system will be elsewhere, and might suffer, so will consider water hardness more thoroughly thanks.

 

[Reloadron]

Sounds to be a pretty interesting project and best of luck with it.

Ron