The purity of air in any habitable enclosure is determined by the amount of CO₂ (Carbonic acid gas) included in its composition. The process of ventilation is that of adding fresh air to the impure atmosphere of houses, until a desirable quality is attained. In the opinion of hygienists, when air does not exceed 6 to 8 parts of CO₂, by volume in 10,000, the ventilation is desirable. Ordinary outdoor air contains about 4 parts of CO₂ to 10,000, while very bad air may contain as high as 80 parts to the same quantity. The quantity of air required for the ventilation of a building is determined by the number of people to be provided. The amount of air required per individual per hour necessary to produce a desired condition of ventilation is determined by adopting a standard of purity to suit the prevailing circumstances.

In hospitals where pure air is considered of greatest importance 4000 and 5000 cubic feet per inmate per hour is not uncommon. The practice of supplying 30 cubic feet of air per person per minute (1800 cubic feet per hour) seems to fulfill the average requirements. It is the amount commonly specified for school-rooms.

The quantity of fresh air required per person to insure good ventilation will depend on the type of building to be supplied and varies somewhat with different authorities. The De Chaumont standard is that of 1 cubic foot of air per second or 3600 cubic feet per hour, for each person to be accommodated. De Chaumont assumed a condition of purity which will permit less than 2 parts in 10,000 of CO₂ over that carried by country air. In considering the same problem from the basis of permissible CO₂, if 6 parts of CO₂ in 10,000 represents purity of the required air, then 3000 cubic feet per person per hour is necessary. Likewise, the varying amounts for different degrees of purity are given by Kent in the following table. The upper line gives the permissible number of parts of CO₂ per 10,000, while below each factor appears the number of cubic feet of air required per hour for each person supplied.

It is generally recognized, that it is possible to live under conditions where no attempt is made to change the air in a building. It is also an established fact that the only preventive and cure for tuberculosis is that of living constantly in an atmosphere of the purest air. The greatest attainable degree of health is enjoyed by those who live in the open air, because oxidation is one of the most efficient forms of prevention and elimination of disease, and an abundance of pure air is the only assured means of sufficient oxidation.

The De Chaumont standard is intended to represent the limit beyond which the sense of smell fails to detect body odors or “closeness” in an occupied room. The amount of CO₂ that air contains is not an absolute index of its purity, but it gives a standard under ordinary conditions, makes possible the requirement of a definite quantity of air. If it were possible to express the amount of oxygen contained in the atmosphere, the same relative condition might be attained.

The ordinary man exhales 0.6 cubic foot of CO₂ per hour. Some forms of lighting apparatus produces this gas in greater amounts. The ordinary kerosene lamp gives out 1 cubic foot of CO₂ per hour. A gas light using 5 cubic feet of gas per hour produces 3.75 cubic feet of CO₂ in the same time. Any form of combustion permitting the products to escape into the air of the room tends to lower the quality of the atmosphere by adding to its content of CO₂.

The prevailing impression that impure air is heavy and settles to the floor is erroneous. Impurities in the form of gases and vapors (principally carbonic acid gas and odors) diffuse throughout the entire space, and the entering fresh air tends to dilute the entire volume.

As a quantative problem, ventilation consists in admitting pure air into an impure atmosphere in amount to give a definite degree of purity. This is accomplished by admitting sufficient air to completely change the atmosphere at stated intervals, or to provide a definite quantity for each inhabitant.

The methods by which ventilation may be accomplished will depend on the type of building to be ventilated and the apparatus it is possible to use. When the use of mechanical ventilation appliances are permissible, any desired degree of atmospheric purity may be maintained at all times, under any condition of climate or change of weather.

In buildings where mechanical ventilation cannot be considered as that of the average dwelling, the problem is one of producing an average condition of reasonably pure air by natural convection. In the average dwelling, ventilation is accomplished by the natural draft produced in chimneys or air flues, by partially opened windows and by the force produced by the movement of the outside air. In some buildings a better condition of ventilation is attained by ordinary means than at first sight seems possible.

The fact that it is difficult to keep a house at the desired temperature during cold weather indicates that a considerable quantity of outside air is constantly entering and heated air is leaving the building. It may be, however, that the ventilation under such condition is unsatisfactory, even though the amount of air which enters the building is sufficient in quantity to produce a desirable atmosphere. If the places of entrance and exit are so located that the entering air has no opportunity to mix with the air of the building, the advantage of its presence is lost.

In the burning of fuel in stoves and furnaces, the amount of oxygen necessary for combustion is supplied by the air which is first taken into the house and thus forms its atmosphere before it can enter the heater. Theoretically, about 12 pounds of air are required for the combustion of a pound of coal, but in practice a much larger amount actually passes through the heater. As given by Suplee, from 18 to 24 pounds of air are actually used in burning 1 pound of coal. If 20 pounds of air per pound of fuel is taken as an average, there will be required 198 cubic feet of air per pound of coal consumed. In a building that requires 10 tons of coal to be used during the winter months, this would necessitate the average use of 1977 cubic feet of air per hour, which must be drawn into the house before it can enter the stoves. This air acts as a means of ventilation and if it is used to advantage would furnish a supply sufficient in amount to produce excellent ventilation, considerably more than enough for two people. The amount of air drawn into the house in this way is further increased by that which passes into the chimney flue through the check-draft dampers, when the fires are burning low.

The aim of architects is to construct buildings as completely windproof as possible, but that such construction is attained in only a slight degree is sometimes very evident during cold weather. No matter how tightly constructed buildings may be, most of the contained air filters through the cracks and crevices of the walls or through the joints of the windows and door frames, because there is seldom any special provision made for its entrance. During extremely cold and windy weather the amount of air that enters the house in this way—because of the air pressure on the windward side—is sometimes sufficient to keep the temperature at an uncomfortably low degree. Under such conditions, the air drifts through the building faster than it can be raised to the desired temperature and the rooms on the windward side of the building cannot be kept comfortably warm.

The common method of ventilation in dwellings is that of partially open windows. The air thus admitted, being colder and consequently heavier than that at the temperature of the room, sinks to the lowest level. In so doing it creates drafts that produce discomfort and act only in the smallest degree to produce the desired effect of ventilation. The effect of window ventilation may be greatly improved by a simple expedient illustrated in Fig. 162. In this, the entering air meets a deflector in the form of a board or pane of glass that directs the cold air upward where it mingles with the heated air with the least production of a noticeable draft. This is the most efficient method of house ventilating where no special provision is made for the admission of fresh air.

The object sought in ventilating a room is to keep up the quality of the air by constant addition of fresh air, and in order to bring about a uniform purification of the entire atmosphere the entering air must be mixed with that already in the enclosure. If the discomforts of drafts are to be avoided, this mixing process must be brought about by admitting the cold air at the upper part of the room.

Warm air rises to the top of the room because it is lighter than the colder air beneath it. The coldest air is always lowest in point of elevation and unless there is some means to stir up the entire volume this condition will always remain the same.

When the easiest means of air for entering and leaving are near the floor, the cold entering air and that which goes out will always be in the lower part of the room, even when the supply is amply large. If no opportunity is given for the fresh air to mix with that already in the room, a poor average quality will result.

In the process of ventilation, the entering air should be admitted at, or directed toward, the highest part of the room, so that the pure cold air may have a chance to mix with that which is warmest. Air is not a good conductor of heat, and in mixing warm and cold air the cold particles will tend to float downward and take up heat from the warmer air with which it comes into contact, and thus produces a more uniform temperature.

The condition most to be desired is that of admitting cold air at a point where it will most readily mingle with the warm air from the source of heat. The reduction in temperature that must take place from this mixture will produce a gravitational circulation. Unfortunately this is not always possible to attain in an old building, but in the construction of a new building air ducts placed to admit air at points near the ceiling and located with reference to the supply of heat will bring about the best effect of ventilation.

The air which enters a room should, therefore, be near the top or so directed that the entering shaft will carry it upward. The air which is taken out of the room should leave from a point near the floor. In so doing it will tend to produce a more uniform quality and a more even distributor of the heat.

In order that the most desirable quality of atmosphere may be attained, there should be a constant supply of pure air entering and an equal amount discharging from the house. In the better-constructed dwelling such a condition is often provided through a ventilating flue that is a part of the chimney. This flue is arranged with registers placed to take air from the parts of the house requiring the greatest amount of air. Such an arrangement is shown in the picture in Fig. 163.

Fig. 164 shows the method of Fig. 163 combined with a direct means of admitting fresh air from the inside. The fresh air ducts should be provided with dampers to control the effect of extreme cold and wind.

Quantity of Air Discharged by a Flue.

—Any change of temperature of air produces a change equal to ¹/₄₉₁ part of its volume, for each degree variation. If a cubic foot of air is raised in temperature 1°F., its volume is ¹/₄₉₁ part larger than the original volume, and its buoyancy in the surrounding air is increased correspondingly. Air that has a temperature higher than that surrounding it will tend to rise because it is lighter. The air rising from a hot-air register or from a heated surface are illustrations of this condition.

Since the change of volume—or what is the same thing, its tendency to rise—increases ¹/₄₉₁ for each degree difference in temperature, the upward velocity of highly heated air will be very great. In warm air that fills a chimney flue or a room, the same tendency exists, the warmest air rises to the highest point and if the air can escape, as in the case of a chimney, a draft will result.

The draft of a chimney, in quiet air, is due to the difference in temperature between the air inside the flue above that outside the house. A chimney that does not “draw” and causes a stove to “smoke,” will often produce sufficient draft after the flue has been warmed. The upward movement of the warmer air in the flue produces a constantly increasing velocity, until it reaches the top of the chimney. This is an accelerated velocity that may be calculated by use of the formula given in physics, to express the velocity of accelerated motion. The well-known formula V = v(2gh) may be modified to express the conditions existing in a flue and permit of the calculation of the quantity of air discharged.

The upward flow of air in a chimney flue being due to the difference in temperature of the air in the flue over the outside air, the flow of air from the rooms will continue as long as the difference in temperature exists. Moreover, the air that is discharged from the rooms will be replenished from the outside, and for the air sent out of the flue a corresponding amount will be brought into the rooms through any openings that exist—door or windows or through cracks or crevices, depending on the completeness with which the house is closed. In no case is a house air-tight. The air pressure registered by the barometer is always the same inside as that outside the building. During cold weather, when the windows and doors are closed, the air is escaping through the chimney and also through every little crack and chink in the top of the rooms where the air is warmest. The colder air is entering at the same time through the joints about windows, door casings, through the crevices in the walls and particularly through the open joints made by the baseboards and the floor. This latter entrance of cold air is one of the commonest causes of cold floors. The shrinkage of the baseboards and floors from the quarter-round moulding which forms the joint leaves openings through which cold air is freely admitted from partitions and outside walls. The cold, heavier air remains near the floor because it can rise only when heated or forced upward by a draft. If the same air were permitted to enter at points near the ceiling and mingle with the warmest air in the room, a more uniform temperature would result, as well as better ventilation. The entering cold air, mixing with the warm air at the top of the room, would be reduced in its temperature and weight. The heavier air in falling would diffuse with the air beneath it and thus improve the general quality of the atmosphere.

It is important to remember that the discharge of air through a chimney flue will depend, in considerable amount, on the rate the new air is able to enter the house. In a new, tightly constructed house, the flue is often capable of discharging air much faster than it can enter, when it must find its way in through accidental openings. In such cases an open door or window immediately improves the draft of the stove.

The ventilation in the average dwelling is and must be accomplished by natural draft that is generated through difference in temperature of the air. The possibility of providing an acceptable system of continuous ventilation is confined to the draft of the chimney or to a flue provided especially for that purpose. Such being the case, the dimensions of flues constructed for ventilation should be the subject of investigation. The work that a chimney or ventilating flue has to do is continuous and will last throughout its lifetime; its proportions should therefore be considered with more than passing care.

It has been stated that the method of calculating volumes of air that will pass through a flue is based on the formula used to express the velocity of accelerated motion. The fundamental formula must be changed to suit the conditions produced when air is heated and made buoyant by expansion.

As has been stated, the change in temperature of air 1°F. causes an increase or decrease ¹/₄₉₁ part of its volume for each degree change. Any portion of air, warmer than that which surrounds it, tends to rise because of its lighter weight; the tendency to rise increases with the difference in temperature. The draft of a flue is caused by this condition of difference in temperature between the air inside the flue and the outside atmosphere.

In order that this general condition may be expressed in the simplest form let: T = the temperature inside the flue in degrees F.

t = the temperature outside the flue in degrees F.
H = the height of the flue in feet.

The quantity (T-t)/491 expresses the difference in temperature in degrees, divided by the change of volume for each degree. This gives the constant upward tendency of the air in passing through the flue. If this quantity is placed in the formula V = v(2gh), so as to exert its influence through the height of flue H, the condition may be expressed:

V = v(2g((T-t)/491)H)

The factor g, representing the acceleration of gravity, is constant and equal to 32 feet per second. The quantity 2g may be removed from under the radical and the formula becomes:

V = 8v(((T-t)/491)H)

The formula may now be used to express the volume of discharge of air from a flue. Suppose such a flue contains an area of 1 square foot in cross-section and that it is desired to estimate the air discharged from the flue per hour. The value of g is given in feet per second, and in order to make the formula express the volume of air discharged in cubic feet per hour, it must be multiplied by the number of seconds in an hour. Volume discharged in cubic feet per hour

= 60 × 60 × 8v(((T-t)/491)H) = 28,800v(((T-t)/491)H)

This formula applies to conditions such as will permit uniform movement of the air in a straight flue, uninfluenced by irregular, odd-shaped passages and rough surfaces. Moreover, it is supposed that the air may enter the house as rapidly as it escapes. The theoretical discharge will, in most instances, be less than the calculated amount, because the air cannot enter the house as fast as it may be discharged by the flue. It is a common custom to consider the theoretical flue only 50 per cent. efficient. As applied to the formula, the constant 28,800 when reduced 50 per cent. will become 14,400, and will be so used in the calculations as follows.

As an illustration of the application of the formula, suppose that the temperature in the house and in the flue is 70°F. and that the outside temperature is 20°F. The height of the chimney is 30 feet. The area of the flue is 1 square foot. Volume = 14,400 v(((T - t)/491)H)
= 14,400v(((70 - 20)/491) × 30)
= 25,140 cubic feet per hour.

Such a ventilating flue would be sufficient in size, under the conditions given, to furnish air at the rate of 25,140 cubic feet per hour or 30 cubic feet per minute to 13 persons, provided of course that the air could enter the building at the rate demanded. Where no provision is made for the air to enter the building it must find its way by the accidental openings. A common illustration of this effect may be noticed in the rate at which the fire of a stove will burn in a tightly closed room. The opening of a door or window causes an immediate increase of combustion, because of the extra air supply. It is evident that in well-constructed houses other means should be provided for admitting air than that of accidental opening.

The following table calculated by the above formula gives the quantity of air in cubic feet per hour discharged through a flue of 1 square foot cross-section. The table shows the calculated discharge from flues of heights varying from 15 to 40 feet, and with temperature differences from 10° to 100° between the outside air and that of the house.

In Fig. 163 is illustrated the form of chimney that is often used for the ventilation of dwellings. This is built with three flues. The flue to the left—marked A at the top—is intended to carry away the smoke and gases from the kitchen range. The flue to the right is that to which is connected the smoke pipe from the furnace. The flue in the middle marked B is for ventilation. Occupying as it does the space between the other two, it is kept warm by the heat of the other flues and the draft is thus increased. Openings to the flue are shown in the different floors at the points R and S. The openings are furnished with registers which may be regulated to suit the weather conditions.

The dimensions of such a flue may be calculated by the formula given or the area may be taken from the table to correspond with required conditions. In all cases flues should be made ample in size, as they must often do their maximum work under the poorest conditions for the production of good draft.

The amount of air discharged from the flue as given in the table is due to the gravitational effect alone. The suction produced by the wind adds in a very large degree to the amount of air discharged. The quantity of air that will flow from a 30-foot flue, by reason of the suction of the wind, blowing 7 miles per hour is equal to the same flue working by gravity with a temperature difference of 20°. With a wind velocity of 7 miles per hour and a temperature as given, the capacity of the flue is doubled. It is easy, therefore, to understand why the rate at which fires burn is so greatly increased by high winds. At the time of very high winds, a chimney flue will carry away three and even four times the volume discharged at the time of atmospheric calm.

Cost of Ventilation.

—The cost of good ventilation is often looked upon as prohibitive, because of the expense in heat necessary to keep the inside atmosphere at standard purity. Cost of ventilation is determined by analysis of the known conditions and calculations made of the amount of extra heat necessary to warm the greater volume of air.

The common practice of estimating the quantity of heat used in any form of heating or ventilation is by reference to the B.t.u. used in producing the desired condition. This unit, as has already been stated, is the amount of heat necessary to change a pound of water, 1°F.

In considering the cost of heating the air for ventilation, it must be borne in mind that the heat used in raising the temperature of the air contained in an enclosure is only a part of that necessary for warming the building. Most of the heat used goes to keep up the loss due to radiation and conduction which goes on from the windows, the walls and other parts of the building that are exposed to the outside cold. The material of which the building is composed must be heated and in turn radiates its heat to the colder outside air.

The quantity of heat necessary to change the temperature of a definite amount of air is easy of calculation. The problem is that of determining the number of heat units required to warm the necessary air to suit the average condition of weather. We will assume that the house is heated to the normal temperature 70°, and that the additional cost of heating the air for ventilation over the amount thus expended is the cost of ventilation.

Assuming that the house is so constructed that it is possible to supply air at the rate of 1000 cubic feet per hour to each person of a family of five, this condition will necessitate 5000 cubic feet of air per hour or 120,000 cubic feet of air per day.

The house is such that 10 tons of coal are required per year, at a cost of $10 per ton. The period of winter weather will be considered 5 months of 30 days each. This will be 150 days, during which the fuel for heating the house will cost 66²/₃ cents per day.

The average temperature of the outdoor air during the entire period will be assumed to be 20°F., thus requiring the air for ventilation to be changed 50° in order to raise it to the normal temperature, 70°.

The weight of a cubic foot of air at 70° is practically 0.075 pound. The 120,000 cubic feet of air used per day will, therefore, weigh 0.075 × 120,000 = 9000 pounds which must be raised 50° in temperature.

In order to express in B.t.u. the necessary heat required to produce the change of air temperature, the quantity of air is best stated in an equivalent amount of water. The specific heat of air is 0.237; that is, the amount of heat necessary to change a pound of air 1° is 0.237 of the amount used in changing 1 pound of water 1°. The 9000 pounds of air expressed as an equivalent amount of water will then be:

9000 × 0.237 = 2133 pounds of water.

This amount of water raised 1° is equivalent to raising 120,000 cubic feet of air 1°. Now the average change in the temperature of the air is 50°, so that 50 × 2133 will be the number of heat units used.

50 × 2133 = 106,650 B.t.u.

That is, 106,650 B.t.u. will be required to heat the air for ventilation one day.

In order to express this amount of heat in terms of fuel consumed, it will be assumed that the coal contained 14,000 B.t.u. per pound, this being a fair valuation of good coal. The average house-heating furnace will turn into available heat about 50 per cent. of the fuel burned. This value is taken from house-heating fuel tests made at the Iowa State College. The available heat in each pound of coal then will be 7000 B.t.u.

106,650 ÷ 7000 = 15.2 pounds of coal.

That is, 15.2 pounds of coal per day must be burned in order to furnish 1000 cubic feet of air per person each hour at the desired temperature.

At $10 a ton of 2000 pounds, the fuel costs ½ cent per pound. The cost of ventilation is, therefore, ½ × 15.2 = 7.60 cents a day, not an extravagant amount for good air.

It is evident that with the use of hot-air furnaces which take their entire amount of air from outdoors, the extra amount of heat necessary for this improved quality of atmosphere is very well expended. The use of ventilating devices adds only a relatively small amount to the total cost of heating and provides for the well-being of the occupants of the house—in the form of good air—an amount of healthfulness impossible of calculation.

The best ventilation is attained where a constant supply of fresh air is admitted to the house at points from which the best circulation may be secured and equal quantities of vitiated air are removed from the different apartments.

It is understood that in the process of natural ventilation the desired condition can only be approximated and that the permissible ventilation appliances are so placed as to give results such as to permit the air to follow the natural laws that must prevail.

If the house is heated by stoves, the outside air is best admitted near the ceiling, so that the cold air on entering may come into contact and mingle with the warmest air in the room. The circulation will by this method be effected by gravity.

In the use of the hot-air furnace, the air supply—as has already been explained in the figures on pages 55 and 58—is brought from the outside, where after being heated it enters the rooms through the registers placed near the floor. Being warmer than the air in the room, it tends to quickly rise. The currents set up by its motion help to produce a uniform temperature and to diffuse the new air through the entire space. The more evenly the air is distributed the more uniform will be the condition of temperature of the room.

In hot-water and steam heating, the direct method of heating in Fig. 29 and the indirect method of Fig. 30 show two forms of apparatus for admitting air to buildings that are quite generally employed for ventilation of dwellings. In the use of all such devices for ventilation purposes, there should be provided means of escape of air corresponding in amount to the fresh air admitted. The exhaust air vent should be located near the floor to bring about the best results. The degree of success attending the use of such apparatus will depend on the amount of care taken, to suit the position of the dampers to the prevailing weather.

The Wolpert Air Tester.

—The purity of air is expressed by quantity of carbonic acid gas included in its composition. In order to determine the degree of purity of any atmosphere the amount of contained gas must be determined. This is accomplished by use of simple apparatus that may be successfully operated by those who are unacquainted with chemical analytical methods. The process is due to chemical action but the manipulation of the required apparatus is purely mechanical.

Fig. 165 shows the Wolpert air tester which is a form of this apparatus that has given general satisfaction. The results attained by its use are approximate but sufficiently exact for all practical purposes. The apparatus consists of a graduated glass tube in which fits a rubber piston mounted on a hollow glass rod, through which the sample of air is admitted to the tube. The chemicals used for absorbing the carbonic acid gas are furnished with the instrument but may be replenished without difficulty. Directions for its use are furnished with the tester that may be readily followed after a trial. The results obtained are read directly from the side of the tube. The tester may be obtained from any dealer in chemical or physical apparatus.

Pneumatic Temperature Regulation.

—Pneumatic temperature regulation is very generally used in large and complicated heating systems, because of its positive action and completeness of heat control. This method of heat regulation utilizes the energy of compressed air, with which to open and close the valves of the radiators. It may be adapted to any mode of heating and can be used with any size of plant, but is particularly suited to extended systems. The radiators, providing heat for any particular space, are under control of separate thermostats, which by means of motor valves admit heat only as required. A motor, operated by compressed air, is attached directly to each radiator valve. Any change in temperature of the room causes the thermostat to correct in the radiator the required amount of heat.

With this method of regulation the temperature-controlling element of the thermostat, like that of the electro-thermostatic system, is a sensitive part, which by expanding and contracting with the heat and cold directly controls the heat in any part of the building. The motive power for opening and closing the valves of steam or hot-water radiators or for operating the dampers in a hot-air system is supplied by compressed air. The air supply is furnished by an air compressor which automatically stores air under pressure in a pressure tank, from which is drawn the necessary energy, as occasion demands. The air is conducted to the motors through small pipes which are connected with the regulating elements and also with the motors. The function of the thermostat is to so govern the air which enters the motor as to correct any change in the temperature of the rooms. This it does by opening and closing the valves as occasion demands.

In Fig. 166 is shown the arrangement of the thermostat T as it appears on the wall. Air from the supply tank is conveyed by the pipe A through the thermostat T to the motor valve V attached to the radiator. The function of the thermostat is that of so controlling the radiator valve by means of the motor V that the radiator will give out just sufficient heat to keep the room at the desired temperature. A closer view of the thermostat is given in Fig. 167.

The thermostat illustrated in Fig. 167 is that employed by the National Regulator Co. The drawing shows the exterior and interior construction of the parts enclosed in the previous illustration. The pipe C at the right and opening P at the left are the same as A in Fig. 169; likewise, the pipe D connects at the opening M of the motor valve in Fig. 169.

Referring again to Fig. 168, the sensitive part consists of a tube A of vulcanized rubber. It is the dark-shaded part in the left-hand drawing. Any change in the air temperature influences the length of this tube. The changing length of the tube effects the air supply to close the radiator valve when the temperature rises above the desired amount and to reopen it when more heat is required. A finely threaded screw passes through the plug H at the top and to this is secured the indicating disc X. The bottom of this screw is cupped to receive the point of the rod K, which connects with the piece L. Any change in length of the sensitive tube moves the valve lever O, and thus opens or closes the air port G.

Air under pressure is supplied by the pipe C, connected to the air supply, flowing into the thermostat through the filter P, the restriction S, the passage T, and the port G. The adjustment of the thermostat for different temperatures is provided for by the screw J through the top plug H, and the indicating disc X. The screw R in the connector Q at the base of the thermostat is a needle valve which opens or closes the connection with the air supply, and is used as an air shut-off valve when it is desired to remove the thermostat. The screw S is a restriction valve which controls the supply of air to the thermostat, and this screw is set so as to allow the air to pass in a restricted quantity.

When the temperature of the apartment has risen so as to expand the thermostatic element A, the pressure on K and L is relieved and the spring N closes the port G. The air admitted through the restriction screw S, since it cannot escape through the port G, accumulates in the passage Y and pipe D, filling the diaphragm and moving the valve into the position to decrease the supply of heat. When the temperature of the apartment has decreased so as to produce pressure on the connecting rod K, through the contraction of the thermostatic element A, the port G will be opened by the valve lever O, allowing the air in the pipe D, together with that which flows through the restriction S, to escape through the passage W to the atmosphere, allowing no air to accumulate in the pipe D, and thus permitting the spring at the diaphragm to actuate the damper or valve for more heat. The amount of air released through the port G by the valve lever O varies the pressure accumulated in the pipe D and produces the graduated or intermediate action desired.

A further application of air pressure in temperature regulation is that of the type of motor shown in Fig. 170. This device is intended to open and close dampers such as are used in the automatic regulation of temperature where heated air is used to warm the buildings. The operation of the motor is the same as that which controls the steam valve. The pressure exerted by the diaphragm is applied at A and the attachment to the damper is made at B. The motors indicated at V and N in Fig. 174 and D in Fig. 175 are examples of its application.

Mechanical Ventilation.

—Draft ventilation produced by open windows, flues and chimneys is influenced by extremes of temperature and by the force and changing direction of the wind; it is, therefore, but imperfectly controlled. The superiority of mechanical ventilation is generally recognized because the amount of entering air may be regulated to suit any circumstance and its temperature and humidity varied to conform to any desired atmospheric conditions. Mechanical ventilating plants are seldom employed in any but the more pretentious dwellings, but their use has extended to a degree that they are occasionally installed in apartment buildings and their further application is likely to grow. Neither the cost of installation nor the expense of operation is prohibitive in dwellings of the better types. Mechanical ventilation is quite generally employed in school buildings, auditoriums, hospitals, public buildings and others where means will permit, and there is a universal recognition of the effects of the agreeably conditioned air.

Mechanical ventilation may be accomplished by power-driven fans, either by exhausting the air from the building or by forcing air into it, and under some conditions a combination of the two methods is used.

The exhaust method of ventilation is that in which air is blown out of the building by a fan; and the supply, to replenish that taken away, is conducted into the building through ducts prepared for the purpose. In some cases the induced air supply leaks into the rooms through the joints in the doors and windows, and through the accidental crevices. In Fig. 171 is shown a simple exhaust fan installed to produce such a change of air. It is suitable for kitchens and other places where it is desired to eliminate smoke or gases rather than to produce a supply of air. With this apparatus the air of the room is blown out by the rotating fan and new air to take the place of that exhausted is drawn in at any convenient opening.

The Plenum Method.

—That form of mechanical ventilation by means of which air is forced into the rooms is known as the plenum method. It is the most positive means of air supply because its action is attended by a slight pressure above the outside air; it is continuous in action and the amount of entering air is under control. The escape of the expelled air is made through vent flues especially constructed for the purpose.

This represents the draw-through or induced-draft type of ventilation apparatus. The air delivered by the fan induces a flow of outside air which is drawn through the heating coils and discharged through the opening E. At this point it enters the main ventilation duct from which it is distributed by branch conduits throughout the building.

The temperature of the air sent out from the fan is regulated by the steam valves of the heater coils to suit the prevailing conditions. Under some installations of this character the ventilating air is made to furnish the heat necessary to warm the building as well as to provide its air supply. As ordinarily used, however, the temperature of the ventilating air is the same as that of the room.

The method of conveying air to the various apartments depends entirely on local conditions. The conduits may be made of sheet iron, placed to suit the existing conditions; but when a building is constructed with a ventilating plant in view as a part of the building equipment, it is customary to make the ducts part of the partitions. In brick buildings the ducts are constructed, so far as it is practicable, in the walls. These ducts are made in size and arrangement to suit the amount of air required for each room. When the plant is put into operation each duct is tested with an anemometer which indicates the velocity of the entering air. The calculated amount of air, determined by the velocity and area of the entering column, when compared with the necessary quantity demanded for good ventilation, gives the efficiency of the system.

Air Conditioning.

—In addition to the possibility of a constant supply of air, a combination of the exhaust and plenum methods admits of air purification. With such a plant, the air may be washed free from all suspended dust or gases and moistened to any degree of humidity. The process of washing and humidifying air is known as air conditioning. Apparatus for air conditioning is made in a variety of forms to produce any desired extent of air purification and any degree of humidity. The plant may be regulated by hand or it may be made entirely automatic in its action. Air-conditioning plants may be arranged to produce air that is purified, humidified and warmed during winter weather and in summer the hot humid atmosphere may be cooled and dehumidified to a temperature and percentage of moisture that is most agreeable.

Conditioned air is often used in manufactories, not for the purpose of supplying good air to the employees but because of the effect of the atmospheric air on the products. The manufacture of textile fabrics often demands a constant atmospheric humidity in order that the material produced may be uniform in grade; this is particularly true in the making of silks. Various manufactories require an atmosphere free from lint and dust in order that the best quality of material may be produced. The air for ventilation in such places is washed free from all suspended matter before being sent into the building.

In Fig. 173 is indicated an application of apparatus similar in construction to that just described. The arrangement of the parts is such as to produce a Plenum hot-air system of ventilation and temperature regulation.

The plant occupies a room in the basement and the drawing shows the method of heating, together with the plan of distribution. The air duct leading to the room above furnishes an example of the manner of admitting the warmed air to the rooms. The dampers C₁, C₂, etc., are controlled by separate motors. The motor M is under the control of the thermostat T in the room above. Any change of temperature in the room is corrected by the damper to admit cold or warm air as is desired.

The power-driven fan F draws in outdoor air from an opening A, through a set of heater coils H₁, in which it is raised considerably in temperature. The heater in this case is a coil of steam pipes. The air—after being warmed—is taken into the fan and from it may be sent through a second set of coils H₂, to receive additional heat, or if already sufficiently warmed the air from the fan may pass under the second set of coils and receive no heat from them. Under the first heater coil is also a bypass which may be opened by the motor N to admit cold air that is drawn directly into the fan. The movement of the air through these bypasses is under control of the thermostat, which causes the motor N to open or close the bypass to suit the temperature of the room. When the bypass is opened the steam is shut off from the heater coils.

Examination of the drawing will show that the air from the fan may pass through a second heater H₂, to the place marked warm air, or it may pass under the heater to the compartment marked cold air. The amount of warm and cold air which enters the duct is regulated by the position of the dampers C.

The position of the dampers C, which is controlled by the motors M, is made to take amounts of cold or warm air to produce the desired temperature in the rooms. The motors C₁, etc., are under control of the thermostat in each room. Any change of temperature in the room will immediately affect the thermostat. The effect on the thermostat will so change the air pressure in the motor that the valve C is moved to correct the difference in room temperature. If warm air is demanded, the motor changes the damper C to close the cold-air supply and take air that must pass through the heater coils H₂. If only cold air is desired the damper will turn to shut off the course through the heaters and admit air directly from outdoors.

Humidifying Plants.

—Mechanical ventilation plants that are intended for washing the air may be made up of parts similar to that of Fig. 173, but in addition to the apparatus shown provision is made for the air to pass through a chamber filled with a spray of water. The air in passing through this spray is washed free of dust and at the same time absorbs water necessary for its desired humidity.

The humidity of air may be increased by the addition of moisture or decreased (dehumidified) by raising its temperature, thereby increasing its capacity for containing moisture. Suppose that air at 50° is saturated with moisture; it will contain practically 4 grains of water per cubic foot. If now the temperature of the air is raised to 70°, the same amount of air is capable of containing 8 grains of water and is, therefore, only 50 per cent. saturated.

Humidification is accomplished in air-conditioning plants through one of two general methods: by the evaporation type of apparatus, in which the passing air absorbs moisture from contact with a large area of water; or the spray method, in which the water is broken into a very fine spray by a specially devised nozzle and thus rendered easy of absorption by the air to be moistened. A third method is sometimes employed, in which steam is introduced into the air supply. Steam is already vaporized water and immediately becomes a part of the air without further change. The steam type of humidifying plant possesses features that limit its application, in that the steam in some cases may possess objectionable odor or includes the vapor of grease, either of which would materially effect its use. Further, the heat of vaporization liberated by the condensing steam is also a factor that influences the temperature of the air and in case of direct humidification must receive special attention.

Vaporization as a Cooling Agent.

—The evaporation of water has a distinct value aside from humidifying the air, in that the cooling effect is in direct proportion to the added moisture. In the process of evaporation the heat necessary to change the water into vapor is taken from the surrounding air and the temperature is thus materially lowered.

In practical air-conditioning apparatus, of the evaporative or spray types, the process consists of drawing the outside air into a chamber filled with falling water that is broken up into drops like rain or spray. In passing, every particle of the air comes into contact with the water drops; the almost invisible particles of dust adhere to the water and are carried away leaving the air washed clean. In addition to freeing the air from dust, the intimate mixture of the air permits of a ready absorption of the water, which is taken up to any per cent. of saturation. After leaving the spray chamber, the moisture-laden air passes through an eliminator in which any unabsorbed moisture is extracted. It is possible for air to become not only completely saturated with water under the conditions encountered in a humidifying plant, but in addition, the movement of the air may carry along unabsorbed particles that are precipitated directly after leaving the spray chamber. For this reason the air is passed through an eliminator.

The eliminator is composed of a series of irregular sheet-metal surfaces so arranged that the air is required to abruptly change its direction several times in its passage of a short distance. The impact of the air against the surfaces and the centrifugal force exerted by the sudden changes of direction throw out the excess moisture and any remaining suspended matter the air may contain.

The saturated air from the eliminator passes through a heater where the temperature is raised to that of the rooms. In the rise of temperature the air which is saturated is rendered capable of absorbing more moisture, and hence has been dehumidified. The rise of temperature and the corresponding decrease in relative humidity is intended to be such as to leave in the finished air the desired percentage of moisture.

Air-cooling Plants.

—The use of air-washing and humidifying plants so far mentioned has been confined to elimination of dust and the addition of moisture to air, under winter conditions. The same type of apparatus, used in summer, becomes a cooling plant, and by observance of the necessary requirements may be used to produce agreeable atmospheric conditions during hot weather.

When used for such purpose the air is washed, by passing it through falling water which frees it from dust and reduces its temperature. It is then further cooled by passing over cold surfaces that take the place of the heaters used in cold weather. The cooling surfaces are pipe coils kept cold by the contained water which comes from the water supply or from a refrigerating plant. The temperature and humidity are thus changed to suit the requirements of the conditioned air.

During the hot weather of summer the most disagreeable atmospheric condition is that caused by humidity near saturation, at a time of relatively high temperature. Under such conditions the cooling plant not only cools the air, but causes a precipitation of the moisture on the cold surfaces which are kept below the dew-point. The air is cooled and dehumidified to a point such that the conditioned air produces an agreeable atmosphere. The regulation of the degree to which the air is cooled is accomplished by the same general methods as are used in heating.

Humidity Control.

—The method of regulating atmospheric humidity in a humidifying plant will be determined by the conditions under which it is intended to work. There are a variety of means employed that may be used to bring about the same effects, each of which is particularly suited to certain requirements. The present object is to describe the essential features of airconditioning plants, by use of illustrations representing each of the three methods mentioned above. That of the ventilation of a school building under winter conditions will be taken as an example.

In Fig. 174 is shown a heating and ventilating system in which the air conditioning is accomplished by automatic regulators for both temperature and humidity. The plant occupies a room in the basement, and a room directly above illustrates the conditions that prevail in all of the other rooms of the building. The principal features of the plant are the fan G, which supplies the air; the hot-air furnace H, which furnishes the heat; and the water spray S, which provides the moisture with which the air is humidified.

The air is drawn in at A to a room in which a motor-driven fan G forces the supply through the heating apparatus into the building. The air after leaving the fan passes through a cold-air duct C to the heating surfaces H to be warmed. The air in passing over the heating surfaces is raised to a degree considerably above the temperature of the rooms. The hot air leaving the heater H enters the tempered air chamber T through the passage K. A damper M provides means for also admitting cold air to the chamber T directly from the fan. The thermostat, located at O, is connected with a pneumatic motor V (similar to Fig. 170) which regulates the supply of cold and hot air from K and M to suit the desired temperature of the air supply for the rooms above. The arm of the motor V is so arranged that an upward movement opens the cold-air and closes the hot-air passages; the downward movement produces the opposite effect. The motor V thus controls the temperature of the air.

In this system the air is humidified by a direct water spray marked S in the drawing. A part of the hot air from the heater H may escape through the damper W and absorb water on its way to the duct D, which takes the air to the room above, where it enters through the register E. This air as it comes from the heater, being hot, will absorb a larger amount of water than the air could hold when cooled to room temperature; for this reason only a part of the air supply is humidified. The supply of the hot humid air is admitted to the duct D in such quantity as will produce the desired degree of humidity in the rooms.

The degree of room temperature is governed by the thermostat, in the room, which, by means of the motor N, controls the damper F. This damper admits hot humid air and the tempered air from the chamber T in proper proportion. At any time the humidity of the air in the room reaches the maximum amount for which it is set, the humidostat, through its motor, closes the valve R, which controls the water supply to the spray nozzle, and the moisture in the air is reduced until a further amount is demanded. With apparatus of this kind the temperature and humidity may be kept practically constant.

Fig. 175 shows another arrangement of a similarly controlled plant in which steam is used for humidifying the air. The air is admitted at A, from whence it passes through a steam-heating coil S, which raises it to a predetermined temperature. The steam jets are arranged at H, for providing the necessary moisture. The humidostat through a motor valve V governs the amount of steam that is permitted to enter the humidifying chamber. A thermostat located in the air duct at B controls the temperature of the air sent to the rooms by regulating the amount of heat given out by the steam coils S. This control is made still more sensitive by use of a cold-air bypass. The damper D is opened by a motor valve to admit cold air at the same time the steam is shut off from the heater coils.

In this plant the ventilating air is not intended to supply all of the heat to the rooms. A thermostat on the wall controls the room temperature by regulating the amount of steam admitted to the radiators. In the ventilating plant previously described, all of the heat for the building is supplied through the ventilating system; in the plant shown in Fig. 175, the heating apparatus which warms the building is entirely separate and may be used when the ventilating system is inoperative.

The humidity is controlled by admitting saturated air to the warmer air of the rooms in such quantity as will produce the desired mixture. The humidostat, on the left-hand wall, regulates the quantity of moisture by opening or closing the steam valve V as occasion requires.

Another example of air-conditioning plant similar in principle to that just described is often called the dew-point system. It depends for its action on a definite dew-point temperature at which the air is saturated with moisture, before being heated to room temperature. The air to be conditioned is first warmed, by passing through a set of tempering coils, to a degree at which it will contain the necessary moisture when saturated. After saturation the temperature is raised by a second set of heating coils to the room temperature, the moisture contained being right to give the desired humidity.

To illustrate, suppose that it is desired to maintain a constant humidity of 50 per cent. saturation at 70°F. in the building. The temperature at which the air must be saturated, to contain 4 grains of moisture per cubic foot, is found in the table on page 199 to be 48°F.

The entering air must first be raised to that temperature by the tempering coils. The air then enters the spray chamber where it absorbs moisture to saturation, by contact with a multitude of water particles. This saturated air now passes through a second set of heated coils and takes up heat sufficient to raise it to the finished temperature.

The dew-point temperature at which the air enters the spray chamber and the final temperature are kept constant by motor-operated valves which supply the heating coils with the necessary heat in the form of steam. The motors are controlled by thermostats, placed to measure the temperature of the air as it enters the saturator and the finished air as it enters the rooms. If these conditions are now kept constant, the finished air will be constantly 50 per cent. saturated.

A plant of this character is illustrated in Fig. 176. The figure shows the exterior of the casings which enclose the tempering coils and saturator at A, the eliminator at B, and the heating coils at C. This is another draw-through type of plant where a fan, enclosed in D, draws the air through the conditioning apparatus and forces it through the sheet-iron ducts E. The passages in the walls—as indicated by the arrows—conduct the air through the register R, into the room. The register S represents the discharge duct through which the vitiated air is forced from the room.

In this system of air conditioning, all of the ventilating air is to be saturated with moisture at a temperature such that when raised to room temperature will contain the desired percentage of humidity. The saturator occupies the space between A and B. A number of spray jets are arranged to fill the entire space with water drops that are moving in every direction. The air, as it passes, must come into contact with the drops again and again, until by repeated impact each particle is completely saturated and at the same time washed free from dust. It has already been explained that the movement of the saturated air through a mass of spray will carry with it a considerable amount of unabsorbed water that must be taken out by an eliminator. A section of the casing is broken out at B, showing the eliminator plates. The irregular surfaces of these plates repeatedly change the direction of the passing air, and the suspended water or remaining solid matter is thrown against the surfaces where they adhere. The moisture accumulates in drops of water that run down the plates to the bottom of the enclosure and finally into the sewer.

From the eliminator the air passes through the heating coils enclosed in C, where it is heated to the necessary temperature for admission to the rooms.

The regulation of the temperature of the tempering coils and heating coils is accomplished as in the other plants described. The thermostats with their motors operate the valves of the heaters to admit steam sufficient to keep constant temperatures at the different parts. The humidity is maintained at a constant amount by saturating the air at a constant temperature and therefore no humidostat is required.