Accidents have occurred in houses owing to ignorance of the full effects
of heating or cooling water from its ordinary temperature. Water at any
ordinary temperature expands when subjected to the action of heat; it
contracts on cooling till it reaches a temperature seven degrees above
the freezing point; from this temperature it expands until it becomes a
solid mass of ice. At still lower temperatures ice contracts.
Let us consider first the effect of heating water. If water at the
ordinary temperature be poured into a vessel which is placed on a fire
or other source of heat the water at the bottom of the vessel will be
warmed and will expand; it will therefore be lighter, bulk for bulk,
than the water nearer the top of the vessel. The cold water will
therefore descend, and the warm water will rise. All ordinary water
contains air; presently the air in the water will become visible as
small bubbles which rise to the surface of the water and escape
noiselessly into the atmosphere. As more heat is applied some of the
water in the bottom of the vessel will be formed into steam, and bubbles
of steam will expand and rise into the cooler water above and collapse
there with a rattling noise which is characteristic of the state known
as simmering. These bubbles of steam rising and bursting aid the
convection currents in stirring and mixing the water so that it
presently becomes of even temperature throughout. When this occurs the
bubbles of steam rise to the surface and burst explosively into the
atmosphere, throwing the water violently about; the water is then
boiling. It is an important point to remember in cookery that boiling
water will not become any hotter with the application of more heat, but
it will "boil away;" that is, it will be completely converted into
steam. The steam resulting from any volume of water occupies a space
1700 times that of the water from which it is produced, but what
concerns the housewife most seriously is that the change of water into
steam is accompanied with the evolution of tremendous mechanical force
that will burst any vessel in which the water is enclosed. It is the
fact of this tremendous exercise of mechanical force that has led to
serious accidents when hot-water bottles have been put into the oven to
keep warm. It has been assumed by some people that if the hot-water
bottle be not completely filled, that if what they consider to be
sufficient room is left for the expansion of the water, no harm can
result from putting the bottle into the oven, but no arrangement can
make such a course safe.
The bursting of the kitchen boiler is an accident resulting from
disregard of the phenomena of heated water. It sometimes happens that
the hot-water supply of the various taps in the house fails. If the
boiler supplying the water is a hand-fed one some one whose duty it was
to fill it has neglected that duty. An empty boiler with a removable lid
will do no harm, but it is not advisable to leave it empty, as the heat
of the fire will destroy the iron of which it is made. No attempt,
however, should be made to fill the boiler while it is hot, as the
result of pouring cold water into it will be the sudden and violent
conversion of the water into steam, and the person pouring in the water
will assuredly be scalded. If the boiler be one that is filled
automatically, one of two things has probably occurred: either the pipes
are blocked by fur--that is to say by sediment from the boiled water--or
the supply-pipe is frozen. In neither case is it safe to light the fire.
If the pipes are blocked by fur steam will be formed in the boiler and
it will burst; if the supply-pipe is frozen the heat may thaw the ice,
and the inrush of cold water will at any rate crack the boiler.
[Illustration: FIG. 3.]
When water expands with heating convection currents are formed in it,
and the hot water rises to any height we please if cold water be
available to take its place. This law of convection is applied to
maintain a circulation of hot water in pipes used for warming a house.
The general arrangement of such a system is shown in Fig. 3. The
furnace heats a boiler in the basement or on the lowest storey of the
house; HB and HL' are parallel vertical pipes connected with a
horizontal pipe H'H at the top of the house; C is a small cold-water
cistern which is furnished with a ball-tap to maintain the supply of
cold water to the pipe H'L if any water is drawn off at any part of the
circuit. The short pipe A acts as a valve for the escape of air from
the pipes. The pipes H'L, H'H, and HB are filled with water. When
the fire is lighted in the furnace, hot water is driven up the pipe
HB by cold water descending through H'L, and this circulation goes
on so long as a difference of temperature is maintained in the pipes;
that is, so long as the fire is burning. Any number of coils of pipes
may be introduced into the circuit between the boiler and the top of
the pipe HB. In filling the pipes with water allowance is made in these
coils for the expansion of the water with heat and for the air which we
have seen escapes from heated water, and a tap is fixed in each coil
for letting out any air that may have lodged in it. If free air
remains in the pipes the circulation of the water will be hindered and
the boiler may become dangerously overheated. It is therefore necessary
when the heating apparatus is in use to examine these taps and see that
water and not air escapes from them.
The installation of a heating apparatus in middle-class houses is fairly
common, and where one is not found many persons use gas or oil stoves in
the passages in the winter, for it is now realised that it is not
possible to heat rooms by means of open fires without creating cold
draughts in them from the cold passages into which they open. And,
moreover, the constant change of temperature encountered in passing from
one warm room to another through cold passages is not only disagreeable,
but is not found to be conducive to health.
Let us turn to the cooling of water. Water expands about one-eleventh of
its volume on becoming ice. This change of state, like that of change
into steam, is accompanied by the evolution of tremendous mechanical
force. If water freezes in pipes it bursts the pipes, and on a thaw
taking place the pipes are found to leak. The appropriate remedy for
this state of things is to protect the pipes from cold or to empty them
when a frost is apprehended. In all properly built houses there is a tap
by means of which the water supply can be cut off from the house, thus
allowing the pipes to be emptied on a frosty night. The custom of
leaving the taps dripping is effective, because the pipe is generally
liable to freeze at some particular point where it is in immediate
contact with the cold air, probably in the unclosed chink where the
pipe passes through the wall; keeping the water moving in the pipe
prevents any part of it getting cold enough to freeze, but the practice
should not be resorted to, as it wastes water.