not absolutely necessary, but they can do no harm if kept clean and in working order. 3. On High-pressure Steam worked expansively. The working of steam expansively is one of the most important subjects to which the engineer can direct his attention. It involves considerations for enquiry into the elementary laws of our present practice, and it leads to a wide field of investigation relative to the varied forms and conditions of any future improvement which we may be called upon to adopt. The difference which exists between high and low pressure steam is, according to our present ideas, the measure of its elasticity and temperature when taken at the extremes at which it is worked, viz. from 10 to 150 lbs. on the square inch. inch. When the steam impinges upon the piston at 10 lbs. on the square inch, it generally follows up the supply and pressure continuously throughout the whole length of the stroke, or nearly so; but when steam of increased density is used, say 50 lbs. upon the square inch, then instead of allowing the steam to act with a constant pressure throughout the whole of the stroke, the communication with the boiler is, at some particular point of the stroke, suddenly intercepted, and the steam, thus cut off, is left to perform the remaining portion of the stroke by its elastic force. In this case, the steam dilates or expands as the piston moves forward, and consequently acts with a constantly decreasing pressure upon the piston, until it arrives at the end of the stroke. This is technically called 'working steam expansively.' Let us now examine the subject more minutely, and endeavour to ascertain the relative values of the two systems of high and low pressure steam. It will be found, that in working high pressure steam expansively, the advantage is greatly in favour of that process, as compared with the non-expansive principle. E C A! FIG. 15. 8 10 20 19 40 D If we take a cylinder, such as is shown in the annexed diagram, of any given diameter, say 5 feet long, and divide it into five equal parts, as represented by the lines CD, EF, GH, we shall then have the different equidistant positions of the piston as it ascends from the bottom to the top of the cylinder. Let us now H suppose the space ABCD to be filled with steam from the boiler at a pressure F of 40 lbs. upon the square inch, and that with this force the piston is moved from AB to CD, a distance of 1 foot. Now it в is obvious, if we cut off or interrupt the flow of steam at this point, that the next foot of the stroke, that is, from CD to EF, will double the space occupied by the steam; and there being no further supply from the boiler, the steam will have to expand itself into double its original volume, and its pressure by this dilatation will be reduced from 40 to 20 lbs. on the square inch. The piston having arrived at EF, with the force thus reduced to 20 lbs. on the square inch, it again moves forward another foot, that is to GH, where the original space occupied by the steam becomes enlarged three times, with a proportionate decrease of pressure in the steam, that is, with a pressure of one-third of 40 lbs., or 134 lbs. acting upon the piston, and so on to the other points of the stroke. The pressure of the steam at the successive equidistant intervals of the stroke will be as follows: 40 lbs. 20 lbs. 134 lbs. 10 lbs. and 8 lbs. These pressures, derived from the law of Marriotte, are no doubt slightly in excess, inasmuch as the vapour suffers a loss of temperature upon expansion. The deductions to be made for this loss of heat and loss of pressure in the process of working can be ascertained by the indicator; but for our present purpose it will be sufficient to assume that there is no loss. In order to find the work performed by the steam in one stroke of the piston, we shall first find the mean pressures of the steam acting through the successive intervals of the stroke; and then from these mean pressures we shall find the total mean pressure. Pressure in the 1st foot of the stroke =40 lbs. Mean pressure in the 2nd foot of the stroke =(40 +20 )=30 =(20 +13) = 162 the mean pressure by the length of the But this Now, when the steam acts uniformly throughout the whole of the stroke, the work = 40 x 5 = 200. work is done with 5 times the quantity of steam that is employed when acting expansively; therefore the work done by an equal quantity of steam is the 5th of 200, or 40. Comparing the numbers 1073 and 40, we find that the steam used expansively performs 2 times the work that it does when it is used non-expansively, or with a constant pressure. This simple method of calculating the work performed by the expansion of the steam gives the result a little in excess. The following method, depending upon Thomas Simpson's rule for finding the area of irregular curved surfaces, is more exact.* * Rule. - To the sum of the extreme pressures (per square inch) add four times the sum of the even pressures, and twice the sum of the odd pressures; then this sum multiplied by one-third the distance between the consecutive points at which the pressures are taken, will give the work done expansively on 1 inch of the piston in one stroke. Work done expansively = {40+9+4(20+ 10) + 2 x 13}} Work done before the steam is cut off = 40 × 1 = 40. ... The total work in 1 stroke 65+40 which corresponds very nearly with the work as before found. = 105, = 65. In this calculation some allowance must be made for the loss of heat, and consequently the loss of pressure, during the process of expansion, which may be ascertained by diagrams taken from the indicator. This loss of heat by expansion is much greater than is generally imagined, as we seldom find general practice to agree with deductions derived from theoretical calculations. In this short Lecture, it will not be necessary to give further examples. I have sufficiently demonstrated the advantages peculiar to high-pressure steam when worked expansively, and it now only remains for me to direct your attention to the closing part of the subject, which demands the most careful attention, as all our instructions and attempts at economy will prove fruitless unless they are supported by a steady and efficient course of management. 4. On Management. It has ever been the province of the philosopher and man of science to investigate and elaborate, for the good of mankind, all those physical and mathematical truths which bear upon the wants of civilised society and the development of those laws which, through a succession of ages, have been handed down to us. These truths have been still further extended by the inventions and discoveries of the mechanician and those men of practical science whose lives have been devoted to the pursuit. To the researches and labours of those benefactors of the human race, we are indebted for most of the comforts and enjoyments we now possess: but these are of no avail unless properly used, and carefully managed; and it is to the management of one of these ingenious discoveriesthe safe and economic production of steam-that I would, in conclusion, direct your attention. To the combined discoveries and inventions of the mechanic and man of science, we are indebted for the steam-engine; and it remains with the possessor to determine to what extent he will make it safe and efficient; for in the management of so docile and so powerful an instrument depends its security as well as its effect. In the faithful discharge of this very important duty, many circumstances concur to render the uses and appliances of steam-power profitable and secure; and I avail myself of this opportunity to enforce upon your consideration the following suggestions, which, if carried into effect, will doubtless secure to the owners the most important and satisfactory results. In the steam-engine, the boiler is the source of all power, and the quantity of work performed depends upon the quantity of water evaporated and the quantity of fuel consumed. Its generative powers, and the way in which those powers are used, are therefore matters of considerable importance; and those who would work with economy, will require to attend to two things-the perfect combustion of the fuel on the one hand, and the transmission as well as the retention of heat on the other. In a wellmanaged concern, we never hear of safety valves and feed-pumps being out of order: there is no tampering with such vital organs of safety; everything is in its place, and the self-acting movable parts of the apparatus, such as valves, stuffing-boxes, and bearings, are kept in the most perfect order, well oiled and cleaned, so as at all times to be ready and fit for service. In the steamengine also the same regularity and system of management are preserved, and the result is a ponderous - |