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Engineering, February 10, 1888, p. 142
We have, on one or two former occasions, illustrated triple expansion engines manufactured by the well-known Central Marine Engineering Company, of West Hartlepool, and in the present issue we are again enabled to put before our readers an engraving taken from a photograph of a very interesting set of engines manufactured by the same firm, and fitted on board the s.s. Maine. This vessel is a large Atlantic cargo steamer, built by Messrs. William Gray & Co., to the order of Messrs. Hooper, Murrell, and Williams, of the Atlantic Transport Line of steamers, and was originally christened Swansea. The management of the line being subsequently turned over to the Baltimore Storage and Lighterage Company, whose London agents are Messrs. Williams, Torrey and Field, the name of the vessel was changed to Maine, so as to be in keeping with the names of the Maryland, Montana, and Missouri vessels, built and engine by the same firms for the Baltimore Storage Company.
Referring to our engraving on page 142 and beginning at the base, we may mention that these engines have the bedplate all in one piece and that the surface is planed all over in the machine so as to enable the engines to be truly erected on the special iron erecting tables, which are in use at the Central Engine Works. The fore and aft condenser at the back of the engine has cast with it three back columns carrying the main guides and supporting the cylinders, the latter being supported in front by three substantial wrought iron columns, set at an angle so as to counteract any tendency to vibration and made with flanges both top and bottom, so as to have bearing surfaces and prevent any bedding into the metal due to end pressure. This arrangement appears to gove the maximum stiffness with exceptional lightness and openness in front of the engine, and we are assured that when running at 63 revolutions per minute, and indicating 1400 horse-power, those engines were throroughly rigid, showing no signs of undue vibration. There are six main bearings all lined throughout their whole surface, as is the practice with these builders, with white metal. The crankshaft is of the built description, the body parts and pins all being of forged steel hammered from the ingot. The shaft is in three sections bolted together by flanges and tapered steel bolts, each section being precisely like its fellows and also equal ended, so as to be both interchangeable and inter-reversible.
The valve gear is of the double bar link motion type made adjustable for wear and for length. The high pressure valve is a piston valve constructed on a very simple plandeveloped by Mr. Mudd, the managing engineer of the Central Engine Works, and is so designed as to obviate all the ordinary objections that have been brought forward against piston valves. It is made without any springs or loose rings of any kind, and is therefore incapable of wearing barrel-shaped, as is often the case with the ordinary piston valves. It is capable of a slight amount of adjustment in diameter, and is found to polish itself and its chamber perfectly bright all round : it is very light to drive and the wear on the valve itself has been found to be only 1/160 the of an inch in the diameter of the valve in twelve months continuous work. This valve and its liners are made of a special mixture of hard durable irons, and out of more than twenty that have been fitted by the Central Marine Engineering Company on this principle, not one has ever given the least trouble or anxiety to the engineers in charge. The mean pressure and low pressure valves are of the ordinary slide valve type and are fitted with double and triple steam edges. The engines are reversed by steam, the reversing engine being conveniently placed on the condenser at the back and working on "all round" gear, the hand wheel for hand reversing and also for the steam handles for reversing by steam being placed in front of the engines, together with the throttle and starting valve handles. This reversing engine is itself reversible and is fitted with a dial and indicating pointer at the front of the engines, so that when reversing rapidly by steam the engineer can see by the pointer precisely in what position his radius links are. All the hand gear, including drain-cock and main engine stop valve, is concentrated on the centre column, where are also the pressure gauges indicating the boiler pressure and the pressure of the various receivers and the vacuum.
We now come to speak of an important feature in point of efficiency in these engines, and that is the special arrangements carried out in connection with the cylinders. It is well known that one of the greatest stumbling blocks in the efficient use of high pressure steam is the loss arising from the excess of condensation during admission to the high pressure cylinder over re-evaporation throughout the later stages of expansion, and whilst doctors differ as to whether this great difficulty can be best overcome by jacketing the high pressure cylinder so as to minimize the initial condensation, or by jacketing the low-pressure cylinder so as to augment the re-evaporation, the designer of these engines has decided on taking a midway course, believing the truth lies somewhere between the two, and that it is possible to get what there is of good in both these methods by a very simple device. Objections have been raised against the jacketing of either the high or the low-pressure cylinder-against the first because the range of temperature in each cylinder in a triple expansion engine is small, and that it has never been clearly proved experimentally that in such a case jacketing reduces initial condensation to any very great extent-and against the second because during one half the stroke the cylinder is common with the condenser, and the heat that is during that period passing from the jacket into the cylinder is in effect only passing into the condenser where it is worse than useless. We ourselves do not hold this latter view, but it is one which has numerous supporters. There is not likely, however, to be any serious objection raised against jacketing the receivers themselves instead of the cylinders, as these receivers are the most consistent feeders of the cylinders following, and it is on this principle that the engines we are here describing are constructed. It has been held, and probably rightly so, that one of the principal causes of the superiority in point of economy of the compound engine is the fact the re-evaporation in the receiver is utilized in the next cylinder : and it seems reasonable that any augmentation of this receiver re-evaporation will be beneficial, because the heat so imparted to the steam is immediately utilized in the cylinders, and not during any portion of the stroke directly thrown away into the condenser.
A glance at our engraving will show those who are acquainted with marine engineering that the high-pressure cylinder of the engines of the Maine is in the middle between the other two, and that all the valves are between the engines, the outer ends of the cylinders being rod and smooth without any protuberances or gear attached thereto whatever. This system of having the high-pressure cylinder and all the valve castings and receivers in the middle part of the cylinder structure, is of itself a great step on the road to economy, as radiation is thereby reduced to its minimum. The arrangements, however, to which we have above referred are as follows : The high pressure cylinder id fitted with a separately cast head liner, and the space between this and the cylinder barrel forms a steam jacket at full boiler pressure : outside of this cylinder barrel there is a third casing of metal divided by cross divisions in such a way that that the high pressure valve casing practically forms a superheating jacket for the first receiver, which almost entirely encircles it, leaving the outer surface of the high pressure cylinder jacket to form a superheating jacket for the second receiver. In other words, the jacket forms a annular layer of stration of high pressure steam, the inner surface of which is preventing the loss of heat from the high-pressure cylinder, and the outer surface of which is giving out heat to the receiver of the low-pressure cylinder, all steam passing from the mean pressure exhaust to the low-pressure seam chest being thrown against the two opposing sides of this superheating jacket, and so dried on its way to the low-pressure cylinder. When the diagrams from these engines are correctly combined on a theoretic curve, they clearly show the effect of these intermediate superheating arrangements, as will be seen by reference to to the set of diagrams we publish above, where it will be observed that the expansion part of the low-pressure diagram comes fully up to and indeed rather overruns the curve of adialatic expansion traced down from the quantity of steam shown on the high-pressure diagram, inclusive of the clearance steam, and that the mean pressure diagram bulges cut a great way beyond this same curve. The lower curve indicates the theoretic expansion adialantically of the quantity of steam received at each stroke from the boiler in the condition of steam, and it is found that the area of the three actual diagrams is 88 per cent. Of the area contained between this lower curve and the vertical line at the left of the diagram and the lines between the top and bottom boundary, the actual limits of pressure registered on the diagram.
These figures appear to indicate a high efficiency of the steam, and the Central marine Engineering Company are to be congratulated on the great success they have obtained. During construction the machinery of the Maine was under the inspection of Mr. A. E. Allen, of Hull, the superintendent engineer to both the Baltimore Storage Company and the owners of the Atlantic Transport Line.
The Maine carries over 4000 tons deadweight across the Atlantic at 10 ½ knots per hour, on 20 tons Welsh coal per day. Her engines are 24 ½ in., 40 in., and 63 in. in diameter and 42 in. in stroke, with 100 lb. boiler pressure. They have now been at sea seven months, have cost nothing at all for repairs, and have not given a moment's trouble or anxiety to the engineers in charge.
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