To determine if there is a discharge valve leak, connect the gauge manifold and start the unit. Run it until the low-side (LO) pressure gauge indicates normal pressure for the unit. Stop the unit. With an ear near the compressor housing, listen for a hissing sound. Also, watch the gauges. When leaking caused by an obstruction is present, the low-side pressure rises, and the high side decreases until the pressures are equalized. A quick equalization of pressures indicates a bad leak that should be repaired immediately or the compressor replaced.

Compressor Lubrication

The oil level in the compressor crankcase should be checked by the procedure in the following manufacturer's manual. This procedure normally includes the following steps:

1. Attach the gauge manifold to the suction and discharge service valves.

2. Pump the system down.

3. Close the suction and discharge valves, isolating the compressor.

4. Remove the oil filter plug and measure the oil level as per the manufacturer's manual.

Compressor Knocks

When the compressor knocks, you may have to disassemble the compressor to determine whether the cause is a loose connecting rod, piston pin, or crankshaft. Sometimes a loose piston can be detected without the complete disassembly. In cases requiring disassembly, you should take the following steps:

First, remove the cylinder head and valve plate to expose the top of the piston. Start the motor and press down with your finger on top of the piston. Any looseness can be felt at each stroke. The loose part should be replaced.

Check the oil level because oil levels that are too high often cause knocks. Always make sure that a low oil level is actually the result of a lack of oil, rather than a low charge.

Stuck or Tight Compressor

A stuck or tight compressor often occurs as a result of poor reassembly after a breakdown repair. In such cases, determine where the binding occurs and reassemble the unit with correct tolerances; avoid uneven tightening of screws or seal covers.

INSPECTION OF COMPRESSORS

An inspection should be performed on a refrigeration unit from time to time for knocks, thumps, rattles, and so on, while the unit is in operation. When any of the external parts have excessive grease, dirt, or lint, they should be cleaned. Before cleaning, you should always ensure the power is off.

A careful check of the entire system with instruments or tools is essential to determine if there has been any loss of refrigerant. NO LEAK IS TOO SMALL TO BE FIXED. Each leak must be stopped immediately.

Some specific conditions to look for during the inspection of a refrigeration system are as follows:

Inadequate lubrication of bearings and other moving parts.

Rusty or corroded parts discovered during the inspection should be cleaned and painted.

Hissing sounds at the expansion valve, low readings on the discharge pressure gauge, and bubbles in the receiver sight glass, all indicate a weak refrigerant charge.

Loose connections and worn or pitted switch contacts result in inoperative equipment or reduced reliability. Thermostats with burned contacts may produce abnormal temperatures in the cooled compartment.

Fans difficult to rotate by hand, with bent blades, or loose or worn belts are a source of trouble easy to locate and correct during inspection.

Air filters clogged with dirt should be cleaned or replaced during the inspection.

Hermetically sealed units should be inspected for signs of leaks and high temperatures and for too much noise or vibration.

Q43. On compressors, refrigerant leaks most often occur at what location?

Q44. Hissing sounds at the expansion valves, low discharge pressure, and bubbles in the receiver sight glass during inspections indicate what possible problems?