IV. AMOUNT OF AIR NECESSARY FOR VENTILATION.—Few subjects present a greater diversity in practice than the amount of air given for ventilation. From 2 to 4 cubic feet per minute for each person have usually been considered a large supply, but this is far beyond the amount commonly allowed. A very slight examination, however, will show that even this amount is often too small. If the process of respiration be accurately examined, it will be observed that a cubic foot of air, or more, is involved or mixed, and contaminated with the air discharged from the lungs, at each expiration, independently of that affected by the skin. Such a supply, therefore, is at least desirable, were the air always at a mild and genial temperature. According to Dr D. B. Reid's experiments, where the effects of variable quantities of air were tried upon numbers included in an experimental apartment, not less than 10 cubic feet of air per minute should always be allowed when it is warm; and to sustain the atmosphere in all its freshness and purity, even a much larger quantity is at times desirable. At the late houses of parliament, from 36,000 to 50,000 cubic feet per minute have occasionally been given in sultry weather in the House of Commons alone, or about 60 feet per minute to each individual in a crowded house. This question of the amount of air necessary for ventilation, taking the deterioration produced by the human frame alone into account, is often complicated and influenced by a vast variety of circumstances, of which the following are the most important. In the preceding and following remarks, a temperature of 65 degrees may be considered as the average most generally desired when there is a steady but uniform and gentle movement in the air. It is not so much the amount of supply with which life can be sustained that is the question, as that which it may be desirable to afford so as to maintain the system in a state of vigour and comfort, even when the air is much warmer.
1. The Purity of the Air supplied.—In general, the less pure the air the greater the amount necessary for ventilation, especially if it be loaded with moisture, and charged with offensive exhalations from the lungs and skin. But rare cases occur, when an atmosphere is so largely charged with external poisonous effluvia, that it becomes an object to use as little as possible to avoid this source of contamination. In one instance, cases of fever are reported to have occurred at a particular period in every room that was largely supplied with air, while, in other rooms in the same building, with a much less supply of air, no disease was noticed.
2. The Temperature of the Air.—This is the most important circumstance affecting the supply necessary for ventilation. When the air is very cold, and the moisture of the breath is condensed in hoar-frost as fast as it escapes from the lungs, a proportion of air extremely small compared with the usual allowance desirable will be sufficient for ventilation. Air, under ordinary circumstances, is below the temperature of the body; it therefore acts as a cooling power. But the higher its temperature, and the more nearly it approaches that of the body, the larger is the quantity required to produce an equivalent cooling power. Further, in warm weather, the air is charged with more moisture than in the cold season, while the body is at the same time more exhalant of moisture. These and other circumstances tend to render the supply of air desirable in warm weather far greater than the mere arithmetical increase in the temperature would, at first sight, appear to demand. But here we must advert to the popular error that the temperature, as indicated by the thermometer, is a proper guide to the quality of air in respect to the warmth that may be most agreeable. The temperature, as a moment's reflection will show, is a very imperfect guide, unless the velocity of movement and chemical qualities of
Ventilation. the air, especially in reference to moisture, be also taken into consideration. A small quantity of air, stagnant, and at 32° Fahr., does not cool the body more than a larger supply at 40°, 50°, 60°, 70°, 80°, or 90°, and, indeed, at any temperature below that of the living system, if brought to act upon it in sufficient proportion. A large quantity of air not so cold may be made to produce the same amount of cooling effect as a less amount of colder air.
3. Moisture in the Air.—Air in winter usually requires the addition of moisture when warmed and introduced into any apartment, as at this period it has deposited a large proportion of the moisture associated with it in summer. When the air is warmed by the approach of summer, it gains moisture from the surface of the earth or of the ocean, and thus acquires more of the pleasing and agreeable qualities which a summer atmosphere presents; but if warmed and introduced into any apartment without the previous addition of moisture, then, having had its power of action upon moisture increased, without receiving a corresponding supply, it absorbs moisture with extreme rapidity from the surface of the body and of the lungs. By taking away an undue proportion, it produces a harsh and disagreeable impression. The injection of steam into the air, or the evaporation of water from shallow pans placed over the heating apparatus, removes the defect.
Moisture is frequently communicated to the air with the view of cooling it by the reduction of temperature attending evaporation.
In some climates, again, a redundancy of moisture forms one of the greatest sources of oppression, and is regarded as one of the most powerful causes in developing the activity of miasma.
The influence of different degrees of moisture in the air is as various in different constitutions as that of different temperatures. Again, some individuals exhale moisture almost solely by the lungs, while in others the skin is equally active. According to the relative condition of the living system, the air, in reference to moisture, may either exhale or absorb moisture from it. A large quantity of air charged with much moisture, but not saturated, may produce as much evaporation from the body as a less quantity of air containing little moisture in solution.
4. Idiosyncrasy and Habit.—A very wide range of experiments has shown that the constitutional peculiarities of different individuals vary as much in respect to the amount of air desired, and the temperature at which it is preferred, as in respect to food or drink. In the British Houses of Parliament, no temperature below 52° or above 76° has been demanded for a series of years. It is often very difficult to determine, however, how much is due to absolute peculiarities of constitution, and what is dependent on the circumstances of the moment, more especially the state of occupation or excitement, the time that has elapsed since any refreshment may have been taken, and the nature and quality of the repast, the clothing in use, the previous exposure, the temperature, moisture, and other circumstances affecting the quality of the air. The brilliancy of the illumination also affects the supply of air required by some constitutions. The force of habit is nowhere exemplified in a more marked manner than in the amount of contamination which different individuals can bear. Some miners pass habitually much of their time in air in which a candle does not burn, though a lamp may be maintained in combustion in the same atmosphere. In public buildings, the great object is to sustain a uniform standard of ventilation suitable to the great majority of the audience, and to reject the complaints of individuals with extreme constitutions in regulating this standard. It is not unreasonable, however, to provide a judge who is confined for a long period to a particular place in a court of law, and others in parallel positions, with more power in varying the atmosphere in their vicinity than is given in any other part of the court.
V. NATURE OF HEATING POWER.—This is necessarily as various as the climate, the fuel available, the building to be heated, and the precise effect desired. Few circumstances require more care in connection with ventilation. The open fire, so much prized from its lively and cheerful appearance, exerts also an agreeable effect upon the animal system by the light radiated along with the heat, and the movement of air which it sustains as it ventilates the apartment in which it is placed; but in point of economy of fuel, or facility of regulation, so as to maintain an equal temperature in large apartments, it is, perhaps, the least desirable of all kinds of heating apparatus. The great beauty, therefore, which its appearance presents, the absolute purity of the heat which it conveys by radiation, and the extreme facility of access which it affords, so important for many different purposes, ought to be contrasted with the attendance which it requires, the dust and ashes which it leaves, and the tendency, when neglected, to produce back-smoke, if the action of the flame be not maintained with proper force. In connection with ventilation, the following points require special attention in the construction of the common fireplace. 1. It should be provided with an independent supply of air entering in its immediate vicinity, to be employed when heat is required in any
apartment without changing much air there, as in warming the apartment before it is occupied, or moderating offensive currents near the fireplace. 2. An open fireplace, unless the air enter at or near the ceiling, often produces little or no ventilation above the level of the chimney-piece, and even then it does not afford the best and purest atmosphere. 3. The air above may be comparatively stagnant, and offensive in the extreme from lights and the products of respiration, while a fresh current moves along the floor to the fireplace alone, if no separate discharge be provided above. 4. The introduction of a valve to regulate or diminish the excessive draught of some chimney-flues often adds more to the comfort of the rooms on which they act than any other measure. Sometimes a reduction in the escape at the top, even to a quarter of the aperture previously given, is found to be a great improvement both in point of comfort and in the economy of fuel.
Of other arrangements for heating, the mild hot-water apparatus, where the water is always under the boiling temperature, affords a very perfect and equal diffusion of heat, when properly arranged,—a point of great importance in producing equality of ventilation. Stoves presenting an extensive surface at a moderate temperature, varying from 100° to 200° degrees, as the Russian, Prussian, and Swedish porcelain stoves, or Dr. Arnett's iron stove, come next in order. Those varieties should be preferred which are provided with chimneys, both for the ingress and egress of air, that have no direct communication with the apartment in which they are placed; otherwise, when worked most economically, they are all liable to evolve carbonic acid occasionally, if they are not adjusted by persons who thoroughly understand their action. Stoves and other apparatus, where the iron is heated to a high temperature, may be in many cases more economical than the preceding varieties; but from the manner in which they affect the air, they are not so conducive to health, and greatly interfere with regularity of ventilation, the very hot air from them ascending rapidly to the ceiling, while a cold atmosphere, almost unaltered, is often left below. The stoves and heaters usually employed for conveying warm air to the principal apartments and passages of houses in very cold countries, generally heat it too much, and are provided with channels that are too small, while they are deficient in arrangements for returning air from the passages and individual apartments, when there is little or no vitiation of the air supplied.
There are three different kinds of steam-apparatus.—1. Mr. Gould's low-temperature steam-apparatus (a recent patent). A variable proportion of steam is admitted into thin but broad iron cases, part of the air only being expelled, according to the temperature required. A somewhat similar effect may be secured by surrounding steam-pipes at 212°, with discs or leaves of zinc or other metals.
2. The ordinary steam-apparatus, in which steam is used at the temperature of 212°.
3. Perkins's high-pressure steam-apparatus, in which a more elevated temperature is maintained, according to the pressure on the water producing it.
With such a variety of resources, great facilities are afforded in regulating the ventilation of public buildings. The more certainly the floor is warmed, the milder the source of heat; the more universal the diffusion of the entering air, the more certain the continuous escape of vitiated air; and the means of altering a valve to adapt the amount of air to the numbers present, the more satisfactory will the ventilation be.
In buildings for public assemblies, subject to great and sudden fluctuations of attendance, hot and cold air-chambers are provided, from which any supply of warm or cold air may be obtained. A mixing-chamber for mingling the various proportions that may be supplied is a great additional convenience.
VI. SOURCE OF MOVEMENT.—An alteration of specific gravity in the air, dependent on an alteration of temperature produced by expired air, and the general warmth of the body, is the great, the more natural, and the most economical source of movement, under all ordinary circumstances. When the openings for ingress and egress are well arranged, any ventilated apartment may be compared to a piece of apparatus in which the current of air, entering rapidly by a narrow channel, expands with the greatest possible diffusion into a slow-moving stream, occupying the principal portion of the area of the place to be ventilated, and gathering together again in a smaller channel by which it escapes, where the velocity of movement increases as the area diminishes. In ordinary apartments, where nothing more can be afforded, two openings at different levels will always give much relief, the one usually admitting cold, and the other, which should be as high as possible, discharging hot and foul air. Where one opening only is made for ventilation, one part of it admitting and the other discharging air, the nearer the ceiling it is placed the more effectually does it act.
A crowded room, in which fresh air enters on every side with the most gentle movement and at a proper temperature, so that its impetus is not perceived, vitiated air escaping in a central stream, and all products from artificial lights being carried away by the same current, presents a perfect system of ordinary ventilation. It is presumed that double glazing is introduced to prevent down-draughts from ice-cold glass.
In public buildings, where long sittings are held, under every variety of circumstances, in different seasons, with ever-varying numbers, by day and by night, and amidst endless changes of the external atmosphere, it is impossible to regulate the ventilation satisfactorily without a power to move the air, and without appropriate valves. For this purpose ventilating chimneys or shafts, worked by fire, fanners, pumps, or screws, driven by a steam-engine or water-wheel, a jet of steam, a stream of air acting as a blow-pipe, a current of electricity, or any other power, may be used. The shaft and the fanner are usually preferred, especially the former.
The moving power of air itself is constantly used both for forcing and exhausting air. The windmill and the cowl are familiar illustrations where this power is resorted to, but the continuity of its action cannot be depended on. A chimney, a shaft, or any other external discharge of vitiated air, acts with increased power when it assumes a conical form at the top. Elaborate experiments on this subject show that this power is increased if a small part of the upper portion be inclined outwards.
A stream of water, descending in one or more tubes or flues, is often used as a ventilating power, especially where it is made to fall as a shower, in condensing poisonous ingredients or other impurities.