SIZING SUCTION LINES

  Suction line sizing is the most critical from a design and system standpoint. Any pressure drop occurring due to frictional resistance to flow results in a decrease in the pressure at the compressor suction valve, compared with the pressure at the evaporator outlet. As the suction pressure is decreased, each pound of refrigerant returning to the compressor occupies a greater volume, and the weight of the refrigerant pumped by the compressor decreases. For example, a typical low temperature R-502 compressor at -40° F. Evaporation temperature will lose almost 6% of its rated capacity for each 1 psi suction line pressure drop.

  Normally accepted  design practice is to use as a design criteria a suction line pressure line drop equivalent to a 2° F. change in saturation temperature. Equivalent pressure drop for various operating conditions are shown in table

  Pressure drop equivalent for 2° F. change in saturation temperature at various evaporating temperature

               Evaporating      pressure drop PSI
               Temperature        R-12          R-22       R-502

                 45° F.                  2.0              3.0         3.3
             
                 20° F.                  1.35            2.2         2.4

                   0° F.                  1.0              1.65       1.85

                -22° F.                   .75             1.15       1.35

                -40° F.                   .5                 .8         1.0

  Of equal importance in sizing suction lines is the necessity of maintaining adequate velocities to properly return oil  to the compressor. Studies have shown that oil is most viscous in a system after suction vapor has warmed up a few degrees from the evaporating temperature, so that the oil is no longer saturated with refrigerant, and this condition occurs in the suction line after the refrigerant vapor has left the evaporator. Movement of oil through suction lines is dependent on both the mass and velocity of the suction vapor. As the mass or density decreases, higher velocities are required to force the oil along.

  Nominal minimum velocities of 700 FPM in horizontal suction lines and 1500 FPM in vertical suction lines have been recommended and used successfully for many year as suction lines sizing design standards. Use of the one nominal velocity provided a simple and convenient means of checking velocities. However tests have shown that in vertical risers the oil tends to crawl up the inner surface of the tubing, and the larger the tubing, the greater velocity required in the center of the tubing to maintain tube surface velocities which will carry the oil. The exact velocity required in vertical lines is dependent on both the evaporating temperature and the lines size, and under varying condition, the specific velocity required might be either greater or less than 1500 FPM.