Document DvqbJG71G92BMMVVwXaMX8QBQ

490 CHAPTER 23 1949 Guide return connections. Boiler header and piping sizes should be based on the total load. CONDENSATE RETURN PUMPS Condensate return pumps are used for gravity systems when the local conditions do not permit the condensate to return to the boiler under the existing static head. The return of the condensate permits the water to repeatedly go through the cycle of vaporization, with subsequent condensation and return to the boiler. During such repeated cycles any incrustants or other substances in solution are precipitated and the water de-activated to a considerable extent so that corrosion of a serious nature is seldom ever encountered where the condensate is repeatedly used. Serious corrosion is more frequently found in systems in which the con densate is wasted and fresh make-up water is continually being intro duced. A generally accepted condensate pump unit for low pressure heating systems consists of a motor-driven centrifugal pump with receiver and automatic float control. Other types in use include rotary, screw, turbine 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 condensate unit is illustrated in Fig. 9. 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 con densate. Direet-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 In general, however, these have been replaced by the automatic motor-driven return line heating pump especially developed for this, service.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 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 passing thS 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 be supplied, In some instances separate air and water pumps may be used. . Steam Heating Systems and Piping 491 For rating purposes* vacuum pumps are classified as low vacuum and high vacuum. Low vacuum pumps are those rated for maintaining 5J in. Hg vacuum on the system, and high vacuum pumps are those rated to maintain vacuums above 5j 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 1000 EDR served. This capacity is at 5| in. of vacuum and with con densate at 160 F. The larger air capacity is for smaller systems and the smaller capacity for the larger systems. Some manufacturers, however, specify more air capacity than stand ard where higher vacuums are desired and where air leakage is sus pected. 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 flash tank or flash leg 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 later in this chapter in section on Drips. Vacuum Pump Controls In the ordinary vacuum system, the vacuum pump is controlled by a vacuum regulator which cute in when the vacuum drops to the lowest point desired and cute out when it has been increased to the highest point, these points being varied to suit the particular system or operating condi tions. In addition to this vacuum control, a float control is included which will start the pump whenever sufficient condensate accumulates in the receiver, regardless of the vacuum on the system. A selector switch is usually provided to allow operation at night as a condensate pump only, also to give manual or continuous operation when desired. There are several variations in the control of the vacuum maintained on the system by the pump. In some sub-atmospheric systems:where orifices are used, the vacuum pump control maintains a pressure difference between the supply and the return piping, which is held within relatively close limits. There are other sub-atmospheric systems which utilize special temperature-pressure actuated controls for maintaining the desired condi tions in the return lines. Where various zones are connected to the same return main, the return vacuum must be controlled to meet the requirements of the zone operating at the lowest steam supply pressure. Piston Displacement Vacuum Pumps Piston displacement return vacuum heating pumps may be either elec tric or steam driven. Their piston speed in feet per minute should not exceed 20 times the square root of the number of inches in their stroke. They are usually supplied with an air separating tank, open to atmosphere, placed on the discharge side of the pump and at an elevation sufficiently