System Lubrication and Purging Flashcards

An operator in an ammonia refrigeration facility notices that the compressor discharge pressure is consistently higher than normal, even on cool days. The condenser fans are operating correctly and the coils are clean. What is the most likely cause of this issue?

Answer: The presence of non-condensable gases in the condenser.

Non-condensable gases, such as air, nitrogen, or hydrogen, do not condense at the operating temperatures and pressures of the refrigeration system. They accumulate in the condenser, taking up space and increasing the total pressure (Dalton's Law of Partial Pressures). This forces the compressor to work harder, resulting in higher discharge pressures and increased energy consumption.

What is the primary purpose of an oil separator in a large industrial refrigeration system?

Answer: To remove entrained lubricating oil from the hot discharge refrigerant vapor and return it to the compressor.

The oil separator is a critical component located in the compressor's discharge line. Its main function is to separate the fine mist of lubricating oil from the high-pressure refrigerant gas. This ensures that the oil is returned to the compressor for lubrication, while preventing it from circulating throughout the system where it can foul heat transfer surfaces and reduce efficiency.

During a routine check, an operator needs to add oil to a screw compressor that is currently in operation. Which of the following describes the safest and most appropriate general procedure?

Answer: Use the pressure differential between the oil reservoir and the compressor's low-pressure side to draw oil into the system.

The standard method for adding oil to an operating compressor is to use the system's own pressure differential. Oil is typically drawn from a container into the low-pressure side of the system (e.g., the crankcase or oil pump suction) by creating a lower pressure in the compressor than in the oil container. This is a controlled method that prevents introducing large amounts of air into the system and avoids the dangers of opening a pressurized system.

Which of the following is a direct consequence of oil accumulating on the heat transfer surfaces of an evaporator?

Answer: A reduction in the system's cooling capacity.

Oil is an insulator and a very poor heat transfer fluid compared to refrigerant. When a layer of oil coats the inside of the evaporator tubes, it impedes the transfer of heat from the refrigerated space into the boiling refrigerant. This reduces the evaporator's efficiency and, consequently, the overall cooling capacity of the system.

When manually purging non-condensable gases from an ammonia system, an operator connects a hose from a purge valve on the high-pressure receiver to a bucket of water. What is the visual indicator that mostly non-condensable gas, rather than ammonia, is being vented?

Answer: A continuous stream of bubbles rises to the surface of the water.

Ammonia has a very high affinity for water and will be readily absorbed with a crackling sound. Non-condensable gases like air and nitrogen are not soluble in water. Therefore, when these gases are bubbled through the water, they will rise to the surface and escape to the atmosphere. A steady stream of bubbles indicates the removal of non-condensables. The purging should be stopped once the bubbling ceases, indicating that mostly ammonia is now flowing.

An operator is tasked with draining oil from a dedicated collection point (oil pot) on the low-pressure side of an ammonia system. After properly isolating the oil pot, what is the next critical step before opening the drain valve?

Answer: Gently warm the oil pot to boil off any trapped liquid refrigerant.

Oil pots on the low side often contain a mixture of oil and liquid refrigerant. Opening the drain valve while liquid refrigerant is present can cause a violent release of pressure as the refrigerant flashes to vapor, spraying oil dangerously. The standard procedure is to isolate the pot and allow ambient heat to gently warm it, or use a dedicated heating element, to evaporate the residual liquid refrigerant, which is then vented back to the system. This ensures only oil is drained.