Thermostat/control unit advice

[alpha]

Hi all, I need some sort of stat/control box for a water chiller.

What I have is a 1/3 hp water chiller chilling off 15 ltr of glycol/water, and what I would like to do is keep this coolant a degree or two lower than ambient, what I do not know is how I would achieve this.

for example, if the ambient is 20degC I would like to keep the coolant at say 18C and when the ambient drops overnight to say to 10C the coolant will be maintained at 8C.

Any ideas?
Any help much appreciated.

[guy in blue van]

try using a time clock with a second t-stat for night set back. as long as you know the load difference you should be alright. the only time ive seen this is with a johnson air rotation unit but it should work for your application as well

[Rob S]

What about a proportional reset controller? It can change the loop temp vs OSA.
Real question is pocket book?

[walden]

I've done this with a little PLC and two temperature sensors. The PLC program then compared the two incoming temperatures and produced an output signal to modulate a cooling valve.

[chillin out]

If you had a electronic stat with a dial, and instead of using the dial to control the temp you could actualy configure a probe to act as the dial,,,

Dial would constist of a variable resistor rangeing from, say, 200k to 2000k.

Make the ambiant probe do the same, less 2 deg c

Connect the real probe (water) to the proper connection and thats it.


Chillin:) :)

[alpha]

Thanks for the insight into ideas guys, I think I have gotten my head around the gist of the ideas you speak of.
Now I'm not too, clued up in so far as making up electronic devices from scratch but would be willing to learn if anyone could provide a few links or further explination into how to, ie.. chilled out's and waldon's suggestions.

I am also quiet interested in rob S suggestion of "proportional reset controller" would you have a manufacturers name/control model that I could have a gander into?

regards.

[Rob S]

The Johnson 350 system is what I was thinking of.
You would need a A350S, S350A, and D350. Then either field supplied 24VAC 40VA or you can use a Y350. All these can be mounted on a Din rail.
Now the A350S reset controller is only good for a Min supply temp of 35-85F. With a Max supply of 45-140F.
The reset ratio can be set from 1:5 to 10:1 or 1:30 to 1:1. The master reset is adjustable between 30-90F.

The other option is a Honeywell T755J1001 single stage reset controller. These have all the same functions built in. But with a min set puint of -40F. And the cost is higher.