CHAPTER II. PRINCIPLES OF VENTILATION. 20. Relation of Ventilation to Heating.—Intimately connected with the subject of heating is the problem of maintaining air of a certain standard of purity in the various buildings occupied. The introduction of pure air can only be done properly in connection with the system of heating, and any system of heating is incomplete and imperfect which does not provide a proper supply of air. The general principles relating to ventilation are considered in this chapter, but the practical methods of securing ventilation are considered in connection with systems of indirect heating. The subject of ventilation often receives very little consideration in connection with the erection of apparatus for heating. 21. Composition and Pressure of the Atmosphere.— Atmospheric air is not a simple substance, but consists of a mechanical mixture of nitrogen and oxygen, together with more or less vapor of water, and almost always a little carbonic acid and a peculiarly active form of oxygen, known as ozone. The nitrogen and oxygen are combined in the ratio of 79.1 to 20.9 by volume, and these proportions are generally the same in all parts of the globe, and at all accessible elevations above the earth's surface. The amount of carbonic acid in the air varies in the open country from 4 to 6 parts in 10,000 by volume. The amount of moisture in the atmosphere sometimes forms 4 per cent of its entire weight, and sometimes is less than one tenth of one per cent. The weight of the atmosphere is measured by the height in inches at which it will maintain a column of mercury in an instrument called a barometer. The pressure of the atmosphere is less as the distance from the centre of the earth becomes greater. For that reason points of different elevation give different average readings of the barometer. The normal reading of the barometer at sea-level, which corresponds to a boiling-point for pure water of 212° F., is 29.905 inches. The weight of the atmosphere, even at the same place, is constantly fluctuating with various conditions of the weather. The variation in barometer-reading from the mean may be 1.5 inches in either direction. The fall of the barometer due to different elevations from the sea-level would be approximately as follows: At 917 feet the barometer sinks I inch. The atmospheric pressure has great effect upon the boilingtemperature of water; thus pure water will boil at the temperatures corresponding to the various barometric pressures, as shown in the following table :* The weight of a cubic foot of air is inversely proportional to the absolute temperature; if freed from aqueous vapor and under a pressure of 30 inches of mercury, it weighs, according to Regnault, 536.29 grains or 0.076613 pound. The rate of expansion in volume or decrease in density is I 460+t degree Fahrenheit, t being temperature above 32°. for each Table VIII in the Appendix gives the weights of air for different temperatures. For the temperature of 60° air is 813.67 times lighter than water. Various other units are sometimes used to measure the head or pressure, and for convenience of reference these equivalents can be arranged as follows: 30 inches of mercury = 14.7304 lbs. =407.07 in. water = 33.92 ft. water = 11985.4 ft. air at 60° Fahr. I inch water = 0.57902 oz. Air contains more or less impurities which are to be found only in places where the ventilation is not perfect. These impurities consist of carbon monoxide, CO, ammoniacal compounds, sulphuretted hydrogen, and sulphuric and sulphurous and nitric and nitrous acids. It also contains some ozone, which is a peculiarly active form of oxygen, and is believed by many to have an important influence in the preservation of the purity of the atmosphere. Authorities, however, differ very widely as to its distribution and action. Lately a new constituent called argon has been discovered. Air contains more or less solid matter in the form of minute particles of dust. The dust particles are thought to bear an important part in the propagation and distribution of the bacteria of various diseases, and also in the production of storms. Air contains microbe organisms, or bacteria, in greater or less numbers. The number of bacteria may be determined by slowly passing* a given volume of the air through a glass tube coated inside with beef jelly; the germs are deposited on the nutrient jelly, and each becomes in a few days the centre of a very visible colony. In outside air the number of microbe organisms varies greatly, being often less than one per litre (61 cubic inches); in well-ventilated rooms they vary from 1 to 20, while in close schoolrooms as many as 600 per litre have been found. Carnelley, Haldane, and Anderson found in their researches in mechanically ventilated schoolrooms an average number of 17 microbe organisms per litre. The results of stopping the mechanical ventilation was to increase the carbonic acid without changing the number of microbe organisms. * Encyc. Britannica, article "Ventilation." 1 22. Diffusion of Gases.-Gases which have no chemical action on each other will, regardless of weights or densities, mingle with each other so as to form a perfectly uniform mixture. This peculiar property is called diffusion, and is of great importance in connection with ventilation, since it indicates the impossibility of separating gases of different densities. Liquids of different densities do not make uniform mixtures, unless they have a special affinity for each other; the heavier invariably settles to the bottom. Perfect diffusion is a process which requires some time, so that the composition of samples from the same room may in some instances be sensibly different. The time required for the diffusion of gases is inversely proportional to the density, and directly proportional to the square root of the absolute temperature. Diffusion is a molecular action, and can be calculated from the kinetic theory of gases. One computation of this character indicates that the time required for the equal diffusion of carbonic acid throughout the atmosphere was 2,220,000 years. Dr. Angus Smith found the following percentages of oxygen present in the air, in samples collected in various. places, which serve to show the variation which may exist under different conditions:* Seashore of Scotland, on the Atlantic.. Sitting-room, feeling close, but not excessively so. Backs of houses and closets. Under shafts in metal mines. When candles go out.... 20.99% 20.98 20.89 20.70 20.424 18.50 17.20 When difficult to remain in air many minutes.. The variation in amount of carbonic acid is equally great, the quantity being as follows: 23. Oxygen.-Oxygen is one of the most important elements of the atmosphere, so far as both heating and ventilation Encyc. Brit., vol. xvI. p. 617; also vol. II. p. 35. is concerned. It is the active element in the chemical process of combustion, and also of a somewhat similar physiological process which takes place in the respiration of human beings. It exists in a free state mixed with about four parts of nitrogen in the air, and is essential not only for the support of any combustion, but for the support of life. It is not to be considered as having any properties as a food, but is rather the necessary element which makes it possible to assimilate and utilize the food. Taken into the lungs it acts upon the excess of carbon of the blood, and possibly also upon other ingredients, forming chemical compounds which are thrown off in the act of respiration. The chemical action of oxygen with the other elements can generally be considered as a sanitary one. In many respects the process of respiration resembles that of combustion; for in both cases oxygen is derived from the air, carbon or other impurities are oxidized, and the products of this oxidization are rejected. In both cases heat is given off as the result of this process. Its weight is sixteen times that of hydrogen. It is sometimes found in a peculiarly active form called ozone. 24. Carbonic Acid or Carbon Dioxide, CO,, and Carbonic Oxide, CO. The first is a product resulting from the perfect combustion of carbon; it is always found in small quantities, 3 to 5 parts in 10,000 in the atmosphere of the country. This gas, although very heavy as compared with that of pure air (22 times that of hydrogen), will, if sufficient time be given, mix uniformly with the air. It is not a poisonous gas, although in an atmosphere containing large quantities of carbonic dioxide a person might die from suffocation or for want of oxygen. While carbonic dioxide is not of itself injurious, yet as it is a product of combustion and respiration, and is usually accompanied with other injurious products, it is regarded as an index of the quality of the air, and the amount of it present in the air is taken as the standard by which we can judge of the ventilation.* In such a case pure air, containing 4 parts of car *J. S. Billings, in his work on Ventilation and Heating, cites an experiment by Carneiley and Mackie, showing that the ordinary theory of increase |