7.2 Oil Separators and Net Oil Pressure

High-Stage Oil Separators

When a compressor operates, the shearing action of the pistons or rotors and the high temperatures of compression cause a small amount of oil to vaporize or form a fine mist. This oil is discharged along with the hot, high-pressure ammonia gas. If this oil is allowed to travel through the system, it will coat the condenser and evaporators, severely reducing heat transfer. To prevent this, a high-stage oil separator is installed in the discharge line between the compressor and the condenser.

Separation Mechanisms

Separators use several physical mechanisms to remove oil from the hot gas stream:

1. Velocity Reduction: The discharge gas enters a chamber that is much larger than the discharge piping. This sudden increase in cross-sectional area slows the gas down. The reduction in velocity allows larger, heavier oil droplets to drop out of the gas stream by gravity and accumulate at the bottom.
2. Directional Change (Baffles): The gas is forced to flow through a series of internal baffles, screens, or centrifugal plates. Because oil droplets are much denser and have more momentum than the gaseous refrigerant, they cannot change direction as quickly. The droplets collide with the metal surfaces, adhere to them, and run down into the oil sump.
3. Coalescing Filters: Modern, high-efficiency separators use glass-fiber coalescing cartridges. As the discharge gas passes through the micro-fiberglass matrix, sub-micron oil mist droplets collide with the fibers and cling to them. As more mist is captured, these tiny droplets merge (coalesce) into larger droplets that gravity pulls down to the separator bottom. Coalescing separators are highly efficient, typically capturing 99% to 99.9% of the oil in the discharge stream.

Oil Return Systems

The oil collected in the separator sump is under high discharge pressure. It must be returned to the compressor crankcase or oil reservoir, which operates at a lower pressure (suction pressure). This is accomplished via:

• Float Valve Return: A mechanical float assembly inside the separator sump rises as oil accumulates, opening a needle valve. The high discharge pressure forces the oil through a return line back to the lower-pressure compressor crankcase.
• Solenoid Return: An electronic level switch (such as an optical or float switch) detects high oil levels and opens a solenoid valve to return the oil.
• Thermostatic/Orifice Return: A fixed orifice or a thermal expansion valve controls oil flow, preventing hot discharge gas from bypassing the condenser.

Net Oil Pressure

In compressors that utilize an internal, positive displacement oil pump (most reciprocating compressors and some screw designs), the pressure shown on the oil pump outlet gauge does not represent the actual pressure lubricating the bearings.

The Concept of Differential Pressure

The bearings and moving parts of the compressor are enclosed within a casing exposed to crankcase pressure, which is roughly equal to system suction pressure. The oil pump draws oil from the crankcase sump and pushes it into the bearings. Therefore, the oil pump must overcome the pressure inside the crankcase to deliver lubricant. The actual driving force pushing the oil through the bearings is the difference between the oil pump discharge pressure and the crankcase pressure. This is defined as the Net Oil Pressure (or differential oil pressure):

$\text{Net Oil Pressure} = \text{Oil Pump Discharge Pressure (psig)} - \text{Crankcase Pressure (psig)}$

For standard industrial reciprocating compressors, the normal net oil pressure range is 30 to 45 psi.

The Oil Pressure Safety Switch (OPS)

If the net oil pressure falls below a safe operating limit (usually 15 to 20 psi), the bearings will experience oil starvation, leading to rapid wear and catastrophic failure. To prevent this, an Oil Pressure Safety switch (OPS) is installed. The OPS measures both the pump discharge pressure and the crankcase pressure. If the differential pressure falls below the setpoint, the switch starts a timer. If the net oil pressure does not recover within a preset delay (usually 30 to 120 seconds), the switch trips and shuts down the compressor. The time delay is critical to prevent 'nuisance trips' during compressor startup, when the oil pump is still priming and establishing flow.

Worked Examples

Worked Example 1: Positive Suction Pressure

Scenario: A reciprocating compressor is operating with a suction pressure (crankcase pressure) of 22 psig. The oil pump discharge pressure gauge reads 65 psig. Calculate the net oil pressure and determine if it is acceptable.

• Formula: $\text{Net Oil Pressure} = \text{Oil Pump Discharge Pressure} - \text{Crankcase Pressure}$
• Calculation: $\text{Net Oil Pressure} = 65 \text{ psig} - 22 \text{ psig} = 43 \text{ psi}$
• Conclusion: The net oil pressure is 43 psi. This falls within the normal range of 30 to 45 psi, so the lubrication pressure is acceptable.

Worked Example 2: Vacuum Suction Pressure

Scenario: A low-temperature booster compressor is operating with a suction pressure of 8 inches of mercury vacuum (8 in. Hg Vac). The oil pump discharge pressure gauge reads 32 psig. Calculate the net oil pressure.

• Step 1: Convert Vacuum to psig: 1 inch of mercury is equivalent to 0.491 psi. Therefore, 8 in. Hg Vac is: $8 \times 0.491 \text{ psi/in. Hg} = 3.93 \text{ psi below atmospheric} \approx -4.0 \text{ psig}$
• Step 2: Apply the Formula: $\text{Net Oil Pressure} = 32 \text{ psig} - (-4.0 \text{ psig}) = 32 + 4.0 = 36 \text{ psi}$
• Conclusion: The net oil pressure is 36 psi. On the exam, remember that vacuum pressure is a negative gauge pressure, so subtracting a negative pressure results in adding the value to the pump discharge pressure.

Troubleshooting Low Net Oil Pressure

If a compressor trips on low oil pressure, the operator should systematically investigate the following causes:

• Low Oil Level: Inspect the crankcase sight glass. If the level is below the minimum limit, the pump will draw air and lose pressure.
• Dirty Oil Filter or Suction Strainer: A high pressure drop across the filter restricts oil flow to the pump inlet, starving the pump.
• Refrigerant Foaming (Ammonia Dilution): When the compressor is off, cold oil absorbs liquid ammonia. Upon startup, the pressure in the crankcase drops rapidly, causing the dissolved ammonia to flash into gas. This makes the oil foam violently. The pump cannot pump foam, resulting in a sudden loss of net oil pressure. To prevent this, crankcase heaters must be energized when the compressor is idle to keep the oil warm and drive off dissolved refrigerant.
• Worn Bearings or Pump Gears: Excessive clearance in bearings or worn pump gears allows oil to flow out too quickly, preventing the pump from building pressure.