=Ventilation=
=Definition.=--The air within an uninhabited room does not differ from
that without. If the room is occupied by one or more individuals,
however, then the air in the room soon deteriorates, until the
impurities therein reach a certain degree incompatible with health.
This is due to the fact that with each breath a certain quantity of
CO2, organic impurities, and aqueous vapor is exhaled; and these
products of respiration soon surcharge the air until it is rendered
impure and unfit for breathing. In order to render the air pure in
such a room, and make life possible, it is necessary to change the air
by withdrawing the impure, and substituting pure air from the outside.
This is _ventilation_.
_Ventilation_, therefore, is the maintenance of the air in a confined
space in a condition conducive to health; in other words, "ventilation
is the replacing of the impure air in a confined space by pure air
from the outside."
=Quantity of Air Required.=--What do we regard as impure air? What is
the index of impurity? How much air is required to render pure an air
in a given space, in a given time, for a given number of people? How
often can the change be safely made, and how? These are the problems
of ventilation.
An increase in the quantity of CO2 [carbon dioxide gas], and a
proportionate increase of organic impurities, are the results of
respiratory vitiation of the air; and it has been agreed to regard the
relative quantity of CO2 as the standard of impurity, its increase
serving as an index of the condition of the air. The normal quantity
of CO2 in the air is 0.04 per cent, or 4 volumes in 10,000; and it has
been determined that whenever the CO2 reaches 0.06 per cent, or 6
parts per 10,000, the maximum of air vitiation is reached--a point
beyond which the breathing of the air becomes dangerous to health.
We therefore know that an increase of 2 volumes of CO2 in 10,000 of
air constitutes the maximum of admissible impurity; the difference
between 0.04 per cent and 0.06 per cent. Now, a healthy average adult
at rest exhales in one hour 0.6 cubic foot of CO2. Having determined
these two factors--the amount of CO2 exhaled in one hour and the
maximum of admissible impurity--we can find by dividing 0.6 by 0.0002
(or 0.02 per cent) the number of cubic feet of air needed for one
hour,==3,000.
Therefore, a room with a space of 3,000 cubic feet, occupied by one
average adult at rest, will not reach its maximum of impurity (that
is, the air in such a room will not be in need of a change) before one
hour has elapsed.
The relative quantity of fresh air needed will differ for adults at
work and at rest, for children, women, etc.; it will also differ
according to the illuminant employed, whether oil, candle, gas,
etc.--an ordinary 3-foot gas-burner requiring 1,800 cubic feet of air
in one hour.
It is not necessary, however, to have 3,000 cubic feet of space for
each individual in a room, for the air in the latter can safely be
changed at least three times within one hour, thus reducing the air
space needed to about 1,000 cubic feet. This change of air or
ventilation of a room can be accomplished by mechanical means oftener
than three times in an hour, but a natural change of more than three
times in an hour will ordinarily create too strong a current of air,
and may cause draughts and chills dangerous to health.
In determining the cubic space needed, the height of the room as well
as the floor space must be taken into consideration. As a rule the
height of a room ought to be in proportion to the floor space, and in
ordinary rooms should not exceed fourteen feet, as a height beyond
that is of very little advantage.[13]
=Forces of Ventilation.=--We now come to the question of the various
modes by which change in the air of a room is possible. Ventilation is
natural or artificial according to whether artificial or mechanical
devices are or are not used. Natural ventilation is only possible
because our buildings and houses, their material and construction, are
such that numerous apertures and crevices are left for air to come in;
for it is evident that if a room were hermetically air-tight, no
natural ventilation would be possible.
The properties of air which render both natural and artificial
ventilation possible are diffusion, motion, and gravity. These three
forces are the natural agents of ventilation.
There is a constant diffusion of gases taking place in the air; this
diffusion takes place even through stone and through brick walls. The
more porous the material of which the building is constructed, the
more readily does diffusion take place. Dampness, plastering,
painting, and papering of walls diminish diffusion, however.
The second force in ventilation is the motion of air or winds. This is
the most powerful agent of ventilation, for even a slight,
imperceptible wind, traveling about two miles an hour, is capable,
when the windows and doors of a room are open, of changing the air of
a room 528 times in one hour. Air passes also through brick and stone
walls. The objections to winds as a sole mode of ventilation are their
inconstancy and irregularity. When the wind is very slight its
ventilating influence is very small; on the other hand, when the wind
is strong it cannot be utilized as a means of ventilation on account
of the air currents being too strong and capable of exerting
deleterious effects on health.
The third, the most constant and reliable, and, in fact, principal
agent of ventilation is the specific gravity of the air, and the
variations in the gravity and consequent pressure which are results of
the variations in temperature, humidity, etc. Whenever air is warmer
in one place than in another, the warmer air being lighter and the
colder air outside being heavier, the latter exerts pressure upon the
air in the room, causing the lighter air in the room to escape and be
displaced by the heavier air from the outside, thus changing the air
in the room. This mode of ventilation is always constant and at work,
as the very presence of living beings in the room warms the air
therein, thus causing a difference from the outside air and effecting
change of air from the outside to the inside of the room.
=Methods of Ventilation.=--The application of these principles of
ventilation is said to be accomplished in a natural or an artificial
way, according as mechanical means to utilize the forces and
properties of air are used or not. But in reality natural ventilation
can hardly be said to exist, since dwellings are so constructed as to
guard against exposure and changes of temperature, and are usually
equipped with numerous appliances for promoting change of air.
Windows, doors, fireplaces, chimneys, shafts, courts, etc., are all
artificial methods of securing ventilation, although we usually regard
them as means of natural ventilation.
=Natural Ventilation.=--The means employed for applying the properties
of diffusion are the materials of construction. A porous material
being favorable for diffusion, some such material is placed in several
places within the wall, thus favoring change of air. Imperfect
carpenter work is also a help, as the cracks and openings left are
favorable for the escape and entrance of air.
Wind, or the motion of air, is utilized either directly, through
windows, doors, and other openings; or indirectly, by producing a
partial vacuum in passing over chimneys and shafts, causing suction of
the air in them, and the consequent withdrawal of the air from the
rooms.
The opening of windows and doors is possible only in warm weather; and
as ventilation becomes a problem only in temperate and cold weather,
the opening of windows and doors cannot very well be utilized without
causing colds, etc. Various methods have therefore been proposed for
using windows for the purposes of ventilation without producing
forcible currents of air.
The part of the window best fitted for the introduction of air is the
space between the two sashes, where they meet. The ingress of air is
made possible whenever the lower sash is raised or the upper one is
lowered. In order to prevent cold air from without entering through
the openings thus made, it has been proposed by Hinkes Bird to fit a
block of wood in the lower opening; or else, as in Dr. Keen's
arrangement, a piece of paper or cloth is used to cover the space left
by the lifting or lowering of either or both sashes. Louvers or
inclined panes or parts of these may also be used. Parts or entire
window panes are sometimes wholly removed and replaced by tubes or
perforated pieces of zinc, so that air may come in through the
apertures. Again, apertures for inlets and outlets may be made
directly in the walls of the rooms. These openings are filled in with
porous bricks or with specially made bricks (like Ellison's conical
bricks), or boxes provided with several openings. A very useful
apparatus of this kind is the so-called Sheringham valve, which
consists of an iron box fitted into the wall, the front of the box
facing the room having an iron valve hinged along its lower edge, and
so constructed that it can be opened or be closed at will to let a
current of air pass upward. Another very good apparatus of this kind
is the Tobin ventilator, consisting of horizontal tubes let through
the walls, the outer ends open to the air, but the inner ends
projecting into the room, where they are joined by vertical tubes
carried up five feet or more from the floor, thus allowing the outside
air to enter upwardly into the room. This plan is also adapted for
filtering and cleaning the incoming air by placing cloth or other
material across the lumen of the horizontal tubes to intercept dust,
etc. McKinnell's ventilator is also a useful method of ventilation,
especially of underground rooms.
[Illustration: FIG. 5.
HINKES BIRD WINDOW. (TAYLOR.)]
[Illustration: FIG. 6.
ELLISON'S AIR INLETS. (KNIGHT.)]
[Illustration: FIG. 7.
SHERINGHAM VALVE. (TAYLOR.)]
[Illustration: FIG. 8.
THE TOBIN VENTILATOR. (KNIGHT.)]
[Illustration: FIG. 9.
McKINNELL'S VENTILATOR. (TAYLOR.)]
To assist the action of winds over the tops of shafts and chimneys,
various cowls have been devised. These cowls are arranged so as to
help aspirate the air from the tubes and chimneys, and prevent a down
draught.
The same inlets and outlets which are made to utilize winds may also
be used for the ventilation effected by the motion of air due to
difference in the specific gravity of outside and inside air. Any
artificial warming of the air in the room, whether by illuminants or
by the various methods of heating rooms, will aid in ventilating it,
the chimneys acting as powerful means of removal for the warmer air.
Various methods have also been proposed for utilizing the chimney,
even when no stoves, etc., are connected with it, by placing a
gaslight within the chimney to cause an up draught and consequent
aspiration of the air of the room through it.
[Illustration: FIG. 10.
VENTILATING THROUGH CHIMNEY. (KNIGHT.)]
The question of the number, relative size, and position of the inlets
and outlets is a very important one, but we can here give only an
epitome of the requirements. The inlet and outlet openings should be
about twenty-four inches square per head. Inlet openings should be
short, easily cleaned, sufficient in number to insure a proper
distribution of air; should be protected from heat, provided with
valves so as to regulate the inflow of air, and, if possible, should
be placed so as to allow the air passing through them to be warmed
before entering the room.[14] Outlet openings should be placed near
the ceiling, should be straight and smooth, and, if possible, should
be heated so as to make the air therein warmer, thus preventing a down
draught, as is frequently the case when the outlets become inlets.
[Illustration: FIG. 11.
COWL VENTILATOR. (KNIGHT.)]
=Artificial Ventilation.=--Artificial ventilation is accomplished
either by aspirating the air from the building, known as the vacuum or
extraction method, or by forcing into the building air from without;
this is known as the plenum or propulsion method.
The extraction of the air in a building is done by means of heat, by
warming the air in chimneys or special tubes, or by mechanical means
with screws or fans run by steam or electricity; these screws or fans
revolve and aspirate the air of the rooms, and thus cause pure air to
enter.
[Illustration: FIG. 12.
AN AIR PROPELLER.]
The propelling method of ventilation is carried out by mechanical
means only, air being forced in from the outside by fans, screws,
bellows, etc.
Artificial ventilation is applicable only where a large volume of air
is needed, and for large spaces, such as theaters, churches, lecture
rooms, etc. For the ordinary building the expense for mechanical
contrivances is too high.
On the whole, ventilation without complex and cumbersome mechanisms is
to be preferred.[15]
FOOTNOTES:
[13] In cerebro-spinal meningitis, tuberculosis, and pneumonia, fresh
air is curative. Any person, sick or well, cannot have too much fresh
air. The windows of sleeping rooms should always be kept open at
night.--EDITOR.
[14] These outlets may be placed close to a chimney or heating pipes.
Warm air rises and thus will be forced out, allowing cool fresh air to
enter at the inlets.--EDITOR.
[15] The ordinary dwelling house needs no artificial methods of
ventilation. The opening and closing of windows will supply all
necessary regulation in this regard. The temperature of living rooms
should be kept, in general, at 70 deg. F. Almost all rooms for the sick
are unfortunately overheated. Cool, fresh air is one of the most
potent means of curing disease. Overheated rooms are a menace to
health.--EDITOR.