In the upper right-hand comer, find the line for 15 tons. Follow this line downward until it intersects the diagonal line for 40°F evaporator temperature. From this intersection point, draw a horizontal line toward the left. This example assumes a 100°F condensing temperature. Now look in the lower left-hand corner and find the horizontal 100°F condenser line. Now locate the diagonal lines that show pressure drop in psi per 100ft. Note where the diagonal 2-psi line crosses the horizontal l00°F condenser line. From this intersection point, draw a vertical line upward. Locate the point where the vertical line meets the horizontal line that you drew before. This point will show you the size of suction tubing to use. However, in this case, the intersection point does not fall exactly on any particular size. It is just below the diagonal line for the 1 5/8 inch tubing.
You can select liquid and hot gas piping in the same manner as suction piping. Liquid-line pressure drop should not exceed 7 psig.
THE HVAC/R TRAINING AUTHORITY