"The Wheal Towan engine, doing 87 millions, had 1248 feet of tube fire-surface, and a similar amount of external boiler surface in the flues. 2½ bushels of coal were consumed each hour, giving about 1000 feet of fire-sides for each bushel of coal consumed per hour, and 50 feet of fire-bars. Those boilers were intended to supply steam for working the engine at ten strokes a minute; a bushel of coal an hour would in that case have had 600 feet of boiler fire-surface.
"Binner Downs 70-inch cylinder, 10-feet stroke, did 41 millions. A fire was then put around the cylinder and steam-pipes, which burnt 5 bushels of coal in twenty-four hours, by which the duty was increased to 63 millions. The surface sides of the cylinder, cylinder-top, and steam-pipes heated by flues was 300 feet, and caused a saving of 41 bushels of coal in twenty-four hours. Another engine in the same mine was tried, having a 42-inch cylinder; when the fire was around the cylinder, she worked 100 strokes without injection-water; the expansion-valve was closed at half-stroke, the steam in the boiler 56 lbs. on the inch above the atmosphere."
It is not easy to deal with the important reasonings flowing from those facts, and influencing the form and economy of the steam-engine, nor to show if Trevithick was right in discrediting the laws laid down by Watt. Newcomen's engine had the interior, as well as the exterior of the steam-cylinder exposed to the cooling atmosphere. Watt, by putting a cover on the cylinder, reduced the loss of the heat from the interior, and by his steam-case hoped to reduce the loss from the exterior, though by it he increased the amount of surface exposed to the cold. In Trevithick's early engines the boiler alone exposed heat-losing surface, and this was further reduced by its own comparatively small size, the engine and boiler complete not exposing one-quarter of the surface of a Watt low-pressure engine of equal power. One object of the Binner Downs experiment was to further curtail this loss of power by increasing the heat of the steam while in operation in the cylinder, since called superheating steam.
This principle of giving increased heat to steam, after it had left its state as water, was made practical by Trevithick's boiler at Wheal Prosper in 1810, where the flues having first been carried around the water portion of the boiler, then passed over the steam portion;[166] and again in the upright boiler of 1815, having the upper end of the fire-tube surrounded by steam above the water line.[167] Those early beginnings of superheating steam and surface condensation culminated in the Binner Downs experiments of 1828, one immediate practical result of which was the tubular surface condenser, enabling steamboat boilers to avoid, in a great measure, the use of salt water, facilitating in a marked degree the application of marine boilers and engines with steam of an increased pressure.
The Binner Downs engine, with a cylinder of 70 inches in diameter, and a stroke of 10 feet when working with steam in the boilers of 45 lbs. to the square inch above the atmosphere, and using the heating flues around the cylinder, required 13 gallons of injection-water at each stroke, and consumed at the rate of 3 bushels of coal an hour, to produce a duty equal to eighteen millions; by removing the cylinder superheating flues, the quantity of injection-water for the same amount of work increased to 15½ gallons, and the coal to 4¼ bushels. Watt's rule for his low-pressure steam vacuum engine doing a duty of eighteen millions, gave 57 gallons of injection-water, and 11¼ bushels of coal.
On the question of coal, this statement agrees very nearly with Trevithick's letters of sixteen years before, when he used the high-pressure boilers in the Dolcoath pumping engine,[168] promising that his high-pressure expansive engine would do the work with one-third of the coal required in the low-pressure vacuum engine.
The high-pressure steam required a less amount of injection-water to condense it than the low-pressure steam, in proportion to the work done, showing the Watt rule and the Watt experience to be inapplicable to high-pressure engines; for instead of 57 gallons of injection-water the Binner Downs engine with steam of 45 lbs. to the inch required but 15½ gallons of injection-water, and this amount was further reduced to 13 gallons by superheating the steam; this roughly agrees with the coal consumed, or in other words, with the amount of heat to be carried off by injection-water: the Watt rule giving 11¼ bushels as the fair allowance for low-pressure steam vacuum engines, while the high-pressure steam vacuum engine burnt but 4¼ bushels. This was further reduced to 3 bushels by superheating. Those facts led to the idea that if the steam pressure was sufficiently increased, condensation might be carried out without any injection-water, by the transmission of the heat in the steam through the metal sides of the condenser. An experiment was at once made by removing the Watt condenser and injection-water, as he had done seventeen years before,[169] using in their stead a thin copper surface-condenser immersed in cold water, producing, within 1/2 lb. on the inch, as good a vacuum as when injection-water was used, leading to the conclusion,—
"It is my opinion that high steam will expand and contract with a much less degree of heat or cold, in proportion to its effect, than what steam of atmosphere strong will do. I intend to try steam of five or six atmospheres strong, and partially condense it down to nearly one atmosphere strong, and then by an air-pump of more content than is usual to return the steam, air, and water back into the boiler again, and by a great number of small tubes, with greatly heated surface sides, to reheat the returned steam."
This, in practical words, is the surface condenser by which the used steam is returned to the boiler in the form of water. The more general use of high-pressure steam of 70 or 90 lbs. to the inch, increasing its expansive force on one side of the piston by superheating it on its passage through numbers of small tubes, and decreasing its expansive force on the other side of the piston by cooling it in passage through similar tubes exposed to cold, is partly effected in steamboats, but has not yet been attempted in engines on the road.
After a month's further consideration he wrote:—