Ventilation Standards and Ventilation Methods
Reprinted from Journal of the Association of Engineering Societies, Vol. LI, No. 3, 1913
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Ventilation Standards and Ventilation Methods.
—by—
R. C. Carpenter.
Editors reprinting articles from this Journal are requested to credit the author, the Journal of the Association, and the Society before which such articles were read.
Association
of
Engineering Societies.
Organized 1881.
This Association is not responsible for the subject-matter contributed by any Society or for the statements or opinions of members of the Societies.
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Ventilation Standards and Ventilation Methods.
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By R. C. Carpenter.[1]
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[Presented to the Sanitary Section of the Boston Society of Civil Engineers, January 7, 1913.]
The ventilating engineer has before him the practical solution of the problem of supplying pure air for all purposes wherever it is required. Incidentally he must, in many cases, install means and methods for purifying the air, during which process he must remove smoke, dust, odors, moisture, organic material and other objectionable matter.
Considering the varied industries and the varying character of the demands made, it will at once be perceived that the problems to be solved are complex in their nature and may involve the installation of intricate machinery and the application of complicated chemical processes.
No Definite Standard.
The ventilating engineer in his work has been greatly handicapped in the past by the lack of a definite standard acceptable to all as to what constitutes good ventilation. It is the engineer's business to design machinery and processes for producing definite results rather than to decide just what character the results must have in order to produce satisfaction. In previous years a theoretical standard for good ventilation was assumed, and this standard, without being subject to any physiological investigations, so far as I can learn, has been almost universally accepted for the last twenty-five years as the criterion of good ventilation.
This standard was based on what was apparently a scientific foundation, although the foundation was merely theoretical. For instance, it has been known for a long time that normal atmospheric air contains about four parts in ten thousand of carbon dioxide. It was known that one of the principal products of respiration was carbon dioxide, or, in other words, that the effect of breathing normal or pure air was to increase the amount of carbon dioxide. An arbitrary limit of carbon dioxide as assumed as the danger point, and a standard of ventilation was based on this constituent. It was assumed, for instance, that ten parts of carbon dioxide in ten thousand of air was the danger limit. On the theory that each person uses one cubic foot of air per minute, which contains normally four parts of carbon dioxide in ten thousand, and that the respired air expelled from the lungs contains four hundred parts in ten thousand of carbon dioxide, a computation would indicate that it would be necessary to supply about 30 cubic feet of air per minute for each person in order to maintain a standard air supply containing ten parts of carbon dioxide in ten thousand. In other words to produce this result the calculations indicate that about thirty cubic feet of air must be supplied for each individual per minute.
This standard of thirty cubic feet per minute has come to be generally accepted throughout the civilized world.
Unfortunately, recent investigations have proved that there is absolutely no scientific basis for the standard which has been so universally adopted and so extensively applied. On the other hand, they indicate that carbon dioxide cannot be taken as the index for proving the unfitness of air for human respiration.
The amount of oxygen in normal air is about 21 per cent. It was generally supposed that the injurious effect of bad air on the human body was largely due to the diminution of oxygen. This constituent was well known to diminish during respiration very nearly as the carbon dioxide increased. Such a deduction is also proved by recent investigation to be erroneous and without any scientific foundation.
A very excellent statement relating to the physiology and processes of respiration is to be found in the twenty-third volume of the eleventh edition of the Encyclopedia Britannica under the heading of "The Respiratory System," and for a full discussion of this subject I would suggest that this article be carefully read.
Sources of Impurities.
The impurities which the air contains are obtained from various sources. Thus, for instance, smoke comes from results of combustion, dust is so universally present that it may almost be considered a constituent of the air. Its sources are, however, exceedingly numerous and may come from manufacturing processes or from winds or various causes. It may be inorganic or mineral in nature, or it may be organic containing bacterial life, depending upon its source. Respiration is a common source of impurities. The air in inhabited rooms is contaminated principally from the products of respiration of the people in the room. The contamination of air by the products of respiration requires the closest investigation on the part of the ventilating engineer.
Respiration Products.
The following table shows the constituents of normal and expired air and is useful in showing the changes brought about in pure air by the process of respiration.
| Normal Air. | Expired Air. | |
| O | 20.90 | 17.3 |
| N | 79.04 | 79.2 |
| CO2 | 0.03 | 3.5 |
| Additional moisture, 6.00 per cent. | ||
This table indicates that by the process of respiration the carbon dioxide is increased by about 3.5 per cent., and the oxygen diminished by 4 per cent. on the basis of dry air. The expired air leaves the body with about 6 per cent, of moisture to be compared usually with about 1 per cent, in the inspired air. It also has temperature approximating that of the human body. The added moisture in higher temperature of expired air makes it decidedly lighter than pure air. The average volume of air inspired per minute by healthy adult men during rest is about seven liters, or ¼ cu. ft. During muscular work the volume of air breathed may be six or eight times as much as during rest. The volume of carbon dioxide given off varies from about ½ cu. ft. per hour during complete rest to 5 cu. ft. during severe exertion, and averages about 0.9 cu. ft. per hour. The volume of oxygen consumed is about one seventh greater than that of the carbon dioxide given off.
It was formerly thought that the process of respiration was practically the same as that of combustion and that the greater the supply of oxygen, the greater the formation of carbon dioxide and the greater the supply of useful energy. The physiological investigations referred to show, however, that a very different process takes place, due to the action of the nerves controlling the respiration. These nerves have a source of control in the brain, and operate automatically to maintain a constant percentage of carbon dioxide in the immediate passage leading to the lungs. The effect of this automatic action is to maintain about 5.6 per cent. of carbon dioxide and about 16 per cent. of oxygen in the storage or alveolar space where it is available for use in the lungs. It consequently follows that within such degrees of variation as usually occur in the worst or in the best ventilated room, the amount of oxygen used by the individual does not vary at all. That is, the lungs extract from air which contains only 16 per cent, oxygen just as much oxygen as from normal air, which contains 21 per cent. If, however, the air supplied falls below 15 per cent. in oxygen, then the regulating apparatus cannot overcome the deficiency.
The physiological investigations also indicate that so far as the effect on the human body is concerned, no harm whatever results in supplying air containing thirty to forty times the amount of carbon dioxide which our old theory assumed to be harmful and injurious.
In the ordinary operation of breathing, the percentage of carbon dioxide in the alveolar air is kept remarkably constant. If the air supply is such as not to increase the carbon dioxide in this space, the effect is not noticeable to the patient. The effect of one per cent. of carbon dioxide in the inspired air is negligible. With four or five per cent. of carbon dioxide, however, much panting is produced, for the reason that the percentage of carbon dioxide rises in the chamber preceding the lungs. As a consequence, headache and other symptoms are produced. This is a condition which is practically impossible to realize unless the space were hermetically closed.
Even if oxygen is breathed instead of air, there is no appreciable change in the percentage of carbon dioxide in the alveolar air. Want of oxygen is thus not a factor in the regulation of normal breathing. It is the carbon dioxide stimulus that regulates the breathing, although with excessive muscular work other accessory factors may come in. Thus, for instance, the barometric pressure either higher or lower than normal has a great effect on the proper regulation of the amount of carbon dioxide and oxygen, but this consideration is not important in connection with ordinary ventilation.
Owing to the unpleasant effects often produced in badly ventilated rooms, it was supposed for a long time that some poisonous, volatile organic matter is also given off in the breath. Careful investigation has not verified this. The unpleasant effects are partly due to heat and moisture and partly to odors which are usually not of a respiratory origin. Carbon dioxide present in the air of even badly ventilated rooms is present in far too small proportions to have any sensible effect. Apparently the unpleasant sensations are principally due to high percentage of moisture and high temperature, the effect of both of which on the action of the heart is injurious.
Various experiments have been made on human subjects enclosed in hermetically sealed boxes provided with windows, which experiments have tended to check or verify the conclusions drawn from the physiological investigations to which I have already referred. Briefly, these experiments have indicated that the human subjects suffered very little or none at all by deduction in the amount of oxygen supplied to from 21 to 16 per cent, and an increase in the carbon dioxide content to nearly 5 per cent., provided the air in the enclosure in which the body is situated is kept in motion, the temperature maintained at less than 74 degrees and the percentage of humidity kept from reaching an excessive amount. If the air is not kept in motion, or if there is excessive moisture or high temperature, suffering is soon evident from the lack of ventilation.
Humidity.
An extremely important property of air is its humidity or moisture content. This is not an impurity, but it needs regulation. Air at a definite temperature has the property of absorbing a certain amount of vapor of water. When the air is so fully charged with this vapor that any increase will be followed by precipitation or rain, it is said to be saturated. Saturated air has the property of coating materials with moisture if the temperature be lowered the very least amount. It would of course for that reason be extremely unpleasant if not unsanitary if introduced into a room for the purpose of ventilation.
The amount of moisture which air will absorb increases very rapidly with an increase of temperature, At very low temperatures the amount is small, say at a temperature of 32 degrees, saturated air contains only 2.35 gr. per cu. ft. At a temperature of 70 degrees, it contains 7.94 gr. per cu. ft.; at a temperature of 100 degrees, it contains 19.12 gr. per cu. ft.
Air in order to be comfortable should contain some moisture. Out-of-doors air is, under usual conditions, from 30 to 70 per cent. saturated and such a degree of saturation is, in accordance with investigations, more sanitary than either extremely dry or extremely damp air. When air is saturated with moisture, water is deposited on all bodies which conduct heat readily and have a lower temperature than the air. On the other hand, if the air is entirely deprived of water vapor, it evaporates moisture from the body, which operation causes an unpleasant sensation. It also takes up a great deal of heat. When the air is saturated, evaporation cannot take place from the body and an unpleasant and depressing effect is produced on the nervous system and the action of the heart. A high temperature in connection with excessive humidity is a frequent source of difficulties with ventilation, and the cause of most complaints as to poor ventilation.
Dust.
A common impurity in the air is dust. This when it exists in large quantities may not be unsanitary but it is certainly a great nuisance, and one of the objects of the ventilating engineer in the purifying of air must be to remove the dust which it contains. The dust may have almost any sort of origin, it may be inorganic or mineral, or on the other hand it may be organic and loaded with injurious bacteria.
Dust has been defined as simply matter in the wrong place, the presence of which had to be tolerated, and it was supposed to serve no useful purpose in nature. Since the year 1880 it has been known to play an important part, and instead of being a nuisance it adds much to the comforts and pleasures of life. Every cloud particle owes its origin to a growth around a nucleus of dust. As a consequence, without particles of dust clouds would be impossible. The presence of dust in the atmosphere allows the condensation of the vapor to take place whenever the air is cooled to the saturation point. If there were no dust present, condensation would not take place until the air was cooled far below that point. Under such conditions, when it did take place it would result in heavy rain drops without the formation of what we know as clouds. This would result in many disadvantages. The super-saturated air having no dust to condense on would condense on our clothes, the inside and outside walls of our dwellings and on every solid and liquid surface with which it came in contact.
Without atmospheric dust, we should not have the glorious cloud scenery which we at present enjoy. We should have no haze in the atmosphere, we should have no twilight. Darkness would come as soon as the sun passed below the horizon.
The relative humidity of the air has a great effect on the dust by increasing the size of the particles of water vapor and so increasing the haze. The number of dust particles rapidly decreases with the amount of moisture present.
Thousands of tests have been made of the distribution of dust over the world, and these tests indicate that in the air over cities like London and Paris the number of dust particles may rise to an amount as great as 100 000 to 150 000 per cu. cm.
Even the purest air contains a considerable number of dust particles. The mean of a number of observations of air over the Atlantic showed 338 dust particles per cu. cm. In the purest country air the number is rarely below 10 000 per cu. cm. (1 cu. cm. = about 1-16 of a cubic inch.)
Smoke.
The air is frequently charged with smoke particles. In a general way, smoke particles are to be considered as a peculiar character of dust. They are peculiarly disagreeable because the particles are generally black in color and consequently render everything on which they settle of a disagreeable black and sooty color. The disagreeable part of smoke from the ventilation standpoint consists of the small particles of carbon which float in the air. Whether or not these particles are unsanitary directly is a proposition regarding which there is difference of opinion. It is, however, certain that they cause an immense economic loss by discoloring buildings and by soiling clothing, house furnishings and everything that pertains to life. Smoke particles are frequently of considerable size and are retained in the nostrils to a considerable extent during the processes of respiration. In that way they may have a deleterious effect on health.
As is well known, smoke is a product of combustion which would not occur, at least to any sensible amount, if the combustion were perfect. It is, therefore, of itself an evidence of economic waste. While I shall not have time to make any discussion whatever of the smoke problem, I would state that it is very largely preventable by the installation of proper appliances and proper methods of operation of plants which produce it.
The remedy for smoke is its "prevention" rather than its removal, and smoke prevention is possible with good devices and good operation. Time will not permit a further discussion in this talk.
Odors.
The air is frequently not only loaded with dust which it may be necessary to remove in order to bring it to a proper sanitary condition, but it may also be odorous or smelly. It a way, odors might be treated as pertaining to the same class of deleterious substances as dust, but they have a property not possessed by ordinary dust of affecting the organs of smell.
Odors vary greatly in character and have great effect upon the nervous organism of the human body. Many odors are, extremely pleasant, and some have an exhilarating effect, while other odors are extremely unpleasant and have exactly the reverse effect. Unfortunately, all people are not agreed as to the character of odors. Gases that smell pleasant to some people are extremely unpleasant to others. As an illustration, the odor of cooking may be pleasant to some, whereas to others it may be extremely unpleasant. Odors are not necessarily harmful, and generally speaking they do good rather than harm. Odors are produced by a great variety of substances, and, as stated above, vary in quality from the most disagreeable nature, which is almost sufficient to cause sickness, to the most pleasant and delightful nature such as characterize our most expensive perfumes. A great majority of the disagreeable odors in chemical composition are complicated compounds of carbon and hydrogen. They may differ from each other by small variations in composition less in amount than the character of the odors would indicate. It is well known that many of the desirable perfumes, such as the odor of wintergreen, of orange blossoms, of violets, are produced artificially by the combination of the required chemical compounds.
In all cases the natural scents are complex mixtures of many ingredients, and a variation in the amount of any one may completely alter the scent. Such mixtures would be difficult to reproduce economically. The perfumer is content with a product having practically an identical odor which may be formed artificially.
The ammonia compounds consisting of nitrogen and hydrogen are extremely odorous and are very penetrating and unpleasant.
The odorous gases, with scarce an exception, are combustible and are converted by the process of combustion into carbon dioxide, water and other non-odorous materials. Certain odorous gases like ammonia are absorbed in large quantities by water, and although by that process the odor is not removed, the air is however very greatly purified.
As with respect to smoke in many cases it may be easier to prevent odors than to remove them.
Air Purification or Conditioning.
Air purification as practiced at the present time removes from the air dust and smoke particles and regulates the proper degree of humidity and temperature. This process is usually called at the present time "air conditioning."
Apparatus for conditioning air as defined above is now installed in many important structures, and the future demand for sanitary results is likely to lead to its almost universal use. It is also successfully installed in special industries which are of such a character as to require definite temperatures and definite degrees of humidity.
I will describe briefly only a few of the features of the air conditioning process as now successfully applied.
Dust particles are to a great extent precipitated by moisture, as may have been noted from the statements which I have already made. Consequently the process ordinarily employed for the removal of dust is a system of washing which consists in the use of a great many extremely fine sprays. The mist produced by the sprays has the property of surrounding every dust particle with sufficient moisture to cause the dust particle to be precipitated and put in a position where it may be removed with the removal of the water. Formerly it was customary to remove dust particles by passing air through cloth filters which were kept continually wet, and such filtering processes may still be found in operation. But I believe that new filtering plants are at the present time not being installed to any great extent, while the spray washing systems are being installed to a great extent.
Air always contains water vapor. The maximum weight of water vapor which can be absorbed by the air is a function of the temperature. The relation between the maximum weight of water vapor and the temperature has been quite recently very carefully investigated by Mr. W. H. Carrier, of Buffalo, N. Y., and has been made the subject of a paper before the American Society of Mechanical Engineers (December, 1911). When the air is loaded with a maximum amount of water vapor, the least fall of temperature will cause precipitation. The amount of water vapor carried by the air in percentage of the total amount is its percentage of humidity. For the reasons stated above, if the temperature of the air be changed without increasing the moisture content, the percentage of humidity will also be changed, but in a reverse direction. External air is found with a variable percentage of humidity, but it is considered in the most desirable condition when it contains from 30 to 60 per cent. of humidity, as explained above, understanding that 100 per cent. humidity indicates saturated air.
Automatic devices have recently been perfected for varying the moisture content of the air automatically so as to maintain it at any desired percentage of humidity. This apparatus works on the principle of a double, differential thermostat, one part of which is moved by the temperature of the wet bulb thermometer and the other by the temperature of the dry bulb thermometer in such a way as to give a differential action arranged to supply or cut off the supply of moisture as desired to maintain a constant percentage of humidity corresponding to a constant temperature difference between a dry and wet bulb thermometer for a given temperature.
The washing of the dust particles from the air frequently charges the air with an excess of moisture so that the apparatus for air conditioning must be provided with means for separating or taking out excessive water if necessary.
Air conditioning usually requires the control of the temperature of the air, and this in turn requires heating coils which are under thermostatic control so as to bring the air passing through to the desired temperature.
Time will not permit any further discussion of air conditioning processes, but enough has been said to indicate that the subject is of itself an extensive one and could profitably be made the entire subject of an evening's meeting.
General Practical Remarks.
The ventilating engineer is under the serious handicap of not having a definite standard to work by which would enable one to judge of the perfection of his work. His work is naturally of a varied character. First, he must introduce the proper quantity of air to supply the necessary amount required for the best sanitary conditions; second, he must purify that air so as to remove from it objectionable dust, smoke or odors and regulate its humidity; third, he must introduce this air at the desired temperature; and fourth, distribute it uniformly in the rooms which are to be ventilated.
I shall not discuss further the standard requirements as to the amount of air necessary for sanitary purposes. I have pointed out to you that the scientific reasons underlying the present standard requirement were unsound. I have not said, however, that any other standard for quantity would have been better nor that it is desirable in the future to change the requirement as to the amount of air to be supplied. Without doubt, it is true that so far as quality is concerned our best standard is the external air surrounding the building to be ventilated. Investigation also indicates that the utilization of the external air for natural ventilation by raising the windows and regularly admitting the air to the apartment to be ventilated is desirable when the conditions are favorable. Generally speaking, it is not desirable to have a system bf ventilation which will not permit the direct communication with the outside air by the opening of windows. It however must be recognized that no supply of a definite amount can be obtained by merely connecting a room with the outside air by opening a window, and that as a consequence a system of ventilation which depends alone on the opening of windows will be certain to fail and will be certain to give air which will differ largely from the external air surrounding the building.
In order to meet the requirements which I have stated above are necessary for good ventilation, in my opinion, the engineer must introduce into the room where ventilation is required the desired amount of air, which air has been purified and put in the most desirable humidity condition for respiration. This air should be introduced at a moderate temperature and at a temperature sufficiently low never to make the occupant uncomfortable and should be uniformly distributed throughout the room.
[Note.—Discussion of this paper is invited, to be received by Fred. Brooks, Secretary, 31 Milk Street, Boston, by October 15, 1913. for publication in a subsequent number of the Journal.]
- ↑ Professor of Experimental Engineering, Cornell University.