It is now readily seen that in general three definite and distinct considerations present themselves in the solution of problems involving the computation of total heat. The first instance is one in which the steam exists in a dry state and at the temperature and pressure at which it is generated from the water. Such steam is known as dry saturated steam. The second instance is that in which the steam is not completely dry, but holds in suspension small particles or globules of water, and in this instance the mixture is known as wet steam. The third instance is of especial importance in modern central station practice and involves what is known as superheated steam. In this case the steam is first formed by evaporation from water into dry saturated steam, after which it is conveyed through pipes that are exposed to high temperatures, thus causing the temperature of the steam to be still further raised, although the pressure practically remains constant.

The complete solution of these three instances for computation of total heats will be found in the chapter on Quality of Steam as stated above. Meanwhile the thorough mastery of the fundamentals of the physical properties of water as herein set forth will be of vast assistance in a clear understanding of this later discussion.

Examples

1. The water entering a feed-water heater is at a temperature of 75°F. and leaves the heater at 190°F., what is the heat absorbed per lb. of water? From the steam tables the heat of liquid at 75°F. is 43.05 B.t.u. and at 190°F. it is 157.91 B.t.u. Hence the heat absorbed per lb. of water is 114.86 B.t.u.-Ans.

157.91

43.05

=

2. Water enters a boiler at 160°F. and is converted into dry saturated steam at 200 lb. pres. per sq. in. abs., what is the total heat required to evaporate each lb. of steam?

The heat in the entering water at 160°F. is from the steam tables 127.86 B.t.u. The total heat of dry saturated steam at 200 lb. pres. abs., is 1198.1 B.t.u. Hence the actual heat necessary to evaporate each lb. of steam is 1198.10 127.86: 1070.24 B.t.u.-Ans.

3. If the heat of liquid for boiling water at 212°F. is 180 B.t.u. and the latent heat of evaporation is 970.4 B.t.u., how much heat is required to evaporate a pound of water from an open water heater which is receiving its supply at 64°F.?

Each lb. of water entering at 64°F. has a heat of liquid of 32.07 B.t.u. Water evaporating into steam at 212°F. has a heat of liquid of 180 and a latent heat of evaporation of 970.4 B.t.u., making a total heat of evaporation of 1150.4 B.t.u. for every lb. of water so evaporated. Hence the net heat required is

CHAPTER VII

THE STEAM TABLES

FIG. 36.-The book of steam tables.

T has already been shown that since no simple mathematical laws have as yet been devised to express the temperature, pressure, latent heat, heat of liquid and other fundamental properties of steam and water that are absolutely necessary in the solution of steam engineering problems, we must resort to carefully compiled steam tables.

Practically all the research and scientific investigation along the lines of pure steam engineering of the last half century have been devoted to the more complete establishment of some of the fundamental constants involved in the steam tables.

The three most important of these are the zero point of the absolute temperature scale, the proper value for a constant employed in the conversion of mechanical energy into heat energy, and the exact determination of the heat required to evaporate. one pound of water from 212°F. into dry saturated steam at 212°F.

Since these values are continually found by more careful and exacting experimental work to be slightly different than formerly held, we find that the steam tables of recent publication are different than those of former years.

The Steam Tables as Adopted in this Discussion.-The Steam Tables and Diagrams as computed by Marks and Davis and published by Longmans, Green & Company, are today universally recognized and are adopted as the standard compilation for the problems cited in this discussion.

In the rear of these steam tables an interesting discussion of the methods employed by these investigators in arriving at the

three fundamental constants mentioned above is given. The result of these investigations shows that the absolute zero is to be taken at 459.6°F., the mechanical equivalent of heat at 777.5, and the latent heat of steam at 212°F. to be 970.4 B.t.u.

Recapitulation of Fundamental Evaluations. These three constants are so important that they should be memorized and for emphasis let us recapitulate their exact interpretation.

The absolute zero is now found to be a point situated at 459.6° F. below the zero point on the Fahrenheit scale or 491.6°F. below the freezing point of water. At such a temperature it is supposed to be impossible to further draw off heat from any substance for at this temperature the heat storage is supposed to be absolutely exhausted.

The mechanical equivalent of heat as given above means that the energy represented by one B.t.u. or British thermal unit of heat is equivalent to 777.5 ft. lb. of mechanical energy. Or if one pound of crude petroleum contains 18,500 B.t.u., it possesses as stated in a previous chapter, sufficient energy to raise a human being weighing 175 lb. a vertical upward distance of over 18 miles.

The latent heat of steam at 212°F. and atmospheric pressure means that the quantity of heat necessary to evaporate one pound of water at 212°F. into dry saturated steam at 212°F. is found experimentally to be 970.4 B.t.u.

Analysis of a Typical Page of Steam Tables.-Let us now proceed to analyze a page of Marks & Davis' steam tables, column by column. The illustration as given is found on page 12 of this compilation and we shall follow across the page the line corresponding to a temperature of 231°F.

Temperatures in Fahrenheit Units. Since all steam engineering computation is based on temperatures represented in the Fahrenheit scale instead of the Centigrade system, the temperatures are here listed in the Fahrenheit units.

Pressures in Absolute Notation. This column means that the pressures here given represent the pressure in pounds per sq. in. at which water will boil when the temperature is that as listed in the first column. Further on in the steam tables an exactly similar table may be found to the one cited except in this latter instance the pressures are made to vary pound by pound and the corresponding boiling temperature of water given.

In this instance, then, we read that a pressure of 21.16 lb.

per sq. in. will be produced before the water boils or the formation of steam begins at 231°F. This pressure, by the way, is in absolute units and would not be the pressure read on the steam gage of a boiler room. Since the steam gage indicates

pressures

above

FIG. 37.-A typical page from the steam tables.

the atmosphere, one must subtract from this reading in the steam tables the atmospheric pressure of the day in order to find the proper gage pressure. Thus, in this instance, if the atmospheric

FIG. 38.-Marks & Davis method of collating data for specific heat of water from three noted investigators.

pressure of the day be 14.7 lb. per sq. in., a steam gage in a boiler room would read 6.46 lb. per sq. in., when the water in the boiler is 231°F.

This precaution is most important and the student should carefully reread the former chapter on pressures if he does not thoroughly understand the conversion of gage pressures, inches of vacuum, inches of mercury, etc., into standard absolute pressure units.

Pressures in Atmospheres. In many engineering computations pressures are given as so many atmospheres instead of pounds per square inch. The pressure of the standard atmosphere is usually taken as 14.7 lb. per sq. in. but for very exact work it is more accurately 14.696 lb. per sq. in. Hence this column is computed by dividing each item in the preceding column by 14.696, which in this instance is found to be 1.440 atmospheres.

When, however, the reading is below that of ordinary atmospheric pressure, such values are often desired in inches of mercury since vacuum pressures for the condenser are given in such units. This particular column is therefore found by dividing the corresponding line in the preceding pressure column by the number of inches of mercury equivalent to one pound pressure per square inch. It is to be remembered that this does not even yet give the reading in inches of vacuum. Pressures in absolute inches of mercury and inches of vacuum cause seemingly endless confusion. A complete discussion of this feature was taken up under the chapter on pressures and its careful review is emphatically recommended if any unsettled question still exists in the mind of the reader.

Specific Volume.-The cubic feet occupied by one pound of dry saturated steam at a given temperature and pressure is known as the specific volume of the steam for that temperature and pressure.

This is a factor often necessary in steam engineering computations. Yet no known means has ever been invented whereby this factor can be accurately ascertained by experiment. The task is indeed one that involves such difficulties as to make its determination by experiment practically impossible. The science of higher mathematics has come to the rescue and here is indeed an instance where purely theoretical deductions have brought about a practical solution of an otherwise unsolvable problem in steam engineering.

This relationship involves the latent heat of evaporation L; the absolute temperature T at which the saturated steam is formed; the ratio of the increase in pressure Ap to the increase in