Document pmd3bjxJNDZzk0Ye1Bmkd3doD

`440 CHAPTER 23 - J ; 1948 Guide i' - ... SIZING PIPING FOR TWO-PIPE LOW PRESSURE SYSTEMS Piping for two-pipe low pressure systems is sized in the same manner as' for two-pipe vapor systems, except that the pressure drop throughout the system can be based on psi to 1 psi drop. SIZING PIPING FOR TWO-PIPE VAPOR SYSTEMS While many manufacturers of patented vapor heating accessories have their own schedules for pipe sizing, an inspection of these sizing. tables indicates that in general as small a drop as possible is recommended. The reasons for this are: (l) to have the condensate return to the boiler by gravity, (2) to obtain a more uniform distribution of steam throughout the system, especially when it is desirable to carry a moderate or low fire, and (3) because with large variations in pressure the value of gradu ated valves on radiators, is destroyed. For small vapor systems where the equivalent length of run does not exceed 200 ft, it is recommended that the main and any runouts to risers that may be dripped should be sized from Column D, Table 5, while riser runouts not dripped and radiator runouts should employ Column I. The up-feed steam risers should, be taken from Column H. On the returns, the risers should be sized from Tables 6 and 7, Column U, (lower portion) and the mains from Column U (upper portion). It should again be noted that the pressure drop in the steam side of the system is kept the same as on the return side except where the flow in the riser is concerned. On a down-feed system the main vertical riser should be sized from . Column.iT, but the down-feed risers can be taken from Column D al though it so happens that the values in Columns D and H for small systems correspond. This will not .hold true in larger systems. For vapor systems over 200 ft of equivalent length, the drop should not exceed psi to psi, if possible. Thus, for a 400 ft equivalent run the drop per 100 ft should be not over J-^psi divided by 4, or M2 psi. In this case the steam mains wouid be sized from Column B, the radiator and undripped riser runouts from Column I; the risers from Column B, because Column H gives a drop in excess of M2 psi. . On a down-feed system, Column B would have to be used for both the main riser and the smaller risers feeding the radiators in order not to increase the drop oyer M2 psi. The return risers would be sized from the lower portion of Column O and the dry return main from the upper portion of the same column, while any wet returns would be sized from Column N. The same pressure ..drop is applied on both the steam and the return sides of the system. Notes on Vapor Systems 1. Pitch of mains should not be less Uian M in. in 10 ft. 2. Pitch of horizontal runouts to risers and radiators should not be less than J^ in. per foot: Where this pitch cannot be obtained runouts over 8 ft in length should be one size larger than called for in the table.- " 3. In general it is not desirable to have a supply main smaller than 2 in. 4. When necessary, supply main, supply risers, or runouts to supply risers should be dripped separately into a wet return, or- may be connected into the dry return through a thermostatic drip trap. SIZING PIPING FOR TWO-PIPE VACUUM SYSTEMS . Vacuum, atmospheric, sub-atmospheric and orifice systems are usually employed in large installations and have total drops varying from to -'Steam Heating Systems and Piping - -' - ' 441 H psi. Systems in which the maximum equivalent length does not.exceed 200 ft preferably employ, the smaller pressure drop, while systems over 200 ft equivalent length of run more frequently are designed for the higher drop, owing to the relatively greater saving in pipe sizes. For example, a system with 1200 ft longest equivalent length of run would employ a drop per 100 ft of x/2 psi divided'by 12, or l/u psi. In this case, the steam main would be sized from Column C, Table 5, and the risers also from Column C (Column H could be used as far.as critical velocity is concerned but the drop would exceed the limit.of l/u psi). Riser runouts, if dripped, would use Column C but if undripped would use Column I; radiator runouts," Column I; return risers, lower part of Column S, Tables 6 and 7; return runouts to radiators, one pipe size larger than the radiator trap con nections. Notes on Vacuum Systems 1. It is not generally considered good practice to exceed M psi drop per 100 ft of equivalent run nor to exceed 1 psi total pressure drop in any system. 2. Pitch of mains should not be less than M in. in 10 ft. 3. Pitch of horizontal runouts to risers and radiators, should not be less than M in. per foot. Where this pitch cannot be obtained runouts over 8 ft in length should be one size larger than called for in the table. 4. In general it is not considered desirable to have a supply main smaller than 2 in. . 5. When necessary, the supply main, supply riser, or runout to a supply riser should be dripped separately through a trap into the vacuum return. A connection should not be made Between the steam and return sides of a vacuum system without interposing a trap to prevent the steam from entering the return line. ' "6.. Lifts should'be avoided if possible, but when-.they cannot be eliminated they "should be made in the manner described in this chapter. 7. No lifts can be used in orifice and atmospheric systems. In sub-atmospheric systems the lift must be at the vacuum pump. SIZING PIPING FOR INDIRECT HEATING UNITS Pipe connections and mains for indirect heating units are sized in. a mariner similar to radiators, but 'the equivalent direct radiation^ must be ascertained for each row of heating unit stacks and then must be divided into the number of stacks constituting that row arid into the number of connections to each stack. = Q X 60 X (ft - k) _ Q X (ft - <e) 55.2 X 240 220.8 (1) where .l EDR = equivalent direct radiation, square:feet. ... , .Q ^volume.of air, cubic feet per minute.. h =- the temperature of the air . entering the row of heating units under con. sideration, Fahrenheit, degrees. ... :;. ft..= the temperature of.the air leaving the row of heating units under considera tion, Fahrenheit degrees. 60 =.the .number of minutes in one hour. : 55.2 =-the number of cubic feet.of air.heated,1 Fahrenheit degree by 1 Btu. 240 = Btu equivalent of 1 sq ft of EDR. ...... . . :: .. ..1. Example 3. Assume that a 3-row heating-unit .shown in this chapter in Fig. 38 is ^handling 50,000 cfm of air and that the rise in the first row. is. from 0 to 40 F\ in the second row from 46.to 65 F; and in the third row from ,65 to 80 F. What is the load iiiEDR on'-each supply'and return connection? '