radiation on the small pipe sizes and more on the large than that by either Baldwin's or Mills' rules. In small plants, or in plants where a large number of small radiators are supplied from the risers, the number of radiators on a given riser may affect its size irrespective of the amount of radiation. Risers less than 1 inch in size are rarely used on a two-pipe system, or less than 1¼ on an ordinary one-pipe system, unless perhaps for only one small radiator. In an overhead system the height of the building and the consequent number of radiators on the riser affect the size, especially at the lower end. In such a system it must be borne in mind that all the water of condensation from the higher radiators is falling down the pipe, passing the connections to the lower radiators. For such systems in high office buildings it is therefore well to make the risers fairly large toward the bottom while the upper portion can be proportioned according to the sizes given in column (d). For buildings about ten stories high, the lower part of the riser should be not less than 2 inches, and if the amount of radiation on the riser is large or the building is over 15 stories high, this may better be 2½ inches. The table given is intended for low-pressure gravity systems and exhaust heating where not more than 2 or 3 pounds back pressure can be carried on the engine.
For high-pressure systems working at 20 or 30 pounds pressure, such as are used sometimes in factories, when engines are used with a condenser, the pipe sizes may be somewhat smaller than those given in columns (e) and (h), although for the smaller connections it is not advisable to reduce them on account of the possibility of water from the radiators backing into the supply pipes.
Pipes for vacuum systems.—In vacuum systems in which the vacuum is maintained on the return side, pipe connections may be reduced very materially, and Table III, given herewith, shows sizes recommended by Messrs. Warren Webster & Company for mains, risers and radiator connections for the vacuum systems which they install. As already described, their system is in principle an ordinary two-pipe system with a vacuum pump on the returns, but also having the special feature, of an automatic thermostatic valve on the return connection, which valve closes automatically when it is heated to steam temperature and opens when it becomes cooler. From the author's experience he would con-
