Steam bending oak. ??

[marcros]

...


So there you go, I wonder what reaction there will be but I think I already know.

yawn?

 

[9fingers]

So there you go, I wonder what reaction there will be but I think I already know.

yawn?

Yes that is usually the reaction when a sound scientific explanation is given.

We will soon have a society when virtually no one knows why anything happens.

Bob

 

[Teckel]

Don't know anything about it except that steam is hotter than boiling water, which maybe makes the difference.

ahahahahaha, that's one of the stupidest things you've ever said and let's be honest, you've come out with a lot.

Steam could be hotter if it's pressurised, but so would the water, e.g. a pressure cooker.

It probably is the temperature that makes the wood soften along with being damp but you have to have a method of transferring the heat from the source to the wood. Air isn't great at transferring heat, better than nothing obviously, but water has much more ability to absorb heat and therefore carry it from the source to the wood.

Aidan

Is there a need to be so condescending towards other forum users???
I don't think so...Jacob is wrong that's fine but don't be so sarcastic about it. We are all here to learn and everyday is a schoolday. It doesn't take much to be civil.

 

[Sgian Dubh]

It probably is the temperature that makes the wood soften along with being damp but you have to have a method of transferring the heat from the source to the wood. Aidan

I'm not interested in all the shenanigans going on about pressurised this or that, supersteam, or whatever, but I think you've just about summarised what happens Aidan, but I'll add a bit to it.

It's the lignin in the wood that needs to soften so that bending can occur. Lignin is a complex substance, but it's often described simply as the 'glue' that binds the wood fibres together. Soften the lignin by heating it one way or another, quickly bend the wood whilst the lignin is soft, and when the lignin cools it will hold the wood in its new shape, more or less. Steam is used to fairly efficiently transfer the heat to the wood's core, and steam prevents the wood burning as it's heated which it would do so if the heat was dry. Small bits, as has been demonstrated in another post about a stringed instrument, can be heated with dry heat. There's always a need to half expect a surprise when steam bending wood, eg, you can see springback and even 'spring in' where the wood takes on a tighter curve than expected when removed from the former or cooling jig.

In most cases the best compromise for steam bending is air dried wood at about 22%± MC. Green or wet wood bends more easily because it's relatively flexible anyway being green, but I think the heat from steaming transmits more efficiently too because of the water already in the wood, but it can distort more as the wood dries down to interior MC levels, although this is much less of a consideration if the end location of the finished piece is outside. Wood kilned to 7% MC (the North American standard) is much harder to steam bend. Dry wood is significantly stiffer than green wood, and once it's developed this stiffness it will never again be as flexible as it was when it was green freshly harvested wood. So air dried wood is generally considered to be the best compromise for steam bent interior parts. Some drying has occurred so if the wood shows problematic drying faults at this stage it can be rejected prior to steaming, but it's not so stiff that it won't bend if prepared properly.

I can only guess how many times I've seen inexperienced people try to bend kiln dried timber of any sort and end up with near 100% failure thus wasting all their wood. Those that pay attention to my suggestions and guidance go and find a source of straight grained air dried wood, and they end up with something like eighty percent of their bends working out okay or better. Slainte.

 

[Jacob]

....

PV=cT or PV/T=c as our such learned chum says.

Yes well done!

.......The only way to increase the temperature would be to keep that volume the same and increase the pressure (i.e. a pressure cooker)... that's that one dealt with.

Wrong again. Once the water vapour has separated from the water it can be heated (depending on the configuration of the apparatus) to any temperature, but still remain at atmospheric pressure. Look at the equation and you might be able to work it out for yourself, but I doubt it..
Hint - the volume and the temperature increase but the pressure remains the same. Have a go at working it out. NB water vapour (not steam) obeys Boyle's Law just like any other gas.

So getting back to the beginning, I wonder if a steam (strictly speaking "water vapour") apparatus might produce higher temperatures than boiling alone. It certainly could do but whether or not this is a factor I do not know.

PS
sorry about this - I can see the fascination of being a primary school teacher! But where Tiddles is wrong is in thinking that as the heated water vapour expands it cools, by virtue of expanding. It doesn't. If it did it'd shrink again and hot air balloons would never fly.
Simple mistake really and quite understandable. 5/10 Tiddles - a good effort. :lol: I hope all that writing didn't tire you out!

PPS I just edited a bit - I understood it when I wrote it but it's gone in the meantime!

 

[dickm]

Oh heck................. why did I start this????????????

 

[Jacob]

Oh heck................. why did I start this????????????

No worries! it's all good and harmless light entertainment!

 

[Harbo]

Like making a good cup of tea at high altitudes?

 

[Andrewf]

At least no tool chatter is involved in this topic.

 

[Phil Pascoe]

.......or A2, O1.........

 

[Jacob]

Don't know anything about it except that steam is hotter than boiling water, which maybe makes the difference.

My fault - I should have said "water vapour can be hotter than the boiling water, depending on the apparatus" etc etc

 

[Teckel]

Where is General Custard gone to sort this steam bending out.
Cussssssssssstaaaarrrrrrrrdddddd are you there?????

 

[Tony Spear]

Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air. At lower pressures, such as in the upper atmosphere or at the top of high mountains water boils at a lower temperature than the nominal 100 °C (212 °F) at standard temperature and pressure. If heated further it becomes superheated steam.

In other words "steam" as most people know it only exists in the brief phase when water vapour is cooled and therefore condenses and returns to the liquid phase.

Theoretically therefore, visible "steam" would be fractionally cooler than the boiling fluid from whence it came!

To get water vapour hotter than 100 C. it has to be contained and therefore the "mist" that we call steam no longer exists.

As I said before, the basic principle of Boyle's Laws means that the vapour will attempt to expand, but as it is contained, the pressure will increase and big bang eventually follows! Alternatively of course, if the container is allowed to expand (e.g. piston in cylinder, the vapour will expand as it tries to get back to atmospheric pressure.

I realise that we've all been banging our heads against a brick wall with Jacob so far and as far as he's concerned nobody else can win and as I know what I know, I'll not bother with this thread any more. (hammer)

 

[Jacob]

......
Theoretically therefore, visible "steam" would be fractionally cooler than the boiling fluid from whence it came!

Visible "steam" does not come from boiling water - it comes from invisible water vapour condensing to form water droplets. It starts life as invisible water vapour.

You haven't heard of latent heat?
Theoretically (visible) steam won't have a lower temperature than (invisible) water vapour (either side of the point of phase change) but will be a lot "cooler" i.e. contain less heat. Nothing fractional about it. Get yer brain round that one if you can!

Come to think it's the latent heat which makes the steam more effective than boiling water as a heat conveyor, which answers the OP (probably). i.e. At boiling, at 100ºC, steam (water vapour, not condensing visible steam) is a lot "hotter" (contains more heat) than water at 100ºC.

If in doubt don't start shouting at me - just look up "latent heat"! It's not my fault you nodded off during physics lessons!

 

[Teckel]

All I wanted to know was "does it bend better with steam or actually putting it down in boiling water.."
I didn't want any science behind it..
Jesus this is gone off on an tangent....close the thread I'm sick to the diddies of it....

 

[Jacob]

Steam!

 

[Sgian Dubh]

All I wanted to know was "does it bend better with steam or actually putting it down in boiling water.."

Thick stuff, eg, anything above about 4 or 6 mm bends better if it's steamed. Thin stuff, below about 4 mm can often be bent successfully if dunked in boiling water for a while. When steaming, common guidance is to allow one hour per 25 mm (1") of thickness in the steamer to get the centre of the wood hot enough to bend successfully. This is only a guide, and times vary quite a bit according to a range of factors, eg, the species, the MC of the wood, etc. Things like stringing or thick veneers at about 2- 3 mm thick and/or square can frequently be bent successfully with dry heat.

I hope that answers your question, and I'm sorry if I offended you by going off on a bit of a tangent earlier about lignin and so on. I'm guessing you probably don't want to hear about plasticising wood by shoving it into a container that is able to withstand high pressure and subjecting it to gaseous ammonia... ha, ha... ha, ha, ha. Slainte.

 

[marcros]

I'm guessing you probably don't want to hear about plasticising wood by shoving it into a container that is able to withstand high pressure and subjecting it to gaseous ammonia... ha, ha... ha, ha, ha. Slainte.

Could be an interesting thing for a new post though?

 

[Sgian Dubh]

I'm guessing you probably don't want to hear about plasticising wood by shoving it into a container that is able to withstand high pressure and subjecting it to gaseous ammonia... ha, ha... ha, ha, ha. Slainte.

Could be an interesting thing for a new post though?

Nah, I'm not going to get into it I'm afraid. Ammonia bending is mostly of interest to researchers-- wood scientists and the like, and only companies with a big budget to set up the process, with appropriate procedures and systems in place to cover H&S issues, are likely to investigate using ammonia to create bent wood forms. Slainte.

 

[thick_mike]

Right, s*d theory, just done an experiment. Flask of water heated from below with a bunsen burner, fed through a still head into a small cardboard box. Temperature measured in the water, at the top of the still head and in the box:

Temperature of boiling (distilled) water - 100C
Temperature of steam at hottest point - 103C
Temperature in the "steam box" - 90C

It is perfectly possible to heat water vapour above 100C without pressurisation as long as the walls of the heated container are above 100C. The vapour will not escape immediately and so will be heated in the vapour phase.