The Locomotive - Lighthouse Survival Blog

1901.1 THE LOCOMOTIVE 125
one vessel was fitted with compound engines, another with high pressure condensing
engines, and the third with low pressure condensing engines. The ratio of expansion
in the compound engine was a little over G, in the high pressure engine, 3.5, and in the
low pressure about 3. The water per indicated horse power per hour was, for the compound
engine, 18.38, high pressure engine, 23.9, and for the low pressure engine, 2G.9.
The steam pressure for the compound and high pressure tests was sixty-seven pounds,
and for the low pressure thirty-two pounds.
What may be called the compound period in marine engineering extended up to
about 1883, and during the twenty years that it lasted the efforts of many talented en-
gineers were devoted to improvement, with the result that there was very decided pro-
gress. Even here, however, the lesson which it would seem Mr. Isherwood had taught
in the Michigan experiments, that expansion cannot advantageously be carried beyond a
moderate amount, had to be learned again. Many of the compound engines were fitted
with ingenious adjustable cut-offs to enable the steam to be used with a very hio-h degree
of expansion. As steam pressures increased, the ratios of the cylinders became
greater, and it was found that in most cases sufficient expansion could be obtained by
the use of the link motion without any additional cut-offs. As a result in the later
compound engines with steam pressures of one hundred and twenty pounds, and havinoa
cylinder ratio of four and one-half, all independent cut-offs were abandoned. The
splendid steamers of the Guion Line, which were the pioneers in fast trans-Atlantic
service, and the later Cunarders, furnished fine examples of the best compound engines.
While steam jacketing had been used to some extent with simple engines, its use
only became general after the compound engine had been well established. Experi-
ments showed very clearly the decided gain in economy from the use of the jackets
and they were also very beneficial in warming up for preventing sudden strains due to
variation of temperature. Doubtless in many cases where there was not careful atten-
tion paid to draining, the full benefit to be derived from their use was not obtained;
but when good automatic traps were fitted so that the jackets were really filled with
steam and not with water, their action was very beneficial.
Reference was made earlier in the lecture to the loss from blowing off. This began
in the days of jet condensers, because the condensation by direct mixture made the feed
practically salt water. There was unfortunately a curious misunderstanding of the for-
mation of boiler scale, so that even after condensing engines became universal, blowing
off was still resorted to. When it was finally found that blowing off did not prevent the
formation of scale, this practice was stopped and the density was allowed to become as.
great as five times that of ordinary sea water instead of about twice. In more recent
days, since the invention of the evaporator, nothing but fresh water is fed into the
boilers. I may remark that the evaporator is merely a small boiler using steam instead
of heated gases for evaporating the salt water. As there must inevitably he a deposi-
tion of scale, the evaporator is so constructed that it has a nest of tubes which can
readily be withdrawn and the scale removed from the tubes. As the loss by blowing
off was often as high as ten per cent., it will be seen that there was a distinct gain in
economy when this practice was abandoned.
The great increase of steam pressures and the improvement in materials led progres-
sive engineers to see that the next step was along the same lines, and as the temperature
ranges with the cylinders of the compound engine had become as great as they previously
were in the simple engine, the next step was obviously expansion by three cylinders in-
stead of two, which meant the triple expansion engine. The credit for this is due to
Dr. A. C. Kirk, the engineering head of Napier & Sons iu Glasgow. There was no