What is flood back in refrigeration

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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Compressor floodback is a common cause of HVAC/R system failure. Floodback occurs when uncontrolled liquid refrigerant enters the compressor during system operation. In an HVAC/R system, the refrigerant exits the evaporator and enters the compressor in a vapor state. But when the refrigerant is not vaporized entirely in the evaporator, a part of it gets sucked into the compressor in its liquid form. Compressor floodback may be misdiagnosed as compressor failure, even though the compressor doesn’t cause this phenomenon.

Causes of Compressor Floodback

Many different factors may lead to refrigerant floodback in an HVAC/R compressor. Most of them could be traced to:

• Problems with the evaporator – Ice buildup on the evaporator coil due to poor ventilation, a damaged evaporator fan belt, and a failed fan motor are some common problems that could result in refrigerant floodback. Incorrect defrost cycles or a less effective method of defrosting could also cause ice to accumulate on the evaporator coil, eventually leading to compressor floodback.   
• Overcharged conditions and/or incorrectly sized capillary tubes – An overcharge of refrigerant or a capillary tube that’s too large and/or too short indirectly means that a larger amount of refrigerant could enter the evaporator. In this case, the boiling point of the refrigerant may never be reached, which could lead to refrigerant floodback. However, floodback can be prevented in systems equipped with expansion valves that are working correctly.
• Defective or misadjusted expansion valves – Because expansion valves regulate the rate at which the refrigerant flows into the evaporator, a malfunctioning or a misadjusted valve could result in refrigerant floodback.
• Refrigerant migration – During the off cycle, a difference in pressure may occur between the refrigerant liquid/vapor within the system and the oil in the crankcase. Because liquids and vapors flow from high- to low-pressure areas, the refrigerant will migrate to the crankcase, which is the lowest pressure point in a refrigeration system. When the compressor starts, the refrigerant settled in the bottom of the crankcase, under the oil (refrigerant is heavier than oil), will start to boil and vaporize, entering small particles of oil and eventually causing compressor failure. Refrigerant migration to the crankcase may also result in liquid slugging during start-up. Although refrigerant migration and floodback can be prevented by adding a suction accumulator to the system, accumulators may also flood in a severe flooding condition.  

Signs of Compressor Floodback

Oftentimes, the amount of liquid refrigerant returning to the compressor determines the type and extent of damage that will occur. Here are a few signs that could indicate refrigerant floodback.

Loss of Lubrication

Once liquid refrigerant gets into the crankcase, it’s only a matter of time before the compressor seizes. Because a part of the oil used to lubricate different mechanical components is entrained by the refrigerant and pumped by the compressor into the system, the level of oil in the crankcase will decrease, robbing important compressor parts of vital lubrication. As a result, certain parts may wear, overheat, seize, or break. As well, if liquid refrigerant is drawn into cylinder bores, it will wash lubricating oil off cylinders and pistons. This will lead to scoring and overheating in the cylinder.

Lower Efficiency

The oil particles entrained by the refrigerator will increase the density of the refrigerant charge. As the refrigerant gets pumped through the cylinder, higher than recommended crankcase pressures will develop. That extra load will cause the compressor motor to draw more current, which will result in unwanted inefficiencies. Additionally, a motor that draws more current could lead to compressor burnout. Oil that gets inside a refrigeration system will also coat the inner surfaces of valves and tubes, further decreasing system efficiency. Clogged capillary tubes, faulty valve assemblies, broken valves, compressor overheating, and repeated breaker tripping may also occur from this phenomenon.

Oil Foaming

When the compressor starts, the pressure of the refrigerant-oil mixture in the crankcase drops suddenly. As a lower amount of liquid refrigerant is required to saturate the oil, the rest of the refrigerant will expand and evaporate into vapor. The sudden expansion of the refrigerant will cause the oil-refrigerant mixture to boil rapidly, generating the excess foaming often associated with compressor floodback. Because oil can also cause some foaming at start-up, only constant foaming should be considered an indicator of refrigerant floodback. A frosted, cold, or sweaty crankcase is another telltale sign of floodback.

A point worth mentioning is that compressor floodback often occurs during low load conditions. In case of a malfunctioning HVAC/R compressor, monitoring the system over a 24-hour period could pinpoint the problems that only occur when the facility is unattended.