Salt pot for heat treating (PID, SSR, and solenoid questions)

S

Saul Kokkinos

Joined
Sep 4, 2015
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Hi, I am in the process of buying the parts needed to build a high temp gas powered salt pot for heat treating knives. I already have the ceramic fibre blanket, and a 1 metre length of 100mm OD 304 stainless steel pipe with a 5.7mm wall thickness.

The part of this build that has me somewhat stumped is the PID temperature controller, the SSR, and which solenoid gas valve to buy.

I’m looking at getting this PID: http://www.auberins.com/index.php?main_page=product_info&cPath=1&products_id=3

I’m going to order an Omegaclad Xl sheathed thermocouple from omega.

Not sure which solenoid gas valve to get that is compatible with the PID. I have found some very cheap ones on ebay, and some very expensive ones on other sites. I understand that I want one with OFF as default and is rated for propane gas. Any help with compatibility or links appreciated.

Also not sure what type or where to get the SSR?

Many thanks!
 
"Newark electronics" sells SSR's and has been easy to deal with.

Omega sells the PID controllers as well as the thermocouple.
 
All I will say is "Be Very Careful". High Temp Salt Pots are nor amateur equipment. Read everything Kevin Cashen wrote about them...then decide.

I no longer give online advice on building high temp salt pots.


Now a low temp 400F salt pot is another thing entirely. Marquenching and straightening become a breeze.
 
Thanks Ron, i'll have a look.

I appreciate your concern Stacy, and i have read everything i could find in relation to safety. I understand that any amount of moister either water, oil or other, can have an explosive effect if introduced into the pot, and i also understand that if you are melting the salt from the bottom when you fire it up, you must have a tapered rod that you can remove from the salt to allow it to expand without blowing the as-yet unmelted top off. I also plan of wearing a full face shield, welding gloves, and leather apron when operating.
 
I'm in England and so have not used Auber. Looks like a decent controller and I gather they are very good for support. If you need to be asking the sort of questions you are asking, you will most likely need someone available at the end of a phone line. Buying from somewhere that can provide this service is well worth a premium.

Usually, DC controller outputs to drive SSRs are used with the SSRs switching large currents for resistive heating loads, particularly those with short output cycle times.

A gas-fired salt pot has lots of thermal mass and does not need the short cycle times. The current drawn by the solenoid valve is small. Personally, I would look for a controller with a relay output to switch mains power to your solenoid valve. The Auber Model: SYL-2342 is basically the same controller with a 10A-rated relay output and looks ideal.

As far as thermocouples go, I'd recommend going type N over type K. Type K tends to "drift" over time spent at high temperatures (over about 1000 degC, 1832 degF). Type N was developed to be more stable than type K at these higher temperatures. The SYL-2342 will accept a type N input.

If you've not already done so, make a cup of coffee, get a notepad, pick up the phone and talk to Omega about the thermocouple. Depending on how hot you intend to run, a heavier sheath in a nominally lower-spec material might be better for you. Maybe even a 5/8" industrial sheath. The Omegaclad XL is a great material and is my choice for a thermocouple sheath to use when I'm messing with forges and need to go hotter than 310 S/S or Inconel is happy at. Otherwise, I like the 310 S/S, largely for the lower price.

I'm not sure how well 304 S/S pipe will stand up to the temperature and cycling compared to 316L, particularly when welded.

A lot of the cheap Chinese solenoid valves have BSPP connections: British Standard Pipe Parallel threads. BSP is a 55-degree Whitworth thread form, rather than the 60-degree of American pipe threads. It's used nearly everywhere except North America as far as I can tell. It comes in both taper (BSPT) and parallel (BSPP) variants. Some sizes have the same pitch for the British as for the American threads and the BSP and NP stuff will screw together despite the difference in thread angle. They'll even seal if a good pipe seal is used (anaerobic for preference). Other sizes have slightly different thread pitches and do not go together well. In your position, I'd avoid the cheap ones off Ebay and buy locally.

You'll need Propane compatibility. I'd go for a 110VAC coil assuming you are mains-powering the setup. Without knowing your burner configuration, it's hard to tell the size you'll need, but I'd expect 1/4" to be enough for most Venturi burners and 3/8" to be enough for a big one or a couple of 3/4"-1" Venturi burners. I usually use a regulator that maxes out at 4 bar (60 PSI) and I'd want a valve rated for at least 10 bar (150 PSI) downstream of it. You are correct about wanting a Normally-Closed (NC) version. Being in England, I can't really make any specific brand recommendations for your location.
 
You say, that You want to make Salt Pot?
Sign me your tools in your testament.

Nothing will help you when this make a "boom".
 
Thanks Tim, I'll go with the controller you recommended. Also good to know that type N thermocouples have lower rates of drift at higher temperature compared to type K. I'll look at getting a type N thermocouple. I've edited my profile to include my location, so BSPP or BSPT threads should be fine for the solenoid.

So for this PID: http://www.auberins.com/index.php?main_page=product_info&products_id=1

Would either of these solenoid valves be compatible if i'm running the PID off 240V AC?

http://www.ebay.com.au/itm/220V-AC-...ir-2W025-08-/291513048920?hash=item43df881f58

http://www.ebay.com.au/itm/1Pcs-1-4...-Closed-N-C-/121718858279?hash=item1c57026227

Many thanks for the responses
 
You don't get voltage FROM the relay, the relay coil is controlled by a voltage, either low DC or line AC.
The relay is just a switch, like your shop light switch, it turns things on and off.


Pablo
 
Voltage is ok. Pressure rating looks fine. Seals are also probably fine for use with Propane. Threads are NPT though, the North American standard. In Oz, I'd expect BSP to be the standard.
 
Thank you very much for your responses Tim, I keep finding valuable info you've posted on this and other forums, so cheers for taking the time. I figure it's cheaper to get two NPT to BSP adapters than to buy a solenoid locally from what i've seen.
 
The NPT should be fine with adapters.

Being the wrong side of the pond to readily use Auber, I tend to use either AutomationDirect Solo controllers

http://www.automationdirect.com/adc...s_Controllers/1-z-16_DIN_Size_(SL4848_Series)

Or the Omega 7800-series

https://www.omega.co.uk/pptst/CN7800.html

I usually use the pulse output versions for use with SSRs on my HT ovens, but there are relay output versions available.

I buy whichever is cheapest at the time, as they are both the same controller, just differently-badged.

Edit:

Apologies, my mind was elsewhere and I was thinking you'd need ramp/soak.

Omega and AD tend to be very good on phone support. AD only have the Solo range, nothing cheap, and Omega have a wide range of controllers, but the cheapest is only about 20% cheaper than the 7800 series over here. For that small a difference, I'd take the ramp/soak capability any day.

There are some very cheap Chinese PID controllers on Ebay. I used a SET-620 a few years ago with a type K thermocouple. Few of the cheap controllers back then would take a type N input (I had access to a free supply of type N thermocouples at the time).

I have since used a Mypin and a Sestos, though I can't remember the model numbers or thermocouple types.

There seem to be a lot of Rex C100s on ebay at the moment. Looking on some of the homebrew and general geeky forums, it seems the input and control type (Heat control is "Reverse", Cool control is "Direct". We want heat/reverse) are part of the factory configuration. Some will allow access to the high-level menu to change them, others will not. It seems to be a bit of a lottery which you get, but it might be worth the risk. Most seem to come set for 0-400 degC on a type K, with pulse DC output and an SSR included in the package. I'm quite tempted to buy one just to play with, since it's dirt cheap and if I can't change the config, it'll still run an electric tempering-only oven.
 
Last edited:
Sorry for the slow reply Tim, and thanks for all the info. I ended up going with Auber instruments as they were extremely helpful when i emailed them. I also managed to get 316L tubing instead of 304. Here's some photos of my progress. It's almost done, just need to line it with ceramic wool and fix on the gas equipment and electronics.

Chimney is designed so that i can pre heat blades in it to remove any moister before going into the salt. Wheels are solid rubber so will not puncture and also have splash guards so they don't catch on fire. SS pot hangs from flange that sits on lid, and is located by 4 tabs.

Salt tube it 100mm or 4" ID and 500mm or 19.5" deep.
Gas tank body is 600mm or 23.5" tall and 300mm or 12" diameter.
The chimney is 100mm * 150mm or 4" *6".
Burner is a 1" venturi.

Any thoughts or suggestions welcome.

PICT0969_zpsclhvlh1w.jpg
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That looks pretty impressive so far, and you've clearly thought about it, which is a good sign.

Preheat in the chimney seems like a good idea but check the temperature with a thermocouple first: I suspect the exhaust temperature will be higher than the salt temperature. Good contact with the outside of the chimney might be enough for preheat though?

I've not actually used a salt pot of any decent size myself, but I've seen a tapered rod advised for the cooldown: it can be pulled after the salts are solid and leaves somewhere for the salt to expand into if it heats/melts from the bottom up. It looks like yours will probably heat from the bottom up.

I'd also fit a pressure gauge and mark up the burner choke sleeve so you can easily return to a setting once you've found it.
 
Saul, would you please take some pictures of the salt pot with the top off? I love that thing!
 
Hey Tim, thanks for the great advice. Yes having done a firing without salt, i think you are right that the chimney exhaust gas will be hotter, but i don't think it will be a problem, just means i'll leave it in there for less time..?

Yes I've almost finished the tapered rod. I'll upload a pic.

I have bought a pressure gauge, and good idea on marking where the burner choke works best, i will definitely do that.

Thanks Hoffman, i'll put a pic up.
 
Here you can see the adjustable stand i made to hang the blades in the pot for heating, and the cover that can swivel over the pot. I will use the cover when firing it up, as the salts will absorb water while not in use, which will cause some spitting as it is heating.
PICT1011_zpsqwqcaltu.jpg
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Here's the solenoid valve and needle vale for idling.


Inside of the body. You can see the support rod at bottom of chamber that sits 5mm below SS tube in case the lid slumps in a bit.


Tapered rod. The rusty piece of steel slides up and down the shaft and will be used as a form of hammer to remove the rod from the solid salt. I still need to weld a cap on the end that i will strike upwards against.
 
That looks like it'll work fine.

With the idle circuit, the pressure gauge is probably best located at the burner, or at least downstream of the solenoid valve and needle valve. It seems to me it would be nice to have a pressure reading when setting your idle circuit: flow through a jet varies as the square root of the pressure, so if you want, for example, a 4:1 ratio between heat input on full and heat input on idle, you'll need a 16:1 pressure ratio (maybe 16 PSI on full and 1 PSI on idle?).
 
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