Valves and Fittings. Gate-valves should always be used in connection with hot-water piping, although angle-valves may be used at the radiators. There are several patterns of radiator valves made especially for hot-water work; their chief advantage lies in a device for quick closing, usually a quarter-turn or half-turn being sufficient to open or close the valve. Two different designs are shown in Figs. 101 and 102.
It is customary to place a valve in only one connection, as that is sufficient to stop the flow of water through the radiator; a fitting known as a union elbow is often employed in place of the second valve. (See Fig. 103.) Mr-Valves. The ordinary pet-cock air-valve is the most reliable for hot-water radiators, although there are several forms of auto matic valves which are claimed to give satisfaction. One of these is shown in Fig. 104. This is similar in construction to a steam trap. As air collects in the chamber, and the water-line is lowered, the float drops, and in so doing opens a small valve at the top of the chamber, which allows the air to escape. As the water flows in to take its place, the float is forced upward and the valve is closed.
All radiators which are supplied by risers from below, should be provided with air-valves placed in the top of the last section at the return end. If they are supplied by drops from an over head system, the air will be discharged at the expansion tank, and air-valves will not be necessary at the radiators.
Pipe Sizes. The size of pipe required to supply any given radiator depends upon four conditions; first, the size of the radiator; second, its elevation above the boiler; third, the length of pipe required to connect it with the boiler; and fourth, the difference in temperature between the supply and the return, As it would be a long and rather complicated process to work out the required size of each pipe for a heating system, Tables XXVI and XXVII have been prepared, covering the usual conditions to be met with in practice.
These quantities have been calculated on a basis of 10 feet difference in elevation between the center of the heater and the radiators, and a differ ence in temperature of 17 degrees between the supply and the return.
Table XXVI gives the number of square feet of direct radiation which different sizes of mains and branches will supply for varying lengths of run. •
Table XXVI may be used for all horizontal mains. For vertical risers or drops, Table XXVII may be used. This has been com puted for the same difference in temperature as in the case of Table XXVI (17 degrees), and gives the square feet of surface which dif ferent sizes of pipe will supply on the different floors of a building, assuming the height of the stories to be 10 feet. Where a single riser is carried to the top of a building to supply the radiators on the floors below, by drop pipes, we must first get what is called the average elevation of the system before taking its size from the table. This may be illustrated by means of a diagram (see Fig. 105).
In A we have a riser carried to the third story, and from there a drop brought down to supply a radiator on the first floor. The elevation available for producing a flow in the riser is only 10 feet, the same as though it extended only to the radiator. The water in the two pipes above the radiator is practically at the same temperature, and therefore in equilibrium, and has no effect on the flow of the water in the riser. (Actually there would be some radiation from the pipes, and the return, above the radiator, would be slightly cooler, but for purposes of illustration this may be neglected). If the radiator was on the second floor the elevation of the system would be 20 feet (see B); and on the third floor, 30 feet; and so on. The distance which the pipe is carried above the first radiator which it supplies has but little effect in producing a flow, especially if covered, as it should be in practice. Having seen that the flow in the main riser depends upon the elevation of the radiators, it is easy to see that the way in which it is distributed on the different floors must be con sidered. For example, in B, Fig. 105, there will be a more rapid flow through the riser with the radiators as shown, than there would be if they were reversed and the largest one were placed upon the first floor.
We get the average elevation of the system by multiplying the square feet of radiation on each floor by the elevation above the heater, then adding these products together and dividing the same by the total radiation in the whole system. In the case shown in B, the average elevation of the system would be 100 + 50 + 25 and we must proportion the main riser the same as though the whole radiation were on the second floor. Looking in Table XXVII, we find, for the second story, that a 1k-inch pipe will supply 140 square feet; and a 2-inch pipe, 275 feet. Probably a 11-inch pipe would be sufficient.
Although the height of stories varies in different buildings, 10 feet will be found sufficiently accurate for ordinary practice.