These articles have not been read before the Institute, but are inserted by direction of the Executive Committee.
Different Types of Circulating Pumps for Surface Condensers.
Few who have experience in the care and management of, the old-fashioned circulating pumps driven directly from the steam piston rods, and with independent pumps as now applied, will hesitate in their preference for the latter.
The following comparison of the two systems as applied to the Alaska and Despatch may be of interest to some:
| Alaska | Despatch |
Number of cylinders | 2 | 2 |
Diameter | 50” | 33” |
Stroke of Piston | 42” | 33” |
Diam. of circulating pump | 18” | Centrifugal, driven by idenpend’t steam cyl. 12in. di. and 8in. st., 130 re. per m. |
Stroke of circulating pump | 42” | |
Indicated horse power | 523.8 | 542.1 |
Pressure at release, (absolute) | 13 lbs. | 17 lbs. |
Mean back pressure | 3.25 | 6.2 |
I.H.P. of pump cards | 35.09 | --- |
I.H.P. of independent engine | --- | 2.29 |
Per cent. of power absorbed by circulating pump | 6.69 | 0.41 |
Square feet of condensing surface | 2465 | 1165 |
Cross area through tubes | 188sq in | 198 sq in |
Temperature of injection | 64o | 45o |
Temperature of discharge | 84o | 79o |
Temperature of feed | 136o | 120o |
Foot lbs. of work expended on pump per thermal unit removal | 5.93 | 0.42 |
In the case of the Alaska, only the work done on the water is accounted for, so that really 35.09 I. H. P. should be increased to about 42. I H. P., which would show still worse for the Alaska. On the other hand the Alaska was running at half power and the Despatch at full power.
Charles H. Manning,
P. A. Engineer, U. S. N.
A New Speed Indicator.
The speed indicator consists of a combination of three independent pus, viz, an instrument for creating a vacuum, a long flexible tube for communicating the vacuum to a gauge on deck, and the vacuum gauge itself. The instrument for creating the vacuum (in itself old) is founded on the principle discovered by Bernoville in the sixteenth century, that water flowing through a cylindrical mouthpiece contracts its vein at a distance of about one-half the diameter of the cylinder from its entrance, and that at the point of greatest contraction a partial WWII= is produced. Further, that the vacuum varies as the square of the velocity of the water, and that it is immaterial whether the water. flows through a stationary tube, or whether the tube is hauled through the water, the latter remaining stationary.
To obtain a more perfect vacuum, I use, instead of an ordinary cylinder, a combination of conical tubes about one foot long, made of sheet brass, shaped like a dice-box, and so constructed that a high velocity is produced at the contracted part. Around the circumference of the tube, at equal distances from each other, are three small brass pipes which enter the vacuum instrument at the most contracted part, whence they converge symmetrically to a point some distance in front of the mouth of the tube and in the prolongation of its axis.
The vacuum instrument is kept at a mean depth in the water by means of a float; which is towed by a wire leading from a collar on the flexible tube; the float is over the instrument and connected to it by means of a wire. The flexible tube is of hard rubber, three-eighths inch external and one-eighth internal diameter. To protect it from chafe and wear, fine copper wire is braided over the outside and blended with it by heat. Through the centre is rove a stout copper wire making fast to the ferules by which the sections of the tubing are joined together; by this means the tubing is made as strong as an ordinary log-line. The length of the tubing varies with the vessel; for yachts fifty feet are required; for boats, fifteen feet; for small steamers, one hundred feet, and for large steamers, two hundred feet. The tubing may be astern or alongside; but it must be rigged out from the side so as to be clear of the eddies.
The flexible tube fits over the top of the gauge on deck; the latter may be of any type, but the mercurial gauge is preferable for many reasons. The glass tube of the mercurial gauge is drawn down to a very fine opening to throttle and regulate the fluctuations of the mercury; the gauge is suspended like an ordinary sea barometer, and fitted so that when not in use the tube is protected from injury.
In all instruments hitherto, in which the speed has been indicated by a vacuum, the vacuum instrument has been rigidly connected to the vessel. The objections to this are that the ship by friction always carries along with it a large body of water, and in rolling the tube is carried to different depths; both causing incorrect indications. There is also the practical objection that if the instrument becomes fouled it is difficult to get at it to clear it.
In using the indicator, the vacuum instrument is thrown overboard and the flexible tube paid out from a reel on deck; when it is out to its full extent, the strain is taken by a wire extending from a collar on the tube near the rail to a stanchion on deck. The resistance offered by the instrument being small, it does not come to the surface, while its conical opening and the flow of water through it serve to steady it. The tube is then connected to the gauge, when the mercury will immediately rise till it attains a height due to the speed of the vessel through the water.
When tried on board the Alarm, with a speed of eleven and three-quarter knots (the highest attained), the height of the mercurial column was nine inches; the fluctuations amounted to only one quarter of a knot at the most, and generally the mercury was' steady. No doubt with better apparatus the results will be improved. The higher the speed with the same fluctuations, of course the smaller the error on account of the increased distances on the scale. In smooth water the instrument will show a speed of one knot, equal to a height of one-eighth of an inch of mercury. The advantages of its use in sailing vessels are especially apparent, as the effect of any sail may be known in a moment.
Chief Engineer Isherwood, in a personal report to me, signed also by Chief Engineers Zeller and Magee (the experimental board), says, "We are of opinion that this instrument, when properly constructed and adjusted and correctly graduated, will give the vessel's speed reliably and to a closer reading, owing to the sensibility of the mercurial column, than any of the patent logs known to us. It is so simple that its use may be entrusted to the least intelligent, and there is nothing in its construction liable to derangement. The above statements are warranted by the practical test to which we saw the instrument subjected on board the Alarm during the experimental trials of that vessel in the Hudson river."
W. S. Hogg,
Ensign, U. S. N.