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 Drop System may be described as a one-pipe with vertical mains, as will be seen by Fig. 67. In this, as with the overhead hot-water system, the supply main is carried up the building to the highest floor that is to be heated, and this main has no work upon it. It should always be covered to prevent loss of heat, and it requires a drip pipe at its foot as shown. At the highest point the branch or sub-mains fall in the direction of the work, and are there branched into what are commonly termed "drop risers," these being the vertical services having the radiators on them.
On the right is shown a service which has a dry return except just where it enters the boiler, and no other is needed here ; while on the left a wet return is given, the object of this being to enable the returns to join without both coming all the way back to the boiler. As with all steam works, for best results the returns must not join above water-line.
The radiators are connected with the same care as is necessary with the one-pipe system, it being essential that the branches swing or give to the push and pull of the mains as they expand and contract.
The sizes of mains and radiator branches are the same as those given in connection with the one-pipe system.
The Cold Supply to all low-pressure steam-heating systems is by means of a plain pipe, 1/2 to 3/4 inch, taken from a main or a house service, and connected directly into the boiler, with a stop cock to control it. With modern gravity installations the condensed water flows back to the boiler, and it might be considered that, once the apparatus was filled and started, no further water would be needed. It is found, however, that there is a trifling waste from air vents and other causes, and the water-line may require elevating an inch now and again (once a week or longer). To do this the stop cock is opened, and, being near the boiler, the water gauge can be watched so that on the water reaching its normal level the cock is closed and the process ended. This could be done when steam was on if the water pressure exceeded the steam pressure (as it must do in most cases), but it is better to choose a moment when the fire is out or very low.
The Automatic Regulating Device referred to earlier, which has done so much to make steam heat possible where the attention is that of unskilled persons, is illustrated in section by Fig. 68. This appliance is connected either directly with the boiler, or with a steam service over the boiler, by a single pipe. It requires no second pipe to secure a circulation. Assuming that the steam comes up the lower pipe, it will fill the lower half of the body of the fitting, and press upwards against the diaphragm which divides the body into two halves horizontally. This diaphragm is of rubber, and the effect of the pressure beneath is to raise the central part and lift the plate and the rod which rests upon it. This action causes one end of the long top lever to be raised and the other to be depressed, and this up-and-down motion is utilised to work two specially prepared dampers, or draught controllers, by the chains shown. In most of the modern boilers there is a balanced door to the front of the ash pit, called the "draught damper," and a balanced door in the smoke nozzle at the rear, called the "check damper." The opening of the former increases the draught and combustion, while the opening of the latter checks them. The steam pressure in the appliance, and the lift it gives to the weighted end of the lever, is made to open the check damper when a certain steam pressure is reached, this at the same time closing the draught damper and checking the fire. In other words, when steam is being made a little fast its increasing pressure at once operates in checking the fire. On the other hand, as the pressure falls so does the weighted end of the lever fall also, and this brings about a reverse process. The result is that the fire then brightens up and recovers from the check. The actual effect of the regulator is to keep the fire in a condition which makes just the volume and pressure of steam required, and thus it regulates steam pressure, steam production, and combustion. It only requires the attendant to put fuel into the furnace, and the regulator attends to the dampers and keeps the pointer on the gauge at a normal figure.