The suction line needs to be sized for a minimum pressure drop through the line while maintaining an adequate refrigerant velocity to ensure good oil return to the compressor. It should be sized so its pressure drop does not exceed an equivalent of a 2°F change in saturation temperature and a minimum refrigerant velocity of 700 fpm through the horizontal sections and 1,500 fpm through the vertical sections.
If the pipe size chosen is too large, the velocity of the refrigerant flow will be reduced and the oil movement through the line will be poor. If the pipe size is too small, oil return will not be a problem; however, there may be an excessive pressure drop through the suction line. This will reduce the system’s capacity and increase the system’s power consumption. Deciding which pipe size to use for the suction line is generally a compromise between ensuring good oil return and maintaining a minimum pressure drop through the line.
These two sizing requirements are in direct conflict with each other. Increasing the size of a suction line will lower its pressure drop, but it will also reduce the refrigerant’s velocity. Reducing the size of a suction line will increase its refrigerant velocity, but it will also increase its pressure drop. In general, suction lines are sized for the maximum allowable pressure drop as long as the refrigerant’s velocity is maintained above the minimum requirements.
Sometimes choosing the correct pipe size alone is not enough to ensure good oil return. If the compressor is located above the evaporator, oil will still have a difficult time moving up a vertical riser. 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 and 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.
Promoting Oil Movement
To promote good oil movement in horizontal sections of the suction line, they should be sloped in the direction of flow with a pitch of at least ½-inch per 10 feet.
Some systems are designed where the capacity can be reduced as the load on the system decreases. This type of system presents a new problem for designers. Now the refrigerant velocity through the suction line is not fixed. On systems where the compressor is above the evaporator and there is a considerable vertical section of suction line, 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. A line properly sized for light load conditions may have too high a pressure drop at maximum load, and if the line is sized on the basis of full-load conditions, then the velocities may not be adequate at light load conditions to move oil through the tubing. 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.
Always follow the manufacturer’s installation guidelines when selecting and installing the suction line. Normally the manufacturer will supply piping charts to reference for selecting the correct diameter pipe and installation directions for proper installation. Following these guidelines is essential to the overall operation of the equipment installed.
Publication date: 10/03/2011
