LECTURE XL

CONTENTS.-Temperature and Pressure of Steam-Marcet's BoilerGraphic Curve of Pressures and Temperatures-Mercurial Pressure and Vacuum Gauges-Bourdon's and Schäffer's Pressure and Vacuum Gauges Pressure Pyrometer or Thalpotasimeter.

Temperature and Pressure of Steam.-When water is confined in a closed vessel, and heated, the pressure of the vapour contained therein continually increases. The precise temperature which corresponds to any particular pressure, has been made the subject of very careful inquiry by Regnault and others.

We shall now illustrate these phenomena by means of a simple apparatus, termed Marcet's boiler. On applying heat from the Bunsen burner, B B, steam is generated from the water, W, and the temperature as it rises is noted by the thermometer, T. Simultaneously, the column of mercury rises in the tube, and the height from the free surface of the mercury may be read off (roughly) on the graduated scale, G S. Since the tube, GT, is open at the top, when the temperature has arrived at 233° F., the mercury will be observed to have risen about 15 inches, corresponding to a pressure of 7'4 lbs. above the atmosphere, or a total pressure of 22 lbs. per square inch. This is usually termed 22 lbs. absolute.* When the temperature arrives at 250° the mercury will have risen to about 30 inches, corresponding to a pressure of 147 lbs. on the square inch (1 atmosphere), or 294 lbs. absolute (i.e., 14.7 lbs. above the atmosphere + 147 for the atmospheric pressure).

If our glass tube had been longer, and the supply of mercury in the bottom of the boiler sufficient, we might have gone on applying heat and registering still higher pressures with their corresponding temperatures, but the limited experiment has been

* Absolute pressures, are pressures reckoned from a perfect vacuum as o or zero; whilst ordinary pressures, as indicated by steam gauges, &c., are pressures reckoned from the atmospheric pressure at the place as zero, and they, therefore, require about 15 lbs. to be added to them in order to convert them into absolute, or total pressures. In all questions relating to the temperature and expansion of steam, absolute pressures are used in this book.

sufficient to show roughly, that a rise in temperature cannot take place without a corresponding rise in pressure. Mercurial gauges, such as that in the Marcet's

boiler, were much used to register the pressure of steam in steam boilers, before the introduction of the Bourdon gauge.

The following curve illustrates the way in which the pressure of saturated steam rises with its temperature. It is plotted out from the table in Lecture XII.

When any two physical conditions (such as the temperature and the pressure of

steam in the case we have been considering) vary with respect to each other, it forms a useful exercise, as well as impresses the fact upon the student, if he plots out graphically the corresponding quantities to scale. In the next

MARCET'S BOILER.

figure, the absolute pressures are divided off to scale on the vertical line from 15 to 100 lbs., while the corresponding temperatures are pitched to scale on the horizontal line. The intersection of the horizontally drawn dotted lines and the ver tically drawn dotted lines from each of the corresponding quanti

H

ties gives us points on the curve. If these intersections are now joined by a firm line we obtain the desired curve.

66

66

Pressure Gauges.-Instruments for indicating the intensity of the pressure of a fluid contained in a closed vessel, are called pressure gauges,' or vacuum gauges," according as they register how much the pressure is above or below that of the atmosphere. As we shall see later on in the case of the "Indicator," that instrument is adapted for registering pressures both above and below the atmospheric pressure.

The Mercurial Pressure Gauge, as seen by the following figure, consists of a bent, U, glass tube, containing mercury, from O to O, round the bend of the tube. One end is connected directly to the closed vessel, or say to a steam boiler, while the other end is connected to a cup, to prevent the mercury being lost when the pressure rises higher than the range of the tube. This cup is open to the air, and consequently the pressure of the atmosphere acts on that side of the mercurial column. A vertical scale is fixed immediately behind the vertical limb connected to the boiler or closed vessel, and it is graduated in any convenient manner-say, for lbs. per square inch of pressure. As the pressure increases, the mercury in this limb is depressed, and rises correspondingly in the other limb. When the pressure in the closed vessel equals that of the atmosphere, both free ends of the mercury should stand at o. The reading on the first following cut shows a pressure of 39 lbs. Nothing could be simpler or more accurate than this arrangement, for, as we saw in the case of the Marcet's boiler, a vertical column of mercury produces a definite pressure of about 1 lb. per square inch for every 2 inches in height. In practice, however, the inside of the glass tube gets coated with a dirty film, owing to the oxidation of the mercury, which prevents the attendant observing the exact position of the depressed end of the mercurial column.

Open to

Air

Such a pressure gauge is, of course, inadmissible on board a ship or on a locomotive, owing to the jerking motion; and further, the length of the tube would have to be very great for the pressures now carried in highpressure steam boilers (about 300 inches, or 25 feet for 150 lbs. on the square inch). For these reasons its use has been discarded in ordinary practice; but, as an exact and standard instrument for scientific purposes, and for testing and graduating the working pressure gauges (which we are about to describe), it is indispensable. In all the best

works where ordinary pressure gauges are made and tested, there is, therefore, fixed a long vertical mercury column or gauge, with which these may be compared, and there the inside of the glass is occasionally rubbed clean by a little cotton-wool dipped in sulphuric acid, and fastened to the end of a wire.

Mercurial Vacuum Gauge. -This gauge indicates directly the absolute pressure inside a vessel such as the condenser of a steam engine, the suction pipe to an air-pump, or the vacuum pan of a sugar-refinery. The simplest form in which it is made is illustrated by the second figure. It consists of a vertical glass tube a little over 30 inches in length, with its lower end open and dipping into an iron pot containing mercury, while its upper end is attached to a brass cock and pipe connected with the vessel or condenser. A scale is fixed behind the glass tube,

PRESSURE GAUGE.

VACUUM
GAUGE.

graduated on the right hand into inches, and on the left hand into millimetres, but it would be more convenient if this latter scale were divided so as to show the absolute or the back pressure in lbs. per square inch due to an imperfect vacuum. The more perfect the vacuum, the higher the mercury rises in the tube, and every 2 inches of rise corresponds to a diminution of about I lb. of back pressure.

It does seem absurd that we should thus continue to register pressures in three or four different ways.

1. In lbs. per square inch above the atmosphere—e.g., in the case of the pressure of steam in a boiler.

2. In inches of mercury from atmospheric pressure downwards, towards a perfect vacuum, or in lbs. per square inch below atmospheric pressure—e.g., in the case of ordinary vacuum gauges.

3. In lbs. per square inch reckoned from a perfect vacuum, or what are termed lbs. per square inch absolute-e.g., in the case of the back pressure during exhaust of a condensing engine.

If we universally adopted the last of these methods, there would be no confusion, and only one way of reckoning pressures—viz., from absolute zero. Condenser vacuum pressures would then range from 0 to 15 lbs., and boiler pressures from 15 lbs. upwards.

Bourdon's Pressure and Vacuum Gauges.-Steam pressures in boilers or pipes are usually indicated by Bourdon's pressure gauges, and negative or vacuum pressures in condensers, &c., by Bourdon's vacuum gauges, or by instruments of somewhat similar design and construction.

The construction of Bourdon's pressure gauge is clearly shown by the figures on the opposite page. Figure C shows the internal mechanism in its earliest form; the small figure, D, to the right, shows a section of the Bourdon tube; and the upper figures, A and B, show a sectional elevation plan and front view of a modern high-pressure gauge, as made by Schäffer and Budenberg, having a quadrant wheel and pinion arrangement, connecting the pointer, P, with the tube, T. The action of the gauge is as follows (see Fig. A): The steam, gas, or water enters by the cock shown in connection with the gauge to the curved metallic tube, T, of hard brass or steel, whose upper end is hermetically sealed or closed. The cross section of this tube, being of a flat, oval form (with its greatest breadth fixed perpendicularly to the direction in which the tube is curved), tends to become more and more circular in section, the greater the pressure within it (above the atmosphere), and consequently, at the same time, the curve of the tube tends to straighten out, thus pulling the link, L, upwards, the motion of which is transmitted to the quadrant, Q, and thence to the toothed wheel, TW, fixed on the same axis as the pointer, P, which latter moves across the scale.