WATER TUBE TYPE OF BOILER.
The special difference between the fire tube boiler and the water tube boiler is that in the former the fire passes through the tubes, while in the latter the water is in the tubes and the fire passes around them.
FIG. 6. FREEMAN VERTICAL BOILER.
In this type of boiler there is an upper cylinder (or more than one) filled with water; a series of small tubes running at an angle from the front or fire door end of the upper cylinder to a point below and back of the grates, where they meet in another cylinder or pipe, which is connected with the other end of the upper cylinder. The portions of the tubes directly over the fire will be hottest, and the water here will become heated and rise to the front end of the upper cylinder, while to fill the space left, colder water is drawn in from the back pipe, from the rear end of the upper cylinder, down to the lower ends of the water tubes, to pass along up through them to the front end again.
This type of boiler gives great heating surface, and since the tubes are small they will have ample strength with much thinner walls. Great freedom of circulation is important in this type of boiler, there being no contracted cells in the passage. This is not adapted for a portable engine.