1. They generally occupy more space and require more masonry than
ordinary boilers.
2. On account of the small quantity of water which they contain, sudden
fluctuations of pressure are caused by any irregularities in supplying
the feed-water or in handling the fires, and the rapid and at times
violent generation of steam causes it to accumulate in the contracted
water-chambers, and leads to priming, with a consequent loss of water,
and to overheated tubes.
3. The horizontal or inclined water tubes which mainly compose these
boilers, do not afford a ready outlet for the steam generated in
them. The steam bubbles cannot follow their natural tendency and rise
directly, but are generally obliged by friction to traverse the tube
slowly, and at times the accumulation of steam at the heated surfaces
causes the tubes to be split or burned.
4. The use of water which forms deposits of solid matter still further
increases the liability to overheating of the tubes. It has been
claimed by some inventors that the rapid circulation of the water
through the tubes would prevent any deposit of scale or sediment in
them, but experience has proved this to be a grave error. Others have
said that the expansion of the tube would detach the scale as fast
as it was deposited and prevent any dangerous accumulation, but this
also has been proved an error. Again, the use of cast iron about these
boilers has frequently been a constant source of trouble from cracks,
etc.