Nicholson's Thermic Syphons.

The application of these syphons (Illustration No. 5) increases the circulation of water throughout the boiler, forming a large, valveless pump, which is automatically operated by the application of heat. The speed of the water in the syphons reaches 5ft. per second, and the syphon functions from the time the firing starts.

Circulation is induced in the following manner:— Water evaporating inside the syphons from the effect of heat applied in the zone of highest temperature, forms a succession of steam bubbles rising from the walls of the syphon. As these are disengaged from the walls they accumulate in large quantities in their passage upward through the body of water within the syphon. The expanding of each bubble and the accumulation of bubbles displaces solid water, which creates a constant unbalanced “head” of water in the barrel of the boiler, due to its greater weight per cubic foot as compared with the much lighter combination of steam and water within the syphon. The action of the force of gravity causes the heavier water in the barrel of the boiler to rush into the syphon through the opening in the lower part of the flue sheet. The power of this pumping, or syphoning, action will be better understood when it is considered that a cubic inch of water, when expanded into steam at 200lbs. pressure, occupies 115 cubic inches of space.

The great features embodied in the adoption of these syphons are:—Increase in firebox heating surface; constant and rapid circulation of water; equal temperature balance maintained through boiler; flues, stay bolts, etc., relieved of unequal strain caused by temperature variations, and a saving of from 10 per cent, to 12 per cent. in fuel is effected on account of increased heating surface and increased steam capacity.