Document 3egedgQ016O0oqOvB89Rx6RKE

American Society of Heating and Ventilating Engineers Guide, 1937 undripped riser runouts from Column I; the risers from Column B because Column H gives a drop in excess of 54 lb. 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 over % lb. 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 be not less than 34 in. in 10 ft. * 2. Pitch of horizontal runouts to risers and radiators should be not less than }4 in in 10 ft. 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., and when the supply main is 3 in. or over at the boiler or pressure reducing valve it should be not than 234 in. at the far end. _4. When necessary, supply main, supply risers, or branches to supply risers should be dripped separately into a wet return. The drip for a vapor system may be connected into the dry return through a thermostatic drip trap. VACUUM SYSTEMS Vacuum systems are usually employed in large installations and have total drops varying from to x/i lb. Systems where 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 go to 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 XA lb divided by 12, or lb. In this case the steam main would be sized from Column C, 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 344 lb). Riser runouts, if dripped, would use Column C but if undripped would use Column /; radiator runouts, Column I; return risers, lower part of Column S; return runouts to radiators, one pipe size larger than the radiator trap connections. Notes on Vacuum Systems 1. It is not generally considered good practice to exceed 34-Ib drop per 100 ft of equivalent run nor to exceed 1 lb total pressure drop in any system. 2. Pitch of mains should be not less than 34 in. in 10 ft. 3. Pitch of horizontal runouts to risers and radiators should be not less than }4 in. in 10 ft. 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. When the supply main is 3 in. or over, at the boiler or pressure reducing valve, it should be not less than 234 in. at the far end. 5. When necessary, the supply main, supply riser, or branch 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 Chapter 31 under Up-Feed Vacuum Systems. 582 Chapter 32--Piping for Steam Heating Systems ATMOSPHERIC SYSTEMS Xhe sizing of the supply and return piping on atmospheric systems is practically identical with the sizing used for vacuum systems and the same notes apply, except that no lift can be made in the return line. SUB-ATMOSPHERIC SYSTEMS Any properly pitched, correctly sized vacuum system without a lift may be used as a sub-atmospheric system when the proper equipment is substituted for the ordinary vacuum pump, traps, and controls. On new systems manufacturers usually recommend a drop on the steam line of between XA and % lb for the total run, and suggest adding 25 ft to the total equivalent length of run to insure that the steam gets through to the last radiator. The same notes apply to these systems as for vacuum systems, except that no lifts can be made in the returns. ORIFICE SYSTEMS The orifice systems can be operated with any piping system suitable for vacuum operation, according to experienced designers. Because these systems vary considerably in detail, it is advisable to consult the manu facturer of the particular system contemplated for recommendations. The same notes apply to these systems as to vacuum systems, except that lifts cannot be made in the returns of orifice systems if a vacuum pump is used. HIGH PRESSURE STEAM When steam heating systems are supplied with steam from a high pressure plant, one or more pressure-reducing valves are used to bring the pressure down to that required by the heating system. It has been con sidered good practice to make the pressure reductions in steps not to exceed 50 lb in each case. For example, in reducing from 100-lb gage to 2-lb gage, two pressure reducing valves would be used, the first reducing the pressure from 100-lb gage to 50 lb and the second reducing the pressure from 50-lb gage to 2-lb gage. Valves are available that will reduce 100 lb in one step, and it is questionable whether two valves are now required for initial pressures of 150 lb or less. The pressure-reducing valve, or pressure-regulator as it is sometimes termed, has ratings which vary 200 to 400 per cent. Some of these ratings are based on arbitrary steam velocities through the valve of 5,000 to 10,000 fpm and it is assumed that the valve when wide open has the same area as the pipe on the inlet opening of the valve. At times it is considered desirable to keep the steam velocity in the high pressure section of the piping and the low pressure section constant. The velocity ' through the valve port is obviously a function of the pressure drop across the valve. It is well known that steam flowing through an orifice increases its velocity until the pressure on the outlet side is reduced to 58 per cent of the absolute pressure on the inlet side, and that with further reduction of pressure on the outlet side little change in velocity will be obtained. As practically all pressure-reducing valves used for steam heating work 583