The oil temperature also affects its movement. As the temperature drops, the oil becomes more viscous, and it becomes more difficult for the refrigerant to sweep the oil back to the compressor. Oil return becomes more difficult in the evaporator and suction line because of the temperature of the refrigerant and the lower pressure.  

Good piping practice is the foundation of reliable oil return. Properly sized suction and discharge lines are essential. Oversized piping may reduce pressure drop, but often lowers gas velocity to a point where oil no longer travels effectively. Undersized piping, on the other hand, leads to excessive pressure drop and higher energy consumption. The goal is to size piping to maintain recommended velocities: a minimum velocity of 700 feet per minute through the horizontal sections of the suction line and 1,500 FPM through the vertical sections of the suction line. 

When a compressor is located above the evaporator, sweeping the oil up a riser becomes more difficult. To promote good oil return, P traps should be installed in the vertical section of the suction line. This will normally be required when a compressor is located more than 3 to 4 feet above the location of the evaporator. An initial P trap should be installed at the base of the vertical riser with additional traps installed every 20 feet of vertical rise, as necessary. Traps should be constructed with a minimum depth and the horizontal section should be as short as possible. This will avoid the accumulation of large quantities of oil in the trap. 

Systems designed with capacity control present an additional issue for oil moving up a riser. Since the refrigerant velocity through the suction line is not fixed, the velocity of the refrigerant may not be sufficient to push the oil up the line during low load conditions, even with the use of P traps. The solution to this problem is the use of two sections of vertical piping. The two lines should be sized so that the total cross-sectional area is equivalent to the cross-section area of a single riser that would have both satisfactory gas velocity and acceptable pressure drop at maximum load conditions. The two lines normally are different in size, with the larger line having a P trap at its base and the smaller line sized to provide adequate velocities and acceptable pressure drop when the entire minimum load is carried in the smaller riser. 

On horizontal sections of the suction line, gravity will help promote sufficient oil return. Suction lines should be installed with a slight pitch toward the compressor. Generally, a pitch of at least ½-inch per 10 feet is recommended. 

Oil return is not just a maintenance consideration; it is a fundamental design requirement for every refrigeration system. Good piping practice ensures the compressor stays lubricated, the system operates efficiently, and the equipment life is maximized. By understanding and using proper piping techniques, technicians and designers can significantly reduce failures and maintain consistent, reliable system performance.