VENTILATION
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.
-----+-----+-----+-----+-----+---+---+--------------------------
6 | 7 | 8 | 9 | 10 | 15| 20| = Parts of CO₂ per 10,000
-----+-----+-----+-----+-----+---+---+--------------------------
3,000|2,000|1,500|1,200|1,000|545|375| = Cubic feet of pure air
| | | | | | | per hour
-----+-----+-----+-----+-----+---+---+--------------------------
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.
[Illustration: FIG. 162.--A simple expedient for the prevention of
drafts and improving ventilation.]
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.
[Illustration: FIG. 163.--A chimney flue used as a ventilator.]
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.
[Illustration: FIG. 164.--Method of admitting cold air into rooms so as
to produce the best condition of ventilation.]
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_ = √(2_gh_) 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_ = √(2_gh_), so as to
exert its influence through the height of flue _H_, the condition may
be expressed:
_V_ = √(2_g_((_T_-_t_)/491)_H_)
The factor _g_, representing the acceleration of gravity, is constant
and equal to 32 feet per second. The quantity 2_g_ may be removed from
under the radical and the formula becomes:
_V_ = 8√(((_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 × 8√(((_T_-_t_)/491)_H_)
= 28,800√(((_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 √(((_T_ - _t_)/491)_H_)
= 14,400√(((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.
--------+-------------------------------------------------------------
Height | Temperature of air in the flue above that of external air
of flue +--------+--------+--------+--------+--------+--------+-------
in feet | 10 | 15 | 20 | 25 | 30 | 50 | 100
--------+--------+--------+--------+--------+--------+--------+-------
15 | 7,980 | 9,720 | 11,280 | 12,550 | 13,800 | 17,820 | 25,140
--------+--------+--------+--------+--------+--------+--------+-------
20 | 9,180 | 11,180 | 13,080 | 14,520 | 15,900 | 20,520 | 29,040
--------+--------+--------+--------+--------+--------+--------+-------
25 | 10,260 | 12,600 | 14,520 | 16,260 | 17,820 | 22,980 | 32,460
--------+--------+--------+--------+--------+--------+--------+-------
30 | 11,280 | 13,800 | 15,900 | 17,825 | 19,500 | 25,140 | 35,580
--------+--------+--------+--------+--------+--------+--------+-------
35 | 12,180 | 14,880 | 17,160 | 19,200 | 21,060 | 27,180 | 38,400
--------+--------+--------+--------+--------+--------+--------+-------
40 | 13,020 | 15,900 | 18,360 | 20,520 | 22,500 | 29,040 | 40,980
--------+--------+--------+--------+--------+--------+--------+-------
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 66-2/3 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 1/2 cent per pound. The
cost of ventilation is, therefore, 1/2 × 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.
[Illustration: FIG. 165.--The Wolpert air tester; an instrument used to
determine the quality of air.]
=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.
[Illustration: FIG. 166.--Thermostat regulator and motor-valve attached
to a radiator.]
=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.
[Illustration: FIG. 167.--Outside view of thermostat as it appears in
use.]
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.
[Illustration: FIG. 168.--Internal construction of the National
Regulator Co.’s thermostatic regulator.]
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.
[Illustration: FIG. 169.--Cross-section of pneumatic radiator valve
showing its internal construction.]
[Illustration: FIG. 170.--Pneumatic motor valve for automatic control
of dampers, etc.]
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.
[Illustration: FIG. 171.--Exhaust fan for induced ventilation.]
[Illustration: FIG. 172.--Ventilation apparatus in which is included
the heater coils, the fan and the motor.]
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.
=Ventilation Apparatus.=--Fig. 172 illustrates the form of apparatus
used for ventilating buildings where no attempt is made at washing or
humidifying the air. Enclosed in a sheet-iron case _C_ is a fan which
is driven by the electric motor _M_. The capacity of the fan, for the
delivery of air, is made to suit the requirements of the building. In
this case the fan is secured to an extension of the armature shaft of
the motor. Connecting with the case which encloses the fan is another
sheet-iron box _H_, containg coils of heating pipe. The heating
apparatus is designed to change the temperature of the entering air to
suit the requirements of the building.
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.
[Illustration: FIG. 173.--Plenum hot-blast heating system with
temperature regulation.]
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.
[Illustration: FIG. 174.--Furnace blast system of heating, with
temperature regulation and humidity control.]
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.
[Illustration: FIG. 175.--Direct steam heating system with mechanical
fan-blast ventilation, temperature regulation and humidity control.]
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.
[Illustration: FIG. 176.--School building section showing a complete
air-conditioning plant.]
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.