TABLE 15.—TABLE OF NUMBER of Pipes of Equivalent Area, also Number Having THE SAME DISCHARge for Same Length (3⁄4 in. to 12 in.)

lower scale for pounds of steam, and the full lines to the upper scale, which is 100 times the lower. The pressure drop per 100 lin. ft. is in pounds per square inch, and dotted lines show equal pressure drop in ounces per square inch per 100 lin. ft. In order to use this diagram and table it is necessary to know (1) the pounds of steam per hour (determined from the heat loss of the building plus 25% divided by the latent heat of evaporation), and (2) the friction head (drop) in pounds per square inch per 100-ft. length (determined from the maximum allowable friction head divided by the length of the circuit). If the piping is arranged in the proper manner, the branch circuits may be designed from the equalization table, Table 15.

126. Size of Return Pipes.-James A. Donnelly has deduced the sizes of return pipes for various systems as shown in Table 16. The theoretical size of return main in any steam heating system will depend upon (1) the allowable pressure drop in the main, usually

FIG. 14.-Low pressure steam, 1-lb. to 5-lb. pressure, 220 deg. F., Schedule "C" (Table 13), overhead down feed. one and two pipe; also one and two pipe up feed.

taken the same as for the steam main, and (2) upon whether the main is wet (run below the water line of boiler) or dry (run above the water line of the boiler). In the case of wet returns the main carries only water. Assuming the same friction pressure loss as for steam and with water flowing in the pipes, the weight will vary as the square root of the densities for same pressure drop and pipe size, or a wet return has 40 times the capacity of a dry return. These capacities are given in square feet of radiation at 0.3 lb. per sq. ft. per hr.as the condensation factor. 12c. Illustrative Problem.-Fig. 14 is a steam piping layout. One-half of the building has a one-pipe up feed and the other half an overhead down feed piping system. Practically 220 deg. average, or 2 to 5-lb. steam pressure is used. From Fig. 7 and Table 13, B.t.u. losses per hour 1,232,000+25% 1,540,000 B.t.u. Divide by the latent heat of steam at 220 deg. F., or 965 B.t.u. 1600 lb. of steam per hr. with 5310 sq. ft. of surface (schedule "C"). The length of the circuit on the overhead system will be 400 ft. The available drop in pressure in feet of water is equal to the difference between the height of the water line in the boiler and the height of thewater in the return main-say 18 in. of water column, or, with 0.43 lb. per ft. = 1.5 X 0.43 = 0.645 lb. or 0.645 X 16 =

1,540,000
965

10.32 X 100

10.32 oz.

400

If there are 5300 sq.

= 2.6-oz. drop in pressure per 100 ft.-say 2 oz. to be conservative. 1600 ft of surface, 1600 lb. of steam per hr., and 2 oz. drop per 100-ft. run, we have = 0.3 lb. of steam per sq. ft. 5300 of heating surface per hr. In Diagram 2, use the line of 2-oz. drop which shows a 5-in, main will be large enough If we assume 1-oz. drop per 100 ft., it will require a 6-in. line. Use a 5-in. line with covered risers, or a 6-in. line uncovered.

As the total heat loss of the building was used, the point of capacity for covered or uncovered pipes is not so important, as the greater radiation will reduce the pressure required to heat the building, increasing the drop and velocity.

2650

=

40.6 X 100
2650

The 5-in. main, being divided into 2 branches, Table 15 shows 1.8 of 4-in, pipes equal a 5-in. pipe and, therefore, two 4-in. branches will be a little larger than the 5-in, main. A 4 in. has 65.5 times the capacity of a 34-in. pipe (Table 15). For one-half the surface, or 2650 sq. ft., 40.6 34-in. pipes or = 1.53 of 4-in. pipes 65.5 per 100 sq. ft. of surface. Riser 1 has 260 sq. ft.; therefore, 2.60 X 1.52 = 4.0 of 34-in. pipes, or one 11⁄2-in. pipe (Table 15). Riser 2 has 305 sq. ft.; therefore, 3.05 X 1.52 = 4.7 of 4-in. pipes, or one 11⁄2-in. pipe, etc. For return pipe, use Table 16, Column F.

For one-pipe up feed system, double the riser connections in size as there will be water and steam flowing in opposite directions-that is, use 3.2 of 34-in. pipes per 100 sq. ft. for riser and 1.52 of 34-in. pipes per 100 sq. ft. for steam main.

Fig. 14 is intended to show diagrammatically different methods of connecting risers and radiators applicable to all steam, vacuum and vapor systems. The use of automatic vents and thermo-traps would form a vapor system when connected directly to boilers; with mechanical air removal and traps and pumps would be a vacuum system; with all appliances left off and directly to boiler would be a gravity return system.

TABLE 16.-MAXIMUM CAPACITY OF STEAM AND RETURN MAINS FOR RUNS OF

MODERATE LENGTH1

In this table the maximum capacity of return mains is given for various percentages of steam carried, as well as the steam rating of pipes from 1⁄2 to 16 in. These quantities are all figured for a drop in pressure of 1 oz. per sq. in., per 100 ft. in straight pipe and are expressed in sq. ft. of radiation with a condensation factor of 0.3 lb. per sq. ft. per hour.

Column "A," steam rating in standard direct radiation for piping within buildings for all classes of systems. Column "B," rating for wet return main of a gravity system.

Column "C," rating for main return of the positive differential system.

Column "D," rating for the main return when it is above the water line in a dry return gravity system. Column "E," rating for the branch returns of the positive differential system, and for the branch returns that are above the water line in a wet return gravi y system.

Column "F," rating for the branch returns of a dry return gravity system.

Columns "G" and "H," rating for gravity systems where the returns have unusual condensing capacity and for the returns in vacuum systems where jet water is used.

If it is desired to apportion the sizes more accurately, follow each circuit and branch with the distances, reading the drops from the discharge on Diagram 2. The sum of the drops for flow on all circuits, drawing a line from boiler through the radiator and back, should all equal the same total sum of 2-oz. drop per 100 ft. or a total of 8 oz. All risers and mains should be dripped and checks or thermostatic traps placed at points marked “A” in Fig. 14.

13. Forced Hot Water System.—Diagram 3 is the result of a comparison of more than seven friction formulas published by the writer in "Power" July 9, 1912. The average seemed to agree nearest to William and Hazen's formula:

If all units are equidistant from the source in the layout of any piping system, the length in the formula may be disregarded as the distances are balanced.

In any case, no matter how complicated the piping system, the head through all circuits and fluid passages will be the same, and the velocity of the flows will adjust itself accordingly. It often occurs that with some layouts the problem becomes indeterminate, and these constructions should be avoided by reducing to a minimum the number of passages for the fluid. Effort should be made to avoid complicated systems requiring accurate determination of the drop and to so plan the circuits that they will give the proportional flow required for each branch, with a minimum of computation.

13a. Pumps for Forced Hot Water Systems. On large plants two pumps should be used, one electric and one steam driven. If these pumps have a rating for the total capacity of the system, they should be placed in series; if the rated capacity is 1⁄2 of the maximum or less, they should be placed in parallel.

The pumps should be double suction with hollow impeller, preferably of brass or bronze, with water packed bearings.

in which h= total friction head plus safety factor in feet.

After calculation of required quantities, the nearest size frame built by the manufacturer should be determined and the layout adjusted accordingly.

The curves of these pumps are such that when the head reaches the maximum point, the discharge falls off to nothing at the rated speed, or the discharge increases to a certain point and cannot be increased except by an increase in speed.

If the friction on the piping for a certain capacity is greater than the rated head on the pump, the discharge will be reduced. If the friction is less, the discharge will increase until there is a balance and the motor is apt to overload due to the increased horsepower.

When the head on the gages shows the same as the rated head of the pump, the pump may not be delivering the There are a large number of installations where the water delivered is not even near the capacity of the It is good practice when using motor driven pumps in places where they receive little attention, to specify a Cutler Hammer overload release so the motor will be thrown out of circuit automatically when overloaded.

water. pump.