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How to Use the Oil Separator Calculator
Choose the refrigerant, then enter the saturated condensing temperature (20 to 60°C) and the saturated evaporating temperature (-50 to 10°C). For blends, use the dew point temperature.
Enter the total refrigerating capacity of all compressors on the separator and the number of compressors. Enter the oil carry-over from the compressor maker, and the separator efficiency from the data sheet.
Enter the maximum gas velocity across the vessel body from the separator manufacturer. The calculator starts at 0.4 m/s.
The results give the minimum diameter, the next size up, the gas velocity in that size, the oil carried out of the compressors, the oil passing the separator and the oil return per compressor.
Oil management in refrigeration systems
Every compressor sends some lubricating oil out with the hot discharge gas. In most systems that oil travels round the circuit and returns with the suction gas, but at low evaporating temperatures, on long or risered suction lines and on parallel racks it can collect in the evaporators instead. An oil separator sits in the discharge line, slows the gas so oil droplets drop out, and returns that oil to the crankcase or an oil reservoir through a float valve.
Where the oil goes and why it matters
Oil that stays in the circuit lowers the heat transfer of the evaporator and the condenser, and oil that never comes back leaves the crankcase short. How readily oil returns depends on how well it mixes with the refrigerant, which is why the choice of oil and refrigerant pair matters. The refrigerant oil miscibility calculator covers that side, and a separator is the usual remedy where miscibility and line velocity are not enough.
What the calculator does with your inputs
Refrigerant mass flow is the capacity divided by the refrigerating effect, which is the enthalpy of saturated vapour at the evaporating temperature minus the enthalpy of saturated liquid at the condensing temperature. The discharge state comes from compressing the suction gas, with 10 K of superheat that is not credited as useful, at 70% isentropic efficiency, and the gas density is read at that state. All of it comes from real-fluid property tables made with CoolProp, so the evaporating temperature changes the answer, as it should. The refrigerant flow divided by the density is the gas flow that the vessel has to handle.
Choosing the gas velocity limit
The vessel area is the discharge gas flow divided by the allowed velocity, and the diameter follows from the area. A separator manufacturer's technical guide keeps the velocity across the vessel body at or below 0.4 m/s and sizes from the discharge flow at the actual suction and discharge temperatures, with 10°C of suction superheat and no liquid subcooling. For a 40 kW R-404A system at -25°C evaporating and 40°C condensing, the discharge flow is 5.6 L/s (20.2 m³/h) at a density of 77.1 kg/m³, so the minimum diameter at 0.4 m/s is 134 mm and a 150 mm vessel runs at 0.317 m/s. At a 0.10 m/s limit the same system would need 267 mm, which rounds to 300 mm, so the limit you choose matters more than any other input. Always compare the answer with the maker's capacity table, because that table also accounts for the pressure drop and the internal design.
Oil carry-over and separator efficiency
The carry-over is the share of the refrigerant mass flow that leaves the compressors as oil, and it differs between compressor types and operating points, so take it from the compressor maker. The 2% default is a planning figure. In the example it means 31.11 kg/h of oil (34.56 L/h at 0.9 kg/L) reaching the separator. One manufacturer of helical separators states that 95 to 99% of the oil is separated when the vessel is sized correctly, so at 95% the oil passing the separator is 1.555 kg/h, or 0.1% of the refrigerant flow. That oil is what ends up in the rest of the circuit, so a few points of efficiency matter on a large rack.
Oil return at low evaporating temperatures
Oil gets thicker as it cools, and at a low evaporating temperature the suction gas is also moving slowly because its density is low. Together these make oil return in the suction line harder, which is why separators are so common on low-temperature racks. Oil viscosity is quoted in different units by different makers, and the viscosity converter turns centistokes into the other scales when you compare oil grades.
Several compressors on one separator
On a parallel rack one separator usually serves all the compressors, so size it for the total capacity of every compressor that can run at once, and put a check valve in each discharge line ahead of the common inlet. The oil return per compressor is shown as an even split, which is 17.28 L/h each for the two compressors in the example. The compressor sizing calculator gives the capacity of each unit at your conditions, and the estimated shaft power in the results, 26.3 kW for the example, is a quick check on the electrical size.
How the refrigerant changes the answer
Refrigerants with a larger refrigerating effect need less mass flow for the same capacity. The R-404A example moves 1,555.3 kg/h, while R-448A at the same conditions moves 1,061.4 kg/h, but its discharge gas is less dense, so the volume flow is about the same (19.4 m³/h) and both land on a 150 mm vessel. R-448A also discharges hotter, with an estimated 91°C against 73°C, so check the compressor's discharge limit as well as the vessel. Canada's Ozone-depleting Substances and Halocarbon Alternatives Regulations prohibit manufacturing products designed to contain HCFC-22, so R-22 is now a service refrigerant for existing racks.
When a separator does not fix the problem
A separator does not cure oil trapped in a badly graded suction line or an evaporator that floods, and a rising oil level problem can come from the compressor, not the circuit. If the crankcase level is low or the discharge temperature is high, work through the compressor diagnosis tool before adding a vessel, and check suction superheat because liquid returning to the compressor dilutes the oil.
Codes and approvals
In Canada refrigeration installations follow CSA B52:23, Mechanical refrigeration code. A separator is a pressure vessel, so check the provincial pressure equipment requirements and the pressure rating against the condensing pressure, which is 18.15 bar(a) in the example, with the authority having jurisdiction.
Frequently Asked Questions
There is no single temperature or line length that decides it. A separator is worth fitting where oil may not return to the compressor reliably, such as low-temperature systems, long or risered suction lines, parallel racks and compressors that pass a lot of oil with the discharge gas. If oil returns in a normal suction line with good velocity and the crankcase level holds, a separator adds a pressure drop and a vessel to maintain without solving a problem.
Use the limit on the manufacturer's data sheet. One separator maker's technical guide keeps the gas velocity across the vessel body at or below 0.4 m/s to avoid excessive turbulence, so the calculator starts there. Some makers rate their vessels by discharge gas flow instead. A limit of 0.10 m/s would need a vessel about twice the diameter, because the area scales with the inverse of the velocity.
Gas passes through the vessel faster than the oil droplets can settle, so more oil leaves with the discharge gas. It collects in the condenser, liquid line and evaporator, where it coats heat transfer surfaces, and the compressor crankcase level falls. Low oil level is the failure to watch for, because a compressor running short of oil wears its bearings quickly. Check the sight glass level and oil return on a new installation.
The calculator works from a saturated condensing temperature. CO2 has a critical point of about 31°C, and in transcritical operation the gas cooler runs above it, so there is no condensing temperature. The gas cooler pressure is set by the control strategy, so the discharge gas density cannot be read from a temperature. Use the compressor and separator manufacturer's selection software for transcritical CO2.