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.
The Cylinder System owes it name to the shape which the hot-water tank usually takes, and differs from that already treated in having the tank beneath all the tap branches instead of above them. It may be mentioned here that the cylindrical form of tank is used because of its pressure-resisting qualities. A square or rectangular tank of good quality, say, 1/8-inch plate, is only tested to 5 lbs. to the square inch, whereas a cylinder made of this plate is tested to 25 lbs. per square inch. Pressure is easily calculated in this class of work by ascertaining the vertical distance from the cold-water level in the house cistern down to the position the tank is to occupy, and then allowing 1 lb. for each 2 feet 4 inches of the distance. Thus a vertical distance of 42 feet causes a pressure of 18 lbs. to the square inch to be felt. This would make the sides of a good quality square tank bulge out and possibly start it leaking, but quite a moderate quality cylinder will bear it without becoming distorted in shape. Most makers' price lists give the test pressures of their tanks, and the working pressures should not exceed these. For good work, one quality better than is absolutely necessary should be used, as this allows for wear and deterioration.
Fig. 42 illustrates the cylinder system of apparatus in it most simple form. The apparatus need never be of more complex form than this when the taps are all grouped near the cylinder, as will be learned directly.
Taking the details in the order adopted when describing the tank system, the boiler connections are arranged in the manner given there. The care to be observed in running the pipes from boiler to hot tank is also the same, and the two pipes terminate at different levels in the tank in much the same manner. The high termination of the flow-pipe serves to deliver the heated water from the boiler to the upper part of the cylinder only, in this case, as no draw-off branches are supposed to be taken from this pipe. Cylinders, as delivered from a maker's stock, nearly always have flanges provided for the flow and return connections, and these, placed one above the other, are situated as near the bottom as possible. This necessitates putting a bend or elbow on the flow connection, inside the cylinder, with a short piece of pipe extending upwards as shown. This internal pipe reaches to a point threefourths the way up the cylinder, but when desirable or convenient the flow flange can be ordered to come three-fourths the way up the cylinder, outside, and the flow-pipe can then be carried directly to it, as shown by the pipe marked "Alternative method of running flow-pipe."
From the top of the cylinder an expansion pipe is carried up to above the level of the house cistern in the manner described with the tank system, but in this case it serves as the main hot-water supply pipe, being branched to the different points as shown. By this means the system obtains its special degree of safety in the fact that the cylinder cannot be emptied except by the emptying cock, which is for workmen's use only. Should there be a shortage of water from any cause, so that the taps yield no water, the cylinder remains full, and consequently no danger need be anticipated if the fire is used, even for a whole day. The contents of the cylinder may be evaporated by boiling, but this must take some time, and as the rule is to use no fire in the range when the hot taps cease to run, the risk due to shortage of water is reduced to a minimum.
The cold-supply pipe comes down direct from the cistern, and is connected in the manner already explained in connection with the tank system. It is customary to put a stop valve in this pipe, near the cylinder, as there is then no need for workmen to traverse the house to shut the water off when boiler cleaning or effecting repairs. A very important matter concerning this pipe is its size. A glance at Fig. 42 will show that there is no store of hot water above the taps, and whatever water comes from them has its issue caused and governed by the pressure and flow of water down the cold-supply pipe. According to the size of this pipe, so the outflow at the taps will be. For small jobs, such as that illustrated, it might be a 3/4-inch pipe (internal size), but in larger works 1 inch and 1 1/4 inch must be used. When the pipe is a long one the friction through its length, through the cylinder, and then up through the expansion pipe and branch, retards delivery quite seriously. This is the fault of the cylinder system previously referred to, for in an extensive apparatus the high taps have an outflow that is so slow as to be usually a source of complaint, and there is no remedy except in converting the system to that which is described a little farther on, which has an auxiliary hot-water tank at a high point. As stated, the cold-supply service should always be as large as possible, and it must not be choked or have its bore restricted at any point. Any stop valve in it must have a full straight way through it equal to the full area of the pipe.
The final detail appearing in Fig. 42 is the emptying cock. Without this the emptying of the apparatus for boiler cleaning, etc., is a considerable task. A 1/2-inch tap will do, and a piece of hose can be slipped on the nozzle to run the water away to a sink or gulley. The tap should have a removable key to be kept by the housekeeper, together with the key of the stop cock in the cold supply.
The emptying cock, which is only used when repairs are necessary, might theoretically be better placed so as to entirely empty the boiler, but this is rarely if ever done, it generally occupying the position shown on the return pipe - leaving a little drop of water in the boiler even when the rest of the system has been emptied by its aid.