High Temp Salt Controls

J

John Frankl

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Oct 16, 2001
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I have a question that I will be asking on various forums—please forgive me if you have to read this more than once. I am setting up a heat controller for a high-temperature salt bath. Everything I have seen has been a fairly simple combination of controller, solenoid, and thermocouple. The people I am thinking about purchasing my controller and thermocouple from strongly suggest including a relay in the package. They only charge $5 for it, so they are not trying to “upsell” me stuff I don’t need. They say the controller will actually last quite a bit longer with this setup. I am no electrician, but they say that the constant signaling on-off is just hard on the controller. Can anyone confirm/deny/clarify this arrangement?

Also, will a 30 centimeter Iconel probe, bent so that 10 centimeters run horizontally and 20 centimeters run vertically, be sufficient for this application? And how thick do I want it so that it lasts in the high-temp salt environment?

Thanks in advance,

John Frankl
 
Hi John-


My first probe from South Eastern Heaters was NOT inconnel sheathed, and it broke in half after a month of use.

My second one broke two days ago, after about 2 years or so of use. I used the salt bath from 1 pm to 2 am non-stop the other day, so yes it does get used.

I really don't know how thick the sheathing is. I think so long as you get one that's covered in inconnel you'll be okay for a good while.

They talked me into the relay... so I can't say how I'd have done without it. But I know I haven't had problems...

:D
-Nick-
 
You definitely want to put the relay between the controller and the heating element.
Running an element through the controller contacts will burn them up quickly.
The relay provides the same function as a switch or thermostat in an appliance, in that it uses a small current to switch a larger current.
I don't think the sheath thickness on a thermocouple will make a big difference, although a really thin one will cause the controller to cycle more rapidly, as it heats and cools at a faster rate than a more massive one.
As for how fast a salt bath will eat a sheath on the thermocouple, I have no experience, as I've only played with kilns and environmental chambers, and mine will last indefinitely in air.
As Nick replied, it lasted him a couple of years in salt.
 
I can confirm that a relay is needed. In my job, I get to mess with these things all the time. Like was mentioned above, the relay in the controller isn't meant for heavy-duty use. The surge put through it will fry it quickly. Whether you are using a counter, temperature controller, or any other indicating device, putting a heavy-duty contact relay between that and the item you are trying to actuate will help everythign last longer and run smoother.

I recommend getting a contact relay with a heat sink on it. If your thermocouple and your controller are a well-matched pair, they'll cycle frequently. A relay with a thermocouple on it will stay cool and do its job a long time. :)
 
You need a relay.You also should install another pyrometer with an alarm on it.This acts as a backup signal to prevent the salt pot becoming a mortar if the controller/thermocouple fails (exploding salt pots are NOT fun).The cheap controllers available work good as a backup alarm.
 
Nick, Thanks alot. How long is your thermocouple, and how much of it do you submerge in the salts?

If all the pros are talking us into relays, there must be a reason.

howiesatwork,

Will do. Thanks. Are you in San Jose? I'm from Santa Cruz and get back summers and winters.

jhiggins,

I'll look into the heat sink. What should it look like?
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Stacy,

Sounds good. I hadn't thought of that. Backups are always good. On the other hand, I plan to run this thing either with a venturi or very small blown burner and make sure the burner itself is topping out pretty low. OTOH, I'm not sure at what temps salts go boom. Any ideas?

Thanks,

John
 
I don't run a relay to my solenoid valve. My contoller has the correct out put to turn the valve on and off. On an electric set up yes, on a burner set up, I don't think one is needed.

I use an un-sheathed thermocouple and have had no problems, but i probably don't do the volume that Nick does. A protected thermocouple is a good idea. Mine is bent at 90° and probably about 8 inches is submerged into the salts.

Salt pots need to be looked after when in use. I would never think of letting mine sit unattended for more than a few minutes. The most dangerous part is when you initially fire it up and are melting the salts. Most salts are hygroscopic (absorb moisture from the air). When the water starts to be driven off by the heat is when it gets scarey. I have had my pot spit a thin stream of molten salt 8 feet into the air. The only thing that stopped it was the ceiling.

I put a fire brick on it now and run at a lower psi setting if the pot has been sitting a while. That way it warms up slower and has more time to drive off the water.

I don't think having a run away salt pot is much to worry about especially if you are baby sitting it. If you plan on lighting it and then coming back hours later, then a back up would be good. When running, she will click on and off at pretty regular intervals, so if an overheat is happening you should notice it. I don't know how hot the salts can go, they are all probably different.

I still think most injuries/accidents happen when you are initially melting the salts, not during use.
 
John:

I run a relay between the controller and the load device. I do believe this would be easier on the controller (it is just a very thin little gold strip that handles all that current in the controller outputs), but it is a very good saftey feature as well. Running the juice through a relay isolates any shorts, overloads or accidents from a $300+ controller. Please don't ask me how I know this or why I am so keen on the idea :(
 
All this talk about relays to operate the propane solenoid valve confuses me. If I want to run a relay as opposed to just having my controller do it, I have to buy the relay and a power supply to run the solenoid. I would think that if you look at the specs for your controller output, and what little the valve needs to run, you are going to find that the draw of the solenoid is well below the limit of the contoller.

The controller I am using now for my salt pot used to be used on my HT furnace. When I did that I had a solid state relay and heat sink set up. I agree that the controller is not going to be able to handle 20A that a relay can. Just like the relay couldn't handle the short circuit I wired in. HeHeHe....blew the relay into two pieces, and tripped the 200 amp main breaker on the house. Ooooops :rolleyes:

Anyway, for $5, you might as well get it, but then you are also going to have to power your solenoid externally.
 
Thank you again.

Laredo, I hear you. But for $5 that's the best insurance I can think of.

Kevin, I won't ask ;) but you are a very large part of the reason I am doing this in the first place. Thanks for all the info on your site, and for your continued help.

John
 
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