CR4® - The Engineer's Place for News and Discussion®


Previous in Forum: Cell Phone Time Is Always Wrong. Any Ideas?   Next in Forum: Can Ultraviolet Light Fight the Spread of Influenza?
Close
Close
Close
17 comments
Participant

Join Date: Feb 2018
Posts: 2
Good Answers: 1

Welding/Installing Stainless Steel Sheets for High Heat Food Drying Application

02/10/2018 12:33 AM

I have a fun topic, please advise. I am not an engineer or an expert welder, so please keep this in mind as you give your answers. I am managing a coconut production project in Pacific island villages. Local "experts" cannot solve my problem, so I turn to you.

To make coconut oil with our process, we need to dry grated coconut to a low moisture content within a half hour. To do this, we have built a long brick oven and flue, and have laid to 4x8 2mm stainless steel sheets down as the surface.

We must use stainless steel to satisfy food grade requirements. And the stainless steel must get very hot (up to 200 degrees Celsius) in order to dry the coconut quickly (the grated coconut is tossed about with stainless steel scrapers to prevent toasting). The stainless steel sheets are heated by fire inside the brick oven underneath. The sheets lay flat on top of a layer of bricks held up by a steel frame.

Our problem with the stainless steel is:

Welding the sheets together without distortion/warping is a challenge in itself, we can do a reasonable job by welding quickly. But when the sheets reach high heat during production, we get mild to extreme warping which makes the producers' job very difficult.

THE PROBLEM TO SOLVE: We need a way to keep the sheets as flat as possible, with minimal to no warping/distortion, when heated to a temperature of near 200 degrees Celsius during production.

Any ideas?

Many thanks.

Register to Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru
Technical Fields - Technical Writing - New Member Engineering Fields - Piping Design Engineering - New Member

Join Date: May 2009
Location: Richland, WA, USA
Posts: 20098
Good Answers: 757
#1

Re: Welding/Installing Stainless Steel Sheets...

02/10/2018 2:12 AM

I don't know if this would be practical in your location, but you might to try to get your stainless sheet in a continuous roll, and then flatten it by passing through a set of rollers.

__________________
In vino veritas; in cervisia carmen; in aqua E. coli.
Register to Reply
Guru

Join Date: Aug 2014
Location: 31°26'S 152°54'E Port Macquarie N.S.W. Australia
Posts: 916
Good Answers: 186
#2
In reply to #1

Re: Welding/Installing Stainless Steel Sheets...

02/10/2018 2:44 AM

My apology Tornado, I'm not replying to your post but for some reason I can't directly reply to the OP

If the sheet does not have to be removed separately from the frame, then consider tack welding it to those supports in the photo. Otherwise you may need to affix stiffeners to the sheet.

If you can turn the entire frame assembly over, then get a bit of heat going in the oven to pre-heat the assembly before welding as this will relieve some of the stresses during the tacking process.

Be sure to support the sheet hard against the frame when welding. A good trick is a strip of copper against the sheet to aid heat removal, start in the centre and move out.

If using a TIG machine then preferably use a pedal, pulsing -ve electrode DC (around 100 pulses per second or the max your machine can manage), a smallish cup to allow you to get close to the fillet and concentrate the gas and also assist cooling, and thin SS filler (takes less heat to melt), taper the electrode to a point - taper length about twice electrode diameter. If MIG or MMI then use SS wire or rods and keep the current as low as possible. Concentrate more heat on the frame than on the sheet.

Keep the weld current down to about 60 - 80 amps and do it quickly to minimise heat in the weldment - get in, get out - no up or downslope.

Get some scrap and practice your art first.

Another possibility is a whole bunch of SS countersunk screws through the sheets into those supports. I specify countersunk so that the heads don't interfere with your spatulas/scrapers.

Register to Reply Score 1 for Good Answer
Guru
Technical Fields - Technical Writing - New Member Engineering Fields - Piping Design Engineering - New Member

Join Date: May 2009
Location: Richland, WA, USA
Posts: 20098
Good Answers: 757
#3
In reply to #2

Re: Welding/Installing Stainless Steel Sheets...

02/10/2018 5:01 AM

No problem. I know of a few ideas, but I'm not an expert on sheet welding. What you say here seems quite sensible.

__________________
In vino veritas; in cervisia carmen; in aqua E. coli.
Register to Reply
Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 20033
Good Answers: 1165
#4

Welding/Installing Stainless Steel Sheets for High Heat Food Drying Application

02/10/2018 11:30 AM

First I would heat the sheets to operating temperature 250° F+ and then weld 1/4" at a time spacing out the welds allowing to cool down a bit between each weld, continue this process by making 1/4" welds all the way across the panel, then starting over and filling in the welds bit by bit...this should minimize warping, but this is always a problem without finishing and annealing...I would probably just beat it with a hammer to massage the metal then roll it out....but don't know what equipment you have to work with...so probably going to take some experimentation...

__________________
Life is like riding a bicycle. To keep your balance you must keep moving. A.E.
Register to Reply
Guru

Join Date: Oct 2008
Posts: 40534
Good Answers: 1601
#5

High Heat Food Drying Application

02/10/2018 5:58 PM

Your problem may be one of differential thermal expansion of the sheet. The "fire" side will be hotter than the "copra" side and this may be causing the warpage.

Does the sheet have room to expand or is it somehow attached. If it can't grow out, it will grow up and warp.

Also, you may want to consider using a butt joint with a backer strip and spot welding the sheets.

Something like the second from the left example, but with another top sheet butted against the first sheet.

Register to Reply Score 1 for Good Answer
Guru

Join Date: Apr 2011
Posts: 6832
Good Answers: 246
#6

re: sheetmetal

02/10/2018 8:30 PM

Lyn is correct. The problem is likely to be heating the sheet to nonuniform temperature. It may look like the same problem encountered when attempting to fabricate, but there may not be any problem with the welds at all.

Likewise, his note that attempts to restrain such deformation by attaching the sheet to supports will not prevent deformation is spot on. Welding stiffening ribs or securing to a heavy frame is not going to keep that material flat unless everything can slowly be uniformly heated together every cycle.

There are a few options.

1. Thicker sheet material will help.

2. Beading or crossbreaking can help to resist deformation.

3. Measures to heat more evenly. Specifically, making sure the outer portions are at least as hot as the center will help.

.

Just to reitterate, room must be left for expansion and subsequent contraction.

__________________
Eternal vigilance is the price of knowledge. - George Santayana
Register to Reply
Guru

Join Date: May 2006
Location: Placerville, CA (38° 45N, 120° 47'W)
Posts: 4725
Good Answers: 174
#7

Re: Welding/Installing Stainless Steel Sheets

02/11/2018 12:46 AM

First, Welcome to CR4!

To quote TNC's post #6: "Just to reitterate(sic), room must be left for expansion and subsequent contraction."

I assume that by "4X8" you mean 4 by 8 feet. Depending on the kind of stainless steel, an 8 foot long sheet of stainless steel will grow around 7.5 mm or 0.3" when heated from 20° to 200°C. Since your ambient is probably higher than 20°C, your expansion will probably be a bit less. Judging by your photos, it looks like you have 3 or 4 of these sheets, so your overall growth in length will be an inch or more.

My answer is DON'T weld it! You may want to tack weld something like a 1" overlap strip under one end of each sheet to reduce loss through the end cracks, as an expansion joint.

Here's my suggestion:

The second nut acts as a locknut, tightened against the first nut such that the screw assembly can slide along the top of the cross beam angle or channel. Ideally, all but one of those cross beams should be free to slide lengthwise. If they are already welded together, you might have to use slotted holes in the beams to obtain sufficient play.

To start, I'd try 5 screws at each cross beam. More might be needed...

If those cross beams are already fixed in place, it will be a tedious job for two people (hopefully a small person inside) to place and correctly tighten all those nuts. An alternative would be to drill and tap a strip of SS under each cross beam instead of using the nuts. Then the screws could be tightened or loosened as needed from above.

__________________
Teaching is a great experience, but there is no better teacher than experience.
Register to Reply
Commentator

Join Date: Dec 2012
Posts: 74
Good Answers: 2
#8

Re: Don't weld, bend

02/11/2018 1:25 AM

I weld a fair amount of stainless steel, and recommend only doing it if you have to, welding always puts stress in the metal, and when you heat it that stress might do unexpected things. Better to mechanically fasten the sheets together if you are going to heat cycle them.

Your setup however looks like you may have uneven heating, and problems controlling the expansion. Does your table need to be flat? If you can curve the surface along the length forming a shallow trough the curvature of the metal might help you control the expansion. Just an inch from edge to center might help to better control the warping. Some things you can't prevent, like uneven expansion, so you work with that and make it a friend.

I wouldn't try to restrain the sheet too much, let it move as it expands and contracts. Since you seem to have already welded the seam, make sure that the weld is ground flush so that the sheet is of even thickness. When you grind stainless never "burn" it as that will cause it to rust. Make sure that your welds are shiny when done.

Register to Reply Score 1 for Good Answer
Guru

Join Date: May 2016
Posts: 1029
Good Answers: 49
#9

Re: Welding/Installing Stainless Steel Sheets for High Heat Food Drying Applicat

02/12/2018 10:53 AM

I think we're barking up the wrong tree.

The issue is more likely to be the center of the sheet expanding more than the edges and going crazy as it buckles. Consider forming the sheet into a slight trough shape with the center either low or else slightly crowned. As the sheet expands, the curvature may change, but it will go in the same direction as everything else and stay a smooth curve. If you are using a spatula or scraper to move the coconut, the edge can be curved to match the trough.

Shrinking of the weld can be fixed by peening the weld against a backing anvil to stretch it to compensate for the shrink. If the coconut doesn't have to flow the length of the table, you could just form an upsticking lip at the joints to prevent liquid from running down the joints and forget the weld altogether. There is an automotive tool called a sheet metal shrinker that would let you shrink the length of the lip to match the curve of the trough. An added benefit to the use of lipped joints is that the length expansion of the table will be relieved at each joint and the whole table will not go into a big curve from end to end. Note that a trough configuration would lift at the ends of trough. A crowned shape would have the center lift up.

Register to Reply Score 1 for Good Answer
Guru
CR4 Admins - CR4 Moderator - CR4 Moderator United States - Member - New Member Popular Science - Cosmology - New Member Technical Fields - Education - New Member

Join Date: Jan 2014
Location: East Greenbush, NY
Posts: 817
Good Answers: 14
#10

Re: Welding/Installing Stainless Steel Sheets for High Heat Food Drying Applicat

02/12/2018 12:30 PM

Peco,

Welcome to CR4! You posed an interesting question and provided enough information to get the problem-solvers started.

Please let us know which direction you decide to follow and how it turns out.

Thanks,

Best in Show (moderator)

Register to Reply
2
Participant

Join Date: Feb 2018
Posts: 2
Good Answers: 1
#11

Re: Response from inquirer

02/12/2018 7:15 PM

Hello all, great stuff everybody, many thanks for sharing your knowledge. This is a problem we are working to solve in many locations - your input is very valuable. I wish you could come out here and knock this out with us!

I think I hear some common threads from your responses big picture-wise:

1) Avoid welding if possible. This is an option we may explore. The downside is having to deal with a seam where a) coconut particles can possibly get down into and go bad, or b) scrapers catch on the seam. I could see underpinning the seam with a SS strip as some suggested to mitigate these problems to an extent.

2) Limit restraint of the sheets. We have definitely made mistakes here, fighting against the sheets expansion. Currently the sheet is restrained by a weld at the middle seam, a weld at the front of the table, and pinched tightly on both edges. I’m thinking that one thing we can try immediately is to remove the pressure on the edges and cut the weld on the front end and form an overlap instead. What do you think? The middle weld would have to stay for now (until our next fresh attempt with new sheets). But I’m curious how freeing up the sheets to expand out might change the way it warps in the center.

3) When welding: a) preheat the sheets to operating temperature b) avoid extreme temperature change by either welding quickly or welding a tiny bit at a time, allowing the sheets to cool back to initial temp in between runs.

4) Form the sheets to a trough. It makes sense to make the expansion your friend. Troughing could be challenging with our current design as the layer of bricks on which the sheets rest for heat distribution is flat, and grinding that layer to suit the reshaped sheets seems difficult. An upward curvature may be better if not too extreme. If we were to try this, would you recommend curving up per 4x8 sheet or curving up the full 16 feet with the weld on the middle seam?

5) I will look into sourcing SS roll to avoid talk of any seam in the middle.

Please let me know if I am off on my interpretation of your great advice! Add anything to the conversation if necessary.

Sincere thanks, Peco

Register to Reply Good Answer (Score 2)
Guru

Join Date: May 2016
Posts: 1029
Good Answers: 49
#12
In reply to #11

Re: Response from inquirer

02/12/2018 7:25 PM

The only thing I would comment on is that the turned up lip on individual sheets was intended to actually be a flange up to perhaps 1" high. If the trough curvature is used, then you have a series of pans running down the table and the scrapers work up to the flange. Drop through should not be an issue although a sealing strip below the joints between pans would prevent combustion gasses from coming up through the joint between pans.

Register to Reply
Guru

Join Date: May 2006
Location: Placerville, CA (38° 45N, 120° 47'W)
Posts: 4725
Good Answers: 174
#16
In reply to #11

Re: Response from inquirer

02/13/2018 2:18 PM

"4) Form the sheets to a trough."

Don't attempt to grind the brick. Curve the supports for the brick. A surface of bricks curved up at the sides would also have at least some tendency to make hot gasses flow to the outside. I'd think this should lead to a somewhat more uniform temperature across the width of the sheet.

I really like the idea suggested somewhere here of having the sheets attached only at the center. In fact I'd carry that one step further, having a single fixed point at the center of the table, and a single slidable slotted point at the center of each end. This is if the two sheets are welded to each other. If you have two separate sheets, then a fixed attachment point for each sheet at the inside center, and a slidable attachment point at the outside end center. These two sheets could expand and contract as necessary without creating a gap at the center.

__________________
Teaching is a great experience, but there is no better teacher than experience.
Register to Reply
Commentator

Join Date: Dec 2012
Posts: 74
Good Answers: 2
#17
In reply to #11

Re: Response from inquirer

02/17/2018 12:48 PM

I wouldn't worry too much about curving the brick bed, if the stainless steel is suspended an inch up the heat will conduct just fine. In fact it might reduce the effect of hot spots as the heat will all transfer through radiation and conduction through the heated air in the space. Just make sure the edge is trapping the heat under the stainless steel. You might be able to rest a steel 2 inch angle (with sides down, angle up like an inverted "V") along the sides of the brick and rest the stainless steel on that, letting the center sag and form your trough. You might be able to cut the angle to make a filler piece for the ends, or stuff them with ceramic wool insulation to keep the heat under the sheet.

Try riveting a strip along the joint to close the gap instead of welding. Here in Boston I can easily order 16ft sheets from my metal distributor. If you can get one shipped to you flat on a pallet, it would certainly make your surface a lot cleaner.

Register to Reply
Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 20033
Good Answers: 1165
#13

Welding/Installing Stainless Steel Sheets for High Heat Food Drying Application

02/12/2018 8:52 PM

So maybe a moveable joint would work....when we used to make sheet metal duct we used sleeve joint connectors, a system where the ends of the duct met would be folded over on each opposing side, then to connect them we had a piece that was folded over on both sides, and this would be hammered on to hold the ducts together...this piece pictured below would slide over both end bends on the opposing duct holding them together, it was a tight fit, but would allow expansion....

__________________
Life is like riding a bicycle. To keep your balance you must keep moving. A.E.
Register to Reply Score 1 for Good Answer
Guru

Join Date: Jan 2006
Location: Tamworth, UK.
Posts: 1574
Good Answers: 36
#14

Re: Welding/Installing Stainless Steel Sheets for High Heat Food Drying Applicat

02/13/2018 6:28 AM

Or abandon the flatbed and consider making a slow tumble dryer. A long slowly rotating sloping cylindrical drum type heater. Moist coconut in one end and dry out the other???

__________________
When arguing, remember mud-slinging = lost ground.
Register to Reply Score 1 for Good Answer
Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 20033
Good Answers: 1165
#15
In reply to #14

Re: Welding/Installing Stainless Steel Sheets for High Heat Food Drying Applicat

02/13/2018 1:50 PM

__________________
Life is like riding a bicycle. To keep your balance you must keep moving. A.E.
Register to Reply
Register to Reply 17 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

BestInShow (1); dkwarner (2); horace40 (1); Jpfalt (2); lyn (1); Peco (1); Solararts (2); SolarEagle (3); spades (1); Tornado (2); truth is not a compromise (1)

Previous in Forum: Cell Phone Time Is Always Wrong. Any Ideas?   Next in Forum: Can Ultraviolet Light Fight the Spread of Influenza?

Advertisement