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page 114, and the return should enter the same pipe at a point below the radiator. A valve affording as little resistance as possible is to be put in each connection. Hot-water heating systems have been erected in which the radiators are joined to the riser by one connection only; and while this system seems to be somewhat slower in heating than that with two connections, it is otherwise quite satisfactory.

In the system commonly employed the main and distributing pipes are erected in the basement, as shown in Fig. 177. An offset from the main to the foot of the riser has usually to be made, which should be done as from the steam main in Fig. 180, and in such a manner as to take the flow from the upper part of the pipe; such a connection is also shown in No. 3. Fig. 183. The connection to the main return may be made on

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FIG. 183.--CONNECTIONS TO MAINS, HOT-WATER HEATING.

the side or at the top, as convenient. In some instances a tee turned at an angle and a 45-degree elbow can be used with good results, as shown at No. 2, Fig. 183. The method of connecting shown at No. I should only be employed in case the room is not sufficiently high for connections, as shown at No. 3, as its use is attended with doubtful success in many

cases.

In taking off branches from the top of a riser a tee should seldom or never be employed, since it will be found that if for any reason the current becomes established in one direction it will be very difficult to induce it to flow in the other. When branches running in opposite directions have to be taken. from the main riser, long-radius tees, as shown in Fig. 52, page 95, should be employed; but unless the riser is long it will in general be better to erect a separate line for each branch. Precautions should be taken in every case that the junction of two currents shall not exert an opposing force which will impede the circulation.

The connections to radiators for this system need to be made in such a way that the horizontal branches which are taken off from the risers will receive a strong current of water. There is a tendency for water to flow directly in the line of motion, and to the highest radiators in the system. This renders it necessary to increase the resistance in the riser beyond the branch a greater or less amount in order to induce circulation into the side connections. This may be done in several ways, as shown in Fig. 184: (1) by connecting

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FIG. 184.-CONNECTION TO RADIATORS, HOT-WATER HEATING,

the radiator to an elbow placed on the main pipe and continuing the main pipe from the side opening of a tee or Y, as shown at A and B; or (2) by using a reducing fitting, as shown at C, and continuing the riser with a reduced diameter. The return connections can be made in a similar manner, but they will in every case work well if the return riser be run in a direct line and the connection be made into the side opening of a Y.

112. Position of Valves in Pipes.-If a valve has to be used on a horizontal pipe it should be located so as to afford the least possible obstruction to the flow of water in the required direction. If a globe valve be used with the stem set vertically, Fig. 185, it will form an obstruction sufficient to fill the pipe very nearly full of water; if the stem be placed in a horizontal direction the flow of water will be less impeded. Globe valves form a great obstruction to the flow in water-heating pipes, and under no circumstances should they be used for that work. In the case of steam-heating they are less objectionable, provided they are located in such a manner as to permit free drainage

of the pipes. In general, angle or gate valves can be used, however, in every place with better satisfaction.

For hot-water heating special valves have been designed,

a

FIG. 185.-ILLUSTRATION OF WATER HELD BY GLOBE Valve.

which when open offer no special impediment to the flow, and which close sufficiently tight to prevent circulation, although not sufficient to prevent leaks. See page 88.

113. Piping for Indirect Heaters.-Indirect radiators have been described and methods of setting them illustrated in Article 69, page 116. These radiators are generally set in a case or box which is suspended from the basement ceiling and made of matched boards lined with tin, Fig. 186. The sides of the casing should be removable for repair of the radiator. The system of pipes which supply the indirect radiators are generally most conveniently erect

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FIG. 186. INDIRECT SURFACE.

ed, like those shown in Fig. 175 or 177 for steam-heating, and like that shown in Fig. 179 for hot-water heating. The heater should be located above the water-line of the boiler a sufficient distance to afford ready means of draining off the water of condensation. In case this is impossible, a style of radiator should

be adopted which can be heated by water circulation. An automatic air-valve should be connected to the heater, and every means should be taken to obtain perfect circulation to and from the boiler. The chamber which surrounds the indirect surface is to be supplied with air from the outside by a properly constructed flue. The air passes up through or over the heater and into the rooms by means of special flues, the sizes of which are given in Chapter X.

114. Comparisons of Pipe Systems.-As to the best system of piping to be adopted little can be said in a general way. The circuit-system, Fig. 173, no doubt gives the freest circulation and is applicable to either hot-water or steam heating. In some respects it is simpler to construct, and it seems quite probable that small errors of alignment, minute cbstructions, and error in proportioning the pipes would not be so fatal to the perfect operation of this system as of the others. It requires, however, that distributing pipes be placed in the top story of a building, and this in many cases will be so objectionable that it cannot be used. Regarding other systems there is little to be said. For steam-heating there seems to be little or no use in making more than one connection to any radiator; and this practice, which is now common, will I think become universal.

115. Systems of Piping where Steam does not Return to the Boiler. For such systems the method of piping and of making connections would be in every case essentially as described; and usually this can be done with less care because of the fact of greater difference of pressure between the supply and the return. Such systems are not often employed except in connection with use of exhaust steam, which is considered in Chapter XI.

116. Protection of Main Pipe from Loss of Heat.-The loss of heat which takes place from an uncovered main steam or hot-water pipe is, because of its isolated position, considerably greater than that which takes place from an equal amount of radiating surface. Unless this heat is actually required it will cause an expenditure of fuel the cost of which is likely to be in a few seasons many times that of a good covering.

The heat lost per square foot of surface from a small uncovered pipe is from 375 to 400 heat-units per square foot per hour in steam-heating, or an amount equal to that required for the evaporation of 0.4 pound of steam. Computing this loss for 100 square feet for a day of 20 hours and for a season of 150 days, it will be found equivalent to the coal required to evaporate 120,000 pounds of steam; this would not be less than 12,000 pounds of coal, which at $5.00 per ton would cost $30.00. The cost per square foot per annum will be found on the above basis to be 30 cents, of which 75 to 80 per cent would have been saved by using the best covering. The loss from hot-water pipes would be about two thirds of the above.

The best insulating substance known is air confined in minute particles or cells, so that heat cannot be removed by convection. No covering can equal or surpass that of perfectly still and stagnant air; and the value of most insulating substances depends upon the power of holding minute quantities in such. a manner that circulation cannot take place. The best known insulating substance is a covering of hair felt, wool, or eiderdown, each of which, however, is open to the objection that, if kept a long time in a confined atmosphere and at a temperature of 150 degrees or above, it becomes brittle and partly loses its insulating power.

inch in thickness, and wrap

A covering made by wrapping three or more layers of asbestos paper, each about 1 inch thick, on the pipe, covering with a layer of hair felt ping the whole with canvas or paper, is much used. This covering has an effective life of about 5 years on high-pressure steam-pipes and 10 to 15 years on low-temperature pipes. There are a large number of coverings regularly manufactured for use, in such a form that they can be easily applied or removed if desired. There is a very great difference in the value of these coverings; some of them are very heavy and contain a large amount of mineral matter with little confined air, and are very poor insulators. Some are composed entirely of incombustible matter and are nearly as good insulators as hair felt. In general the value of a covering is inversely proportional to its weight-the lighter the covering the better its

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