Fan question...

[ppwcanada]

Hi all, I am a long time lurker, first time poster. Thanks to everyone who made this such an excellent forum!!!

My question is this, it seems that there are 2 trains of thought as far as air circulation goes. 1 is to simply place a fan inside to push the air arround, the other is to place a curculation system pulling from the top, and ducted to the bottom. I am guessing the later is the more optimal, but just wanted to be sure. As well, i have seen a few builds include a small vent to the outside, is that needed?? My thinking is to keep it airtight and a vent is somewhat counterproductive. Any thoughts?? Thanks!

[nuttyman]

Re: Fan question...

you need to vent it & moving the air from top to bottom is indeed optimal.

[ppwcanada]

Re: Fan question...

Thanks Nuttyman! That helps alot...

[nuttyman]

Re: Fan question...

no problem bro. if you're gonna take the time to do it you might as well do it right!

[baz]

Re: Fan question...

why do you need to vent?

[GeneNewcomb]

Re: Fan question...

I'm putting a 700CFM fan in a 64 CF oven. In theory it will move all the air almost 11 times a minute. I think thats sufficient. The only profesional oven maker I have talked to says 10X per minute is good. His cheapest oven is $28,000.
I can't see how pulling it from the top is at all neccesary unless your burners are up there.

[Popeye]

Re: Fan question...

ppw,
You've got to have a vent and a circulation fan. The vent gets rid of the nasty smoke and whatnot when the powder is curing and the circulation fan should keep the temps in the oven even.
Our oven has air inlets in the bottom and a vent in the top and a circulation fan.

Gene
I'm not sure I agree with the 11 air changes a minute, WAY to much. If you figure a normal cycle time of 30 minutes.. even at 10 acpm you'll turn the air over 328 times in 30 minutes, and thats way more than needed. IMHO

[chromo]

Re: Fan question...

2 things.
1: you don't want a hard wind in there blowing your powder off your parts! You just need to have a gentle flow top to bottom to even things out a bit.

2: you want to draw the heat off the top and blow back under/near the elements. Heat rises so your hottest oven temps are near the top if you don't circulate well. You want them to be as near even from bottom at heat source to top where heat collects and stands the most, so circulate top to bottom.

A small oven this is no worry really, bigger the oven the more it matters.

[GeneNewcomb]

Re: Fan question...

I know the fan is not actually blowing that much. That is the Grainger catalog specs but I have it 1" off the ball of the oven so I'm sure it is not near that. Its not really windy in there. I've got to call the www.spraytechsys.com guys this week and I'll confirm the air changes they suggest.
The first oven I made had two rows holes every 8 inches or so to test the temp, I stuck a pyrometer through the holes to the inside. The temp just did not vary that much throughout the box! A few degrees max WITHOUT a fan on. IMHO you put burners near the bottom and fan near the top and you have no worries about stratification of the temps.

[FigureLLC]

Re: Fan question...

i'm not sure what a proper vent looks like, to be honest. i've seen a lot of pics on this forum that have done it the wrong way (if i posted pics of my oven, they'd show you what a **** poor vent looks like. i built a pretty tight box, and smoke has no problem getting out pretty much anywhere but the vent (i used a 1" pipe with a ball valve). i've seen some pro ovens with ventilation ducts coming out of them, so i still don't know what those vents looked like.

[jw450r1]

Re: Fan question...

has anyone installed a circulation fan assembly in there oven by a big hole in the bottom and top of the oven?.if so any pics,and how did it work out for you?.....

[ed_denu]

Re: Fan question...

has anyone installed a circulation fan assembly in there oven by a big hole in the bottom and top of the oven?.if so any pics,and how did it work out for you?.....

If you pm me your email address I can send some pictures of my oven under construction. I installed a hi-temp fan at the top with the recirculated air coming out the bottom.

[GatorHyde]

Re: Fan question...

not to hijack ...but if it is best to move the air from top to bottom...and one must be careful not to blow the PC off the item...would it not be best to have a fan at the bottom of the oven and the vent at the top...and run the fan in reverse drawing the air from top to bottom?

If I am wrong, please clue me?

Alex

[ed_denu]

Re: Fan question...

I don't turn the fan on until the powder has melted. At that point it's not going to blow off. I've also got the recirculated air coming in at the very bottom of the oven.

[FigureLLC]

Re: Fan question...

try blowing the powder off a part with a blast of shop air... at 90psi, you've got to be pretty close distrupt the powder. you'd have to have a serious amount of air flow and bad (failing to follow common sense) ducting to mess up a part. since i didn't really know this, i installed a floor register (like you'd find in your house) at the bottom back wall of the oven. that way it directed the air towards the floor instead of the parts and if i had problems i could choke the flow with the adjustment. i considered using a solid state controller to adjust fan speed from the control box, but since the fan motor relies on a heat slinger to keep the heat off the motor, i figured it'd be bad to reduce the speed of the fan, bc the heat slinger wouldn't do its job as efficiently.