Fig. 3267.
[Fig. 3267] represents the arrangement of Field tubes in a boiler.
A boiler of this form may for a given capacity be made lighter and smaller than in any other of the ordinary forms, while the rapid circulation acts to keep the tubes clean.
The inner tubes may be thin, because they are under pressure both inside and out, while the outer tubes may be thin, because they are under a bursting strain, whereas a fire tube is under collapsing pressure.
Fig. 3268.
A design of high rate boilers, in which the uptake does not come into contact with the water, and water tubes are employed, is shown in [Fig. 3268]. In the fire box is an inclined tube which promotes the circulation, and is very effective heating surface, and in the combustion chamber are a number of vertical water tubes.
Two manholes give access for cleaning purposes.