7.3 Low-Side Oil Logging and Management

Oil Migration Through the Refrigeration Cycle

No oil separator is 100% efficient. Even high-efficiency coalescing separators allow a tiny fraction—typically 0.1% to 1.0%—of the circulating oil to pass through as an aerosol mist or vapor. In large industrial facilities with high compressor runtimes, this tiny percentage translates into significant volumes of oil escaping the engine room over weeks of operation. This oil migrates through the system as follows:

1. Condensation: The oil vapor travels with the hot discharge gas to the condenser. As the ammonia gas cools and condenses into a liquid, the oil also cools and condenses, forming a mixture of liquid ammonia and liquid oil.
2. High-Pressure Storage: The liquid ammonia and oil mixture flows into the high-pressure receiver. Because they are immiscible, they do not blend, but they travel together in the liquid stream.
3. Expansion and Low-Side Feeding: The mixture passes through the system's expansion devices and liquid feed valves into the low-pressure side of the system (evaporators, surge drums, accumulators, and recirculator receivers).

The Physics of Low-Side Oil Logging

Once the oil reaches the low-pressure side, the environmental conditions change drastically. The pressure is low, and the temperature is extremely cold, typically ranging from 0°F down to -40°F or lower. These low temperatures alter the physical behavior of the immiscible lubricant in two major ways:

Viscosity Spike

Refrigeration oil viscosity is highly temperature-dependent. As temperature drops, the oil becomes increasingly thick, sticky, and sluggish, behaving like heavy molasses or grease. At -40°F, naphthenic mineral oil is barely fluid, making it difficult to flow or migrate.

Phase Separation and Settlement

Because ammonia and hydrocarbon lubricants are immiscible, they remain in separate liquid phases. Furthermore, their densities are significantly different:

• Liquid ammonia has a density of approximately 41 to 43 lb/ft³ at typical low-side temperatures.
• Refrigeration oil has a density of 53 to 56 lb/ft³.

Because the oil is denser (heavier) than liquid ammonia, the liquid ammonia floats on top, and the heavy, viscous oil settles by gravity to the bottom of the evaporators, accumulators, and recirculator vessels. This accumulation of heavy, stagnant oil in the low-pressure vessels and evaporator coils is called oil logging.

The Insulating Effect of Oil Logging

Oil logging is one of the primary causes of reduced efficiency and capacity loss in industrial refrigeration systems.

Refrigeration oil is an excellent thermal insulator. Its thermal conductivity is approximately 0.07 Btu/hr·ft·°F, which is hundreds of times lower than that of steel (26 Btu/hr·ft·°F) or copper (220 Btu/hr·ft·°F) pipes. As oil accumulates inside the evaporator tubes, it coats the inner pipe walls, forming a thick, stagnant film. This insulating film creates a thermal barrier that resists the flow of heat from the warm room air or product outside the coil to the liquid ammonia inside the coil.

System-Wide Impacts

• Reduced Evaporator Capacity: The rate of heat transfer (overall heat transfer coefficient, U) drops. Liquid ammonia inside the coil cannot absorb heat effectively and cannot boil off.
• Lower Suction Pressure: Because the ammonia in the oil-logged evaporator is not vaporizing, the compressor draws down the suction pressure in an attempt to maintain the room temperature.
• Reduced Efficiency: Operating at a lower suction pressure increases the compressor's compression ratio. This decreases volumetric efficiency and increases the electrical power required to run the compressor, leading to high utility costs.

Symptoms of an Oil-Logged Evaporator

Operators must monitor system parameters to diagnose oil logging before it causes product spoilage or equipment damage. Key symptoms include:

• Elevated Space Temperatures: Room temperatures remain high or continue to rise despite the evaporator fans running, liquid feed valves open, and normal refrigerant flow.
• Abnormally Low Coil Pressure: The suction pressure at the evaporator outlet drops lower than normal because the refrigerant is insulated from the heat load and cannot boil.
• Low Temperature Difference (TD): The difference between the air temperature entering the coil and the air temperature leaving the coil is abnormally small, indicating little to no heat is being absorbed.
• Uneven Frost Patterns: Frost forms only near the liquid inlet of the evaporator coil, while the rest of the coil remains bare, wet, or dry, indicating that the oil is blocking the refrigerant from distributing through all the circuits.
• Compressor Oil Level Drop: The compressor oil level continuously decreases, requiring frequent oil additions, despite the absence of external oil leaks. The missing oil is trapped (logged) in the low side.
• Erratic Level Controls: Oil pooling in accumulators or recirculators can coat float switches or electronic level sensors, causing false readings that lead to dry evaporators or liquid carryover (slugging).

Low-Side Oil Management and Oil Pots

To manage oil logging, low-pressure accumulators and liquid recirculator vessels are designed with a vertical collection chamber at their absolute lowest point, known as an oil leg or oil pot (sometimes called a drain accumulator). Because oil is heavier than liquid ammonia, it drains into these pots by gravity. To facilitate draining, some oil pots are equipped with electric heaters or warming coils (using hot gas or warm high-pressure liquid). Warming the oil pot boils off any liquid ammonia trapped in the oil and lowers the oil's viscosity, allowing it to flow easily and safely when drained.