CARO Compressor Operations and Maintenance 4 — Questions and Answers
Question 1: Before opening a compressor for maintenance on an ammonia system, which step must be completed first?
- Drain the oil while the unit is running
- Increase the condensing pressure to purge ammonia
- Isolate, lock out/tag out, and pump down or evacuate the compressor to a safe pressure (Correct answer)
- Remove the discharge valve plate while pressurized
Correct answer: Isolate, lock out/tag out, and pump down or evacuate the compressor to a safe pressure
Lockout/tagout plus isolation and pump-down ensure no energy or refrigerant is present before opening the compressor.
Question 2: Worn or broken suction valves on a reciprocating compressor most commonly produce which symptom?
- Rapidly rising oil level
- Higher compressor amperage with more capacity
- Lower discharge temperature with increased capacity
- Higher than normal suction pressure with reduced capacity (Correct answer)
Correct answer: Higher than normal suction pressure with reduced capacity
Leaking suction valves let gas flow back to suction, raising suction pressure and lowering pumping capacity.
Question 3: What is a typical sign of a leaking discharge valve on a reciprocating compressor?
- Unusually hot cylinder head and reduced capacity as hot gas re-expands into the cylinder (Correct answer)
- Discharge head runs cooler than normal
- Oil pressure increases sharply
- Suction line frost disappears entirely
Correct answer: Unusually hot cylinder head and reduced capacity as hot gas re-expands into the cylinder
Hot discharge gas leaking back into the cylinder is recompressed repeatedly, overheating the head and cutting capacity.
Question 4: When adding oil to an operating ammonia compressor, why must the oil be the type specified by the manufacturer?
- Only the color of the oil matters
- Incorrect oil can have improper viscosity or compatibility, causing poor lubrication, carbon, or sludge (Correct answer)
- Any oil works because ammonia is not miscible
- Oil type only affects the oil separator
Correct answer: Incorrect oil can have improper viscosity or compatibility, causing poor lubrication, carbon, or sludge
Using the wrong oil can cause poor lubrication, varnish, or incompatibility with the refrigerant, damaging the compressor.
Question 5: Why is oil drained from low points such as the bottom of ammonia evaporators and receivers, rather than returned automatically like in halocarbon systems?
- Ammonia chemically destroys oil
- Oil vaporizes in ammonia systems
- Mineral oil is heavier than and not miscible with liquid ammonia, so it settles at low points (Correct answer)
- Ammonia makes oil float on top
Correct answer: Mineral oil is heavier than and not miscible with liquid ammonia, so it settles at low points
Common mineral oils are denser than and largely immiscible with ammonia, so they collect at low points and must be drained.
Question 6: An operator records that compressor motor amperage has gradually increased over several weeks while suction and discharge pressures are unchanged. What might this indicate?
- Improved compressor efficiency
- The compressor is unloading more frequently
- Lower condensing pressure
- Mechanical wear or friction such as failing bearings (Correct answer)
Correct answer: Mechanical wear or friction such as failing bearings
Rising amps without a change in load conditions suggests added mechanical friction, such as worn bearings or misalignment.
Question 7: What is the main reason for checking coupling alignment between a compressor and its motor?
- To prevent vibration, bearing wear, and seal failure (Correct answer)
- To adjust the slide valve position
- To increase refrigerant flow
- To raise oil pressure
Correct answer: To prevent vibration, bearing wear, and seal failure
Misalignment causes vibration and side loads that wear bearings, couplings, and shaft seals prematurely.
CARO Compressor Operations and Maintenance 4
Before opening a compressor for maintenance on an ammonia system, which step must be completed first?