Document DveMDQvRE1OY5oZgxjZ3790r4

American Society of Heating and Ventilating Engineers Guide, 1924-25 cortKecronsto storagetwik WITH ORPglAgV KEATTHG COIL RCTURnPlPCSX TO DROPBTTO HAM RE7U8M THROUGHKOBTOflAL SWVtt CHECK YALya. Fig. 32. Connections to Coils in Tanks coniiKnoraiOKiTaicnKiicsTAUiiwTrixTuiiu. cflHrgcnao hi hospidm. subiuzk. Fig. 33. Typical Connections to Kitchen and Hospital Equipment Chapter VIII HOT WATER HEATING HOT water heating may be divided into two classes, forced and gravity circulation. The first is used on large units and other installations where it would be difficult to obtain uniform heating by gravity circulation. In many cases, approaching the natural dividing line between the two systems,' a combination of forced and gravity cir culation is installed, in which a'pump is used as an auxiliary to circulate the water during periods of heavy firing. GRAVITY HOT WATER SYSTEMS Gravity hot-water heating may be divided into several classes, de pending on the method of running the flow and return mains. For residences, the two-pipe, up-feed system generally is used. This means that the flow and return mains are run on the ceiling of the basement, with branches and risers feeding the serveal radiators from below. Where attic space is available, and in shops and factories the down-feed system often is used. This may have either" single or double drops, although the single drop is preferable. The chief advantage of the down-feed system is that it raises the mean effective circulating head, and obviates many of the fine adjustments that are required with the up-feed system. Sometimes, when there is plenty of circulating head available, a twopipe up-feed system with a single-pipe basement circuit is installed. This system, while it cuts down the amount of piping in the basement, increases the size of the mains because it divides the available circulating head into two circuits. The basic principle of gravity circulation is shown in Fig. 34. As the water in column A is heated, and its density decreases it is over balanced by the cold water in B. The water in column B, being colder, always outweighs the water in column A, and thus circulation is estab lished. As the temperature difference between these two columns is increased, the difference in weight increases, with a consequent increase in the velocity of circulation. The velocity of circulation also varies with the difference in elevation betweeen the heater and the radiators. A circuit similar to that shown in Fig. 35 will not circulate, because the hot water is not overbalanced by the cold. Adding pressure to the water will not increase the velocity of circulation in any way, except that it may allow greater temperature differences to exist between the flow and return columns. The velocity of circulation cannot be increased or decreased by changing the pitch of the mains. Material for this section was prepared especially for The Gjidb by William Hutton, Winsted. Conn., in collaboration with Philip Parker. Woburn. Mass., W. P. Elder. Sherman Parker and A. F. Karlson, Fitchburg, Mass. Greenhouse heating by F. P. Elder, Irvington-on-Hudson, N. Y. 97