As most direct heating systems, and especially those in schoolhouses, are made up of both radiators and circulation coils, an efficiency of 300 B. T. U. has been taken for direct radiation of whatever variety, no distinction being made between the different kinds. This gives a slightly larger pipe than is necessary for cast iron radiators, but it is probably offset by bends in the pipes, and in any case gives a slight factor of safety. We find from a steam table that the "latent heat" of steam at 20 pounds above a vacuum, (which corresponds to 5 pounds gage-pressure) is 954 + B. T. U., which means that for every pound of steam condensed in a radiator 954 B. T. U. are given off for warming the air of the room. If a radiator has an efficiency of 300 B. T. U., then each square foot of surface will condense 300/ 954 =.314 pounds of steam per hour, so that we may assume in round numbers a condensation of 1/3 of a pound of steam per hour for each square foot of direct radiation, when computing the sizes of steam pipes in low-pressure heating. Table XIII has been calculated on this assumption, and gives the square feet of heating surface

 TABLE XIII. LENGTH OF PIPE 100 FEET. Square Feet of Heating Surface. Size of Pipe. 1/4 Pound Drop. 1/2 Pound Drop. 1 80 114 1 1/4 145 210 1 1/2 190 340 2 525 750 2 1/2 950 1350 3 1550 2210 3 1/2 2320 3290 4 3250 4620 5 5800 8220 6 9320 13200 7 13800 19620 8 19440 27720

which different sizes of pipe will supply, with drops in pressure of 1/4 and 1/2 pounds, in each 100 feet of pipe. The former should be used for pressures from 1 to 5 pounds, and the latter may be used for pressures over 5 pounds, under ordinary conditions. The sizes of long mains and special pipes of large size should be proportioned directly from tables X, XI and XII.

Where the two-pipe system is used and the radiators have separate supply and return pipes, the risers or vertical pipes may be taken from table XIII, but if the single pipe system is used, the risers must be increased in size as the steam and water are flowing in opposite directions and must have plenty of room to pass each other. It is customary in this case to base the computation on the velocity of the steam in the pipes rather than on the drop in pressure. Assuming as before, a condensation of one-third of a pound of steam per hour per square foot of radiation, the following tables have been prepared for velocities of 10 and 15 feet per second. The sizes given in table XV have been found sufficient in most cases, but the larger sizes, based on a flow of 10 feet per second, give greater safety and should be more generally used. The size of the largest riser should usually be limited to 2 1/2" in school and dwelling house work unless it is a special pipe carried up in a concealed position. If the length of riser is short between the lowest radiator and the main, a higher velocity of 20 feet or more may be allowed through this portion rather than make the pipe excessively large.

 10 Feet Per Second Velocity. Size of Pipe. Sq. Feet of Radiation. 1 30 1 1/4 60 1 1/2 80 2 130 2 1/2 190 3 290 3 1/2 390
 TABLE XV. 15 Feet Per Second Velocity. Size of Pipe. Sq. Feet of Radiation. 1 50 1 1/4 90 1 1/2 120 2 200 2 1/2 290 3 340 3 1/2 590

Example. - Compute the size of pipe required to supply 10,000 square feet of direct radiation, where the distance to the boiler house is 300 feet and the pressure carried is 10 pounds; allowing a drop in pressure of 4 pounds.

Steam required = 1/3 X 10,000 = 3333 pounds per hour

= 55.6 pounds per minute.

From Table XII we see that this corresponds to 55.6/ .57

= 97.5 pounds for a pipe 100 feet long. From Table XI the discharge factor is 1.14. Hence in Table X we look for 97.5 =

1/14

85.5 under 4 pounds drop, and we find that a 5-inch pipe must be used.

 Dia. of Steam Pipe. Dia. of Dry Return. Dia. of Sealed Return. 1 1 3/4 1 1/24 1 1 1 1/2 1 1/4 1 2 1 1/2 1 1/4 2 1/2 2 1 1/2 3 2 1/2 2 3 1/2 2 1/2 2 4 3 2 1/2 5 3 6 3 1/2 3 7 3 1/2 3 8 . 4 3 1/2 9 5 10 5 4 12 6 5