Document jyYqgzEk60d4M6962YJYq7gop

w American Society 0/Heating and Ventilating Engineers. 4. The pipe system must be installed sothat each circuit has its correct frictio To bring this about, it is necessary in some cases to minimize the friction, i.e to "ea^! ftthiAe npiinpae lliinnoe nacs eshhnolr+t naos rpVoiesesii'bKlIeil nann/d4 ftno nprr-omvriiVdJefl Oaso ffeawiv ffiitftfi!nngrrso naes npoius^silbLlte.;1 an'i*l Win) tllalrA cases it is necessary to increase the length of the pipe and the number of fittings so fk for every circuit, the friction head will be equal to the available pressure head The connections from the boiler to the mains should be short and direct,.to redu iT fiction head. It is frequently possible to avoid an elbow and to reduce the/length of th* pipe by running the pipe in a diagonal direction, either in aTiorizontal or in a vert 1 plane. \. J "tical The mains and branches should pitch up and away from the neater; generally less than I in. in 10 ft. The flow main should always be covered; the return main shonM be covered except where it is to provide the heating surface for the basement. _ The connections from mains to branches and to risers should be such that circulatio through the risers will start in the right direction. Hence, in a one-pipe system the 00 connection must be nearer the heater than the return connection. In a cor^ectlv, designed two-pipe system, the pressure in the flow main is higher than that in the return Fig. 11. Method op Connecting^Radiator to Allow for Expansion of Pipe mainland a slight variation in:the distances of the flow and return connections from the heater is not material; but it is generally best to have the two"connections about equally distant from the heater. In some cases it may be advisable to take the flow connection off the top of the main and the return connection from the side, but in most cases both connections should be at an angle of 45 deg.. This method shortens the lines, and. substitutes 45-deg ells for 90-deg ells. Preferably, connection of the flow riser to a radiator should be to the upper tapping, and connection of the return riser to a radiator should be to the lower, tapping. When hot water enters at the top of a radiator it will distribute itself along the entire length of the radiator, and as it cools it will settle gradually to the bottom; the cool water may then be taken out of .the radiator at either end. , With forced circulation and high velocities, it is advisable to let the water enter at the top of the radiator and leave at the bottom of the opposite end; With gravity circulation andlow velocities it makes little difference whether the water, leaves at the end at which it enters-or. at the opposite end. The connections of the risers to the radiators should'be such that provision is made for the vertical expansion of the risers. This can be accomplished: as indicated in Fig; 11 by using one-tee and two ells-for each-connection. These connections should be pitched upward or downward, whichever may be necessary to prevent1 the formation of air pockets and to permit draining. 576 Chapter 33--Hot Water Heating Systems and Piping PROBLEMS IIV PRACTICE l causes the circulation of water-in hot water heating systems? ty systems, circulation is caused by the difference between the weight of the cool wna?teTr- ;inn tthnee rreetiuur,n,.r--is--e--r--a--n-d---that of the hot water in the flow riser. forced circulation systems, circulation is produced primarily by a pump, and second ly by the difference in the weights of the water in the return and flow risers. However, h secondary effect is so small when compared with that of the circulating pump that ------- in most cases. ___ _____ _ ... 2 What tends to prevent or to retard the circulation of water in hot water beating systems? [n both gravity flow and forced circulation systems, the friction which must be overcome when the water is flowingThrough pipes, fittings, valves, heaters, and radiators tends to prevent or retard circulation. For a given pipe the friction varies approximately as the 1.7 power of the velocity, and for given fittings, valves, heaters, and radiators, the friction varies approximately as the square of the velocity. It is therefore sufficiently accurate to express the friction in fittings, valves, heaters, and radiators in terms of the friction in one standard elbow, as shown in Table 1. 3 In the elementary heating system* Fig, 8, what is the pressure head main taining the circulation if the water in the return riser is at 180 F and that in the flow riser is at 200 F? It is found, from Table 8, Chapter 1, that 180 F water weighs 60.61 lb per cu ft and 200 F water weighs 60.13 lb per cu ft. The pressure head is independent of the size of the pipe. If the two risers were each 1 ft square, the water in the flow riser would weigh 601.3 lb and that in the return riser would weigh 606.1 lb. Thus the water in the return riser would weigh 4.8 lb more than that in the flow riser. Consequently, the resulting pressure "head is 4.8 lb per square foot. Pressure heads are generally expressed in feet, or inches, or milinches of water of a given temperature. In this case we are dealing with water at both 180 F and 200 F, so the pressure head is expressed in terms of 190 F water. Such water weighs 60.39 lb per cu ft, and to secure a pressure of 4.8 lb per square foot, it is necessary to have a column of water having a weight of 4.8 divided by 60.39 = 0.0795 ft, or 0.9540 in., or 954 milinches. This is the pressure head which maintains the circulation. 4 In the elementary system of Question 3, if the radiator dissipates 14,000 Btu per hour, what is the velocity of the water in the pipe line, if the pipes are 1 in. in diameter? What, if they are ^ in. in diameter? Since the temperature drop through the radiator is from 200 F to 180 F or 20 F, every pound of water flowing through the radiators delivers 20 Btu; consequently, 14,000 divided by 20 = 700 lb of water, or for 190 F water, 700 divided by 60.39 = 11.59 cu ft of water must flow through the radiator and through the pipe lines every hour. The interior area of a 1-in. pipe is 0.864 sq in. The velocity in the 1-in. pipe is 11.59 divided by 0.864 and multiplied by 144 = 1932 ft per hour or 6.44 in. per second. For %-in. pipe, the interior area is 0.533, and the velocity is 6.44 multiplied by 0.864 5 If, in the elementary heating system of Question 3, a 1-in. pipe line is used, what would be the friction head? f the radiator is connected as shown in Fig. 11, with the heater connected to provide reedom of expansion, the heating circuit may be assumed to consist of a heater, 25 ft of ripe, 8 elbows, 1 radiator valve, and 1 radiator. From Table 1 it appears that the heater nd radiator are equivalent, in friction, to 6 elbows; hence, the circuit may be placed qual to 25 ft of pipe and 14 elbows. 577