GENERAL PRINCIPLES OF VENTILATION.
=The Amount of Air required.=—Ventilation is chiefly concerned with
the removal of the products of respiration, just as sewage is chiefly
concerned with the removal of the solid and liquid excreta.
In a less degree it is required for removing the impurities produced by
the burning of gas, candles, and lamps. The main problem, however, is
the removal of the respiratory products.
The amount of carbonic acid in air is usually fairly proportional to
that of the other respiratory products. It may therefore be taken as
a measure of the impurity of the air. There are, however, certain
fallacies in this test. In soda water manufactory, for instance, there
would be a comparatively harmless excess of carbonic acid. In dirty
rooms, and in hospitals and other institutions where rooms are not
vacated for a considerable period, the amount of organic matter present
is often in excess of what would have been anticipated, judging by
an estimation of the carbonic acid. This is strikingly shown by some
valuable researches at Dundee, which are summarised in the following
table. If we take the average amount (in excess of outside air) of
carbonic acid, organic matter, and micro-organisms respectively in
houses of four or more rooms as unity, then in one or two-roomed houses
or tenements we have as follows:—
┌──────────────────┬──────────────┬──────────┬──────────┐
│ │HOUSES OF FOUR│TWO-ROOMED│ONE-ROOMED│
│ │ ROOMS │ HOUSES. │ HOUSES. │
│ │ AND UPWARDS. │ │ │
├──────────────────┼──────────────┼──────────┼──────────┤
│_Carbonic acid_ │ 1 │ 1·5 │ 2·0 │
│_Organic matter_ │ 1 │ 1·6 │ 4·4 │
│_Micro-organisms_ │ 1 │ 5·1 │ 6·7 │
└──────────────────┴──────────────┴──────────┴──────────┘
It is evident that in these cases the carbonic acid did not increase in
the same proportion as the organic matter and micro-organisms, and that
it alone does not form a sufficient test of the impurity of any given
atmosphere. The amount of carbonic acid, however, is a valuable and
convenient test of the condition of the air of a room, and the problems
of ventilation, of which examples are given on page 137, are based on
its amount.
The =Standard of purity= is somewhat difficult to fix. The external air
ought only to contain 4 parts of carbonic acid to 10,000 parts; but
it is almost impossible to maintain this degree of purity in inhabited
rooms. The experiments made by Drs. Parkes and De Chaumont showed that
when the carbonic acid is ·06 per cent., or in the proportion of 6
parts in 10,000 of air, the air begins to be perceptibly stuffy (page
125); this may therefore be taken as the limit of impurity. Pettenkofer
has adopted the limit of ·07 per cent.[7]
The problem then is to discover the amount of pure external air
(containing ·04 per cent. of carbonic acid) that will be required to
pass hourly through a room, for every person in that room, in order to
keep the carbonic acid at the ratio of ·06 per cent.
This may be ascertained by actual observation of the air of rooms in
which a given number of persons are placed; or by calculations from
physiological data.
As _the result of numerous experiments_ on the atmosphere of prisons,
barracks, etc., where the amount of fresh air supplied per hour is
exactly known, it is found that in order to keep the carbonic acid at
·06 per cent., 3,000 cubic feet of pure air are required per head per
hour; 2,000 cubic feet keep the carbonic acid at ·07 per cent.; 1,500
cubic feet at ·08 per cent.; and 1,200 cubic feet at ·09 per cent.
For the removal of the products of combustion of gas, an additional
supply of air is required, for the amount of which, see page 116.
Where a number of sick persons are collected, as in hospitals and
workhouses, a much freer supply of air is required. Much depends,
however, on the cleanliness of the wards, and on whether the
ventilation is constant in character. In St. Thomas’s Hospital, the
space allotted to each ordinary patient is 1,800 cubic feet, and to
each patient in the fever wards 2,500 cubic feet. Thus, by changing the
air of the wards twice in the hour, an abundant supply of fresh air is
ensured. The mortality after operations, and in all fevers, is much
diminished by a free supply of air.
Soldiers are allowed 600 cubic feet of space per head in their sleeping
rooms, which involves a change of the air five times per hour, in order
that the carbonic acid may be maintained at ·06 per cent. The limit of
overcrowding for lodging-houses is usually fixed at 300 to 500 cubic
feet, but this is too little.
The amount of pure air required in order to keep the carbonic acid in
a room at ·06 per cent., may also be ascertained from _physiological
data_.
An average adult expires 3∕5 (·6) cubic foot of carbonic acid per
hour. Now as the carbonic acid in air to be breathed must not contain
more than two parts in 10,000 (·02 per cent.) in excess of what is
present in external air (·04 per cent.), it follows that if _x_ = the
amount of fresh air required by an adult per hour in order to keep the
carbonic acid in the room down to .06 per cent., then:—
·02 : ·6 :: 100 : _x._
_x_ = 3,000 cubic feet.
=Relation of Air Required to Cubic Space of Room.=—If we accept 3,000
cubic feet of air as the amount required per head per hour, this
may clearly be furnished by having a large room with comparatively
little circulation of air, or by having a small room with frequent
interchanges. Thus, supposing the cubic space allowed to each
individual is 1,000 cubic feet—that is, 10 feet in every direction—the
atmosphere will require changing three times per hour.
Now, it is found that when a current of air, at the temperature of
55°-60° Fahr., is moving at the rate of less than one mile per hour, it
is not perceptible—that is, produces no draught. The rate of a breeze,
which is just perceptible, is 18 inches per second, or one mile per
hour. As draughts are objectionable, ventilation, in the best sense of
the word, means the supplying of abundant fresh air at a rate of less
than one mile per hour, or warmed air at a higher rate. Air moving
at the rate of 2½ miles per hour, or 3½ feet per second, is
perceived as a slight draught by all, at the average temperature of our
climate (about 50° Fahr.)
Where natural ventilation is employed, the difficulties of thoroughly
ventilating a small space, without draught, are very great.
A change of air three or four times in an hour is all that can be
borne under ordinary conditions in this country, and this necessitates
a supply of 1,000 or 750 cubic feet of space respectively for each
individual. And a change of this frequency is commonly not effected;
the ventilating apparatus may fail temporarily, or may be wilfully
stopped up, or there may be no means of ventilation; it is essential
therefore to have as large a cubic space as possible. A large cubic
space, does not obviate the necessity for efficient circulation of air.
It is, however, advantageous, not only on account of the initial longer
time before the air reaches the limit of impurity, but also because
there are less draughts, and there is a larger wall surface and larger
windows for unperceived ventilation.
=Common Errors as to Ventilation.=—(1) In relation to the cubic space
of a room, it is most important to note that a _lofty ceiling does
not compensate for deficiencies in floor-space_. One hears, “lofty”
and “airy” rooms spoken of as though the two terms were necessarily
synonymous. This is by no means the case. The impurities produced by
respiration tend to accumulate about the persons who have evolved them,
although it is true that in rooms heated by gaslight, a large amount
of hot and impure air collects near the ceiling. The necessity of an
abundant floor-space is shown by the fact that a space enclosed by
four high walls and without a roof, will, if crowded, speedily become
offensive. Twelve feet is quite high enough for large rooms in schools,
hospital wards, etc., and nine feet suffices for the rooms of a private
dwelling-house. There is no objection to a greater height, if it is
remembered that in reckoning the _practical_ cubic dimensions of a
room, the height should only be reckoned as twelve feet. Supposing 500
cubic feet is the amount allowed per individual, then the floor-space
should be forty-two square feet, which would be furnished by a room
about 8½ feet long and 5½ feet wide. In barracks, soldiers
are allowed fifty square feet of floor-space. In school-rooms the
Education Code requires that at least ten square feet of floor-space,
and at least 120 cubic feet shall be allowed for each child in average
attendance.
(2) It is commonly supposed that a large room compensates for a
deficient circulation of air. _The cubic space of a room is really of
less importance than the capacity for frequent interchanges of air._
Even the largest enclosed space can only supply air for a limited
period, after which the same amount of fresh air must be supplied,
whether the space be small or large. Thus, supposing that as large a
space as 10,000 cubic feet per head were allowed, the limit of purity
would in the absence of ventilation be reached in three hours, and
after that time an hourly supply of 3,000 cubic feet of air would be
just as necessary as if the space were only 200 cubic feet.
(3) It must not be overlooked that the _furniture in a room_ must be
deducted from the breathing space, as the amount of air is diminished
by the space occupied by the furniture. About 10 cubic feet ought to
be allowed for each bed, and 3 to 5 cubic feet for each individual in
a room; projecting surfaces must be allowed for by subtraction, and
recesses by addition. The deductions to be made for furniture are not
of any great consequence, if there is a free interchange of air; as the
cubic space is of less importance than free ventilation.
=General Rules respecting Ventilation.=—The two great objects in
ventilating being to remove all impurities from the air, and to avoid
draughts, it is important that—
1. _The entering air should be, if possible, of a temperature of 55°
to 60° Fahr._ Whenever the temperature of a room differs from the
external temperature by 10° Fahr., a draught is certain to ensue. It
is impossible at all times to ensure the incoming air being of the
temperature of 60°, without some artificial means of warming it. In
this country it is seldom necessary to cool the incoming air, but this
may be managed in artificial systems of ventilation by passing the
incoming air over ice, or by using compressed air which becomes cooled
on expansion, or by passing the incoming air through subterranean
tunnels.
2. _The entering air should be pure._ When a room is hotter than
the passages and kitchens, air from the latter, whatever may be its
character, is drawn into the room. Similarly the ground-air under the
kitchen-floor or the air from ash-pits may be drawn into the house,
when no other means of ventilation are provided; and this is often
followed by evil results.
3. _No draught or current should be perceptible_ from the incoming air,
except when it is wished to flush the room with air, by opening the
windows wide. It is a common complaint that a room is draughty, and, to
remedy this, keyholes are stopped up, and mats are placed at the bottom
of the door, etc. The draught can often be remedied by increasing the
size and number of the openings through which air is admitted, so that
the current of air is not concentrated and rapid. When this does not
remedy it, the incoming air should be warmed. A feeling of draught is
very often due to the radiation to and from a window, and disappears
when a curtain or screen is placed between the radiating surface and
the occupant of the room.
4. _The entry of air should be constant, not intermittent._ The
occasional opening of a window or door will not compensate for the lack
of a constant interchange of air, although it forms a very valuable
adjunct, especially in the removal of organic particles which do not
follow the law of diffusion.
5. _An exit should be provided for impure air_, as well as an entrance
for pure air. The chimney furnishes this in most living-rooms, and
diminishes the necessity for other means of exit.
If the openings in a room for entrance and exit are properly regulated,
a rate of 5 feet per second (about 3½ miles per hour) will provide
sufficient air without any unpleasant draught in a room. For instance,
if the opening measure 1 square foot, then a rate of 5 feet per second
will give five cubic feet of air per second, that is, 18,000 cubic feet
per hour. But as only 3,000 cubic feet are required, it follows that
an opening one-sixth this size, _i.e._ 24 square inches, is sufficient
for each individual. Reckoning the same amount for means of exit, 48
square inches is the size of the ventilating orifices required by each
individual.
6. _A number of small divided openings are not collectively equal in
ventilating power to one large one having the same area._ Thus, when
a ventilating orifice is divided into four parts, which have the same
collective area as the original orifice, it is found that only half as
much air passes through these as through the original orifice. In order
to obtain as much air, therefore, each opening must be equal in size to
half the original opening. This is in accordance with the rule that the
friction for air passing through openings is inversely to the diameter
of these openings, _i.e._ inversely to the square-root of the area of
the openings.
7. The most important requirements of perfect ventilation may be
recapitulated as follows:—
1st. The maximum impurity of air vitiated by respiration should not
exceed 6 parts carbonic acid per 10,000 volumes.
2nd. To ensure the maintenance of this standard, 3,000 cubic feet of
pure air must be supplied per head per hour.
3rd. In order to supply this amount of pure air, with ordinary means
of ventilation, 1,000 cubic feet at least must be allowed per head in
buildings always occupied.