Compressor Failures: Refrigerant Flood Back

Refrigerant flood back occurs during the running cycle of the machine. This is where large volumes of liquid refrigerant are returned to the running compressor down through the Suction pipe work, in an uncontrolled manner. Depending on the degree of liquid returning to the compressor, determines as to what damage will occur. This can often be seen as foaming in the compressors’ oil level sight glass.

If liquid is drawn into the cylinder bores, the lubrication to the cylinders and pistons will be washed away causing scoring and overheating in the cylinder, which in turn will lead to small metallic particles contaminating the inside of the machine. This liquid will also dilute the lubricating oil in the compressor sump. As the oil becomes more and more diluted with liquid refrigerant, its ability to lubricate becomes compromised. When this liquid rich oil is drawn up through the crankshaft to lubricate the bearings, con rods, cylinder walls, etc due to friction the refrigerant in the oil starts to Flash Off into a vapour, this then prevents the oil being able to lubricate the necessary parts, and typically the main bearing and the con rods, furthest from the oil pick up point, will quickly dry out and then seize. Sometimes the upper main bearing will wear so badly that it can cause rotor/stator contact and a burnout, or more often, the upper most con rods seize onto the crankshaft and as the motor continues to rotate the crank, thereby breaking the aluminium con rods, and eventually the pistons as well. When liquid has caused con rods to seize onto the crankshaft the crankshaft journal will be found to have aluminium from the con rod welded onto the bearing surface of the crankshaft. This is caused by the liquid refrigerant virtually exploding from the oil due to the heat of friction from the bearing surfaces. As the con rods and pistons are broken, large pieces of debris are thrown around the compressor shell and motor area, these can cause motor winding insulation damage and motor burn out. Very often the initial diagnosis is the motor burn out, but the actual failure was refrigerant flood back.

Flood back often occurs during night time operation when the duty requirements are low, and the equipment has excess capacity. Often expansion valves are oversized for the system duty and each time the compressor starts or large product loads are introduced to the plant, the expansion valve will be forced to open, and over feeding of the evaporator, even more so if the superheat setting has been set too low, will be the result. Thermostatic expansion valves are forced to open by the suction pressure falling which causes a fall in pressure under the expansion valves’ diaphragm. This pressure fall reacts faster than the charge in the expansion valve bulb and initially the expansion valve is driven open. Until the bulb charge reacts and the bulb pressure starts to fall to help close the valve, liquid refrigerant will rush into the evaporator in a virtually uncontrolled manner (the bigger the valve the larger the liquid quantity). Once this liquid reaches the suction pipe work its next port of call is back to the compressor. Monitoring the expansion valve superheat setting under all conditions and setting the superheat to a point that is stable and can be maintained under all load conditions will often cure the problem. Fitting a smaller capacity orifice may be needed to achieve this stable control. Upgrading to an electronic expansion valve or by fitting a suction line accumulator may be necessary in systems that cannot be controlled by older conventional controls.

Causes of liquid flood back:

  • Low evaporator loads
  • Oversized equipment
  • Cold room product distribution (poor air circulation in cold room caused by lights, racking etc.)
  • Faulty evaporator fans
  • Oil logging in evaporator
  • Poor evaporator defrosting or defrost schedules (e.g. iced up evaporator, no air flow or poor heat transfer)
  • Oversized expansion valve orifice
    • Wrong type expansion valve
    • Expansion valve equalisation tube restricted or blocked
    • Expansion valve bulb strap loose or bulb in the wrong position on the suction pipe
    • Superheat setting too low

Preventative measures to avoid refrigerant flood backs:

  • Expansion valve duty must be checked for the correct size orifice
  • Superheat setting must be set for a minimum 6-8 Degree K Superheat (thermostatic) – can be lower if electronic EEV used
  • Fit correct size Suction Accumulator with proper oil return function
  • Check and reset defrost control as necessary
  • Fully check the system operation or re-commission the plant

Please note:

Liquid Flood back very often occurs during LOW LOAD conditions, which tends to be during the night, due to the lack of activity such as Door Openings, product movements, fork truck and the cold room Personnel which all give an increase to the plant duty. Therefore 24 hour logging of the plant operating conditions will often help to pinpoint plant problems that only occur during the night operation when the plant is unattended.

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