HVAC Technologies-Understanding Oil Return in Refrigeration Systems
Posted on Tue, Mar 15, 2011
Part 4 - Lubricant Issues with Unitary Systems
By Dick Cawley - GEA Consulting
Ed Keuper, a GEA colleague, has presented three (3) very informative postings concerning oil return in refrigeration systems. His emphasis was on chillers and other refrigeration systems that use flooded evaporators and depend on oil separators at compressor discharge as well as a means for returning lubricant from the system to the compressor sump. His key points are (1) that the compressor must always have enough lubricant in the sump and (2) oil must not be present in the heat exchangers, mainly the evaporator, to the extent that heat transfer is significantly degraded.
This sequence deals with direct expansion unitary systems that are designed to move lubricant around the system and back to the compressor sump by momentum, in the case of refrigerant gas transport (Suction and discharge), and solubility where refrigerant is in liquid form. Another way of stating the design objective for this type of refrigeration system is that if, say, 1 % (oil in refrigerant) of lubricant is discharged from the compressor during stable operation, that ratio must be present anywhere in the system at any given time so that the 1 % is safely returned to the sump. One can view this arrangement as two (2) fluid streams – oil and refrigerant – traveling side by side from the compressor, through the condenser and evaporator, and back to the compressor sump after reaching equilibrium.
This series is presented as an overview at best. Many details for piping design to provide oil entrainment and circulation and to prevent oil drainage into places while a system is dormant can be found in the 2010 ASHRAE Handbook of Refrigeration, Chapter 1 – Halocarbon Refrigeration Systems.
(Stay tuned for more)