Heat Treatment Oven

[daleyd]

That's interesting to know, sort of a cross between a Regenerator in a old-school Blast or Open Hearth Furnaces, and a Recouperator in a Thermal Oxidiser, or Fired Heater.

The glass industry is one of the very few I haven't had very much contact with, so it's cool to learn more about it.

I worked for 9 years for a company that made thermal oxidisers, yeah very similar. We used to sell a variant that was just a burner chamber - even with preheating combustion air it used a lot of gas to keep the temperatures up.

The RTO type with ceramic beds were very very efficient - especially if you had a process where there was enough solvent to run itself. Once up to self sustain temp then the burner got switched off and the process air was enough to run the oxidiser, if not then natural gas was injected into the process air stream and burned in the beds, which again was more efficient (by a long way) than using a burner and combustion air.

RTO typically use the same arrangement of flow reversing to prevent the heat from being pushed out of the beds.

 

[Misterg]

Hmmm... okay, I'll have a think. I made it that way to avoid having unsupported bricks. If I make it symmetrical there won't be as much support on each side (38 mm instead of 76 mm), but I guess it would probably be enough.

You've only got 38mm of support either side as it is? Better to have the 'lintel' as an un-molested brick (IMHO).

Yes, definitely. I haven't designed that bit yet. There will be a frame (probably made out of angle iron, possibly with some sheet of some sort in the gaps) with ceramic wool blanket separating the sides from the bricks.

Are you aware of the rigid insulation board? It may be easier to build with than the fibre blanket. (thinking of 25mm 1260 grade). Having said that, though, I wouldn't bother with additional insulation - just build the bricks dry and clamp them together with a 25mm angle frame (our expectations may differ, though).

Recognise that the insulation bricks are something of a consumable - they'll last a long time with hobby use, but will eventually need replacing.

Also, I've just seen a Youtube video of someone who made a similar oven and cracked all their bricks by whacking it straight to 800°C - when you've built the oven, give it a nice, gentle drying out: Start at 80°C and ramp it up to ~120°C over a few hours. If it's steaming, back the heat off. Once it's dry, take the temperature up slowly for the initial heating, and give it a chance to adjust: 100°C/hr to 900°C sort of thing. It'll hold together better for it.

 

[Dr.Al]

You've only got 38mm of support either side as it is? Better to have the 'lintel' as an un-molested brick (IMHO).

As the design is, there's 76 mm of support either side. If I go to having am "un-molested" lintel then I was thinking it would reduce to 38 mm, but that's obviously nonsense (the support will always be brick size minus cavity size divided by 2, regardless of how the bricks are cut). I was obviously having a senior moment!

Are you aware of the rigid insulation board? It may be easier to build with than the fibre blanket. (thinking of 25mm 1260 grade). Having said that, though, I wouldn't bother with additional insulation - just build the bricks dry and clamp them together with a 25mm angle frame (our expectations may differ, though).

I've never heard of rigid insulation board, I'll have a look at that, thanks.

Recognise that the insulation bricks are something of a consumable - they'll last a long time with hobby use, but will eventually need replacing.

Thanks. Yes, I think someone mentioned that earlier in the thread. I'm hoping to design the steel frame such that the bricks can be removed without too much hassle (for replacement). I'm hoping that won't need to happen much (if ever) given the infrequent use but it's definitely better to plan for it in advance.

Also, I've just seen a Youtube video of someone who made a similar oven and cracked all their bricks by whacking it straight to 800°C - when you've built the oven, give it a nice, gentle drying out: Start at 80°C and ramp it up to ~120°C over a few hours. If it's steaming, back the heat off. Once it's dry, take the temperature up slowly for the initial heating, and give it a chance to adjust: 100°C/hr to 900°C sort of thing. It'll hold together better for it.

I was already planning for a very, very slow heat up. I'm surprised someone making a youtube video of building an oven wouldn't have read/seen that advice somewhere before starting. I'd been thinking I'd ramp it up very slowly over the course of a day (an advantage of a fancy controller is I can pre-program this and do it a bit more automatically).

 

[—Tom—]

Worth a watch

 

[Misterg]

Worth a watch

That was the one where he cracked all his bricks on the first firing...

(Normally I enjoy his videos, but IMHO he massively over-complicates the whole thing this time.)

 

[Jelly_Sheffield]

(Normally I enjoy his videos, but IMHO he massively over-complicates the whole thing this time.)

It's not super inconsistent that he got himself tied up in knots with this one, his overall style is that of someone with a really solid technical understanding, and just enough practical experience to be dangerous, trying to go straight into quite challenging projects.

The real issues he faced were in trying to make that very aesthetic sheet metal case with no experience of sheet metal forming, and rushing to meet a production schedule and get the video out...

His design and execution of the furnace itself is pretty standard, although I note he didn't electrically bond the casing to earth (or test the impedance of the earthing from the casing to the earth connection), which seems like an oversight given its riveted, rather than screwed or welded.

 

[not done it yet]

A good idea for a project.

I bought a Paragon SC2 kiln a lot of years ago (well, over ten, I think). It comes in handy at times.

It was actually bought for silver clay and lamp work hobbies, neither of which it gets used for, these days.

 

[Dr.Al]

I'm feeling a bit confused regarding mortar. There seem to be several different types and I'm struggling to understand which is best.

I think it probably needs to be one of these two:

Silcas M-White Refractory Mortar 1430°C

VITCAS Silcas M - Air Setting Ready Mixed Refractory Mortar for lightweight firebricks and insulation materials up to 1430°C / 2600°F. It is a white refractory mortar so may be used where white mortar is needed for visual and aesthetic purposes. Used for setting and jointing of bricks and other...

shop.vitcas.com

or

HPM - Heatproof Mortar

VITCAS Heatproof Mortar is an air-setting refractory cement which is designed for indoor applications and can withstand temperatures up to 1400°C / 2552°F. The ready-mixed formula ensures strong adhesion and forms smoke and gas-tight joints.

shop.vitcas.com

However there are also some Fire Cements (which I don't think are right, but thought it was worth checking), e.g. https://shop.vitcas.com/vitcas-premium-fire-cement.html & https://shop.vitcas.com/black-fire-cement.html

Also, does anyone have a feel for roughly how much of the stuff is likely to be needed? I'd rather order too much than too little, but having a vague idea would be helpful.

I'd like to get on and order the Vitcas stuff fairly soon (so I can try to start the build over the Christmas break) but I'm dithering as I try to make sure I'm getting the right stuff.

My current shopping list for Vitcas is:

• Bricks
• Mortar of some sort (see above)
• Ceramic fibre blanket (to go round the outside of the bricks)
• Fireproof rope (for the door)

Anything else I've forgotten? Do I need this rope seal stuff?

 

[oldbikedavey]

I'm feeling a bit confused regarding mortar. There seem to be several different types and I'm struggling to understand which is best.

I think it probably needs to be one of these two:

Silcas M-White Refractory Mortar 1430°C

VITCAS Silcas M - Air Setting Ready Mixed Refractory Mortar for lightweight firebricks and insulation materials up to 1430°C / 2600°F. It is a white refractory mortar so may be used where white mortar is needed for visual and aesthetic purposes. Used for setting and jointing of bricks and other...

shop.vitcas.com

or

HPM - Heatproof Mortar

VITCAS Heatproof Mortar is an air-setting refractory cement which is designed for indoor applications and can withstand temperatures up to 1400°C / 2552°F. The ready-mixed formula ensures strong adhesion and forms smoke and gas-tight joints.

shop.vitcas.com

However there are also some Fire Cements (which I don't think are right, but thought it was worth checking), e.g. https://shop.vitcas.com/vitcas-premium-fire-cement.html & https://shop.vitcas.com/black-fire-cement.html

Also, does anyone have a feel for roughly how much of the stuff is likely to be needed? I'd rather order too much than too little, but having a vague idea would be helpful.

I'd like to get on and order the Vitcas stuff fairly soon (so I can try to start the build over the Christmas break) but I'm dithering as I try to make sure I'm getting the right stuff.

My current shopping list for Vitcas is:

• Bricks
• Mortar of some sort (see above)
• Ceramic fibre blanket (to go round the outside of the bricks)
• Fireproof rope (for the door)

Anything else I've forgotten? Do I need this rope seal stuff?

Castree Kilns are your friend.....

 

[Dr.Al]

Castree Kilns are your friend.....

In what way? I tried to search for them and all I found was a facebook page (I don't have a facebook account) & few sites linking to a website that doesn't work.

 

[oldbikedavey]

In what way? I tried to search for them and all I found was a facebook page (I don't have a facebook account) & few sites linking to a website that doesn't work.

01994 232760

 

[ajlelectronics]

In what way? I tried to search for them and all I found was a facebook page (I don't have a facebook account) & few sites linking to a website that doesn't work.

You generally don't need a Facebook account to view public pages, even though it gives that impression. Just scroll past the login page.

 

[Kram]

In what way? I tried to search for them and all I found was a facebook page (I don't have a facebook account) & few sites linking to a website that doesn't work.

I have used them before for fire clay, they have an ebay shop.

Items for sale by castreekilnshop | eBay

Shop eBay for great deals from castreekilnshop!

www.ebay.co.uk

As to the question, I would likely choose alumina grit mixed with sodium sillicate as a mortar/adhesive. The problem however, alumina is quite expensive unless you buy it in bulk.

 

[timgunn1962]

I tried mortar on my first HT oven. It didn't work for me and I've made the last 8 without it. Good IFBs are pretty square and a steel frame will hold them together tightly enough not to leak heat.

The problem I had was that the IFBs sucked all of the moisture out of the mortar almost instantly and the mortar wouldn't squidge to an even layer. YMMV.

I also wouldn't wrap the outside in blanket. I did some heat transfer calculations (involving old textbooks, lots of coffee and migraine) and decided there wouldn't be much benefit for the considerable hassle it would entail.

Evenheat actually offered a "Bill Burke" option on some of their HT ovens, back around 2016, that had an extra layer of blanket around the outside of the IFB. It seems to have disappeared pretty quickly from their catalog. Evenheat use a stainless outer wrap for their HT ovens. I think the idea behind the Bill Burke option was primarily to reduce the skin temperature so folk wouldn't burn themselves when they touched it.

I had made my first HT oven without an outer skin, intending to see what temperature the outside reached and pick an appropriate cladding. I was thinking of plastic coated steel sheet, Aluminium sheet or Stainless steel sheet in increasing temperature range and cost. In the event, I found that heat transfer between the surface of the IFB and human skin was so poor that there wasn't much danger. Putting a metal skin on would actually increase the chances of injury massively, due to the faster heat tranfer to skin. I stuck with the IFB surface for all the rest.

 

[Misterg]

Good IFBs are pretty square and a steel frame will hold them together tightly enough

^This.

Putting a metal skin on would actually increase the chances of injury massively, due to the faster heat tranfer to skin.

^ and this.

You can hold one side of an IFB with your bare hands when the other side is at 1000°C. I think the only reason to add wool / fibreboard is to reduce the power requirements from the heating element.

 

[Dr.Al]

I tried mortar on my first HT oven. It didn't work for me and I've made the last 8 without it. Good IFBs are pretty square and a steel frame will hold them together tightly enough not to leak heat.

That's really good to know, thanks. Out of interest, why did you build 9 heat treatment ovens? What do you use them all for?

The problem I had was that the IFBs sucked all of the moisture out of the mortar almost instantly and the mortar wouldn't squidge to an even layer. YMMV.

The advice I read on-line somewhere or other was to mix some really watery mortar, spread it on the bricks, leave them a little while to suck the moisture out, then put another layer of un-watered-down mortar on to actually stick them together. I guess a bit like spreading PVA onto porous things before trying to glue them together.

Of course, if I can get away without mortar then that can only be a good thing, so thank you for the advice.

I guess the disadvantage of not having bricks might be that the steel frame needs to be a little more comprehensive. With the brick layout I currently have and a very simple frame consisting of angle-iron on the six edges of a cuboid, there would be several bricks that aren't held in place by anything other than friction to the bricks around them. That being said, it should be fairly easy to stick a few diagonal braces across to fill in the gaps. It should be easy enough to make sure that the bricks can't drop into the enclosure, although I might have to be a little careful with the door.

Either way, it sounds like the sensible option is to buy just the bricks and some fire rope for the door seal and see how I get on. If I later decide I need to buy mortar, fire cement or similar, the delivery cost won't be as much as it will be when ordering lots of bricks.

I also wouldn't wrap the outside in blanket. I did some heat transfer calculations (involving old textbooks, lots of coffee and migraine) and decided there wouldn't be much benefit for the considerable hassle it would entail.

That'll definitely make it a lot easier (and save a bit more money!) Thanks again.

Evenheat actually offered a "Bill Burke" option on some of their HT ovens, back around 2016, that had an extra layer of blanket around the outside of the IFB. It seems to have disappeared pretty quickly from their catalog. Evenheat use a stainless outer wrap for their HT ovens. I think the idea behind the Bill Burke option was primarily to reduce the skin temperature so folk wouldn't burn themselves when they touched it.

I would have thought that a good option for me for protecting my hands from getting burnt when I touch it would be "don't touch it". I'm usually quite wary when it comes to hot things in the workshop! I keep a little IR temperature gun (the sort of thing you point at something and it tells you how hot it is) for when I'm welding and I always point it at anything I'm about to touch before doing so. It aways astonishes me quite how long steel can stay hot so I err firmly on the side of caution.

I had made my first HT oven without an outer skin, intending to see what temperature the outside reached and pick an appropriate cladding. I was thinking of plastic coated steel sheet, Aluminium sheet or Stainless steel sheet in increasing temperature range and cost. In the event, I found that heat transfer between the surface of the IFB and human skin was so poor that there wasn't much danger. Putting a metal skin on would actually increase the chances of injury massively, due to the faster heat tranfer to skin. I stuck with the IFB surface for all the rest.

Sounds eminently sensible. Thanks. I guess one thing that might be sensible is to have some sort of extra insulation between the bricks/steel frame and the control enclosure. I think I've got some aluminium coated fibreglass heat shield sheet somewhere (something like this: https://amzn.to/49d6La2). I guess that could be used in isolation (not in contact with the steel work) or either side of a small bit of ceramic wool near the control cabinet, but from what you've said it sounds like something I can think about after building the thermal chamber and seeing how hot the air around it gets.

I want the control cabinet to be detachable (for ease of transportation and for the possibility of using the same control cabinet in future for a different shape or bigger chamber) so the other option is just to make the wires longer and have it further away!

You can hold one side of an IFB with your bare hands when the other side is at 1000°C. I think the only reason to add wool / fibreboard is to reduce the power requirements from the heating element.

That really is quite incredible.

Thanks again for all the advice I'm getting on here.

 

[Dr.Al]

Time is running out! Okay, that's a bit of an exaggeration as there isn't really a dead-line for this project. However, I've got two weeks off work over the Christmas break. Most of one of those will be occupied with family stuff but I'd like to use the second week to start building this oven. I doubt I'll finish it in that time, but I'd like to make some solid progress if possible.

Given that most suppliers will be closed over the Christmas break, that means I need to start ordering stuff very soon if I'm going to get it before the break.

Following the conversations above, I now feel like I'm getting close to a shopping list of stuff I need to acquire:

Heat Zone Stuff:

• Insulating Fire Bricks, Grade 23 (plus spares!)
• Fire rope (for the door seal)
• Kanthal Wire (I already have this)
• K-Type Thermocouples (3)

Note: I'm not going to buy high temperature mortar or ceramic wool as both are apparently unnecessary.

Control Cabinet Stuff (some of this is probably overkill, but overkill can be fun!):

• PID Controller
• Thyristor / controller of some sort (I don't think I'll do anything very fancy with this, just set a target current).
• Two SSRs (and heatsinks if they need them)
• Mains voltage relay (for over-temperature safety trip)
• Something to measure temperature from a thermocouple and trip the relay (I'm not sure what options there are for this at the moment, but hopefully something fairly simple and reliable)
• A current transformer (not strictly necessary as current monitoring will mostly be for interest)
• Some DIN rail
• Cable glands and such-like
• Fuses & fuse holders
• Switches (including a couple of mains-rated ones), potentiometers, LEDs (I've probably already got some of this)

Other Chamber Stuff:

• Angle iron for the frame (I've got loads of box section, so I had pondered cutting it in half diagonally; that would result in a lighter frame as it's 2 mm wall rather than the 3 mm typical of angle iron, but it's probably a lot more faff than I can be bothered with)
• "Bullet" hinges for the door
• Door interlock switch(es)
• Ceramic terminal blocks for the electrical connections to the chamber
• Thermocouple extension cables?
• High temperature rated power & signal wire

I think that's everything at the moment. I'll have to make a control cabinet but I've got some sheet steel to use for fabricating that. I'm sure there are other minor bits and bobs I've forgotten but hopefully that lot will get me most of the way there.

Anything you can think of that I've forgotten?

I'm working on the premise that the control cabinet will be relatively big, but I'm planning to make it a separate piece to the main chamber rather than having the cabinet permanently fixed to the side of the chamber. There are two reasons for that:

1. It will make it much easier to re-use the control cabinet with a different chamber if I decide I want a different cavity size or something like that.
2. It will make it easier to transport to/from storage as I can carry the chamber in one trip and the cabinet in a second.

The main disadvantage of that approach (that I can think of) is that I'll have to make sure that all the connections (thermocouples and heater) to the chamber are relatively easy to disconnect/reconnect and extremely easy to identify (so the right connection goes back to the right place).

 

[Dr.Al]

I've been gradually accumulating some of the stuff that will go into the heat treatment oven build. Most of this is shown above, although there are plenty of other things not shown, either because I've already got them buried in a drawer somewhere or because they're still to come (most of the remaining stuff is the control gear, which an exceptionally generous member of this forum has offered to donate).

In the picture above, you can see:

• An insulating fire brick
• A selection of (blue) interlock switches for the door
• Some bullet hinges
• Some (white) thermocouples and (green) thermocouple connectors
• The heating element wire
• Some ceramic terminal blocks for connecting to the heating element
• Some Wago connectors and mounting blocks (orange)
• Some fire rope to seal the door
• Some DIN rail to mount control stuff on
• A heavy duty (black) connector pair to connect the heating element to the control cabinet

The last one is there so that I can have a separate control cabinet to the oven (rather than trying to integrate it into the unit). I explained the reasons for that in the last post, so I won't echo them again here.

I only showed one insulating fire brick in the photo, but there's actually a rather large stack of them piled up in the dining room, along with a box with some thick 3-core wire:

I bought **way** more bricks than I'll need for the build. The delivery cost ended up being quite high and I really didn't want to pay it twice. I also like the idea of being able to make more than one chamber, so having excess bricks should make that possible. The extras also give me a bit of resilience to mistakes!

In the top photo, there were two different sizes of (weld-on) bullet hinges. You can see them a bit clearer in this photo:

I originally bought the smaller ones, having read the dimensions on a website and thought they sounded about right. When they arrived they looked a bit weedy. I could have returned them but I thought they'd be useful for the control cabinet (which I'm planning to make from scratch rather than doing the sensible thing and buying a project box). The bigger bullet hinges will be used for the chamber.

The little orange plastic pieces that are (in the top photo) sitting on top of the Wago connector set are used to allow mounting of Wago connectors on the DIN rail:

I suspect that the wiring of the control cabinet could be quite complex, so this will hopefully help with keeping things under control.

In the next post, I'll try to cover the control cabinet system implementation in a bit more detail. It has got a lot more complicated than originally envisaged as a result of the fancy control gear I've been offered, but more on that in due course...

 

[Dr.Al]

Back in my second post in this thread, I talked a bit about the four options I had for a PID controller for the oven build. What I said then was this:

Choosing a PID controller is an important decision. As I see it, I've got four options:

1. Pick a basic PID controller (circa £15–20) and accept that it can't do anything fancy like controlled temperature ramps.
2. Pick a fancy PID controller (circa £90) that can do temperature ramps / soaks etc.
3. Pick a mid-range PID controller (perhaps £30–40?) with an RS485/Modbus input. This would work much like the basic controller, but I could potentially add an external microcontroller to send commands over RS485 and implement temperature ramps / soaks myself if I ever felt the need.
4. Implement the whole lot from scratch using a microcontroller and a home-designed circuit board.

It turns out there's a fourth option that came out of the blue. An extremely generous member of the MIG welding forum read my posts about the heat treatment oven and offered to send me (free of charge) a selection of very fancy control gear for me to use in the oven. This is what came in the package he sent:

I'll talk through what all of that stuff is and does shortly, but for the impatient, there's a very, very fancy PID controller, a very fancy thyristor current controller, a contactor, a power supply, an temperature monitor, some circuit breakers, an emergency stop button and various connectors / fuse holders.

I have been completely blown away by the generosity shown (from someone I've never met).

The negative thing about this gift package (and believe me when I say this isn't a complaint!) is that my aim to document this heat treatment oven build in a way that will hopefully be helpful to others attempting the same thing will be slightly hindered in that most people won't have access to such fancy control gear. As such, I'm also going to try to write-up how I would have done it without all the posh kit!

Anyway, back to the posh-kit version of the build...

With the selection of control gear I now have available to me, I can revisit the schematic of the control cabinet (I also had a lot of help from my generous benefactor with working out the best way to do this). This is the new schematic:

While it's a bit prettier than the previous drawings I've done, I'm conscious that it's also a lot more complicated so I'm going to try to explain it bit-by-bit in the hope that it helps.

Let's look at the power circuit first, by deleting everything else from the schematic and just focusing on that bit:

It's still a little complicated I think due to the layout being better suited to a complex diagram than this simplified version, so let's lay it out in a simpler way. Hopefully by comparing the previous image with the next one, you'll be able to see that they represent the same circuit.

The power from the mains supply first goes through a circuit breaker. That will protect the system from massive overcurrents. A little bit later in the circuit (after the heater switch) I've also added a fuse (the thing that looks like two circles joined with a cross - that's a slightly old-fashioned symbol for a fuse, but I think it's a bit prettier than the modern one that looks like a resistor with a line through it). The fuse is arguably a bit pointless with the circuit breaker there to protect things. However, I thought I'd include the fuse as an extra layer of protection and also as dedicated protection for the heating element (the circuit breaker also provides power to the control electronics).

As well as the overcurrent protection devices (circuit breaker and fuse) there are couple of linked switches in the diagram. One of these is a simple toggle switch, the other is a contactor (basically the same thing as a relay, just a bit bigger). I haven't shown the contactor coil in this diagram but you can see it if you look at the first schematic in this post.

The switch is arguably unnecessary with the contactor there; however, I quite like the idea of being able to hit a switch and know for certain that the mains connection to the heater has been removed.

The contactor is there to cut the power to the heater in the event of any fault in the system. I'll talk about that a bit more in the next post when we come onto the safety circuit. The one thing to note (from the main schematic) is that the contactor isn't a switch that the user would ever touch, the switches are closed as a result of current flowing through the contactor's coil.

Finally, there's the thyristor pack. Unlike the solid state relay (SSR) used in the simpler version of the circuit (and the version shown in the schematics in earlier posts), the thyristor pack can control the level of current (rather than just relying on the mains voltage, the heater resistance and Ohm's Law) as well as turning it on and off. This means that the heater can be run at a lower power if desired/required.

That's it for the power circuit. It looks complicated but there aren't that many components in there. In the next post, I'll talk about the safety circuit in detail and then revisit some of the other features of the design. I'll then discuss what a simpler (cheaper) version of this circuit would look like.

 

[Dr.Al]

In the last post, I introduced this schematic:

I then went on to discuss the power circuit in a bit more detail. This time I'm going to talk about the low voltage safety circuit used in the new design. If I redraw the safety circuit in an alternative layout (showing only one of the switches in the contactor), you can hopefully see that it's really quite simple:

The box with the switch, the vertical lines and the inductor (four semicircles in a line) is a contactor. Remember that the term "contactor" basically means "big relay". A relay is an electromechanical component with a coil and one or more switches. The switches are opened or closed through the application of current through the coil. The current in the coil causes it to act as an electromagnet, pulling the moving part of the switch one way or the other in order to open or close a switch. In this case, the switches are all normally-open, i.e. when current isn't flowing in the coil, the switches are open and when (sufficient) current flows in the coil, they close (and allow current to flow through the relay's switches).

The power supply produces 24 V (from a mains input); the output of the power supply is fused in case of problems. There are then a series of switches, each of which will open in the event of a problem. For now, ignore the switch marked RESET and the resistor/diode to the left of the contactor.

With those ignored components gone, the current from the power supply goes through four switches, then through one of the contacts (switches) inside the contactor, then through the coil of the contactor, then back to to the power supply.

If any of the switches open, the contactor will deenergise (as there will no longer be current flowing in the coil) and the contactor's switches will all open. Since two of the switches (not shown in the safety circuit diagram) are in the heater power circuit, the heater will be rendered safe whenever the contactor deenergises.

The reset button is there to resolve a simple problem with the circuit layout. While it's great (i.e. safe) that the contactor will be deenergised when power is initially provided to the system (as one of the contactor switches in the circuit with the coil), to turn the system on, you need to be able to bypass the switch inside the contactor. The reset push button does that: with power applied and all the other safety switches closed, press the reset button and current will flow through the coil causing the contactor to energise. The reset button can then be released (as the contactor switch will be closed, maintaining the connection).

The last part of the circuit is a resistor and LED. In practice, this will be a single component (a 24 V lamp), but I prefer to draw it showing what's happening inside that lamp. When the various safety interlocks are closed, the lamp will be on; if any of them trip, the lamp will turn off. That'll help make it obvious whether power is available to the heater.

In the next post, I'll talk a bit about the simpler version and then I'll come back to this schematic and talk about some of the complexities of all the fancy new control gear I now have.