Air Circulation/Recirculation

[justshapes]

I am building an 8 x 8 x 8 foot (out side dimension) oven. Using this outside dimension for a fudge factor. The oven should require 21,094 Watts of power to provide 450 degrees above ambient temperature.

My current plan is to have six 2,600 Watt oven heating elements mounted on a raised circulation box along the base of the sides.

In addition two 227 CFM (each) high temperature blowers forcing air from the top/center of the oven through two 5,000 Watt close dryer heaters on either lower side and discharging the (super) hot air back on the floor area.

Total Watts available: 25, 600

In my thinking the air circulation system should maintain the oven temperature after the initial heat up cycle.

By the way: I plan on using two PIDs. One for start up temperature and one for the maintenance temperature with the circulation system.

My basic parts for powder coating are aluminum, cast arm sides that weigh 17.5 lbs and bench seats that are 70" in length and weigh 51 lbs. If that matters...

My questions are:
1 - Is this too much air circulation?
2 - Is the air flow great enough for the close dryer heaters?
3 - Are the total Watts available adequate?
4 -Where do I go for the powder recovery system advise?


In closing am looking at a Nordson "Vantage" powder coat system which is about two years old. It comes with two 55 lb and one 7 lb fluidized hoppers. What is it worth

And what other suggestions or referrals on powder coat systems are out there

[baz]

25000 watts , at 240 volts means over100amps plus 20% for safety thatvs about 125 amps for the elements alone ,is your electrical supply up to it?
can you explain why 2 pid ,i don t get what you mean by one for start up temp and one for maintenance temp.
i use one pid and it gets my oven up to temp then it switches on and off to keep it at the desired temp so why use 2 pids .
are you sure the clothes dryer will last ,because they are not designed like oven elements . i have been told that they are not meant to operate under that kind of heat they are supposed to be cooled by incoming fresh air passing over them

[ed_denu]

I agree with baz. I don't understand the reasoning for two pid's, one for initial heating and one to maintain temperature. It's at initial startup that you need the most heat. As baz noted, once the oven reaches temperature, it's the design of the pid to keep the temperature constant.

I don't think you will have to much air circulation, but probably to little. I'm betting that the cfm figures are free flow volumes that will be decreased significantly by static pressure as the air is passed thru duct work. For example, I'm using a 227 cfm fan on approximately a 30 cu ft oven and the air flow is adequate, but not overwhelming. Your oven is going to be 500+ cu ft.

Lastly imo even 25000 watts is not sufficient to heat an oven this size. You're probably aware of this, but most large ovens rely on a gas heating source, versus electric. Perhaps someone here with a large oven using electric resistive heating can share their wattage figures.

[justshapes]

In response to "BAZ":

First of all let's not worry about available AMP’s.

We are using electric, because of our very restrictive fire codes on a two level structure. (I don’t want to add a sprinkler system; just for a gas heated powder coat oven.)

More thinking:

I plan on all of the radiant/indirect heating sources to bring the oven up to temperature rapidly.

After reaching; let’s say 425 degrees the radiant elements shut down and the circulating elements remain active to maintain the desired temperature.

I want to reduce the radiant (broiling effect) to the outside or element side of the product being cured.

The castings weigh 17.5 lbs and the seats weigh 51 lbs…

With circulation and the added 10,000 Watts of heat source I should have a convection type of oven?


My main guestions are?

1) How much air circulation is required for an 8 x 8 x 8 oven and how much is excessive? – “CFM”
2) What CFM is required for a 5000 Watt dryer heater?



In response to “ed_denu”

I would like to circulate the heated air around the parts and at the same time I do not want to disturb the powder.

Again.. I am painting heavy castings/large parts. The radiant/indirect heating should be very important, because of the cure time.

The company that coats my stuff currently slows down their line about 50% due to the weight.

Their oven heat source is indirect gas.

With a radiant source I would come out with a blistered exterior and a flakey or uncured interior part.

We can add more CFM to the circulation, but how much

I have more questions and no time left today….

[ed_denu]

In response to “ed_denu”

I would like to circulate the heated air around the parts and at the same time I do not want to disturb the powder. What I do is to not turn the fan on until the parts begin to flow out. In my oven with most parts, the flow out begins when the oven reaches @200 degrees. I use the pid temp alarm to turn the fan on automatically at this point.

Again.. I am painting heavy castings/large parts. The radiant/indirect heating should be very important, because of the cure time.

The company that coats my stuff currently slows down their line about 50% due to the weight. Cure time always should be when the parts reach stated temperature. If you are doing repetitive work then calculating the cure time should be fairly easy and require consistent bake times.

Their oven heat source is indirect gas.

With a radiant source I would come out with a blistered exterior and a flakey or uncured interior part.

We can add more CFM to the circulation, but how much There are calculations that compute cfm flow based on duct size, bends, etc. But you might want to look at convection fans, versus recirculated.

I have more questions and no time left today….

You might consider investigating how large commercial ovens are constructed and get some ideas from them.

[baz]

i understand your reason for electric . i als think you might need more watts or btu s due to the size of the oven but also due to the heavy mass that you will be heating up.

i still don t get the idea of two heating system .
if you build your oven on a convection basis and with adjustable fan speed you would make things much easier .
the way my oven is done the air is picked up by the fan and pushed inside a plenum chamber toward the rear ,that same plenum covers the rear wall and my elements are inside that plenum in the rear wall so the air gets pushed from the top to rear and down over the elements and out from the bottom .
my fan is driven by a 1/2 hp motor indirectly driven with 2 pullys and a belt
so by changing the pully size i ended upwith the right air movement that i needed without having hotspots on the side facing the plenum chamber.
1 pid ,1 fan ,1 set of elements
ed s advice on having a look on how industrial ovens are made and also on the amount of btu s needed to heat it up could save you lot s of work