Most modern boilers used in power generation roles are water tube boilers. With a water-tube design, water tubes are heated until the desired steam is produced. Such boilers have two key advantages: safety and they can often handle higher pressure.
A book published in 1901 simply titled Water Tube Boilers provides some insight into the many different designs and the frequent failures of water tube boilers. This book's copyright has long expired and the entire book is available here, compliments of Google Books.
Water tube can raise a head of steam quickly; and that feature alone can be compelling in many applications.
More than 100 years ago, a boiler called the Yarrow boiler became popular especially in marine applications. After reading about them in the book noted above and various web sites, I decided to use my CAD software to make a similar, model-size boiler. This CAD model isn't complete just yet, but it will give you the essential idea of the Yarrow boiler.
The vegetable burner shown under the model here is one designed to use wicks to help burn ordinary vegetable oil. I've built a similar burner and I've started construction on the model shown here.
Pat J commented, on November 18, 2011 at 5:15 p.m.:
I don't think you want to use a flat plate on a boiler unless you use stays.
The flat plates you have on the lower drums don't appear to have stays, and I believe these flat plates will flex and crack, as it the reason flat plates were not used on boilers witout stays.
The water tubes entering the flat plates will not stop the plates from flexing.