Steam in kiln

[Coltbodi]

I am going to be building a kiln and have been looking at different designs online. I have noticed that some large kilns add steam. Why is this? They have heaters, so the steam isn't to heat the kiln. If you are trying to dry the lumber why would they add moisture to the air?

[Kbeitz]

Use dry steam...

[Brad_bb]

I'm no expert, but I think it has something to do with relaxing tension in the outer fibers.  They steam, then they dry.

[DaveinNH]

One purpose I have heard about, is steaming walnut to darken the sapwood.

[Kbeitz]

I do know that super heated steam is dry steam...

[boardmaker]

The steam is injected at the end of the cycle to relieve the tension between the outer layers of the lumber and the core.  When lumber is dried the outer layer dries faster than the core.  At the end of the process the lumber gradient between the core and shell(outer) is equalized by injecting steam for a period of time to raise the kiln humidity(wet bulb).  Each kiln has their own schedule or recipe but that's the basis of it.

Most kilns that have steam available also use it to heat as well.  You have a steam radiator(heat exchanger) in the kiln to transfer the heat.

[boardmaker]

In response to Dave:

Some companies also inject steam into a walnut chamber to darken the sapwood of walnut.  It leaves a nice uniform chocolate color to the walnut.  I've never seen this done in a dry kiln although from appearance a steamer does resemble a kiln.  The steaming process would be extremely hard on the dry kiln chamber.  Anything that's not stainless or aluminum will experience rapid deterioration.  And I do mean rapid.  If you left a standard grade 5 steel bolt in the chamber, it would be gone without a trace in a month give or take.  I've never actually tried it, but I have seen the damage after just 4 days.  Concrete doesn't hold up well either.

The process isn't so much about the steam injection.  You really just need to get the walnut above 200 degrees for several days and keep the humidity as close to 100% as humanly possible.  It will lighten the heartwood slightly, but your sapwood will be a nice chocolate color.  It also makes the boards much more uniform in color. 

Some love it, some hate it.  I like walnut both ways.  They both have their place.

[Coltbodi]

Oh ok. That makes since. It will be easy to add a steam line in since I will be heating my kiln with a boiler and radiator.