This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol3", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
Fig. 61 illustrates the general details entering into an apparatus on this system. In the first place, it will be seen that the installation is practically sealed at all points. This is to prevent loss of steam, which would mean loss of heat and fuel; also loss of water, which would spoil the best feature of the gravity system, its constant water-line in the boiler. The apparatus being sealed, however, causes it to come under the heading of a rather vexatious rule in the London Building Act, it being required that a steam installation without a "free blow-off" shall have its pipes and parts kept 6 inches from inflammable material. It is awkward, yet in the majority of places where steam heat is used it does not usually prove a very difficult rule to adhere to. The rule is vexatious because a low-pressure hot-water apparatus, which often has its pipes (near the boiler) hotter than low-pressure steam pipes, is free of all restrictions, while a high-pressure hot-water apparatus, with its water at 300° to 400% need, under this rule, be kept only 3 inches from woodwork.
The main, which in this work is called the steam-supply main, it will be observed, makes a circuit wholly above water-line ; and, to get the steam and condense water to travel together in the same direction, the main is always carried to its highest point as near over the boiler, that is, as quickly as possible, and then run with a fall all the way out and back again. This is a rule always observed.
The branches, it will be seen, depart from the desirable plan of getting the steam and water to travel the same way within them, as it is impossible to obtain this result in a single-pipe branch. The trouble, however, is not so great a one in branches as in mains, and it is overcome by using pipe of sufficient size to prevent the steam and water coming into conflict. All branches are connected by tees looking upwards, and are given a rise from this point. The minimum fall in mains is 1 inch in 20 feet, but these figures do not apply to the rise given to branches, in which the fall of water is retarded by the flow of steam. The rise must be at least 1 inch in 3 feet, and always more if possible. In America they have a fitting for this work called a "pitch elbow," which is very useful, as it is not easy to get a sharp rise with an ordinary elbow or bend.
As with hot-water work, the radiator branches must be made to " swing " to any movement of expansion or contraction in the mains. This provision is, in fact, more necessary with steam, as the range of temperature (from cold to 3 lbs. steam) is greater, and it occurs with comparative suddenness. The connection is made like that on the flow-pipe of a low-pressure hot-water system, and its general appearance can be judged from Fig. 61. The main, in order to provide this swing, is never run immediately beneath the radiators, but should it come so in one or more cases, then the swing connection is obtained by taking the branch to the distant end of the radiator, as is shown with the radiator at the foot of the branch which goes to a higher floor (see Fig. 61). To reach radiators on a second floor above the boiler, a branch can be carried up, but this is then known as a "riser." Risers can be taken to any height and carry any number of radiators, but, as will be shown, a riser which has much water coming down it is best drained or " dripped" into a separate main return.
To those inexperienced in this work it may seem possible, on examining such a sketch as Fig. 61, to have two branches to each radiator and so get over the difficulty of using one branch to carry both steam and water. The first branch might carry steam, it is thought, while the second branch could be made to deliver the water to the main. This does not work out correctly in practice, however, owing to the simple fact that, the steam pressure being equal where both branches leave the main, the steam would try to flow up both. One would quite defeat the other, leaving the radiator badly served.
When risers have a large area of radiation on them, and consequently deliver a fair volume of condense water at their lower ends, it is desirable that they be "dripped,"as it is called, if this is in any way possible. If only one riser needs so treating, its "drip" can be arranged as shown in Fig. 62, this return being an overhead pipe until it descends and joins the return of the main circuit. (It may be noted here that there are no objections to returns joining one another in this work, as there is with hot-water circuits, provided that they join below water-line.) If, however, several risers are dripped, or there are other drips to be dealt with, then they must either all return separately to join the main return below water-line or they can be given a "wet" or submerged return of their own, as in Fig. 63. The term "wet" or "submerged" return occurs frequently in dealing with steam-heating works, and it refers to a return which is run below the water-line of the boiler, and is consequently always full of water. Its purpose is to water-seal the lower ends of drips, or other pipes taking the functions of returns, so that steam shall not pass up them and occasion troublesome results by approaching radiators from wrong directions. If two or more drips were to join an overhead or "dry " return, steam would probably work down one and find its way up the others, needless to say with ill results.
In Fig. 63 it will be noticed that the risers, as soon as they leave the main, are given a fall to their drip connections. This is the best plan to adopt, as it quite prevents any water getting into the main from the riser. It is an even better plan, when a riser is to be dripped, to make the connections as shown in Fig. 64. In adopting this plan the tee in the supply main has its outlet looking downwards.
In the illustration (Fig. 63) there will be seen a dip in the supply main, presumably to pass an obstacle. There is no objection to this, except that it forms a pocket for condensation water, but this can be disposed of by a drip as shown. Sometimes a flow main extends so far that, in giving it a proper fall, it comes too low to, say, pass over the head of a doorway. In this case the main can be carried up vertically a little way - "jumped-up," as it is expressed, before it continues its run, provided its low point is dripped.
The valves shown on Fig. 61 are all of the angle kind, and it is a good plan to use these when possible, as the condensed water drains through them easily and they have a way, or bore, equal in area to that of the pipe which they are connected to. If straight valves have to be used, then the ordinary design of screwdown or globe valve should be avoided, for it has an up-and-down way through it, and, at the best, this way seldom exceeds in area one-half that of the pipe. It is argued that the up-and-down passage through the body of the valve can be made a level way by fixing the valve on its side, but this does not give it a full bore. Straight valves should only be of the gate or Peet's pattern, which has a full-sized straight opening through it, like a continuation of the pipe it is attached to.
The final detail to be explained, concerning Fig. 61, is that of the air vents on the radiators. Before steam can fill a radiator the air must be driven out, and although steam at low pressure (this pressure being above that of the atmosphere) will do this, it is not so effectively done as with radiators which are filled with water. Apart from the question of pressure, steam and air are somewhat akin in weight or gravity, and consequently they are inclined to mix instead of keeping separate as air and water do. Air is the heavier of the two, and it would be assumed from this that the air vents would do best at the bottoms of the radiators, but for general purposes the best position is found to be about one-third the way up the radiator from the bottom, and at the end farthest from the steam connection. If the steam entered the top of the radiator, then the bottom might be the best position for the air vent, but as the steam usually enters close to the bottom, radiators are always tapped in the higher position for the vent.
Air pipes are, of course, quite out of the question for this work, while air cocks would prove troublesome by having to be opened almost daily. The automatic air vent is always used, this being commonly called an " aspirator," as it has air passing both out and into it. When heating up the air escapes by it ; then, when cooling, it opens and allows air to enter. Fig. 65 illustrates, in section, an ordinary type of this vent, the description being as follows :-A is the end which is screwed into and through which the air leaves the radiator. B is a screw plug, inside the fitting, which carries the "pencil" C. The cap D is merely to prevent B being interfered with once it is set. The pencil C is of a composition which expands rapidly with heat, and contracts as rapidly when the heat goes down, and by its expansion it closes the end of the passage A. To set these vents the radiators are first heated up, and then the plug B is screwed down until the pencil C, which is hot and expanded, just closes the opening A.
It then follows that on steam going down the pencil will contract and open the vent, and so operate automatically in opening and closing. This is considered to be a reliable type of vent, and there are many modifications of it. With some a small cup can be attached to receive any drips of water that may appear.
Sizes of Main Pipes (with suitable Fall) for the One-pipe System of Low-Pressure Steam-Heating Apparatus.
Size of Pipe.
Will serve this Area of Radiation.
up to 200 sq. feet.
2 inches .
,, 400 ,,
2 1/2 "
" 650 ,,
,, 900 ,,
3 1/2 "
" 1250 "
" 1600 ,,
A common rule for finding the radiation which a main pipe will carry in this work is to square the size of the pipe and call the result hundreds. Thus 3 + 3 = 900.
Sizes of Risers and Radiator Branches which rise from the Mains.
Size of Pipe.
Will serve this Area of Radiation.
up to 25 sq. feet.
1 1/4 "
" 60 "
1 1/2 "
" 100 ,,
" 160 ,,