Document 4v5yeaQxDZD1BrVXY7KZnbLVG

T able 7. R eturn Pipe Capacities Capacity Expressed in Square Feet o f Equivalent Direct Radiation (Reference to this table w ill be by column le tte r U through E E ) T his table Is based on pipe size data developed through the research Investigations of the Am er ic an Society of H eating an d V e n t ila t in g E ngineers. ' I. CAPACITY OF RETURN MAINS AND RISERS --~ HEATING VENTILATING AIR CONDITIONING GUIDE 1944 286 CHAPTER 15. PIPINC FOR STEAM HEATING SYSTEMS Equivalent Length of Run All tables for the flow of steam in pipes, based on pressure drop, must allow for the friction offered by the pipe as well as for the additional resistance of the fittings and valves. These resistances generally are stated in terms of straight pipe; in other words, a certain fitting will produce a drop in pressure equivalent to so many feet of straight run of the same size of pipe. Table 5 gives the number of feet of straight pipe usually allowed for the more common types of fittings and valves. In all pipe sizing tables in this chapter the length of run refers to the equivalent length of run as distinguished from the actual length of pipe in feet. The length of run is not usually known at the outset; hence it is necessary to assume some pipe size at the start. Such an assumption frequently is considerably in error and a more common and practical method is to assume the length of run and to check this assumption after the pipes are sized. For this purpose the length of run usually is taken as double the actual length of pipe. TABLES FOR PIPE SIZING2 Factors determining the size of a steam pipe and its allowable limit of capacity are the direction of the flow of condensate, whether against or with the steam. Tables 6 and 7 are based on the actual inside diameters of the pipe and the condensation of Ji lb (4 oz) of steam per square foot of equivalent direct radiation3 (abbreviated EDR) per hour. The drops indicated are drops in pressure per 100 ft of equivalent length of run. The pipe is assumed to be well reamed without unusual or noticeable defects. Table 6 may be used for sizing piping for steam heating systems by determining the allowable or desired pressure drop per 100 equivalent feet of run and reading from the column for that particular pressure drop. This appiies to all steam mains on both one-pipe and two-pipe systems, vapor systems, and vacuum systems. Columns B to G, inclusive, are used where the steam and condensation flow in the same direction, while Columns H and I are for cases where the steam and condensation flow in opposite directions, as in risers and runouts that are not dripped. Columns J, K, and L are for one-pipe systems and cover riser, radiator valve and vertical connection sizes, and radiator and runout sizes, all of which are based on the critical velocities of the steam to permit the counter flow of condensation without noise. Sizing of return piping may be done with the aid of Table 7 where pipe capacities for wet, dry, and vacuum return lines are shown for the pressure drops per 100 ft corresponding to the drops in Table 6. It is customary to use the same pressure drop on both the steam and return sides of a system. rpipe ffjvy tables in chapter have been compiled in simplified and condensed form for the convenience of the user: at the same time all of the information contained in previous editions of The Guide has been retained. Values of pressure drops, formerly expressed in ounces, are now expressed in fractions of a pound. 'As steam system design has materially changed in recent years so that 240 Btu no longer,expresses the heat of condensation from a square foot of radiator surface per hour, and as present day heating units have different characteristics from older forms of radiation, it is the purpose of The Guide to gradually eliminate the empirical expression square foot ofequivalent direct radiation, EDR, and to substitute a logical unit based on the Btu. The new terms to express the equivalent of 1000 Btu (Mb), and 1000 Btu per hour (Mbh), have been approved by the A.S.H.V.E. 287