This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol3", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
While it would be possible to treat the subject of Ventilation alone, it would not be feasible to attempt dealing with Warming Buildings by Heated Air without going somewhat deeply into ventilation. Consequently the two are included in this chapter. This will not be found to cause any confusion, and it will prevent much repetition, as would occur were the subjects dealt with separately. Remembering, too, that the Plenum System of ventilation usually provides for ventilation and warmth in winter and ventilation alone in summer, it will be seen how closely associated the two are.
The extent to which ventilation is required is determined by the extent to which the air is vitiated. It is not the deprivation of oxygen with which the air of rooms suffers that is considered, so much as the addition of impurities. Oxygen is needed, of course, but before this valuable element is used up to a serious extent the air requires changing for the purpose of removing the undesirable elements that are produced in various ways. Some authorities, quite good ones, consider the sense of smell, if the nose be normally keen, sufficient to tell if the air is pure enough or otherwise ; for though ventilation is provided to remove an odourless product of respiration and combustion - carbonic acid gas (carbonic dioxide, bearing the symbol C02) - this is so regularly accompanied by an odour of vitiation (in occupied places) that the latter is considered to be a good index to the presence and volume of the former. In a paper communicated to the Royal Society an eminent authority, a doctor of science and medicine, went to considerable lengths to show this, he conducting his own experiments and subsequent analysis of the air. His figures may be summarised as follow:-
Odour in the Air.
Humidity over that of outer
Carbonic Acid Gas per 10,000 Volumes of Air.
None (quite fresh) .
Slight smell .
Disagreeable smell .
Offensive smell and very close
From this it will be seen that the increase in carbonic acid gas bore a very distinct relation to the smell in the air. To judge by smell, however, it is necessary to be at least fifteen minutes in the outer air before entering the room ; and, as will be understood, judgment by smell is not resorted to if means for analysis are available, or if it appears to be necessary.
The analysis of the atmosphere, to a precisely accurate extent, is not usually undertaken by the engineer himself, but it is possible to ascertain the volume of carbonic acid gas within a satisfactory degree of correctness in the following manner: - Obtain six white glass-stoppered bottles, dry and clean, capable of holding from 40 to 80 cubic inches of air ; also a measuring glass that will show 3 cubic inches of liquid. Now take a bottle holding 40 inches of the air to be tested and put 3 cubic inches of clear freshly made lime-water into this and shake well with the contained air. If the water becomes cloudy it will show that the carbonic acid exceeds 12 parts in 10,000 of air. In 50 inches of air, cloudiness in the same quantity of lime water will indicate 10 (or more) parts of carbonic acid per 10,000 of air. In 60 inches, 8 parts. In 70 inches, 7 parts. In 80 inches, a little under 6 parts. Thus if the 3 cubic inches of lime water, when shaken in 70 cubic inches of air, became cloudy, but did not do so in 80 inches, it would indicate that the air contained between 6 and 7 parts of carbonic acid gas per 10,000 of air. This latter, as will be seen directly, is an allowed degree of impurity in most cases.
It should be stated, in relation to judging the degree of vitiation of air by its odour, that the humidity of the atmosphere is a controlling factor. The degree or volume of moisture in air does not vary to any great extent in our country, consequently it does not need much consideration if the warming of a place is done by fires or direct radiation ; but if the Plenum System of combined warming and ventilation or any method of warming by heated air be adopted, as it largely is in public buildings and institutions, then the humidity, or rather a want of it, may make the smell test less reliable. The figures just given were obtained with air of about normal humidity, namely, a humidity of 73 per cent, with a temperature of 63 Fahr. With an increase of humidity an odour is more quickly obtained, while the reverse applies with dry air. Any method of warming by which air is made to pass over heated surfaces makes it dry (compared to its temperature), and if moisture is not added, or is added but to an insufficient extent, then, as stated, the smell test must be less trustworthy.
The atmosphere can carry a varying quantity of moisture according to its temperature. Not only can it carry this, but if the quantity is not in proportion to the warmth the air will be dry. Thus air at, say, 35° Fahr. will contain about 2 grains of water per cubic foot, to be normally humid (it can carry 2 1/2 grains) ; but if this air is heated to 60° without obtaining added moisture it will be dry, as it then needs about 4 3/4 grains per foot to be normally moist. This added degree of moisture is usually picked up naturally in the ordinary way, but when air is heated by passing over enclosed batteries of heating surfaces, and from these delivered to a room in large volumes, an artificial means of adding water must be provided.
The Quantities Of Water That Air Is Capable Of Absorbing To Become Saturated Are As Follow
Grains per Ft.
Grains per Ft.
The air composing the outside atmosphere, however, is seldom saturated, the usual degree varying from 73 per cent in summer to 89 in winter, the average for the year working out at 82 per cent. In moistening the air artificially therefore, as is necessary with an apparatus delivering warmed air, the added moisture should keep the air at, at least, 65 per cent of the saturation figure for its temperature, as below this it will be dry.
The customary method of ascertaining the humidity of the air is by means of the dry-and-wet bulb hygrometer. This is a pair of thermometers, fixed side by side, one of which acts in the usual way, while the other has a piece of absorbent material (like wick) covering its bulb and extending down into a small jar of water. The water travelling up the wick on to the bulb might be thought to become of the same temperature as the air, but instead of this it is continually evaporating, the degree of evaporation agreeing with the dryness of the air. The evaporating process has a cooling effect on the bulb, with the result that this thermometer registers a lower temperature than the dry one, and the difference in temperature indicates the degree of dryness. A proper degree of humidity exists when the two thermometers show a difference of 6 to 8 degrees, while 10 degrees verges on dryness. On the other hand, there should not be less than a 4 degree difference, as then the air will verge on being too moist. If the two thermometers both indicated alike, it would show that the air was completely saturated, no evaporation occurring.