When first designing this engine, I did not know how much power I might require from it. I thought that in some cases it might be necessary to allow the high-pressure steam to enter the low-pressure cylinder direct, but as this would involve a considerable loss, I constructed a species of an injector. This injector may be so adjusted that when the steam in the boiler rises above a certain predetermined point, say 300 lbs. to the square inch, it opens a valve and escapes past the high-pressure cylinder instead of blowing off at the safety valve. In escaping through this valve, a fall of about 200 lbs. pressure per square inch is made to do work on the surrounding steam and to drive it forward in the pipe, producing a pressure on the low-pressure piston considerably higher than the back pressure on the high-pressure piston. In this way a portion of the work which would otherwise be lost is utilised, and it is possible, with an unlimited supply of steam, to cause the engines to develop an enormous amount of power.
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Boiler Experiments.—The first boiler which I made was constructed something on the Herreshoff principle, but instead of having one simple pipe in one very long coil, I used a series of very small and light pipes, connected in such a manner that there was a rapid circulation through the whole—the tubes increasing in size and number as the steam was generated. I intended that there should be a pressure of about 100 lbs. more on the feed water end of the series than on the steam end, and I believed that this difference in pressure would be sufficient to ensure a direct and positive circulation through every tube in the series. This first boiler was exceedingly light, but the workmanship, as far as putting the tubes together was concerned, was very bad, and it was found impossible to so adjust the supply of water as to make dry steam without overheating and destroying the tubes.
Fig. 90.—Steam boiler employed in my experiments. With this boiler, I had no trouble in producing all the steam that I could possibly use, and at any pressure up to 400 lbs. to the square inch.
Fig. 91.—The burner employed in my steam experiments. This produced a dense and uniform blue purple flame 20 inch deep.
Before making another boiler I obtained a quantity of copper tubes, about 8 feet long, 3⁄8 inch external diameter, and 1⁄50 of an inch thick. I subjected about 100 of these tubes to an internal pressure of 1 ton per square inch of cold kerosine oil, and as none of them leaked I did not test any more, but commenced my experiments by placing some of them in a white-hot petroleum fire. I found that I could evaporate as much as 261⁄2 lbs. of water per square foot of heating surface per hour, and that with a forced circulation, although the quantity of water passing was very small but positive, there was no danger of over-heating. I conducted many experiments with a pressure of over 400 lbs. per square inch, but none of the tubes failed. I then mounted a single tube in a white-hot furnace, also with a water circulation, and found that it only burst under steam at a pressure of 1,650 lbs. per square inch. A large boiler, having about 800 square feet of heating surface including the feed-water heater, was then constructed. It is shown in [Fig. 90]. This boiler is about 41⁄2 feet wide at the bottom, 8 feet long and 6 feet high. It weighs with the casing, the dome, the smoke stack and connections, a little less than 1,000 lbs. The water first passes through a system of small tubes—1⁄4 inch in diameter and 1⁄60 inch thick—which were placed at the top of the boiler and immediately over the larger tubes—not shown in the cut. This feed-water heater is found to be very effective. It utilises the heat of the products of combustion after they have passed through the boiler proper and greatly reduces their temperature, while the feed-water enters the boiler at a temperature of 250° F. A forced circulation is maintained in the boiler, the feed-water entering through a spring valve, the spring valve being adjusted in such a manner that the pressure on the water is always 30 lbs. per square inch in excess of the boiler pressure. This fall of 30 lbs. in pressure acts upon the surrounding hot water which has already passed through the tubes, and drives it down through a vertical outside tube, thus ensuring a positive and rapid circulation through all the tubes. This apparatus is found to work extremely well. A little glass tube at the top provided with a moving button, indicates exactly how many pounds of water per hour are passing into the boiler. By this means, the engineer is not only enabled to ascertain at a glance whether or not the pumps are working, but also to what degree they are working.
Water may be considered as 2,400 times as efficient as air, volume for volume, in condensing steam. When a condenser is made for the purpose of using water as a cooling agent, a large number of small tubes may be grouped together in a box, and the water may be pumped in at one end of the box and discharged at the other end through relatively small openings; but when air is employed, the tubes or condensing surface must be widely distributed, so that a very large amount of air is encountered, and the air which has struck one tube and become heated must never strike a second tube.
In order to accomplish this, I make my condenser something in the form of a Venetian blind, the tubes being made of very thin copper and each tube in the form of a small aeroplane. These were driven edgewise through the air, so that the actual volume of air passing between them is several thousand times greater than the volume of water passing through a marine condenser. I find that with such a condenser I can recover the full weight of the copper tubes in water every five minutes, and if I use aluminium, in half that time. Moreover, experiments have shown that a condenser may be made to sustain considerably more than its own weight and the weight of its contents in the air, and that all the steam may be condensed into water sufficiently cool to be pumped with certainty.