Document mpqegR18gGVZYX8mQJBdExY6d

748 CHAPTER 70 1962 Guide And Data Book Fig. 20 .... Suction ond Hot Gas Headers for ' Multiple Compressors1 to form crosses with the branch suction lines to the compres sors. The suction header should be run full size along its entire length. The horizontal take-offs to the. various compressors should be the same size as the suction header. No reduction should be made in the branch suction lines to the compressors until the vertical drop is reached. Fig. 20 shows suction and hot gas header arrangements for two compressors operating in parallel. Fig. 21 shows py ramidal or yoke type suction header for maximum equalization of pressure at each of three compressor suction inlets, piped in parallel. An acceptable alternate, although not as good with regard to equal pressure drops to all compressors, is to have the suction line from the evaporators enter at one end of the header instead of using tha yoke arrangement, as shown by the broken line. Take-offs to each compressor from the common suction header should be horizontally from the side, to assure equal distribution of oil and to prevent accumulating liquid refrig erant in an idle compressor in case of slop-over. Discharge Piping Hot gas piping at paralleled compressors should be ar ranged to prevent excessive vibration and to prevent oil and liquid refrigerant from draining back to the head of any of thecompressors. The branch hot gaslines from the compressors should be connected into a common header run at some level below that of the compressor discharge connections. For con venience, it is often at the floor. This is equivalent to the hot gas loop for the single compressor. The hot gas loop accomplishes two functions: 1. It prevents gas which may condense in the hot gas line, during the off cycles, from draining back onto the heads of the compressors, eliminating compressor damage from this poten tial hazard. 2. It prevents oil which leaves one compressor from draining' down onto the head of an idle machine. The solid lines in fig. 22 show.correct hot gas piping from two paralleled compressors. Each branch line has a muffler for reducing hot gas pulsations and drops down into the hot gas header at the floor, before rising up and going to the condense; Piping shown by the broken lines in Fig. 22 is incorrect and can result in three kinds of trouble. 1. The use of a bull-headed tee at the junction of the two branches and the main will greatly increase the gas turtmt^-. and pressure drop at this point. Hammering in the line m&y also result. 7 2. Any gas condensed in the discharge riser, during an off cycle, can run down and accumulate on the heads of the com. pfeasors, causing possible valve breakage on 6tart-up. 3. Oil leaving one operating compressor will be thrown into branch line of the other compressor which may be idle, and drain down onto the head of that machine. Interconnecting Piping In addition to properly designing the suction and hot gas piping at paralleled compressors, there are also certain other piping connections required between compressors and con densing units for proper parallel operation. Interconnection of Crankcases When two or more compressois are to be interconnected, they should be placed on foundations such that all oil equalizer tappings are exactly level. An interconnecting oil equalization line should connect all of the crankcases in order to maintain uniform oil levels and adequate lubrication in all compressors. The oil equalizer may be run level with the tappings, or, for convenient access to the compressors, it may be run at the floor. Under no con ditions should it be run at a level higher than that of the tappings. For the oil equalizer line to work successfully, it is neces sary to equalize the crankcase pressures. This is accomplished through the' use of a gas equalizer lina which is installed above the oil level. .This line may be run (Fig. 23) to provide head room or run level with the tappings on the compressors. It should be piped so that oil or condensed vapor will not be trapped. Both lines should be the same size as the tappings on the largest compressor and should be valved so that any one machine can be taken out for repair without shutting down the entire system. Neither line should be run directly from one crankcase into another without forming some sort of U-bend, or hairpin, to absorb vibration. ' Condensing Pressure Equalization When multiple condensing units are interconnected as shown in fig. 23, it is also necessary to equalize the pressure / Fig. 21 .... Suction and Hot Gas Headers for Multiple Compressors* Refrigeran* Piping Systems for Refrigerants 12, 22 and 500 749 . the condensers to prevent hot gas from blowing through ^ of the condensers and into the liquid line. To do this, a condensing pressure equalizer line as shown. If the jng is looped as shown, vibration should not be a problem. ^ bottoms of ah condensers should be at.the same level to BteVent backing liquid into the lowest one. *^Tbe condensing pressure equalizer line between units should ^ the size as the largest hot gas line that it connects into. PIPING AT VARIOUS SYSTEM COMPONENTS flooded Fluid Coolers flooded cooler design usually involves a refrigerant shell funtaming bundles of water tubes. Refrigerant surrounds the tabs in the shell and water or other fluid flows through the tubes in one or more passes through the cooler, depending opon the baffling arrangement in the heads. The fluid tube bundles fill up the lower portion only of the shell, leaving the upper volume to serve as a gas-liquid separation space so that the suction gas taken off the top of the cooler will not contain refrigerant in liquid, form. The separation space in the top of the cooler is most efficient when eliminators are used in this space, allowing higher loading of the cooler without liquid carryover into, the suction line. The design of flooded coolers to prevent liquid carryover into the suction line creates the problem of getting trapped oil out of the cooler. This is handled with a continuous liquid bleed line from some point, in the cooler shell, below the liquid level, into the suction line. This continuous bleed of refrigerant liquid and oil prevents the oil concentration in the cooler from getting too high. The location of the liquid bleed connection on the shell depends on the refrigerant used. With Refrigerant 12 the connection can be anywhere below the liquid level, because of the high miscibility of the oil and re frigerant Refrigerant 22 is less miscible with oil, however, and there is a tendency toward stratification, with the oil con centration being the richest toward the top of the refrigerant . bath. This becomes more noticeable as the evaporating tem perature drops. With Refrigerant 22 coolers, therefore, the bleed line is usually taken off the shell just slightly below the Squid level. Sometimes there is more than one valved bleed connection at slightly different levels so that the optimum point can be selected during operation. This bleed of refrigerant and oil out of the cooler is by gravity. It is usually drained.into the suction line so that the fig. 23 .... Interconnecting Piping for Multiple Condensing Units* oil can be returned to the cooler with the suction gas, after vaporizing the accompanying liquid refrigerant in a liquid- suction heat interchanges ' Sometimes the refrigerant-oil bleed is not drained into the miction line but into an electrically heated receiver where the refrigerant is boiled off to the suction tine and the oil drained back to the compressor. Flooded coolers can have several different types of refrig erant feed, such as: 1. Low pressure float valve, direct acting. 2. Low pressure float switch and solenoid valve. . . 3. Low pressure pilot operated valve (Fig. 24). . 4. High pressure float valve. - For further information on these valves, see Chapter 36 of the 1961 Guide And Data Book. The low pressure float twice controls the flow of refrigerant in direct response to the liquid level in a flooded evaporator. It is called low pressure because the pressure existing in the float chamber is substantially the same as that in the evap orator. Because of its greater flexibility, the low pressure float is very widely used. It is not critical with respect to the refrigerant charge or number of evaporators in the system. The application of a low pressure float switch is similar to thatfor thelow pressure float valve except that the refrigerant