The importance of suction-line design and sizing cannot be overemphasized. Lubricating oil does not mix well with the cold refrigerant vapor leaving the evaporator(s). It must be returned to the compressor either by entrainment with the refrigerant vapor or by gravity.
Traps and areas where oil may pool must be kept to a minimum. This is because large quantities of oil may become “lost” in the system. Piping should be level or with a slight pitch in the direction of the compressor.
Suction-line evaporator takeoffs should be designed so that oil cannot drain into idle coils. The common suction for multievaporator coils should be lower than the lowest evaporator outlet. Where an application requires that a common suction be above one or more of the coils on a multievaporator coil application, a suction riser with top loop connection is recommended.
Systems requiring a suction riser are more difficult to design. Sizing the pipe for minimum gas velocity at minimum system capacity (minimum displacement and suction temperature) may result in excessive pressure losses at full load. Excessive pressure losses in a suction riser may be compensated for by increasing pipe sizing in horizontal or down runs to reduce total system pressure losses. It can also be compensated for by using double suction risers. In comfort air conditioning, the use of double-suction risers is the exception, rather than the rule. However, where necessary, it proves a valuable tool. (See ASHRAE Guide Book for details.)