W 136. Illustrative Problem.-Overhead Piping (Fig. 15).-210 deg. F. average-20-deg. drop. = 59.8 lb. per cu. ft. Total length of circuit is 450 ft. (From Table 2, p. 1081, Column 6). Table 13, schedule "A" gives 5788 square feet of 3-column radiators for this layout. Water per sq. ft. = 162.0 FIG. 15.-Overhead forced hot water system iayout, 210 deg. average temperature, 20 deg. drop, Schedule "A", (Table 13). Diagram 3 shows 160 gal. in 3-in. pipe at about 7 ft. per sec. A 3-in. main will be ample by easing up the rest of the circuit and have plenty of power in the motor to spare. This makes 2.82 or 3 gal. per min. per 100 sq. ft. for the radiation. Using Table 15 for equalization of pipes, a 3-in. pipe is equivalent to 32 34-in. pipes and two 212-in. pipes are equivalent to 36 4-in. pipes, so the two branch supply mains in roof and returns with basement will be 21⁄2 in. each which will reduce the head slightly to make up for the slight excess head in the 3-in. cap using the same drop per 100 ft., in this case 7 ft., and from the gallons we can size pipes from Diagram 3 and Table 15. Riser 1 has 7.8 gal., and as all distances are alike, they can be neglected. From Diagram 3, riser 1 is 1 in. Riser 2 requires 8.4 gal. and will be 1 in. as a 1 in. gives over 9 gal. per min. for 7 ft. loss in head per 100 ft. (Diagram 3), etc. Continue as indicated. The pump should be 2% or 3-in. double suction with brass impeller ring bearings and water packed stuffing boxes on the pump. A 3-valve by-pass should be arranged so the water may be circulated by gravity at times. Air traps and air valves should be placed at all points necessary to relieve the air, as shown in Fig. 15. The expansion tank (Fig. 15A) should be supplied with pop valve set at 10 lb. It should also be supplied with an automatic water feeder and swing check valve, to break any vacuum. Care must be exercised to see that the city water pressure to the automatic water feeder is sufficient to overcome the static head of 70 ft. plus the 10-lb. pop valve or a total pressure in the basement in this case of 45 lb. per sq. in. The return from the heating system forms the suction of the pump and the discharge is led into the return openings of the boiler. With down draft boilers, all connections should be tied together. It is well to make a double connection to the supply so as to reduce resistance. FIG. 154.-Arrangement of expansion tank and fixtures for closed hot water heating systems operating with temperature above 210 deg. F. 14. Gravity Hot Water Heating.-The same formulas for discharge of pipes do not hold for lower velocities than 2 or 3 ft. per sec., as occur in gravity water heating. I. V. Serginsky in the Heating and Ventilating Magazine, November 1913, translated and gave the formulas of Dr. R. Biel. There are two critical velocities: Vi the lower critical velocity; and V2 = 0.158 1.382 the upper critical velocity. The proper selection of the formulas (A), (B), and (C) Vd given below is made, using (A) for velocities below V1, (B) for velocities between V1 and V2, and (C) for velocities above V2. Both V1 and V2 are in feet per second and d = internal diameter of pipe in inches. h head in inches of water at the average temperature of that in the system (for head in feet of water divide by 12). = These formulas from which Diagram 4 was constructed, are as follows: As various temperatures and heights may occur in hot water heating, the writer arranged Diagram 4 to read in feet head drop per 100-ft. length of standard pipe. The discharge is in cubic feet per hour for the average weight per cubic foot. The chart is universal for any temperature condition as the teet head of water will be proportional to the weight per cubic foot. This is not true where specific conditions of temperature are named. The total head available for gravity heating is the difference in weights per cubic foot at the final and initial water temperatures divided by the weight per cubic foot at the average temperature multiplied by the average height of the system. To determine the cubic feet per hour, divide the heat loss multiplied by 125% by the product of the assumed temperature drop by the weight per cubic foot at the average water temperature. The result will be the cubic discharge for the whole system. Using Diagram 4, the main may be directly determined. For other circuits, divide the total cubic feet by the total square feet of radiation and determine the size for subsidiary circuits in the same manner. If mains are laid out so radiators are equidistant from the boiler, the rest of the sizes can be read from equalization Table 15 in proportion to the square feet of heating surface installed. This applies to all gravity hot water heating plants of all descriptions. 3.0 20 2.0 Drop in head in feet per 100 feet of pipe at average temperature of the discharge water 8 .06 .05 .04 .03 .02 20 0.2 0.5 0.4 0.3 0.6 .01 0.2 per sec 0251 04 0.9 Cubic feet of water "Average weight of water X temperature drop DIAGRAM 4. 3. Table 15 is an equalization table based on the friction formula, p. 1106, used for Diagram Tests give higher values for the smaller size pipes, so there is a safeguard of about 10% where the larger size pipes are in terms of the smaller. The light face figures are the number of pipes of equivalent area; the heavy face figures, the number with the same drop in friction head for the same discharge per unit of time. The heavy lines drawn diagonally and figures on the margin give the ratio on area increase for the number of smaller pipes and also the ratio of the decrease in velocity. All water systems with a temperature over 180 deg. maximum should be closed systems with a pop valve set at least 5 lb. above the pressure corresponding to the maximum temperature, i.e., 220 deg. requires a 10-lb. pop valve. All water systems below 180 deg. maximum temperature may be open systems, but it will be found that all hot water systems circulate better under some pressure. Objection has been made to closed water systems as being dangerous due to likelihood of explosion. This is erroneous provided proper safety valves are used. There is just as much danger of blowing all the water out of the open system and cracking the cast-iron boiler if cold water enters, as there is from a possible rupture from a water temperature of 220 deg. with a proper safety valve. Safety valves on all heating systems should be proportioned to the grate area of the boiler as this is the source of the maximum available energy. FIG. 16.-Gravity hot water heating system layout, 210 deg. average temperature, 30 deg. drop, shunt system, single main in basement. 14a. Illustrative Problem.-Closed Gravity Hot Water System.-Fig. 16 is a layout for a gravity hot water system for the same problem as Fig. 15 with the main below, all risers shunted, and coils on the fourth floor. Water is taken at 210 deg. average temperature with 30-deg. drop. This will have to be a closed system with the expansion tank of the same type as the forced hot water system (Figs. 15 and 15A). This will give a maximum temperature in the system of 225 deg. or the same as 5-lb. steam, and requires a 10-lb. pop valve on the expansion tank. Heat loss plus 25% as before = 1,540,000 B.t.u. (Table 13) Average height of system-schedule "A," Table 13. 4th fl. 1360 X 55 74,800 3rd fl. 1230 X 40 2nd fl. 1230 X 20 = = 49,200 = 24,600 1st fl. 1645 X 8 13,160 = From Table 2, p. 1081, Column 6, W at 210 deg. F. average temperature = 59.88 lb., at 225 deg. F. and at 195 deg. F. = 60.24 lb. From Diagram 4, with a drop of 0.1236 ft. per 100 ft. and a discharge of 862 cu. ft. per hr., this figures a 6-in. main because the distance is shorter and the drop is greater than in Fig. 17. Each of the two branch mains should be (from Table 15) two 5 in. as 1.62 of 5-in. mains equal one 6-in, main. In this system each riser and radiator is connected directly off the main and the power of circulation is derived from the cooling of the radiators. The connection practically increases the size of the main. The head will have to be decreased through the risers and radiators as the temperature falls along the line; there will be only a small part of the drop in temperature through any radiator. We have for the main 0.1277 ft. drop per 100-ft. and a 6-in. dividing into two 5-in. branches. In this method there are no reducing fittings and every temperature increment is added and none are minus so the velocity will be greater with less friction from fittings reducing on the run. One 6-in. main is equivalent to 193 34-in. pipes (see Table 15). With 5500 ft. surface take 11⁄2 times for the first floor, 1 for the second and third, and 34 for the fourth floor. Fig. 17.-Overhead gravity hot water open tank system layout, 170 deg. average temperature, 25 deg. drop. = 0.0351 34-in. pipes per sq. ft. of radiation = 3.51 34-in. pipes per 100 sq. ft. or, from Table 15, equivalent to a 14-in. pipe per 100 sq. ft. of radiation. 193 X 1 5500 For fourth floor: 193 X 0.75 5500 = 0.0263 34-in. pipes per sq. ft. of radiation = 2.63 4-in. pipes per 100 sq. ft. or, from Table 15, equivalent to a 14-in. pipe per 100 sq. ft. of radiation. From the above and the sketch, the pipe sizes may be easily calculated. |