# Steam

## pressure, temperature, heat, water, cubic, lbs, volume, latent, nearly and ft

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When a cubic in. of water is converted into steam at the ordinary pressure of the atmosphere, its volume is increased to 1.645 cubic in.—i.e., a cubic in. of water becomes nearly a cubic ft. of steam of one atmosphere. If the steam is produced at any greater pressure, its volume will be very nearly inversely as that pressure; at two atmospheres, it would occupy about 855 cubic in.; at four atmospheres, about 457 cubic inches.

When water is boiled in an open vessel, neither the temperature of the 'Ater nor that of the steam rising from it ever rises higher than 212°, however hot the fire; the heat as It enters is carried off in a latent state in the steam. But under pressure, the tempera ture of both can be raised to any degree. If, when the water and steam in a, fig. 1, came to 212°, the application of heat were still continued, more steam would continue to rise, and the pressure on the under side of the piston being now greater than that of the air above it, the piston would begin to ascend; but suppose it held in the same position. by force, the upward pressure of the steam would be found rapidly to increase, until it would soon require a weight of 14.7 lbs. per sq.in. to keep it down, showing that the pressure of the steam was now equal to twice that of the atmosphere, or to 29.4 lbs. per su.ineh. If at this point the temperature of the water and steam were examined, it would be found to be very nearly 250° Fahr. When the absolute pressure of the steam reached 50 lbs., its temperature would be 281°; at 100 lbs., at 150 lbs., 360'; and so on.

From the numerous experiments made on this subject some very important general conclusions may be drawn. Of these, one—which will be evident from the tigures just given—is, that the pressure of steam increases at a far higher rate than the temperature (doubling the temperature increases the pressure nearly 23 times), which shows the extreme danger o' continuing to apply heat to a vessel from which the steam is not allowed to escape. The bursting force would soon become such as no vessel could resist.

Another general eonclusiou of great importance is, that for every temperature there is a corresponding density of steam produced. This steam contains a fixed amount of latent heat, and exerts a certain uniform pressure on every side of any vessel in which it may be contained. The following table shows the relation between these values fat steam of several different temperatures: • T. p. H. v. v.

32° 0.085 1091.8 3390.0 211,516 104° 1.00 1113.7 312.8 19,519 158• 4.51 1130.1 8302 4,993 212• 14.7 1146.6 1,645 248° 28.33 1157.5 14.0 874 293• 60.4 1171.2 6.992 430 3560 145.8 1190.4 3.057 191 401° 250.o, 1204.1 1.338 115 1', Temperature in degrees Fahrenheit. This corresponds to the sensible heat of the steam. p, Pressure in pounds per sq.ln. of the steam at that temper...tire.

'total heat of the vapor above 32° Fahr. at that temperature (according to Regnault's hypothesis') in thermal units. A thermal unit (772 foot-pounds) is the quantity of heat which will raise 1 lb. of water 1° Fahr. at or near its temperature of greatest density. 39.1° Fahr. The specific heat of water increases slowly as the temperature rises, so that 1 thermal unit will not raise 1 lb. of water

quite so nm], as 1° at 14.1;;h temperature; but for the purposes of this article we need not take this Into account.

V, Volume in cubic ft. occupied by 1 lb. of steam. ' v, Number of times which volume of steam exceeds that of same weight of water.

The relations between temperature and pressure in the foregoing table apply only So long as the steam is in contact with the water from which it is generated. Once away from the water, its temperature may be raised without altering its pressure. Steam which has received additional heat in this way is called superheated steam. It approxi mates to the condition of a perfect gas, and therefore follows nearly what is known as Boyle's or Mariotte's law (q.v.); its volume varying always inversely as its pressure. By this law, steam which occupied 1 cubic ft. at 20 lbs. absolute pressure, would occupy 4 cubic ft. at 5 lbs., and half a cubic ft. at 40 lbs. absolute pressure.

Steam, however, as commonly used in the steam-engine, is not superheated, but used under the conditions given in the table. It is then called saturated steam, and differs sensibly from the condition of a perfect gas. If the pressure (p) be given in pounds per sq. in., and the product (in) of pressure and volume iu foot-pounds, then the formula, log. (pr) = 4.675 + .061 log. p,* gives results accurate enough at all ordinary pressures, and can be very easily applied. The vtlume, instead of increasing inversely as the pressure, increases less rapidly; the difference, though not very great, is large enough to be taken into account in all calcu lations as to the efficiency and behavior of steam in a steam-engine.

Another fact regarding the constitution of steam requires attention; from its impor tance in point of economy. It would naturally be expected that it would take much more heat or fuel to convert a pound of water into steam at is higher than at a lower tem perature and pressure. In reality, however, the difference is very slight. Referring back to the table, it will be seen that it requires 1146.6 units of heat to raise a pound of water from 32° to 212°, and evaporate it at that temperature; of these, 180 are expended in raising the temperature, while 1146.6-180, or 966.6 units, become latent iu the steam. It only requires 1171.2 units, however (261 sensible, and 910.2 latent), to raise the water to 293°, and evaporate it at that temperature; for the latent heat falls nearly es fast as the sensible heat rises. The additional heat required• is thus only a little over 2 per cent, while the pressure—which is, saris paribus, a measure of the work time steam will do—is more than quadrupled. In this way, a large increase of power in any engine may be obtained by a small additional expenditure of fuel, and consequently steam of a high pressure is now being used for all purposes, its economy and advantages being fully recognized by engineers. It was thought for a long time that the total heat of steam— that is, the sum of the sensible and latent heats—was constant at all temperatures; but this is not strictly the ease, although the table shows that the difference, for ordinary ritflges of pressure, is but trifling. See IltAT and STEAM-ENG1NE.

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