Document YqREJwGKkmzyNqngK55dGOqk

438 CHAPTER 23 ' 1946 Guide and automatic float control. Other types, in use include rotary, screw and reciprocating pumps with steam turbine or motor drive, and directacting steam reciprocating pumps. The receiver capacities of these automatic units should be sized so as not to cause too great a fluctuation of the boiler water, line if fed directly to the boiler and at the same time not so small as to cause too frequent operation of the unit. The usual unit provides storage capacity between stops in the receiver of approximately 1.5 times the amount of condensate returned per minute and the pump generally has a delivery rate of 3 to 4 times the normal flow. This relation of receiver and pump size to heating system condensing capacity takes account of the peak condensation rate. A typical, installation of a motor driven automatic condensation unit is illustrated in Fig. 17. a Steam Heating Systems and Piping . - _______________________ _______________________________________439 ' furnished complete with receiver, separating tank and automatic controls mounted as.an integrated unit on one base. There are also special steam turbine driven units which are operated by piassing the steam to be used in heating the building through the turbine with only a 2 to 3 psi drop across the turbine required for its operation. Under special conditions such as installations where it is necessary to return the condensate to a high pressure boiler, auxiliary water pumps may be1 supplied.. In some instances separate air and water pumps may be used. For rating purposes 3 vacuum pumps are classified as low vacuum and ; high vacuum. Low vacuum pumps are those rated for maintaining 5}/j in. Hg vacuum on the system, and high vacuum pumps are. those rated to maintain vacuums above 5J4 in- Manufacturers of vacuum pumps specify that the standard capacity of pumps shall be 0.3 to 0.5 cfm of air removal and 0.5 gpm of water per VACUUM HEATING PUMPS On vacuum systems, where, the returns are under a vacuum, and subatmospheric systems, where the supply piping, radiation and the returns are under a vacuum, it is necessary to use a vacuum pump to discharge the air and non-condensable gases to' atmosphere and to dispose of the condensation. Direct-acting steam-driven reciprocating vacuum pumps are sometimes used where high pressure steam is available or where the exhaust steam from the pump can be utilized. In general, however, these have been replaced by the automatic motor-driven return line Heating, pump especially developed for. this service. Steam turbine drive is .also frequently used where steam at suitable pressures is available, the steam being used afterward for building heating. The usual vacuum pump unit consists of a.compact assembly of exhausting unit for withdrawing the air-vapor mixture and discharging`the air to atmosphere and a water removal unit which .discharges, the condensate to the boiler. They are Fig. 18. Method of Discharging High-Pressure Apparatus into Low-Pressure Heating Mains and Vacuum Return Mains through ' a Low Pressure Trap 1000 EDR served. This capacity is at 5J- in. of vacuum and with con densate at 160 F. The larger air capacity is for sinaller systems and the smaller capacity for the larger systems. Some manufacturers, however, specify more air capacity than standard where' higher vacuums are desired and where air leakage is suspected. The vacuum that can be maintained on a system depends upon the relationship of the air leakage rate into the system to the operating air capacity of the hydraulic evacuator when operating at any given return line temperature. The hotter the returns, the lower will be the possible vacuum for a given air leakage rate into the system.. It is particularly essential on high vacuum installations to see that the entire system is tight in order to reduce the amount of inward air leakage and, further more, to see that relatively higher temperature steam is prevented from entering the vacuum return lines through leaky traps, high pressure drips, etc. It is for.this reason that the condensate from equipment using steam at high pressures should not be connected directly to a vacuum return line, but should drain to a receiver through a high pressure trap.; The receiver should have.an equalizing connection to a low pressure steam main and drain through a low pressure trap to the vacuum return main as indicated in Fig. 18.