CARO System Lubrication and Purging 2 — Questions and Answers
Question 1: In an ammonia refrigeration system, where does carried-over compressor oil tend to collect?
- At the top of the condenser
- At low points of vessels and evaporators, below the liquid ammonia (Correct answer)
- In the discharge line near the compressor
- Evenly mixed throughout the liquid ammonia
Correct answer: At low points of vessels and evaporators, below the liquid ammonia
Mineral oil is denser than liquid ammonia and does not mix with it, so it settles at the low points of vessels and coils.
Question 2: Why is mineral oil considered essentially immiscible with ammonia?
- It dissolves completely in liquid ammonia
- It reacts chemically with ammonia to form a sludge
- It does not mix with ammonia and separates into a distinct layer (Correct answer)
- It evaporates along with the ammonia in the evaporator
Correct answer: It does not mix with ammonia and separates into a distinct layer
Ammonia and mineral oil stay largely separate, which is why oil must be drained from low points rather than returned with the refrigerant.
Question 3: What is the main job of the oil separator in an ammonia compressor discharge line?
- To remove noncondensable gases from the refrigerant
- To cool the discharge gas before it enters the condenser
- To separate oil from the discharge gas and return it to the compressor (Correct answer)
- To filter moisture out of the ammonia
Correct answer: To separate oil from the discharge gas and return it to the compressor
The oil separator captures oil carried out of the compressor with the hot gas and returns it to the crankcase or oil system.
Question 4: Before draining oil from an ammonia vessel, what should the operator do first?
- Raise the vessel pressure as high as possible to push the oil out faster
- Isolate the oil pot and let its pressure drop so ammonia boils off from the oil (Correct answer)
- Open the drain valve fully and walk away until the oil stops
- Heat the drain line with a torch to thin the oil
Correct answer: Isolate the oil pot and let its pressure drop so ammonia boils off from the oil
Isolating the oil pot and lowering its pressure lets the trapped ammonia boil off so less ammonia comes out with the oil.
Question 5: Which valve is recommended on the final drain point of an ammonia oil drain?
- A spring-loaded self-closing (dead-man) valve (Correct answer)
- A check valve
- A globe valve chained in the open position
- A pressure relief valve
Correct answer: A spring-loaded self-closing (dead-man) valve
A self-closing valve shuts automatically if the operator lets go, which limits ammonia release during draining.
Question 6: Noncondensable gases in an ammonia system tend to collect mainly in which area?
- The bottom of the low-pressure receiver
- The suction line near the evaporator
- The high side, especially at the top of condensers and the high-pressure receiver (Correct answer)
- The compressor crankcase
Correct answer: The high side, especially at the top of condensers and the high-pressure receiver
Air and other noncondensables are carried to the high side, where they gather at points with low gas velocity such as condenser tops and receivers.
Question 7: What is a common sign that noncondensable gases are in a refrigeration system?
- Lower-than-normal head pressure
- Higher-than-normal head pressure for the condensing temperature (Correct answer)
- Frost forming on the discharge line
- Lower compressor amperage
Correct answer: Higher-than-normal head pressure for the condensing temperature
Noncondensables add their own partial pressure, so condenser pressure goes above the saturation pressure for the actual condensing temperature.
CARO System Lubrication and Purging 2
In an ammonia refrigeration system, where does carried-over compressor oil tend to collect?