662 Modern Refrigeration and Air Conditioning Copyright Goodheart-Willcox Co., Inc. unit briefly to determine how much of a vacuum it will pull. It should pull between 25 in. Hg and 28 in. Hg (7 kPa and 17 kPa) of vacuum. Stop the compressor. If the pressure gradually drops to 20 in. Hg (34 kPa) of vacuum, then rises toward 10 in. Hg (68 kPa) of vacuum, the compressor is not holding vacuum. This indicates the exhaust valves of the compressor are leaking. The compressor must be replaced or overhauled. If the compressor holds a vacuum, the refrigerant needs to be recovered, and the part of the suction line that was pinched must be replaced. Diagnosing Capillary Tube Problems Capillary tubes must be correctly sized. Their inside diameters (ID) and lengths must be correct for the capac- ity of the system and the desired evaporator temperatures. Figure 25-28A shows a capillary system of correct design. The undersized capillary tube in Figure 25-28B creates too much resistance. It is either too long or it has an undersized inside diameter. Note that the improp- erly sized capillary tube causes a starved evaporator. A starved evaporator could also be caused by a partially clogged filter-drier or capillary tube. The amount of refrigerant in a capillary tube sys- tem is critical. Refer to Figure 25-29. Notice the change Condenser A B Condenser Evaporator Evaporator High-pressure vapor Low-pressure liquid High-pressure liquid Low-pressure vapor Run Cut-in A B C Cut-in Cut-in Cut-out Cut-out Cut-out Low-side Low-side Low-side Pressure Condenser Condenser Condenser Idle Run Idle Time Idle Run Normal cycle Overcharge Undercharge Goodheart-Willcox Publisher Figure 25-28. The effects of properly sized and improperly sized capillary tubes. A—A capillary tube that is properly selected for capacity prevents excess liquid from backing up into the condenser and keeps the evaporator adequately filled with liquid. B—A capillary tube that does not have enough capacity results in too much resistance in the tube. Liquid backs up into the condenser and the evaporator is “starved.” The discharge pressure may be abnormally high and the suction pressure may be abnormally low. Liquid line refrigerant should have high subcooling. Goodheart-Willcox Publisher Figure 25-29. Pressure-time diagram showing the refrigeration cycles of capillary tube systems charged with different amounts of refrigerant. A—Proper charge. B—Overcharge. Overcharge will usually cause a frosted or sweating suction line. C—Undercharge.