PDA

View Full Version : Heat Transfer Question



chriskat
11-22-2009, 02:06 PM
My Church has a commercial stove (Vulcan I think) with six burners. I don't have the BTU rating but could probably get it if necessary. A few times a year we have a pancake breakfast fund raiser. There is a grill to the side of the stove but it's relatively small and hard to keep up with the crowd.

Question is, if I bought a sheet of aluminum (or another material) sized to fit over the entire stove top and say .75" thick could I effectively make a larger grill? Would the heat spreat effectively enough to reduce hot spots and make cooking possible?

Thought someone here would know.

Thanks,
Jeff

Carld
11-22-2009, 02:11 PM
I think a sheet of steel would be better but I know your going for something you can handle easy. The heat will be localized at the burners unless you can do something to spread the heat out.

Warping will be an issue as you know.

ckelloug
11-22-2009, 03:07 PM
I think what you really want is a sheet of aluminum with a thin sheet of steel welded to it using an explosive plane wave generator. Unfortunately, I doubt this would be cost effective but it would be extremely fun to build;)

Seriously, In general, the thicker you make the plate, aluminum or steel, the more the heat will be evened out. Unfortunately, it will take longer to heat up and be heavier. The electric griddle I grew up with had a steel top. If you used .75 aluminum or even .5 aluminum I'd guess without proof that it would probably distribute the heat nicely since cooking pots and stove griddles aren't even that thick.

Lodge makes griddle pans out of cast iron so that may be worth a look either to buy some or to get ideas for a build. You could definitely make a griddle adapter out of cast iron, steel, or aluminum although I don't really like aluminum personally since it can react with food. Covering the entire cooktop could put heat into the stove itself where you don't want it (maybe not a big problem on a true commercial range) so I'd be a bit careful though I don't know enough to say what might be affected.

Looking at it on a cost basis, it might be easier to buy a couple electric griddles or some griddle pans for the stove than to conjure up something application specific. Aluminum isn't exactly cheap these days.

I doubt my above inane ranting helps but best of luck.

--Cameron

Evan
11-22-2009, 03:15 PM
There is a simple principle for working with dissimilar metals to avoid warpage. If everything is symmetrical it won't warp. You don't want to cook directly on an aluminum griddle unless it is hard anodized and a thin sheet of steel won't conduct heat well enough. However, a three layer sandwich of .125" steel, aluminum and then steel again fastened with flush tinner's rivets every 6 inches will stay flat and will heat very evenly without weighing half a ton.

Use standard tin coated steel tinner's rivets and put the head on the bottom side. Countersink the top sheet and buck the rivet down with a hammer against a solid steel plate below. Leave the bucked rivet slightly proud and grind it flush. Make sure you use the tin coated rivets and not the galvanized variety.

When you drill for the rivets make the holes in the aluminum a bit oversize. That will permit it to expand without causing a problem. Start riveting in the centre and spiral outward.

darryl
11-22-2009, 07:11 PM
It sounds like you will have six heating areas and want a grill large enough to cover them all at once. They do make cast iron grill tops, but I don't know where to find or at what price. We used to own a restaurant and had a cast iron grill- seems to me it would have been about 2ft by 3ft and one piece. You could use just one burner or all burners (think there was 4 in total).

They were straight burners going front to back, so they were arranged side by side to cover the width of the grill. Sounds to me like your six burners would heat the thing fairly evenly, so yes you could cook on the entire surface.

The grill could be lifted straight off for maintenance, and had a slot near the back where scrapings could be pushed to and would fall into a tray which you cleaned now and then.

Buy an existing cast iron grill, or build your own from scratch- I'd probably want to check with used restaurant equipment suppliers as a start anyway.

aostling
11-22-2009, 07:57 PM
When you drill for the rivets make the holes in the aluminum a bit oversize. That will permit it to expand without causing a problem. Start riveting in the centre and spiral outward.

Evan,

I'm grateful for your reply (which I cannot improve on). As one of the forum's degreed thermal engineers I was thinking I might be duty-bound to respond, considering that heat transfer is in the title.

Have you made one of these three-sheet sandwiches before? Your advice about the size of the rivet holes suggests some experience.

Evan
11-22-2009, 08:26 PM
I have made very similar items but not as a cook top. Many years ago when I worked in aircraft repair the shop I was at would take in any metal work they could find in the summer. Summer is when everyone is flying. One job I did was to build a very large fireplace hood for an upscale restaurant. It called for pure copper trim on the lower front edges about 4 inches high riveted to the steel hood. The CLE of copper is about halfway between steel and aluminium but is plenty enough to turn the hood into a bimetal thermostat. I explained to the fellow that designed it what would happen and suggested that the copper be duplicated on the inside of the hood which would balance the forces. He agreed and it worked out well.

In designing my telescope and other items that have to work outside I must take into consideration the coefficient of linear expansion of the materials. The moving parts as well as the frame must be able to operate over a 100 degree F temperature range without binding or loosening and falling apart. That was the main reason I used graphite-epoxy struts for my scope as it has a zero CLE and will stay in focus on an all night observing session.

J Tiers
11-22-2009, 09:06 PM
The other obvious question is where are the products of combustion going to go, and will they impinge on anything that won't like that?

Most burners are made for heat spreading up to the size of a biggish frypan or soup kettle. I don't know what the effect of a hot griddle over the entire top might be, nor what componentry may be under that top, and potentially be fried by the heat. Things like ignitors and the gas valves that always seem to be tied into them nowadays..... Away from the burners the top is usually relatively cool, even with the oven on.

if it's an older stove it may have pilots in which case the worst thing may only be the pilots maybe getting put out by the flow of combustion products.

chriskat
11-22-2009, 09:15 PM
Thanks to everyone. I hadn't thought about used restaurant equipment, that would be a good way too go. That is assuming that the heat transfer would be as good using a steel or cast iron grill. I would think there would be more evenly dispersed burners under the "real' grill.

The stove definitely has pilots. Judging by the look of it alone I don't think there are any electronic parts in it at all. Maybe not even anything electrical. I'll have to look and see if there is any electrical supply too it.

Thanks again.

Jeff

chriskat
11-22-2009, 09:26 PM
Incidentally, this is the same model. We can only make about 8 pancakes at a time on the griddle.

http://home.comcast.net/~chriskat/Pictures/%21BT13sK%21CGk~%24%28KGrHgoOKiEEjlLmf%29k0BKKm01K 1mw~~_12.jpg

Jeff

Carld
11-22-2009, 10:09 PM
You could put spacers under the grill to allow the gases to escape but you might want to block the front so the hot gases can't burn your hands and arm as you cook.

Fasttrack
11-22-2009, 10:19 PM
Mom's pancakes were always made on a cast iron griddle that sat ontop of two gas burners on her stove. Just wait until a sprinkle of water sizzles and then you know you're read to cook. It was basically just a large cast iron plate with little feet to keep it above the burners. The burners were ordinary gas burners that were spaced pretty far apart, but never had any issues.

I don't see why you couldn't make a few yourself (or one giant one if you think you can lift it up there ;) ). It's cooking - not rocket science! :)

darryl
11-23-2009, 01:32 AM
I have somewhere an aluminum square that I take camping at times. I just prop it up over the fire and it works pretty well. Pancakes is the application, and we've done bacon and eggs on it. It's about 1/8 thick and about 18 inches square. Doesn't warp much. I haven't used it for years now, but the concept proved out. These days I'm concerned about the aluminum as in 'throw out your al pots and pans, they're killing you'.

dp
11-23-2009, 01:45 AM
Build one of these bad boys: http://www.selectappliance.com/exec/ce-product/ar_ambg-36

dockrat
11-23-2009, 01:58 AM
Build one of these bad boys: http://www.selectappliance.com/exec/ce-product/ar_ambg-36

Holey chit dennis!!!!!!!! 14k list price??? scuse me while i mop the rum outa my keyboard!

dp
11-23-2009, 02:04 AM
Holey chit dennis!!!!!!!! 14k list price??? scuse me while i mop the rum outa my keyboard!

That's a mid-priced version. That's why you have to make your own! There's one for sale in Craig's List with an original price of $25,000 :eek:

Paul Alciatore
11-23-2009, 03:00 AM
I think I would get six cast iron slillets, square ones if you can find them. One on each burner. Size them to allow at least one inch inbetween. This will allow exhaust products to escape and the stove will be used as intended when designed. And it will probably be just as cheap or even cheaper than a large sheet of aluminum, with or without steel covering. Also easier to clean and for the workers, many of whom will probably be of the gentler sex, to handle. Also gives you versatility to use as many or as few as you need at any given time.

Evan
11-23-2009, 04:12 AM
A little searching turns up a purpose designed commercial style 4 burner griddle designed for Vulcan stoves at the reasonable price of $207. Don't know about shipping but with 4 sq ft of cooking area it will be hard to beat the price making your own.

http://ixian.ca/pics6/griddle.jpg

http://www.ccfse.com/ProductDetails.asp?ProductCode=MC24

chriskat
11-23-2009, 07:42 AM
Thanks once again. Always a great resource.

Jeff