4th. No surfaces to be used which necessitate the use of stays.

5th. The boiler supported independently of the brickwork so as to allow
freedom for expansion and contraction as it is heated or cooled.

6th. Ample diameter of steam and water drums, these not to be less than
30 inches except for small size units.

7th. Every part accessible for cleaning and repairs.

With these points having been determined, No. 20 was designed. This
boiler had all the desirable features just enumerated, together with a
number of improvements as to detail of construction. The general form of
No. 15 was adhered to but the bolted connections between sections and
drum and sections and mud drum were discarded in favor of connections
made by short lengths of boiler tubes expanded into the adjacent parts.
This boiler was suspended from girders, like No. 15, but these in turn
were carried on vertical supports, leaving the pressure parts entirely
free from the brickwork, the mutually deteriorating strains present
where one was supported by the other being in this way overcome.
Hundreds of thousands of horse power of this design were built, giving
great satisfaction. The boiler was known as the "C. I. F." (cast-iron
front) style, an ornamental cast-iron front having been usually
furnished.

[Illustration: No. 21]

The next step, and the one which connects the boilers as described above
to the boiler as it is built to-day, was the design illustrated in No.
21. These boilers were known as the "W. I. F." style, the fronts
furnished as part of the equipment being constructed largely of wrought
iron. The cast-iron drumheads used in No. 20 were replaced by
wrought-steel flanged and "bumped" heads. The drums were made longer and
the sections connected to wrought-steel cross boxes riveted to the
bottom of the drums. The boilers were supported by girders and columns
as in No. 20.

[Illustration: No. 22]

No. 22. This boiler, which is designated as the "Vertical Header" type,
has the same general features of construction as No. 21, except that the
tube sheet side of the headers is "stepped" to allow the headers to be
placed vertically and at right angles to the drum and still maintain the
tubes at the angle used in Nos. 20 and 21.

[Illustration: No. 23]

No. 23, or the cross drum design of boiler, is a development of the
Babcock & Wilcox marine boiler, in which the cross drum is used
exclusively. The experience of the Glasgow Work