FORCED-CIRCULATION SYSTEMS

Forced-circulation hot-water distribution systems have several advantages. They permit the use of smaller pipe sizes and allow the installation of radiators at the same level as the boiler, or below, without impairing water circulation. By using a circulation pump, a positive flow of water is assured throughout the system. In larger installations, especially where more than one building is served, forced circulation is almost invariably used. With the development of a circulation pump of moderate cost, the forced-circulation system is being used more in small heating installations.

Even as in gravity systems, forced-circulation systems can consist of a one-pipe or a two-pipe, upfeed or downfeed, and can be equipped with a direct or a reversed return. Although these systems usually have closed expansion tanks, they may have open tanks.

One-Pipe, Closed-Tank System

The general arrangement of a one-pipe, closed-tank, forced-circulation system shown in figure 4-66 is similar to the one-pipe gravity system, but with the addition of a circulating pump.

The circulation to individual radiators is improved by special supply and return connecting tees. These tees, by an ejecting action on the distribution supply main and an ejecting action on the return, combine to use a portion of the velocity head in the main to increase circulation through the radiators. Tees of this type also aid stratification of hot and cold water within the distributing main. They are designed to take off the hot-test water from the top of the main and to deposit the colder water on the bottom of the main.

Figure 4-67. - A two-pipe, closed-tank, forced-circulation system.

Two-Pipe, Closed-Tank System

The general arrangement of the piping and radiators for the two-pipe, forced-circulation distribution system is the same as that for the two-pipe gravity system. The relative locations of the compression tank relief valve and the circulating pump are shown in figure 4-67.

DISTRIBUTION SYSTEM COMPONENTS

The component parts of a hot-water distribution system are similar to that of steam heating systems as described in chapter 3. They include the following: pipelines, radiators, convectors, unit heaters,

Figure 4-66. - A one-pipe, closed-tank distribution system with a circulating pump.

circulating pumps, reducing valves, flow-control valves, and special tlow fittings.

Pipelines

The piping system constitutes the closed passageway for the delivery of hot water to the points where it is used. Pipelines are made of lengths of pipe fastened by screwed, flanged, or welded joints. They have valves and fittings, such as tees, unions, and elbows, according to the needs of the installation. Pipelines are supported by hangers and fastened by anchors. Expansion joints or loops allow for expansion.

Mains and branches of the pipeline should be pitched so the air in the system can be discharged through open expansion tanks, radiators, and relief