Friday, March 22, 2013

Steam Power

Pegs on which to lay the work to be steamed.
A while back I put together a pair of wooden bicycle fenders. These were made from hardwood ash slats that I soaked and bent by hand. That worked OK and gave a partial bend but the reality was that I needed a perfect bend, such that there was no spring to the fender that could work loose from the hardware affixing it to the bike. I knew that the real answer was to steam bend the pieces. Steam bending is a process by which wood is immersed in a steam bath for enough time to raise the temperature of the material to about 212 degrees Fahrenheit. The hot steam helps to open the poors of the wood and soak water into the core, making the wood pliable and easy to bend rather than breaking. Bending wood or in reality, bending anything involves stretching outer portions of the material and compressing inner portions of the material. The harder and drier the wood, the more reluctant it is to bend rather than break.

Because I need to bend long thin strips of wood, I needed something in which those strips can be housed and then bathed in steam sufficient to raise the temperature above 200 degrees. I remembered seeing an old This Old House or the New Yankee Workshop program years ago where Norm used a chamber made from PVC to steam some work. A quick Lougle search and there was tons of info on such apparatus as well as steam production units. In terms of steam production units, the easiest approach would be to take a hot-plate and a tea-kettle and plumb the steam into the steam chamber. There were hundreds of variations of that premise out there using everything from wood-fired rigs to turkey-friers. I decided to purchase a self contained unit that was specially designed for this and came with everything required except the chamber.

Fitting for steam hose and steam delivery.
For my chamber I used 4" Schedule 40 PVC pipe. This stuff is your standard waste water pipe that is used in most modern homes. Rather than cut a section from the plumbing in our house I decided to head to the Depot and pick a piece up. It comes in 10' lengths and cost about $19. For fittings on the ends of the pipe I had some choices. The easiest was a 4" cap but since PVC components apparently use a material based price model rather than how intricate the piece is, a simple end cap (which did have a mess of plastic in it) cost more than a clean-out and cap combined. I liked and purchased the latter as it affords easier access though it does slightly reduce the ID of the chamber entry.

One note on principle here when dealing with steam. Captured steam is really, really dangerous. For one it is hot, above boiling as we all know. It also pressurizes and can generate enormous power. Remember, they used it for powering locomotives and also recall the first steam heater that they produced without safety blow-off valves, which would trip when the pressure got too high, lead to come catastrophic failures. I think Mythbusters did a segment on them as well. So, this means your steam chamber should not, ever, be air tight. There should be no pressure building in it at all. Rule of thumb is that if you have a 1" hose delivering the steam to your chamber, you should have at least a 1" hole to expel the pressure.

Clean-out end with removable vent peg and thermometer.
Back to the design, I decided to go with a 6' long chamber with the thought that if I needed more space at some point I could pick up a coupler and tack the remaining 4' of pipe back on. After cutting the pipe to size I then measured and drilled 7/16" holes along the center of the pipe spaced equally along the length through both sides of the pipe. Into these holes I inserted 3/8" wooden dowels that I had cut about an inch over the OD of the pipe. These dowels would serve as a rack for the work to be laid across so it would be completely exposed to the steam. To be creative I also drilled smaller holes in each end of the dowel through which I inserted small sections of smaller 3/16" dowels. These acted as stops for the bigger dowels so they wouldn't fall out of the holes as the chamber unit was moved around.

Finished unit, non-drive end.
For the end caps, I simply pressed the caps onto the ends of the PVC pipe dry rather cement them in place. They dry fit tight enough anyhow, dry they would act as an emergency blow-off should things go wrong and it also makes for easy access in case I want to expand the unit. If they start falling off, I may affix them more permanently. In one of the clean-out caps I bored a 1/2" hole and threaded the brass fitting that the steam generation unit came with, This is the end onto which the hose from the unit will be attached. In the other end cap I bored a 7/16" hole straight through the square cap and inserted a removable wooden dowel. This would serve as the steam pressure outflow when the dowel was removed.

Why did I put the dowel in? I had one chunk left over and it isn't in my nature to leave it unplugged. It just felt wrong. Maybe I'll put a leash on the dowel and attach the other end to the chamber so that when removed, you can't lose it. All about closing the loop. I'm thinking that I may redo that and put a threaded fitting for a garden hose on, which I could then run out the window so the unit could be used indoors without releasing all of the steam inside. Putting all of that moisture in the air would make for a wet mess. The flip side though is that you want to chamber to be hot and trying to keep it warm outside in the New England winter could be a problem.

Just because I could, I added a thermometer to the unit so I could tell what the temperature was inside the chamber. This was a simple BBQ grill thermometer that was intended to bolt onto the top of the grill. To affix it to the pipe I simply bored a hole near the far end of the chamber that was slightly smaller than the OD of the threads on the thermometer. I then screwed it into place on the outside of the pipe.

Hung in place for storage.
The last step was to make some simple feet for the unit so it would sit flat. For this I just used some scraps of 2x4 which I traced the OD arc of the pipe onto and then cut on the band-saw. To hold the blocks to the tube I just used a short sheet-rock screw run into the pipe through the bottom of each foot. I then ripped a 2x4 into a couple of 1/2" thick strips which I screwed to the outside of the feet. This was done to make storage of the unit easy. I cut some flat 'hooks' from scrap 7/16" OSB on the band-saw which I screwed to the exposed joist in the basement. These went right up against the main duct-work trunk so the unit can hang against the duct and unexposed. Important simply so you don't slam your head into it.

I've yet to use the unit as I don't actually have ant material to steam at the moment. I think that the next time I may try some different wood, depending on what I can come across. Maybe some maple. I'm not sure how well oak will bend though I've seen plenty oak furniture with loops built into it. Hopefully it all works out. Pretty simple design so I'm sure that it will.


Anonymous said...

I did a project with pvc and steam a few years ago. I had problems with the pvc becoming too soft at elevated temperature and switched to lexan. Be cautious around the melting point.

Yvette Scheffel @ ALB Industrial Supplies said...

"Captured steam is really, really dangerous." -- With this in mind, I have to say that I'm quite impressed that you took on such a project when there are various steam traps readily available in the market. The question left now is whether or not your device works the way you want it to. Have you tried it out yet?

mkr said...

Finally had the chance to use the steam chamber. Had trouble getting it up to temp (212 degrees goal) so had to insulate it and close the vents down some. It is not sealed and the fittings are not glued together. Additionally, I have vents built in that I can open up or close down. With the vents all closed it built enough pressure to push the cap off. With the vent open and insulated I generated just over 200 degrees, which was enough to bend the 3/16" thick ash and maple stock I had.