7.1 Lubrication Principles and Oil Types

Functions of Compressor Oil

In industrial ammonia refrigeration compressors, lubrication is not a simple matter of reducing wear. The lubricant injected into the compressor must perform four critical functions simultaneously to maintain system safety, efficiency, and reliability:

1. Lubrication (Wear Reduction): Oil reduces friction between moving contact surfaces. In reciprocating compressors, this includes piston rings, cylinder walls, crankshaft bearings, and wrist pins. In rotary screw compressors, this includes the male and female rotors, journal bearings, and thrust bearings. Reducing friction prevents heat buildup, galling, and mechanical failure.
2. Sealing: The lubricant acts as a physical sealant between areas of high and low pressure. In screw compressors, oil is continuously injected into the rotor casing to seal the microscopic clearances between the meshing rotors and between the rotors and the casing wall. This prevents high-pressure discharge gas from leaking back to the suction inlet (a phenomenon known as 'gas slip'), thereby maintaining the compressor's volumetric efficiency. In reciprocating compressors, the oil film seals the piston rings against the cylinder liners.
3. Cooling: Compression of ammonia gas generates a substantial amount of heat (the heat of compression). The injected oil absorbs a significant portion of this heat, along with the heat generated by mechanical friction. The hot oil is then circulated out of the compressor to an external oil cooler (which may utilize water, air, or thermosyphon liquid ammonia) before being filtered and reinjected. This cooling action keeps compressor operating temperatures within safe limits, preventing thermal degradation of the oil and mechanical warping of the rotors or pistons.
4. Noise Dampening: The viscosity of the oil creates a mechanical cushion between moving parts. This cushion absorbs shock loads and dampens the vibrations and high-frequency noise generated by the high-speed rotation of screw rotors or the reciprocating motion of pistons.

Refrigeration Lubricant Types

Lubricants used in ammonia systems are broadly categorized into mineral oils and synthetic oils. Selecting the proper oil type is critical because mixing incompatible oils or choosing the wrong base chemistry can cause severe operational issues.

Mineral Oils

Mineral oils are refined from crude petroleum. They are categorized based on their chemical structure into naphthenic and paraffinic oils:

• Naphthenic Mineral Oils: These are the traditional choice for industrial ammonia refrigeration. Naphthenic oils lack paraffin wax, which gives them a low pour point (typically between -30°F and -45°F). They remain fluid at normal low-side refrigeration temperatures and do not deposit wax on cold system components.
• Paraffinic Mineral Oils: These contain dissolved paraffin wax. When exposed to low temperatures in the system's evaporators, the wax precipitates out of the oil (a process called 'waxing out'). This precipitated wax forms a sticky, solid residue that clogs expansion valves, control orifices, and heat-transfer surfaces. For this reason, paraffinic mineral oils are strictly avoided in refrigeration systems.

Synthetic Oils

Synthetic lubricants are engineered chemicals designed to provide superior performance under extreme conditions:

• Polyalphaolefins (PAO): PAOs are synthetic hydrocarbons that share a similar molecular structure to mineral oils but contain no wax. They offer extremely low pour points (down to -60°F or lower), exceptional thermal and oxidative stability, and low volatility. Low volatility means less oil vaporizes into the discharge gas, reducing oil carryover into the piping network. PAOs are highly compatible with seals designed for mineral oils.
• Alkylbenzenes (AB): ABs are synthetic lubricants that provide good thermal stability and chemical resistance in the presence of ammonia. They have lower pour points than mineral oils and are compatible with mineral oil seals, making them excellent for retrofitting systems.
• Polyalkylene Glycols (PAG): Specialized PAG lubricants are unique because they can be formulated to be miscible (soluble) with ammonia. This is discussed in detail below. PAGs are highly hygroscopic (absorb moisture from the air). In the presence of moisture, PAG oil reacts with ammonia to form ammonium hydroxide, a highly corrosive alkaline substance that damages steel and copper-bearing components. PAGs must never be mixed with mineral oils or PAOs; mixing them causes the oils to separate and form sludge.

Miscibility of Lubricants with Ammonia (R-717)

Miscibility is the physical property of two liquids to mix in all proportions, forming a single homogeneous solution. The miscibility relationship between the refrigerant (R-717) and the compressor lubricant is a primary design driver in industrial refrigeration.

Immiscible Systems (Mineral Oil, PAO, AB)

Traditional ammonia refrigeration lubricants (mineral oils, PAOs, and Alkylbenzenes) are immiscible with ammonia. Ammonia is a highly polar compound, whereas hydrocarbon lubricants are non-polar. As a result, they do not dissolve in each other and will separate into two distinct layers when present together in a liquid state.

Keep in mind the significant density difference between liquid ammonia and refrigeration oil:

• Liquid ammonia has a density of approximately 42.6 lb/ft³ (682 kg/m³) at its boiling point of -28°F.
• Standard refrigeration lubricants have a density of 53 to 56 lb/ft³ (850 to 900 kg/m³).

Because the oil is denser (heavier) than liquid ammonia, the liquid ammonia floats on top of the oil, and the oil settles to the bottom of the vessel. In a flooded evaporator, accumulator, or recirculator receiver, the oil will migrate to the lowest point. This requires the installation of oil collection legs (oil pots) at the bottom of low-side vessels so the accumulated oil can be periodically drained or recovered.

Miscible Systems (Specialized PAGs)

In specialized applications, such as direct expansion (DX) ammonia systems, miscible synthetic Polyalkylene Glycol (PAG) lubricants are used. In a DX system, the oil must travel with the liquid refrigerant through the evaporator and return to the compressor dissolved in the suction gas. If an immiscible oil were used in a DX system, it would coat the evaporator tubes and never return, starving the compressor of oil. While miscible PAGs solve the oil return problem in DX designs, they require strict moisture control to prevent corrosion and chemical breakdown.