steam gun

[Badger]

   The microwave topic got me thinking about my steam gun project which has been on the drawing board for so many years now. I have always wanted to build a closed container like a large pipe filled with water and equiped with at least two pressure reliefe valve for safety and a very small hose comming off it for spot steaming bows, making curves etc. Not sure how hot the steam would be if under about 60# pressure but would be easy to look up and probably act a lot like dry heat but no scorching. A long pipe turned into a pressure cooker is another thinh I have wanted to work on, these things can be very dangerous if precautions are not used though. Steve

[Justin Snyder]

I have plans to make a long pipe pressure cooker using pressure regulating parts from an actual pressure cooker.  I will also put a valve that I can quick release the pressure.  Once the pressure is out I hope I can remove the lid safe and quickly so I can pull the wood out and bend it while still hot.  I have tried to use my wife's steam cleaner but it doesn't produce the kind of volume we need.  Justin

[islandpiper]

My pressure cooker runs with up to 20 lbs pressure , I would stay way back, WAY BACK, from anything with 60 lbs unless a certified boiler inspector had seen it first and told me to fire it up.   Seriously. 

Men are cut in half with steam leaks at high pressures. 

Microwave:   ends only.......no pressure. 

But, I agree, if you could heat the whole stave to the temps that 60# will give you it would be a noodle when you took it out.   You'd have 45-seconds to get it into the form, maybe a bit more, and drop all the clamps on it and man, you could write your name with a 2x4!   ( if you have a short name)

piper

[El Destructo]

60 pound Steam would be approximately 307 degrees F........we run 300 pound(421deg) and 600 pound (488 deg)Steam in the Refinery I work in....we use the Low Pressure Steam in a 55 gallon Barrel full of water to boil Oyur Deer Heads and other Trophies.....as long as the Company dont find them..... ........I wonder what a piece of Osage would look like after a couple Minutes in 600 pound Steam at 488 degrees............

[islandpiper]

I gave my handbook away when I moved south so I can't look up the numbers any more.   I thought my 20# pressure cooker ran at 280 F or so.  Seems like 60 would be more.  I'm still pretty careful around steam at most any pressure.  Lots more energy than boiling water. 

To paraphrase an old saw: there's old bowyers and there's bold bowyers, but there's dam few OLD BOLD bowyers. 

piper

[El Destructo]

                        http://www.indpipe.com/images/PDF/steam_temperature_pressure_table.pdf

[GraemeK]

Hi Steve

60 psi is definitely 307 deg F.  
The problem with using 300deg F to steam bend wood is that  the max temp wood can stand without any detremental  change is 150 deg F and it is considered that at 195 deg F even for short periods that you get cellular damage and significantly reduced strength ( Bootle et al. 2005)

I guess what you need to keep in mind is that the lignin that you want to soften so you can bend the wood becomes plastic at about 140 deg F and the biggest difficulty is getting the core of the wood to the same temp as the outside because it is good insulator -- so my money would be steam at about 150deg F and enough time to let it heat soak evenly which means the outside can not get over the 150 which is considered safe

Graeme

[Justin Snyder]

My pressure cooker runs with up to 20 lbs pressure , I would stay way back, WAY BACK, from anything with 60 lbs unless a certified boiler inspector had seen it first and told me to fire it up.   Seriously. 

piper

Your pressure cooker is also aluminum.  I have no intention of using aluminum.  Justin

[Badger]

Graeme, that is good info! I had no idea those temperatures damaged the wood. Could dry heat possibly harden the lignin back up?  Steve

[David Long]

I've thought a little about this type of device, and I've concluded that perhaps it is better to build an oven for baking the bow. If I could fit my wood in our oven, set the temp at 200F, 220F or whatever, and keep it in long enough for temperature to be that high in the middle of the wood, I'd be happy. Wouldn't a nice long skinny oven be a good way to go? Dave

[Badger]

David baking belly lams is something I think would be worthwhile. Heat applied to a backing does more harm than good. I think I would bake at about 3oo till wood had a nice light brown look to it. Steve

[Pat B]

I don't like the idea of using steam on dry wood without sealing the wood from the moisture first. I know shellac will work as a sealer even when using steam. I personally would rather use dry heat(heat gun) and protect the wood with veg oil. I also think the oil helps distribute the heat evenly and help to hold the heat longer...and the heating process takes less time with a heat gun.     Pat

[Badger]

Pat, I tend to agree with you for the most part, I haven't used steam in over 5 years now. But after I retire next year money will be tighter and I am thinking I would have more staves to choose from if I wasn't so fussy. I am mostly thinking about straightening stves that would otherwise be junk. Steve

[Pat B]

Steve, That's about all I have to work with anyway...staves that would otherwise be junk! ...and my heat gun cost $5 at a garage sale many years ago. I can usually get tips hot enough to straighten in about 5 minutes and if I need to work the entire stave(near bow size) it usually takes about 30 minutes. After I have the stave relatively straight and clamped to the caul I re-heat the entire stave and let it rest until tomorrow. Sometimes I have to make slight adjustments after that but not too often.
   When I was steaming wood it would take 30 to 45 minutes to get the wood hot enough but it would cool down quickly so I could only do a small section at a time. I have spent a few days with steam when I can get the same results with a heat gun in an hour or a bit more...and I don't have to let the wood dry out for a week or more.     Pat

[GraemeK]

Hi Steve

The way it works is that lignin is an organic polymer and when you heat it a little you make it plastic by releasing some of the bonds which reestablish when cooled. If you heat it a lot like when you heat treat a belly you do a few  things - firstly you drive off organic volatiles that would normally not be removed except over decades of aging   secondly you cause cross linking of molecules within the lignin similar to decades of aging and some that would just not happen ever at ambient temperature and thirdly you cause oxidation reactions which can  be seen externally as darkening of the wood or charring. All these tend to make the wood more brittle and less elastic but stronger in compression and as long as you only have a thin layer on the belly and the rest of the bow is still unaffected and elastic the you generally improve the performance.
It I was intending to fix a bunch of staves I would build a skinny oven and use dry heat at about 150 deg F - the heat soak time is the important thing and my guess is it probably take the best part of an hour for a largish stave to be heated right through -- you would need to do some testing with a electric thermometer like you can get for baking which you could put the probe in the center of a stave by drilling a hole to check the times. I can not see any advantage to using steam as the heating medium unless you are working with wet wood where you are trying to keep the humidity high to regulate the drying rate. With dry wood it really just causes  problems  by raising the MC in the outer layers and causing the grain to open and check -- also it is more difficult to build and potentially dangerous.

Graem