Water Temp Control Help

[SchoonerMan]

Hi All,
Not sure if this is the correct forum but here goes.....
I have a customer who needs to keep a certain temperature (+ - 1 or 2 deg's) for some of their process water and to do it as simple and as cheaply as possible. The process is filtered water used for organically grown sprouts. I've tried thermostatic mixing valves but the temp fluxuates too much when the flow changes and this can shock the plants.
I'm thinking the easiest way would be with an electronic temp controller with an in-process thermal couple and a 4-20ma three-way control valve. (Compressed air is not available). Used controller is easy on eBay but problem is finding a cheap control valve. 1" should suffice.
Suggestions anyone?
Thanks,
LC

 

[quark]

Try the control valves from HVAC control manufacturers like Honeywel, Belimo, Siemens and Sauters etc.

 

[DickRussell]

Is this a recirculation system, where the water going to the plants is recirculated, with some fresh makeup? Or is it an off/on timed flow? Is the temperature controlled by blending in some warmer water, steam, electrical input?? What sort of volumes and flow are we talking?

 

[SchoonerMan]

Quark, thanks for your input... was hoping for something more specific but I'm looking at what they offer.

Dick, this is not a recirculation system (we attempted that but they were not impressed nor did they want to experiment further because of costs). It is an on/off timed flow which varies from process to process. They use 6 hydoponic sprouting drums with timed misters but not always all at once depending upon need. My guess without testing would be each drum uses about 1 GPM while misting which occurs for approx 10 secs every 10 min but that also changes depending upon product.

The other process using the same water is for keeping the sprouts moist after they've been harvested from the drums and are on racks awaiting packaging and cooling. This too involves variable amounts of timed water depending upon the amount of racks being used. The racks are split into two timed sections of 6 shelves each so estimate the flow to be about 6 GPM each rack.
We have a large gas fired hot water tank used in other stages of the process so feel it's best to use that blended in with cold water to produce the necessary water temperatures.

I was actually the guy who engineered the entire plant a few years ago (electrical, mechanical & process). They are a small time outfit with new owners at the time of opening the plant and I need to keep costs down. At the same time I'm trying to make them understand the need to upgrade some of the old stuff while finding ways to re-use valuable and expensive natural resources. They are now planning to move again so I could start out fresh to have the new plant entirely ready for their needs. I have plenty of R&D process experience but it's been more than 10 years since I worked in the field.

 

[DickRussell]

How about a second tank (surge), in which you would mix the hot and cold water. A small pump would both pump net outflow to the process and recirculate for mixing. As long as the holdup time is sufficiently large, fine control by addition of hot water should be easily accomplished. HTH

 

[SchoonerMan]

I was just thinking.....
What about an electric hot water heater using a digital controller to open and close a relay on the heater circuits?
When I worked at AMOCO's R&D lab we used to use electric heaters for many processes to avoid open flames.....
I could even add a small recirculating pump for mixing.
Thoughts anyone....
LC

 

[jmw]

One of the problems with heaters and heat exchangers is response time. They can deliver lots of heat but can prove difficult to use to obtain a +/- 1degC.

Using steam injection is one way to get better control as you use temperature to control the steam rate.

The alternative is to split the cold water flow, part through the heater or heat exchanger where it is heated to above the target temperature and part by-passes it and is recombined downstream with the hot stream in a static mixer. Of course, this requires two temperature control circuits, one to control the heater (slow response) measuring in the heater outlet, and the second in the recombined stream where it controls the flow split; the fast response side. I used this technique in a sample loop for temperature control to an analyser; temperature stability to better than 1degF is possible.

JMW

http://www.ViscoAnalyser.com

 

[quark]

I think hosedown stations by Spirax may be good. The hosedown station will have its own temperature controller. You add the two streams to it and set the temperature. These are generally used to produce hot water from steam and cold water but can very well be extended to hot water and cold water. Check the link,

HosedownStation

Regards,

 

[SchoonerMan]

Hi Quark,
Thanks for the reply.
I've already tried a thermal type mixing valve which the hosedown station is. It has a tendancy of fluctuating temperatures as flows change and the sprouts get shocked.
That's why I was looking for something more sensitive + or - 1 deg. Add to that a long pipe run from the hot water heater and it's just too inaccurate a method.