FORCED HOT-WATER LATION While the gravity system of hot water heating is well adapted to buildings of small and medium size, there is a limit to which it can be car ried economically. This is due to the slow movement of the water, which calls for pipes of excessive size. To overcome this difficulty, pumps are used to force the water through the mains at a comparatively high velocity.
The water may be heated in a boiler in the same manner as for gravity circulation, or exhaust steam may be utilized in a feed-water heater of large size. Sometimes part of the heat is derived from an economizer placed in the smoke passage from the boilers.
Systems of Piping. The mains for forced circulation are usually run in one of two ways. In the two-pipe system, shown in Fig. 109, the supply and return are carried side by side, the former reducing in size, and the latter increasing as the branches are taken off.
The flow through the risers is produced by the difference in pressure in the supply and return mains; and as this is greatest nearest the pump, it is necessary to place throttle-valves in the risers to prevent short-circuiting and to secure an even distribution through all parts of the system.
Fig. 110 shows the single-pipe or circuit system. This is similar to the one already described for gravity circulation, except that it can be used on a much larger scale.
A single main is carried entirely around the building in this case, the ends being connected with the suction and discharge of the pump as shown.
As the pressure or head in the main drops constantly throughout the circuit, from the discharge of the pump back to the suction, it is evident that if a supply riser be taken off at any point, and the return be connected into the main a short distance along the line, there will be a sufficient difference in pressure between the two points to produce a circulation through the two risers and the connecting radiators. A distance of 8 or 10 feet between the connections is usually ample to produce the necessary circulation, and even less if the supply is taken from the top of the main and the return connected into the side.
Sizes of Mains and Branches. As the velocity of flow is inde pendent of the temperature and elevation when a pump is used, it is necessary to consider only the volume of water to be moved and the length of run.
The volume is found by the equation R E T' in which Q = Gallons of water required per minute; R = Square feet of radiating surface to be supplied; E = Efficiency of radiating surface in B. T. U. per sq. foot per hour; T = Drop in temperature of the water in passing through the heating system.
In systems of this kind, where the circulation is comparatively rapid, it is customary to assume a drop in temperature of 30° to 40°, between the supply and return.
Having determined the gallons of water to be moved, the required size of main can be found by assuming the velocity of flow, which for pipes from 5 to 8 inches in diameter may be taken at 400 to feet per minute. A velocity as high as 600 feet is sometimes allowed for pipes of large size, while the velocity in those of smaller diameter should be proportionally reduced to 250 or 300 feet for a 3-inch pipe. The next step is to find the pressure or head necessary to force the water through the main at the given velocity. This in general should not exceed 50 or 60 feet, and much better pump efficiencies will be obtained with heads not exceeding 35 or 40 feet.
As the water in a heating system is in a state of equilibrium, the only power necessary to produce a circulation is that required to overcome the friction in the pipes and radiators; and, as the area of the passageways through the latter is usually large in comparison with the former, it is customary to consider only the head necessary to force the water through the mains, taking into consideration the additional friction produced by valves and fittings.
Each long-turn elbow may be taken as adding about 4 feet to the length of pipe; a short-turn fitting, about 9 feet; 6-inch and 4-inch swing check-valves, 50 feet and 25 feet, respectively; and 6-inch and 4-inch globe check-valves, 200 feet and 130 feet, respec tively.