Thermosyphon oil cooling removes heat from compressor lubricating oil using liquid refrigerant that circulates by natural convection, with no pump. Liquid refrigerant from a dedicated or combined thermosyphon receiver flows down to an oil cooler, boils as it absorbs heat from the hot oil, and the resulting liquid-vapor mixture rises back to the receiver by density difference alone, where the vapor separates and returns to the condenser. The receiver’s elevation above the oil cooler is what drives the circulation — there is no mechanical pump in the loop.
Why thermosyphon cooling instead of a pump or water loop
Screw compressors reject a large fraction of their input power as heat into the oil, and that heat has to go somewhere before the oil recirculates back into the compressor. A thermosyphon loop uses refrigerant already in the system (instead of a separate glycol or water loop) as the cooling medium, and relies on the natural density difference between liquid and the vapor-liquid mixture in the riser to drive flow, so there is no circulating pump to maintain. The trade-off is that the loop only works if the receiver is elevated enough above the oil cooler, and if the receiver stays adequately full of liquid at all times, which is why receiver sizing and elevation are both engineering-reviewed items, not assumptions.
What drives receiver sizing for the loop
The thermosyphon receiver has to hold enough liquid to keep the downcomer (the pipe feeding the oil cooler) submerged under the worst-case combination of oil-cooling duty and system liquid level, plus enough vapor space above the liquid to let the returning liquid-vapor mixture separate cleanly. The oil-cooling duty itself is usually estimated from compressor [ELI-REVIEW: brake horsepower, converted to Btu/hr using roughly 2,544 Btu/hr per bhp] and a heat-rejection fraction that is compressor- and manufacturer-specific, then used to size the liquid flow the downcomer and riser piping need to carry. A vessel that is too short of liquid, or has a downcomer that runs dry under load, can starve the loop and let compressor oil temperature climb.
Run it
ColdCalcs' HPR / Thermosyphon Sizer handles dedicated thermosyphon receivers and combined HPTSR vessels, and shows the sizing and [ELI-REVIEW: ASME] wall thickness on screen, free.
Open the HPR / Thermosyphon Sizer →FAQ
Does a thermosyphon loop need its own receiver?
It needs a liquid source elevated above the oil cooler, which can be a dedicated thermosyphon receiver or a combined vessel that also serves as the system's high-pressure receiver (a combined HPTSR). Which arrangement is used is a plant-layout decision, not a fixed rule.
What happens if the thermosyphon receiver runs low on liquid?
If liquid level drops below what the downcomer needs, the loop can lose circulation or start passing vapor into the oil cooler, which sharply reduces cooling capacity and can let compressor oil temperature rise above its safe operating range.
Is thermosyphon cooling only used with screw compressors?
It is most associated with oil-injected screw compressors, where oil cooling duty is largest, but the same natural-circulation principle can be applied anywhere a system needs to reject heat using refrigerant instead of a separate cooling medium.
How is the receiver elevation determined?
Elevation has to provide enough static head to overcome the pressure drop through the downcomer, oil cooler, and riser piping at the design flow rate, which is a piping and vessel layout calculation specific to each installation.