How do you size the fan for circulating air in your oven

[Wilsoncs3980]

Whats the rule of thumb for determining the size of fan for circulating air inside your oven for better temperature control. My oven is approximately 15 cubic feet and it doesn't have a fan. I thought about just adding on of the high temp blowers that Grainger sells but it seems to be overkill to me. I hate to spend a hundred bucks on a squirel cage blower that will need a bunch of ducting built. I was hoping to buy a small motor on ebay and putting a fan blade on it. I was thinking all I have to do is mount the motor on top of the oven, drill a hole thru the top for the fan shaft with a shaft extender, put the blade on and put a wire guard around it. Am I simplifing this too much? Sorry if these are stupid questions. I'm a mechanical designer but all my experience is with precision gauging. I'm an idiot when it comes to oven design. I bought an old industrial oven from a scrap dealer and now I'm trying to retro-fit it and get it up and running so I can do some hard gun coatings and maybe a little powder coating.

[ed_denu]

The issue with non high temperature blowers or motors is that heat transfers up the shaft and overheats the bearings, windings, etc. A high temperature blower negates this by having an external propeller blade to help cool the shaft and hi temp bearings.

Some people have had success with low cost blowers and possible someone can recommend a good option. Either a recirculating fan or a shaft mounted propeller will work.

I went with the recirculation fan option which definitely helps average temperatures though the oven.

[Zigeuner]

I was wondering about that as well. but also, when do you determine whether you need a fan? Is one mandatory or just a good idea?

Thanks in advance.

[ed_denu]

IMO the larger the oven the greater the need. My oven is 24X30X68 and in doing temperature readings in different areas of the oven it was obvious that there was uneven heating. The recirculating fan helps a lot. Some of the differences can also be attributed to radiant heat from direct view of the elements.

I also use a small kitchen type oven that doesn't have any type of fan. In it I just shield the element from the general oven area.

[Zigeuner]

IMO the larger the oven the greater the need. My oven is 24X30X68 and in doing temperature readings in different areas of the oven it was obvious that there was uneven heating. The recirculating fan helps a lot. Some of the differences can also be attributed to radiant heat from direct view of the elements.

I also use a small kitchen type oven that doesn't have any type of fan. In it I just shield the element from the general oven area.

That's interesting. Is the fan mounted inside in the heat or somewhere else?

Also, on my other post, CarWiz mentioned the Ir or radiant heat issue. He suggested some perforated metal sheets. Is there any particular kind? I checked one of the home stores and what they have is aluminum and they want an arm and a leg for a small sheet. I would need two pieces of 24 X 36 to cover the four elements I'm using. Any such sheets can be added later by making them so that they simply drop in over the elements.

It sounds like a good idea but I've got to locate some less expensive material or I'll be in the poor house along with my oven. LOL.

[ed_denu]

The hi temp fan I use is a Dayton that mounts outside the oven. It's this one:

Blower,216 CFM,115 V - Small Centrifugal Blowers - Blowers - HVACR : Grainger Industrial Supply

A radiant heat shield is best if it isn't significant enough so that it becomes a source of radiant heat itself when it reaches temperature. I used heavy duty aluminum foil with the edges wrapped over a heavy wire frame. The foil allows for convection heat but blocks the radiant effect. I did some temperature measurements with and without it in place on parts mounted in the lower area of the oven. There's no doubt it works. There are probably many ways of accomplishing the same thing, just don't totally enclose the elements with anything to thick and heavy .

[Zigeuner]

The hi temp fan I use is a Dayton that mounts outside the oven. It's this one:

Blower,216 CFM,115 V - Small Centrifugal Blowers - Blowers - HVACR : Grainger Industrial Supply

A radiant heat shield is best if it isn't significant enough so that it becomes a source of radiant heat itself when it reaches temperature. I used heavy duty aluminum foil with the edges wrapped over a heavy wire frame. The foil allows for convection heat but blocks the radiant effect. I did some temperature measurements with and without it in place on parts mounted in the lower area of the oven. There's no doubt it works. There are probably many ways of accomplishing the same thing, just don't totally enclose the elements with anything to thick and heavy .

I located a steel supply house today that has all sorts of perforated metal in stock. They usually have scraps so i'll go over soon and take a look. I'm thinkiing maybe I'll use some thin steel with larger perforations. There will be two pieces 24" X 36" with a handle on each for removal and cleaning. They will be a drop-in.

At first I will keep it simple and go with the presently-installed definite-purpose contactor. If I decide to use some manner of temperature control, I have plenty of room inside my control panel or in another one nearby to add that in.

Likewise a fan can be an add-on once the unit is working.

Thanks.