This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
The system of heating by low-pressure steam is very similar to that of heating by low-pressure hot water, except that, instead of having an apparatus quite full of water and open to the air, so that it is not possible to produce steam above atmospheric pressure, the apparatus is closed and never allowed to get full of water. Strain is generated in a special boiler, - which is placed below the lowest point to be heated, - and then passed into a system of pipes, which are carried into the parts of the building to be warmed, and either themselves give off the heat, or feed apparatus specially designed for that purpose. It is obvious that in the passage of steam through a system of cold pipes a great deal of condensation must take place; the water thus condensed must be carried oft' as fast as it is formed, and should be used over again in the boiler. For this purpose special pieces of apparatus known as steam-traps are used, which allow free passage to the hot water, but prevent the exit of the steam. All the water passing out has at one time been steam, and is therefore perfectly pure, and if
1 in the boiler will cause no incrustation; besides this, it holds a large portion of the heat which has originally been in the steam itself.
It may now be advisable to recall to mind a few facts relating to steam, ineluding its formation, and the heat which is contained in a given quantity. The British "thermal unit" is defined to be that quantity of heat which will raise one pound of distilled water 1° Fahrenheit in temperature; thus the work of rawing one pound of water at 32° F. to 212° F. would be 180 thermal units. But to change one pound of water at 212° F. to one pound of saturated steam at 212 F. and atmospheric pressure, will require 966 thermal units, and the beat required to raise one pound of this steam to a pressure of one pound above the atmosphere will be 03 thermal units. The large quantity of heat which is absorbed in the change from the liquid to the gaseous state, is called "latent heat"; this heat is given back during the- change from vapour to the fluid state. When steam, therefore, is used for heating purposes, the heat due to its temperature is made use of, and when it condenses in the pipes it gives off the latent heat, and is taken bank into the boiler as hot water
Then is a very considerable difference between steam and hot-water heating in the following respect: the quantity of heat contained in the pipes of a hot-water apparatus when full of water is vastly greater than that contained in a steam apparatus when full of steam, although the perceptible temperature of the latter, when tested by a thermometer, may be considerably higher than that of the former. It must be remembered that the capacity of the water for heal is much greater than that of the gaseous steam; the result therefore is that, if steam be shut oft', it takes but a short time for the pipes to become quite cold, while, in the case of hot water, the heat is retained for a very considerable time. Heating by hot-water pipes is not subject to such rapid fluctuation- as may be the case with steam-pip In hot-water systems the pipes must have a good fall back to the boiler, otherwise the circulation will be impeded; the air also must be got out of the pipes. In the case of steam systems it is even more important that the condensation or return pipe should fall towards the boiler, otherwise pockets of water will be formed, which will be blown out suddenly by the steam with loud crackling noises. Air must of course also be got out of the pipes, but in the case of steam at low-pressure the air would be heavier than the steam, and would therefore need to be drawn off at the bottom of the apparatus, and not at the top, as would be the case with a hot-water apparatus
The principal points which require attention in the design of a low-pressure steam heating-apparatus are - firstly, that the whole of the parts are amply strong enough to l>ear the pressure to which they will bc subjected, and secondly, that the pipes are so laid that the water produced by condensation passes away freely under the influence of gravitation.
The boilers used for low-pressure steam-work closely resemble those used for low-pressure hot-water heating, and the latter types of boiler are usually made of sufficient strength to enable them to be used for low-pressure steam-heating. Ordinary cast-iron radiators are often used with steam of 25 or 30 lbs. pressure per square inch, and sometimes, indeed, with steam direct from high-pressure boilers working at 50 lbs. pressure; but I strongly object to putting them to such severe tests, and certainly do not consider that more than 5 lbs., or in exceptional cases 10 lbs. pressure per square inch, should be used in private houses. Throughout the following description, it may l>e taken that steam of about 5 lbs. pressure is alluded to. Boilers for steam-heating should never be built of castiron. They should have ample steam and water spaces. I consider that no type of boiler is so suitable for this work as the ordinary Cornish boiler, that is, a boiler with cylin-drical shell and one cylindrical flue, with or without cross tubes, and set in fire-brick with proper side flues. Such a boiler is shown in Fig. 546. The grate is inside the furnace-flue, and the products of combustion go from the flue down underneath the boiler, so that the bottom of the boiler receives the hottest gases, and then the draught is split. and the gases pass along the sides and go out into the chimney-stack. Some people, however, prefer to use the awhile type of boiler, with a much deeper crown than in the case of the hot-water boiler, Of a waggon boiler, but both these types are, in my opinion, inferior to the plain Cornish boiler.
Fig. 546 - View and cross Section of the "Trenthmm" Cornish Boiler.
Plan. Fig. 547. - Elevation, section, and Plan of the "Majestic' Independent Boiler for Steam-heating.