I Took the Plunge - New Oven

[Joe Jones]

Put a pipe in the top for the vent
The one in the base is to draw in air that replaces the vented air
You can put a ball valve on one or both to regulate/tune the air flow

Without the second pipe you wont get any fumes out
The fumes from the powder cooking can cause lots of surface and adhesion issues

When your into the testing phase you need to work out how even the temps are.
Cut a bunch of 63mm dia disks from 8mm steel.
Suspended them from the top center in a chain with about 100mm spacing
Once he oven was up to temp, open the door and check the temp of each disk
If you get differences you need to work on your air flow
Once you have that stable move the chain to the sides and corners and repeat

Thank you. That test makes perfect sense. I do not believe I will not have air flow issues, as there will be two ceiling fans that return the heat to four registers at the bottom of the oven, either at the upper level or the lower level. The door will have two small inspection windows so I can use a laser temp probe to check the temperatures of the various items hanging inside.

Joe



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[34by151]

You will need to open the door to check temps otherwise your laser temp will measure the glass window.
I never open the door as I know the temps are constant over the oven and how long it takes to get the items up to temp

[SegoMan DeSigns]

I think the widow will be in a small door so he can check temps..

[Joe Jones]

You will need to open the door to check temps otherwise your laser temp will measure the glass window.
I never open the door as I know the temps are constant over the oven and how long it takes to get the items up to temp

I will be more specific. The door will have two small PORTS that open to allow a laser to take the temperature of the various metal pieces from different angles.

Joe

[Joe Jones]

I think the widow will be in a small door so he can check temps..

Right.

Joe


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[adbuch]

You will need to open the door to check temps otherwise your laser temp will measure the glass window.
I never open the door as I know the temps are constant over the oven and how long it takes to get the items up to temp

[adbuch]

I think the widow will be in a small door so he can check temps..

[34by151]

This oven will be electric, with two banks of four 3Kw burners.

That's a lot of amps
I assume you are using US power @110v
That's 220 amps

It would still be 110 amps here in OZ for 220V

Assuming you have the supply current available let see how fast it heats up (rise time)
Id be planning on more that 24kw (8 x 3kw elements) of heating to get the rise time below 10mins
I like to see it at 5-7 mins

This is why I went gas using 45kg bottles and 40kw heater
Much cheaper to run with a lot more heat
Upgraded that to a 120Kw 2 stage diesel heater (Riello 40)

This is cheaper again to run and rise time from ambient is about 3mins on full

90% of the time I fill the oven cold run it for 20min before removing the parts
Im using powder with a 12min cooking time
So that allows 5 mins extra for the parts to get to temp

That would be a lot longer with less heating
Also I tend to make the same parts in batches
I have a chart on the oven for times that dont fit that general rule
Ie when batching one product I have 10 parts that are 20mm steel. This runs for 23mins

Some smaller stuff say 4mm thick runs for 18mins

To give you a comparison 24Kw electric would cost about $10 per cycle
My diesel burner is 1$

Part of the higher cost is the fact you are running the heaters a lot longer
The heating cost and faster speeds make the payback on the burners very quick

[adbuch]

34by151 - Lots of great information there! I expect he will be using 240Vac power source, but still quite a load. I like the idea of using gas burners, especially if the oven is to be used often for many parts. I know that Robert Johnson has several large ovens that are gas fired. He is running a business and does lots of commercial work - large signs and such. For myself, I was planning to build an oven using electric heating elements. I know it costs more to run, but I would only use it occasionally and I do have a 200 amp service.
Thanks for you input.
David

[34by151]

One critical factor is the rise time
If you observe an oven the temp rise is not linear
The hotter you get the longer it takes to rise one more degree

To go faster you need more watts

Lets suppose you time the temp rise in your oven empty
For the sake of an exercise lets say it takes 5 secs to go up from 30c to 40c
By the time you get to 170c the next 10deg will take 10mins
The last 20deg to 200c will be over 30mins

Now If you add a part and do the same the rise time will be slower
This is because the part is acting as a heat sink (soaking up the heat)

So adding a small 1mm thick sign might not change the time time much but add 10mm parts and it will slow a lot

Why is this important
Most of the powders will begin to flow around 170c
You start your timer once the parts (not the oven) is at 200c

So the time (with your parts load in the oven) to get from 170c to 200c is fairly critical
The goal is to get the part (not the oven) from 17c to 200c as fast as possible

The time it takes to do this greatly effects the adhesion of the powder to the metal
There are other effects as well but this is the big one.

Ideally you want the rise time from 170c to 200c to be under 2 mins.
You aiming for an empty oven to get to 200c under 10mins to do this
The parts should get to 200c less than 2 mins behind the oven

So going back to the example 30 mins for the oven to get from 170c to 200c
The part would take around 45mins to get to 200
So your powder is flowing for 45mins you start the cure (cooking) cycle for 12-15mins

Ill bet the result looks great but adhesion is poor
When it cools bash on it with a hammer, drop it on the floor ect
Ill bet the powder dings and chips maybe even cracks and flakes off

Most of my powder is custom colours to match some OEM colours.
Im ordering one orange mix is 100kg lots
I always run a test part on a new batch
Both to test the match and the quality.
Once I confirm the colour match I bash on it with a hammer, drop it on the floor a few times to make sure.
So that test is not a silly at all

[Joe Jones]

This oven will be electric, with two banks of four 3Kw burners.

That's a lot of amps
I assume you are using US power @110v
That's 220 amps

220 Amps? Not by my calculations.

It is two banks of elements, but only one bank would be used to heat the oven. Either the top bank of four (12,000 watts) with the center shelf for a smaller over that heats up faster, or by flipping a switch, the lower four elements (also 12,000 watts) would heat the full oven.

The controller is a 50A 240VAC controller. Amps x Volts = Watts 50 x 240 = 12,000 watts, which is the rating for the controller.

Assuming you have the supply current available let see how fast it heats up (rise time)
Id be planning on more that 24kw (8 x 3kw elements) of heating to get the rise time below 10mins
I like to see it at 5-7 mins

I will see how long it takes to get to temperature with four elements. I am guessing 20 minutes.

This is why I went gas using 45kg bottles and 40kw heater
Much cheaper to run with a lot more heat
Upgraded that to a 120Kw 2 stage diesel heater (Riello 40)

This is cheaper again to run and rise time from ambient is about 3mins on full

I am beginning with the electric elements. If they fail, the oven can be converted to another heat source.

90% of the time I fill the oven cold run it for 20min before removing the parts
Im using powder with a 12min cooking time
So that allows 5 mins extra for the parts to get to temp

It will be a learning process for sure.

That would be a lot longer with less heating
Also I tend to make the same parts in batches
I have a chart on the oven for times that dont fit that general rule
Ie when batching one product I have 10 parts that are 20mm steel. This runs for 23mins

Some smaller stuff say 4mm thick runs for 18mins

To give you a comparison 24Kw electric would cost about $10 per cycle
My diesel burner is 1$

I will certainly look at a diesel heater if these electric elements fail. I have read many websites about building these. I also have clients with electric ovens, and they all seem to work. So before I uncap a volcano, I will see how this first build works for me.

Part of the higher cost is the fact you are running the heaters a lot longer
The heating cost and faster speeds make the payback on the burners very quick

That makes sense. Only completing it and testing it will give me the true numbers.

Joe



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[Joe Jones]

I have thought about adding simple propane burners for the initial warm up. No thermostat. Just turn them on and watch the temp. When it gets to where it should be, the propane burners would be shut off, and the electric elements would them maintain the temperature. Alternatively, I could connect a turbo propane heater for the initial warm up and just blast heat into the box for a couple of minutes when needed.

Joe


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[34by151]

So your only going to run 4 of that 8 elements at any time (12Kw) not 24Kw?
You are on 240v power?

You may need an extra SCR or Contactor as you are planning to run it at max current for hours at a time
I suspect it will burn out
Better to plan on the SCR/Contactor has double the current

What is the internal dimensions of the oven?

Assuming its 4x4x8 12Kw would take a min of 30min to heat that volume
That's assuming no heat loss, so it would be more
The slower the oven gets to temp the longer it takes to get the parts to temp, its not a linear relationship

Changing to 24Kw would bring that down to 13min
It would take 45Kw to get that down to 7min

You would be close at 24Kw and probably get good results on thin parts
Id be upping the heat source for thicker stuff

[adbuch]

So your only going to run 4 of that 8 elements at any time (12Kw) not 24Kw?
You are on 240v power?

You may need an extra SCR or Contactor as you are planning to run it at max current for hours at a time
I suspect it will burn out
Better to plan on the SCR/Contactor has double the current

What is the internal dimensions of the oven?

Assuming its 4x4x8 12Kw would take a min of 30min to heat that volume
That's assuming no heat loss, so it would be more
The slower the oven gets to temp the longer it takes to get the parts to temp, its not a linear relationship

Changing to 24Kw would bring that down to 13min
It would take 45Kw to get that down to 7min

You would be close at 24Kw and probably get good results on thin parts
Id be upping the heat source for thicker stuff

[SegoMan DeSigns]

Joe is allowing for a divider in the oven for small parts runs, I will prob do the same in my smaller one (25 cu ft) also. Is the diesel costs at the current pricing for #2 or are you running red fuel? I see a propane tank in his pics that might be an option as well. Propane is a plus for me as the stripper plant is less than 10 miles from my shop and might be cheaper than the diesel fuel or nat gas to operate.

[Joe Jones]

So your only going to run 4 of that 8 elements at any time (12Kw) not 24Kw?
You are on 240v power?

Correct.

You may need an extra SCR or Contactor as you are planning to run it at max current for hours at a time
I suspect it will burn out
Better to plan on the SCR/Contactor has double the current

What is an SCR/Contactor? The controller I purchased claims to be able to handle the load.

What is the internal dimensions of the oven?

Assuming its 4x4x8 12Kw would take a min of 30min to heat that volume
That's assuming no heat loss, so it would be more
The slower the oven gets to temp the longer it takes to get the parts to temp, its not a linear relationship

The interior is 4x4x75" tall, so I don't have to hunch over to walk into it. It will have a center shelf to divide the oven into two sections. That way, the top bank of burners would only heat 4x4x36" tall compartment, and the lower portion would not be heated at all.

I have the option of installing another controller, and running BOTH banks of burners to heat it up. That would be 100 AMPS of service for the warm up. Then when it gets to temperature, one bank can shut down, or I can run both banks simultaneously via two controllers.

Changing to 24Kw would bring that down to 13min
It would take 45Kw to get that down to 7min

Most ovens have the typical 4" wall thickness. My walls are 6" thick, and the ceiling and the door will also be 6" thick, all rock wool insulated. I will assume your calculations are correct, but the proof is in the pudding! I will have to put it through its paces to see the results.

You would be close at 24Kw and probably get good results on thin parts
Id be upping the heat source for thicker stuff

I live in a rural part of Kentucky. The single set of wires that runs across a neighbor's farm power three homes and four shops, for a total maximum of 1600 Amps of service divided among 7 buildings. Of course we are not all maxed out at all times, so I am not too worried about starving for power.

I have met other people with ovens. I respect your numbers, but I also see them doing magnificent powder coating jobs with far less than you claim I will require. So I will build this oven, with one, or perhaps TWO controllers, and see what happens.

Look at it this way. If four burners aren't enough to cook a 4x4x75" oven efficiently, I can always build in a permanent shelf up off of the floor, to reduce the oven volume to calculated acceptable levels, and just use the space beneath for the wheels of a rolling cart to roll into, like an engine stand. I have back issues, so if the oven needs to be smaller, I will come up from the floor, rather than build a smaller oven that I need to bend down to access.

Robert is silent on this topic, but I also respect his knowledge and experience. I would like to know if he concurs with your calculations, or if he stands somewhere BETWEEN "Easy Bake Oven" and "Nuclear Volcano."

I know that when a person gives advice, that advice sometimes comes with a dash of "macho" for added flavor. For example, a woman asks about buying her first gun for self-defense. I can think of a dozen guys who would immediately tell her that she needs to carry a Desert Eagle with Black Talon loads, because "...anything less isn't a REAL GUN." I tell her, "Test different guns at a range, and buy, practice with, and carry the one that helps you HIT your target EVERY TIME. I would rather see you HIT an attacker three times with a .22LR, than MISS him three times with a .44 Magnum..." When someone talks about buying their first motorcycle, there is the "..Anything under 1500 CC's is a toy!" crowd. I tell new riders, "Try different bikes, and buy and ride the one that give you the most confidence and enjoyment, even if it is a scooter ..."

So I HEAR what you are saying about how this oven should be built, but I also hear a bit of, " ... if it can't roast an elephant in three minutes, it isn't a REAL powder coat oven." So the only way for ME to know, is to try it.

Joe


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[Joe Jones]

Joe is allowing for a divider in the oven for small parts runs, I will prob do the same in my smaller one (25 cu ft) also. Is the diesel costs at the current pricing for #2 or are you running red fuel? I see a propane tank in his pics that might be an option as well. Propane is a plus for me as the stripper plant is less than 10 miles from my shop and might be cheaper than the diesel fuel or nat gas to operate.

Right. I am building this from scratch. I have created the ducting, so that I can ADD additional heat from an external source if it is needed. I can either use one of those 75,000 Watt torpedo propane heaters to blow additional heat into the air ducts, or I can install a couple of propane burners on the outside, with baffled ducting to send that additional heat into the oven as needed. I could place a square of steel tubing at the base around three sides of the oven, and I could light that for the initial warm up, and then shut off the gas and let the electric elements maintain the temperature for the bake time. Or I could just leave the propane burners ON, and let the electric controller supplement THAT heat via the elements and PID controller.

I am also thinking of making a VERTICAL wall, for tall parts that do not require a 16 sq. ft. footprint. I could put the wall in, perhaps 10 or 12" from one side, and make my over 48"x12"x75" using the four burners on that side of the oven, via switches or relays. I can even build a vertical divider that sits on top of the center shelf, to make an even smaller oven with two burners operational.

I like to think OUTSIDE of the box, and in this case, that is literally what I am doing

Joe



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[adbuch]

Joe is allowing for a divider in the oven for small parts runs, I will prob do the same in my smaller one (25 cu ft) also. Is the diesel costs at the current pricing for #2 or are you running red fuel? I see a propane tank in his pics that might be an option as well. Propane is a plus for me as the stripper plant is less than 10 miles from my shop and might be cheaper than the diesel fuel or nat gas to operate.

That is a great idea with the divider. I was thinking of doing something similar for my build. Why heat the entire oven for just a few smaller parts.
David

[Joe Jones]

Before life reminded me that I am not Elon Musk, I envisioned a 40-foot container with a moving internal wall, like a bicycle pump piston. I could move the wall closer to the door or deeper into the large box as needed, and a fresh bank of propane heating elements and circulating fans would kick in when the wall moved every (n) inches deeper into the cavity.

Well, that dream is dead. So much got in the way. I am currently trying to talk myself out of a Scotchman Turret 55 TON Ironworker. The financing makes it absolutely affordable, but I have truck payments, and motorcycle payments, and I am helping some friends through financial hard times with monthly checks ....

Money is like dry ice. When it is cold hard cash it is great ... but it can vaporize overnight!

Joe


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[SegoMan DeSigns]

Before life reminded me that I am not Elon Musk, I envisioned a 40-foot container with a moving internal wall, like a bicycle pump piston. I could move the wall closer to the door or deeper into the large box as needed, and a fresh bank of propane heating elements and circulating fans would kick in when the wall moved every (n) inches deeper into the cavity.

Well, that dream is dead. So much got in the way. I am currently trying to talk myself out of a Scotchman Turret 55 TON Ironworker. The financing makes it absolutely affordable, but I have truck payments, and motorcycle payments, and I am helping some friends through financial hard times with monthly checks ....

Money is like dry ice. When it is cold hard cash it is great ... but it can vaporize overnight!

Joe


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Yeah and I talked you out of it with the logistics that went with it , I think my big oven is going to be 4'W x 6'T x 12'D due to space limitations & cutting dimensions on the big table. This will allow me to handle most of my sign projects and the ability to say nope it won't fit the big & heavy projects people want to bring in saving what's left of my back..

[34by151]

The interior is 4x4x75" tall, so I don't have to hunch over to walk into it. It will have a center shelf to divide the oven into two sections. That way, the top bank of burners would only heat 4x4x36" tall compartment, and the lower portion would not be heated at all.

So for that size it would be around 22mins assuming no heat loss and no parts

There is a basic calculator here
https://forum.caswellplating.com/forum/ ... calculator

Make sure you change the watts (in your case to 12000) and the final temp to 400

I dont think the time with your shelf in would be half as you will have heat loss around the shelf plus you are heating the shelf itself
Id guess about 30% faster not 50% say 22mins down to about 15mins in comparison running both sets of elements (24kw) time is down to 11mins
That's getting close

[34by151]

Is the diesel costs at the current pricing for #2 or are you running red fuel? I see a propane tank in his pics that might be an option as well. Propane is a plus for me as the stripper plant is less than 10 miles from my shop and might be cheaper than the diesel fuel or nat gas to operate.

I dont know what red diesel is here in OZ we just have diesel

Diesel here is $2.08 Aud per litre that up about 20c/l with the Ukraine thing.
Thats pump price, I have an onsite tank that is cheeper
Electricity is 24c/Kwh
LPG in 45 Kg bottles is $170 each
We dont have town gas so dont know a price

I have a 600L diesel tank on site

I can lookup the riello manuals but LPG was about 40% more fuel per L than diesel
I guess it all depends on fuel prices in your area

[rdj357]

Sorry to have been so silent - I often utilize the KISS method on my projects - Keep It Simple Stupid. There is a lot of talk of over-engineering and that’s cool if it’s your thing! I considered a divider for my larger oven but just don’t think it’s worth my time or effort when I consider the operating cost.

My background is 30 years in the HVAC/R trade. It would take a tremendous amount of typing to arrive at the numbers I use but I’ll try to touch on the concepts. Essentially it is a combination of loads in BTU and then confining it to a time. I realize that I’m not being entirely accurate especially if you preheat the oven and take away some of that initial load before loading parts. Calculated at 4’x4’x75” lined with 14 gauge metal sheet and insulated to R-19 (.0526 U) with rock wool. 50F ambient and 50F entering part temp. 400 degree cure temp. Curing the equivalent of a full 4x8 sheet of 12 gauge parts.

First you have to add up all the ambient loads to get your box loss.
* Air in the oven (pretty negligible but I’ll put it for fun) 100 cu.ft.=8.07lb of air x .24 BTU/lb (specific heat of air)=1.9368 BTU x 350F increase = 678 BTU
* Interior skin - 132 sq ft of 14 gauge = 420.7lb of steel x .120 BTU/lb = 50 BTU x 350F increase = 17,500 BTU
Total static load: 18,178 BTU and if we want to preheat in 15 minutes then times 4 so 36,356 BTU/h

Add up dynamic loads:
* Heat loss through all oven walls (simplified to not take into account the varying delta T as oven warms on startup) .05626(U) x 132sq.ft x 350F (delta T) = 2599 BTU/h
* Air changes due to ventilation - assume 1 complete air change per hour so 678 BTU/h
* Calculate parts to cure load - 504# steel x .120 BTU/lb= 60.5 BTU x 350F increase = 21,175 BTU If we want to have part at temp in 30 minutes then we need to multiply that x 2 to get BTU/h so that is 42,350 BTU/h
Total dynamic loads: 45,627 BTU/h (walls and air changes plus parts to temp in 30 minutes)

Total oven load: 81,983 BTU/h

Convert that to watts for electric heat (divide by 3413 BTU/kWh) so 24 kWh is the requirement for ~30 minutes to part cure temp.

As you can see, the walls, ceiling, and floor and air changes are a pretty small part of the load (biggest part is heating up the internal skin) so I don’t worry about walling anything off on my ovens. Hope this helps - the rule of thumb you can find online recommends 150 watts per cubic foot and I find that to be very low and insufficient for my needs. My large oven is 500 BTUh/cu.ft. And small oven is 1,450 BTUh/cu.ft.

[Joe Jones]

Okay Robert, I admit that a lot of that is absolutely over my head. So here is my next thought ...

We never seem to talk about preventing the oven from becoming TOO HOT. All of the discussion is about not being able to produce enough heat to maintain a 400 degree oven, and the need to get to temperature QUICKLY at the start.

Given that an oven loses heat by virtue of its construction, and this 48x48x75" oven has 6" walls with rock wool insulation as well as in the door and ceiling panel, and there needs to be some means of measuring and regulating the temperature of the oven interior, I would THINK that the four electric elements could maintain that top temperature range without too much difficulty.

It seems to me that one option would be to add a three-sided pipe around the base at the floor, and drill it for a pretty blue propane flame array. The electric elements and PID controller would then only be responsible for "topping off" the heat as needed, because there would be a constant heat source from the flame, a source that is apparently insufficient on its own to EXCEED the heat requirements of the cooking process, even if left ON. Am I correct?

I am building this oven with eight elements, but the 240V/50A controller only handles four of them. so I am thinking that I could add another controller, and run both independently in the same oven, right? The controllers do not care WHERE the heat comes from. They only turn the elements on and off based on temperature readings. If all eight burners are being run by two controllers, would that cause any conflict? I could bring two separate 240V feeds to the oven from the main panel. That would be 100 AMPS of power feeding eight elements

I can put to 1500 Watt milk house electric heaters in a room, and they will both heat the room by kicking on and off independently, based on their individual temperature readings. So why couldn't two PID controllers operate two banks of elements in the same oven, in order to produce this Hellish quantity of heat that seems to be necessary? The circulating fans should eliminate any ability for one level of the oven to exceed the temperature of the other level.

Ah, well. Life is a learning process. I will begin with the four elements, and add additional heat using propane or another electric feed if necessary.

If I have to, I will raise the floor two inches, cover it with expanded metal, and line it wall-to-wall with these!

Joe

FLOOR BURNERS.jpg

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[Joe Jones]

Should I line the interior walls with something other than steel? The Space Shuttle is coated with 1" thick porcelain tiles, and it survives much higher temperatures during re-entry. I suppose I could fine some porcelain tiles and line the interior with them. What does that do to your calculations?

Joe



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