Is too much wattage a bad thing?

[bikejohn]

Greetings from even another Noobie.

I'm assembling my oven--2.5X4X6 inside dimension, 4 inch walls--and the btu/wattage calc tells me somewhere around 5-6000 watts. I'm planning 9000 watts (3 elements), but is there any reason that I shouldn't add a fourth element and boost it to 12,000?

This is a fantastic resource and I've really learned a lot. Shoot I should--I've only been lurking a couple of years now...

Thanks
John

[JTW37]

I don't think wattage is so much of an issue as is the amps that it pulls. If your wiring can handle the amps you should be good to go and would ramp to temp pretty quick. Total wattage divided by voltage tells you the amps draw. You will be pulling 50 amps at 12000 watts. My Shop is wired for 200 amps with a 50 amp breaker for the oven.

[defed]

seems to me, tho i do not know if it holds true in practice, if you have 6000watts and it heats up in 30 mins or 12000watts and it heats up in 15 mins, the cost to run should be the same. and once to temp, they should cycle on/off to maintain temp, which again, should be half as long w/ 2x the power.

i put 2 sets of 2 elements in mine (2800w each). i can either run all 4 or just two, depending on how well it works.

[Banditperformance]

Yup, electricity is charged in KW/Hr Double the wattage, but half the time = same cost. To large of elements increases the setup costs though, The contactor size, wire size, all play a role too. If I were to stress anything from building my own ovens, is Insulate, insulate, and insulate some more. The better insulated the oven is, the faster it will heat up, adn the less power it will take to keep it at cure temp. My large oven has 10" thick walls.

[BottleFed70]

Extra wattage can actually save you a litle.

BUT, you don't want you oven to come up to temperatature TOO quickly. You want the oven to heat the part evenly, and have the part come up to temperature evenly. I wouldn't go with more than 3 elements for your oven.

[BikerBill]

But, the extra BTU's from higher wattage elements aid in curing heavier items.

That BTU clac. shows the number of watts\BTU's to heat "air".. But we are more interested in heating metal.

Think about it.. a Heat gun will get your oven to 400 if it's well insulated, but willl you be able to cure parts with it ??

If you put a 5 lb chicken in the oven and wait 10 min after the stat say's it reached 200 degress, would you eat it ? Now compair that to a 20lb Turkey.. it takes all day for the friggin thing to reach 200 in the center. Now do it again with frozen birds and look at the times.. Burnt in some areas and frozen in others..

You wouldn't need to many BTU's to cure , say a motorcycle fender as compaired to an engine block.

I would think it would be best to go with as many BTU's (Watts) as possible, uses a convection fan, (Convection oven can bake a cake in half the time as a regular oven) ramp your temps depending on the weight and type of metals you are cooking and take lots of notes...

In the Powder coating hand book (I think it is the one in the sticky notes) there is a calculation for lbs/btu. I would look at that to be a more reliable method of determining BTU's need rather than the Home heating calc.

An advantage we have over the conveyor line industrial coating guys is that we can (and often have to) cure our parts one at a time.. therefore we can dial in the best time and temps for each item.. Or use the info to run the ovens as effeciantly as possible based on the weight, materials and btu's at hand.. Not just 500 degress for 30 min once around the conveyor


Bill

PS.. if someone is good with excell Id really like to see them make a spread sheet using the info from the BTU/weight section of that manual.

[CarWiz]

I agree. Ramping up the temperature too fast will cause the powder to melt before the item surface gets to temp. Poor bonding is the result. However, having enough wattage to heat the space efficiently helps the controller maintain an accurate temperature during curring.

Unless you're familiar with the PID controller concept and programming, setting the parameters can be quite complicated. Most simply use the Auto function. In this mode, the controller needs to see temp changes within a certain time frame based on it's own calculations. Lower wattage ovens will suffer with lower accuracy and can even timeout. The best design is to use available calculators to determine the wattage needed to accurately heat the oven space within about 30 minutes. This gives most items time to reach adequate surface temperature.

[defed]

if getting to temp too fast will cause the powder to melt too fast (before part is hot), how does that differ form putting the part into a pre-heated oven?

if the oven is already at 400, wouldn't the powder melt right away before the part even begins to warm?

[BikerBill]

>if getting to temp too fast will cause the powder to melt too fast (before >part is hot), how does that differ form putting the part into a pre-heated >oven?

>if the oven is already at 400, wouldn't the powder melt right away before >the part even begins to warm?

Exactly... Oven temp is not as important (to a degree).. You dont want the part to come to temp too fast on the "surface". Picture putting a torch to a freshly coated (uncured) part. the powder will melt, but once the rest of the peice starts to pull the heat from the surface the curing stops..

Go back to the chicken anology.. do you cook a chicken at 200. You probably could, but how long would it take for it to reach temp.. No you run it at 400 to 500 degrees.Now cook the same chicken in a toaster oven.. Same chicken and same temps, but because the toaster oven dosent have the BTU's that the regular oven has it takes almost twice as long to cook.. Both oven hold the chicken, both ovens reach the desired temp.. But only one of them have enough power to cook it in a timely matter.

And yes.. I know our parts dont need to be heated to the core like a chicken, but they do have to be saturated enough to hold the temp at the surface with the given heat source long enough to cure the powder.

The oven temp doesnt play nearly as important as the temp of the part.. with out enough BTU's to maintain the parts at proper temp you could ruin the finish by having it only "start" to cure or never fully cure.

Now Im sure that it is possible to amp the btu's or wattage up to get the same effect as a torch heating just the surface.. but that would take ALOT of heat. maybe the same effect it the parts were to close to the source..

[BikerBill]

I did a copy and paste from the powder coating book that is in the sticky notes..

It is a bit much to read, but look this over figure what you think your heaviest parts would be and go from there... I never bothered to check but I bet there would be a big differance over what the home heating calc says you need.

Again, I would love to see this in excell.. Im not good at it or I would make my own spread sheet, but is someone here is I would be glad to work any of the formulas out with them to make a copy avalable to put here.


Formula: Sq. Ft. of Oven Surface x Panel Loss Factor x Temp. Rise
Example:
Oven Size: 20’0” W x 50’0” L x 10’0” H
Operating Temperature: 300 °F
Oven Start-up Temperature: 70 °F
Panel Thickness: 4”
determine sq. ft of oven surface
sides: 10 x 50 = 500 x 2 sides = 1,000
ends: 10 x 20 = 200 x 2 ends = 400
top & bottom: 20 x 50 = 1,000 x 2 = 2,000
add all surfaces: 1,000 + 400 + 2,000 = 3,400 sq.ft
find panel loss factor from table 11-1
(.35)
determine temp. rise in degrees F
300 – 70 = 230
calculate panel loss factor
3,400 ft2 x .35 panel loss factor x 230 temp. rise = 273,700 Btu’s
Product Loading
To calculate the Btu’s required to maintain the set temperature in
production, multiply the total load, (parts, rack, conveyor chain and
trolleys), Table 11-2 x 0.12 (Btu’s required per lb.°F) x temperature
rise. The factor 0.12 is used for steel. Different materials will require a
different multiplier. Table 11-3 shows the specific heat value for different
metals.
Panel Thickness Loss Factor
3" .40
4" .35
5" .30
6" .25
8" .20
Table 11-1 – Panel Loss Factor
XI
Powder Coater’s Manual 1/98 XI /5
Chain Weight
Conveyor per Foot Trolley Weight
Enclosed Track 3.50 lbs. .5 lbs (H-Attach)
X348 2.14 lbs. 2.34 lbs.
X458 3.14 lbs. 5.18 lbs.
X678 6.39 lbs. 16.50 lbs.
Gas Use in Btu’s
Example:
Chain - X348
Trolley Centers 12”
Racks - 5 lbs. on 12” centers
Parts - 5 lbs. on 12” centers
Line Speed - 10 FPM
Oven Temp. - 300 °F
Oven Start-up Temp. –70 °F
determine chain lbs./ft.
2.14 + 2.34 = 4.48 lbs./ft.
determine part & rack load
5 + 5 = 10 lbs./ft.
determine total load
4.48 lbs. chain & trolleys + 10 lbs. part & racks = 14.48 lbs./ft.
Table 11-2 – Chain & Trolley Weights
Table 11-3 – Specific Heat of Different Metals
Metal Specific Heat
Steel 0.120
Aluminum 0.226
Iron 0.175
Zinc 0.0931
XI
XI /6 Powder Coater’s Manual 1/98
Gas Use in Btu’s
determine total product load/min.
14.48 lbs./ft x 10 feet per minute = 144.8 lbs. per min.
determine product load/hour
144.8 lbs. per min. x 60 mins. = 8,688 lbs./hour
determine temp. rise
300 – 70 = 230
calculate Btu’s
8,688 lbs./hr. x .12 Btu’s x 2300 temp. rise = 239,789 Btu’s
The parts that are processed may not be made of steel and
may require a different factor from the list on page XI/3.
Exhaust Loss
The exhaust loss is determined by multiplying the cubic feet per minute
of exhaust (CFM) x 60 minutes x 0.075 pounds per cubic foot x .24
Btu’s per pound °F x Temp. rise.
Formula: CFM x 60 x 0.075 x 0.24 x Temp. Rise
Example:
Exhaust Volume: 2,000 CFM
Set Temperature: 300 °F
Oven Start Temp.: 70 °F
determine temp. rise
300 – 70 = 230
calculate exhaust loss
2,000 x 60 x .075 x .24 x 230 = 496,800 Btu’s
• 0.075 is the density in lbs. of standard air
• 0.24 is the specific heat of air
To determine the total Btu’s consumed, add a, b and c. In the above
examples this total would be:
273,700 + 239,789 + 496,800 = 1,010,289 Btu’s