Source of information: http://en.wikisource.org/wiki/Natural_and_Artificial_Methods_of_Ventilation
" Ventilation is a science, and it requires the study of a lifetime to master properly all its intricacies. The greatest engineering skill is necessary in the arrangement of tubes and the supply offresh air." —PARKES.
There is perhaps no other subject with respect to which there is greater diversity of opinion than that of ventilation. Ventilation is the most difficult of all sciences to practically deal with in such a manner as to satisfy every one. Indeed to do so is well-nigh impossible, owing to the varying idiosyncrasies and temperaments of different individuals.
It will, however, be admitted that the method which secures the requisite change of air in the simplest and least objectionable manner, is the one most likely to prove generally acceptable.
That that method is the natural one when properly applied is conceded by the highest authorities on the subject. It possesses the advantage of being easily installed at a moderate cost, and, requiring no special attention, it can never get out of order or break down as so often happens with mechanical ventilation.
It is also admitted that the proper place from whence to extract the warm expired air is at the ceiling, or upper part of a building, to where it naturally ascends.
Dr. Parkes, than whom there is no higher authority, says: "As the ascent of respired air is rapid, on account not only of its temperature, but from the force with which it is propelled upwards, the point of discharge should be above.
" By some it has been argued that it is better that the foul air should pass off below the level of the person, so that the products of respiration may be immediately drawn below the mouth, and be replaced by descending pure air. But the resistance to be overcome in drawing down the hot air of respiration is so great that there is a considerable waste of power, and the obstacle to the discharge is sometimes sufficient, if the extracting power be at all lessened, to reverse the movement.
" This plan in fact must be considered a mistake. The true principle is that stated long ago by D'Arcet : 'In the case of vapours or gases the proper place of discharge is above.'
" Three forces act in natural ventilation, viz., diffusion, wind, and the difference in weight of masses of air of unequal temperature.
" In temperate climates in most cases natural ventilation is the best. Incessant movement of the air is a law of nature. We have only to allow the air in our cities and dwellings to take share in this constant change, and ventilation will go on uninterruptedly without our care.
" The evidences of injury to health from impure air are found in a larger proportion of ill-health—i.e., of days lost from sickness in the year—than under any other circumstances ; an increase in the severity of many diseases, which, though not caused, are influenced by impure air, and a higher rate of mortality."
Another authority says : " Anything which has passed through the human body ought to be treated as excreta and rejected ;—just as sewage was thrown away into the drains, so air that had passed through the human lungs should be got rid of at the earliest possible moment without allowing it to go through the lungs of someone else. It was as unreasonable to breathe the same air twice, or twelve or twenty times over, as was the case in many places, as it would be to go to the sewer for drinking water."
The old fashioned formula which used to be employed to determine the amount of ventilation necessary for a given number of people by the height and area of a shaft or flue is now obsolete, the calculation being based upon the supposition that a column of air equal to the area and length of the shaft would always pass up and out ; but it has been recognised that this is a fallacy, as at times the air passed down as freely as it passed up, and that occasionally no current, either way existed at all.
Atmospheric air per 1,000 volumes is composed of oxygen 209.6, nitrogen 790.0, carbonic acid 0.4. Expired air contains about 5 per cent. less of oxygen, and a little more than 4 per cent. more of carbonic acid than that which is inhaled. From 25 to 40 ounces of water is passed off from the lungs and skin in twenty-four hours.
Carbonic acid gas is not in itself poisonous, but as a product of respiration, particularly in connection with an excess of moisture, its presence is always an index of contamination of the air by other impurities, such as organic matter, and it is therefore employed as the standard to determine the vitiation.
Carbonic acid diffuses in the air and does not gravitate to the lower levels.
One cubic foot of gas consumes the oxygen of about 8 cubic feet of air and produces 2 cubic feet of carbon dioxide.
The average number of respirations made by an adult is 20 per minute, 30 cubic inches of air being inhaled at each respiration, or 600 cubic inches per minute, or about 20 cubic feet per hour.
With a temperature of 70° F. the temperature of the air expelled from the lungs is from 85° to 95° F.
It is the organic matter suspended in the watery vapour expelled from the lungs and exhaled from the body wherein the real danger lies. As this vapour is in a heated state and immediately ascends, it should be drawn off at the highest point and not permitted to return to be rebreathed as happens with downward ventilation by propulsion, a method which has justly been described as "a standing menace to the health of society."
With mechanical ventilation the air is propelled or drawn in at a high velocity ranging from 5 feet to 20 feet per second, destructive of diffusion, and fatal to the effective or comfortable ventilation of a building. As one authority says, "Engine-driving columns of air through a building is not ventilating it."
With downward ventilation by propulsion from three to four times the volume of air is required that is necessary with upward ventilation, the cost being proportionately increased.
Extraction by fans has not been more successful, and they have in many cases to be discarded owing to the unbearable draughts they create.
Hot-air aspirating shafts are a costly and unsatisfactory method of ventilation, and are now seldom employed.
Ventilation by dilution necessitates the supply of a very large volume of air to keep the air in a building in anything like a healthy state. When the ventilation is by extraction the vitiated air is removed as fast as it is generated, and healthy ventilation is secured, with but a fraction of the volume of air required for dilution by propulsion.
One of the greatest dangers to health is where the fresh air supply is raised to a temperature such as is required to effectively heat a building, as it is thereby seriously deteriorated and rendered unfit for healthy breathing purposes.
The heating of a building should always be kept separate and distinct from that of the ventilation. Radiant heat is the healthiest.
The velocity of the air supply should never be greater than 2 feet per second, if draughts are to be avoided. The fresh air should be admitted at about breathing level in an upward direction through a number of small tubes distributed round the room to secure the most perfect diffusion. Where the air is warmed by passing through radiators a lower level is permissible.
The great majority of the bacteria found in air are not only perfectly harmless but are beneficial to human beings, they acting as so many scavengers of the air, and to get entirely rid of them would be prejudicial to health.
" As the authorities on the subject hold such widely divergent views as to the amount of air required for healthy ventilation, and there is no accepted standard, each system must of necessity be adopted on its own merits"—VENTILATION.
" With the many diverse methods of ventilation which are at present in use there can be no fixed standard for determining the volume of air necessary to secure effective ventilation ; one system ensuring with a given volume a healthy atmosphere, to secure which with another system several times that volume may be required.
" At a time when the science of ventilation, and the natural laws which govern it, were but imperfectly understood, it was held by many, and still is by some, that a very large volume of air was necessary to dilute the vitiated air in a building to a healthy standard, from 2,000 to 6,000 cubic feet per hour per person being recommended. Experience, and a more enlightened knowledge of the subject, have, however, taught us that with the more perfect forms of ventilation by extraction from the upper parts of a building, to where the warm expired air naturally ascends, considerably less than the least of these estimates is sufficient for all ordinary requirements. A high authority fixes 100 cubic feet per hour per person as quite sufficient with this method of ventilation, as the average of 20 cubic feet of air which is respired by an adult per hour is expelled from the lungs at a temperature of 80° to 100° F., and ascends, along with the heated exhalations from the body, to the upper parts of the building, and is there at once drawn off, so that no part of the vitiated air can be returned to be rebreathed, as is the case when the ventilation is effected by dilution by impulsion, particularly when on the downward principle.
" As with effective upward ventilation the ascending vitiated air is drawn off as fast as it is generated, an equivalent supply of fresh air being admitted at or below breathing level, it is obvious that a considerably less volume of air would be required with this method to secure efficient ventilation than where the foul air is merely diluted to a given standard, as is the case with all forced systems of ventilation by impulsion, necessitating the introduction of large volumes of air."
Professor R. H, Smith says : " The commonly adopted basis of calculation of so many cubic feet of space in each room per person meant that the object aimed at was the slowing down to a standard time-rate of the vitiation of a stationary quantity of air. From this idea was derived that of supplying per hour between 30 and 200 times as much as was actually inhaled by the inmates of the room. The true idea of perfect ventilation is evidently to inject and extract only a moderate excess, say five to ten times as much, over that actually used, and to do so in such a manner that (1) the exhalations do not mix with the fresh air supply, and (2) the inflow is properly diffused and does not pass direct to the outlets in merely local currents or draughts."
With downward ventilation by propulsion from three to four times the volume of air is required that is necessary with upward ventilation.
Professor Smith, in a recent article in the Engineer, states, "In order to keep down the percentage of pollution to a non-dangerous degree, under this system [forced downdraught] arises, therefore, the necessity of admitting for ventilation fresh air in quantities many times greater than that actually used, and also a correspondingly extravagant expenditure of heat if this supply be artificially warmed. Thus the only ideally perfect ventilation consists in inducing a regular up-current from a level below that of the human head up to the extraction outlets at the ceiling. Under this system the bulk of fresh air required to be admitted is immensely reduced, as is also the expense of warming it to any degree considered desirable."
" Engine-driving columns of air through a building is not ventilating it"
To secure the most perfect diffusion, equable ventilation, and freedom from draught, the fresh air supply should be admitted at a low velocity. The highest authorities fix the maximum speed at from 1,5 feet to 2 feet per second. The air should be admitted directly through the walls into the room at a low level and in an upward direction.
The Builder (April 3oth, 1898), referring to a mechanical system of ventilation, says :—" Mr. . . . lays it down as an axiom that the rate of air motion through the air inlets into the room should not be more than 5 feet per second. This is, in our opinion and in that of good authorities, too much ; and 2 feet per second is far preferable."