Liquid Refrigerant In Compressor: 9 Important Concepts

We all know that the heart pumps blood throughout the human body; in the same way, a Compressor enables the flow of the refrigerant throughout the refrigeration cycle.

In general, low pressure and low-temperature gaseous refrigerant from the evaporator enters the Compressor and gets compressed inside it to high pressure and temperature gas.

A piston moves inside a cylinder to help in and out of gas in the Compressor. The high-pressure hot refrigerant gas from the Compressor is then pushed into the condenser. The refrigerant comes out from the condenser as a high-pressure liquid that enters the evaporator through the expansion valve. Refrigerant  in Compressor
Image: Air conditioning-Basic Refrigeration Cycle

What happens if liquid refrigerant enters the Compressor

Compressors are meant to compress gasses as gas is a compressible fluid. On the contrary, liquids are incompressible. If an incompressible fluid enters the Compressor, it can potentially damage the Compressor internals.

When in an HVAC system, the refrigerant is not entirely vaporized inside the evaporator, then liquid refrigerant directly touches the crankcase of the Compressor. This situation mainly occurs in an operating condition frequently faced by the servicing technicians, popularly known as Flooding.

  • Liquid refrigerant in Compressor dilutes the lube oil resulting in wear and tear of different parts
  • Entering the cylinder liquid refrigerant may damage the reed valve, piston connecting rods, crankshaft, etc. It even leads to total Compressor failure.

Can liquid refrigerant in Compressor damage it

Analysis reveals that entry of liquid refrigerant in Compressor during the running cycle is one of the primary reasons for Compressor failure.

The variety and expanse of damage depend on the amount of liquid refrigerant that enters into the Compressor.

Major damages caused by refrigerant flood back or Flooding due to entry of liquid are:

  • Poor lubrication of Compressor parts
  • The lower efficiency of the system
  • Oil Foaming etc.
  • Since the Compressor motor draws more current, it may lead to compressor burnout.

State of refrigerant entering the Compressor

For the smooth running of an HVAC system, the entry state of the refrigerant must be gaseous.

In normal conditions, the refrigerant enters into the Compressor in a gaseous state from the evaporator. But due to certain factors, the liquid refrigerant returns to the Compressor in large volume through the suction pipe.

The evaporator is responsible for the refrigeration effect in the HVAC system. The compressed hot refrigerant liquid is cooled in the condenser and sent through the expansion valve into the evaporator. At the inlet of the evaporator, the refrigerant is a mixture of gas and liquid at low pressure. The refrigerant takes heat from ambient air (causing the cooling effect) in the evaporator and transforms it to vapor form. It enters into the compressor suction in the vapor form.

Image credit: Schematic diagram of a typical vapor compression refrigeration cycle

What is Compressor flood back

The Compressor is the most critical component in the refrigeration system, and its failure becomes the most expensive problem. Compressor Flood back or Flooding is one of the significant reasons for compressor failure.

The continuous flow of liquid refrigerant into the Compressor in oil droplets instead of superheated vapor is known as Compressor Flood back.

Liquid refrigerant in Compressor mixes with the lube oil present in the crankcase of the Compressor and reduces its viscosity. Inefficient lubrication leads to wear and tear of Compressor parts and overheating. In a compressor, Flooding can be detected through the crankcase sight glass, where the oil appears to be foaming during operating conditions.

Main Causes of Compressor Flood back

Technicians should properly aware of the causes which may lead to Compressor Flood back.

To find the root of the problem is essential to prevent Flood back of Compressor from happening again and again. Proper knowledge of the causes and symptoms also help in identifying between Slugging and Flooding.

Main causes of Compressor Flood back are listed below:

  • Problem with an expansion device. The expansion valve bulb strap is not insulated correctly, or the bulb is in the wrong position on the suction pipe. 
  • Superheat setting inside the evaporator is too low.
  • Defective evaporator fan.
  • Improper adjustment of the expansion valve. The proper adjustment of the expansion valve is necessary to regulate the appropriate quantity of refrigerant to keep the refrigerant in vapor form while entering the Compressor.
  • Incorrect sized capillary tubes send more refrigerant to the evaporator, a large amount of refrigerant couldn’t reach the boiling point resulting in Compressor Flooding.
  • Low load situation.

What is liquid Slugging in a compressor?

Liquid Slugging is the term associated with failure of a reciprocating compressor due to carryover of liquid in its suction.

A compressor is designed to pump refrigerant in its vapor form, but if liquid refrigerant returns to the Compressor and passes through the suction valve, it may bend or break the suction valve. A loud knocking sound arises due to Slugging.

As it enters the cylinder, liquid refrigerant dilutes the lubricant oil present in the crankcase, creating an oil and foamy liquid mixture. This slug of liquid (oil droplets+ refrigerant) gets up and reaches the top of the piston. Since the piston fails to compress the slug, high pressure is created inside the cylinder and destroys the piston crown. Significant damages due to Slugging are:

  • Damaged suction and discharge valve reeds
  • Broken piston crown and crankshaft
  • Damaged con rod
  • Broken head gasket
  • Busted connecting rod etc.

Image Credit: Broken Discharge Reed Valve & Compressor Burn out

Symptoms of a Flooded Compressor are

During servicing of Compressor, one of the most commonly encountered issues by the technicians is Flooding.

It is indispensable to identify a flooded Compressor to prevent excessive damage.

Main symptoms of a flooded Compressor are as follows:

  • The Absence of superheat is an indication of Flooding. Too low superheat allows the presence of liquid droplets in the refrigerant vapor.
  • Broken pistons and cylinders.
  • One of the symptoms of Flooding is oil foaming which can be observed through the oil level sight glass of the Compressor.
  • Broken center as well as rear bearings.
  • Cold, sweaty, and frosted crankcase.
  • Higher current consumption is also a sign of a Flooded Compressor.

Image Credit: Signs of Flooded Compressor

How to fix a flooded compressor?

There are certain measures that can be taken to prevent the Flooding of a Compressor.

Some of them are as follows:

  • Maintain proper superheat of the evaporator as well as Compressor.
  • Installation of accumulator in the suction line after the evaporator.
  • Defrost control system should keep under regular supervision.
  • Modify low load condition.
  • Expansion devices like TXV should be appropriately located and insulated.

Suction Line Accumulator

Image Credit:

Frequently Asked Questions(FAQs)

Q.What causes oil to foam in a Compressor ?

Ans: Oil foaming occurs due to the mixing of lubricating oil with liquid refrigerant, which can be detected through the compressor crankcase sight glass.

If the Compressor is restarted with liquid refrigerant in the crankcase along with lubricating oil, the liquid refrigerant-oil mixture begins to vaporize rapidly due to a rapid decrease in pressure and increase in temperature. This phenomenon causes foaming.

Oil foaming results in carryover of oil with the refrigerant. The carried-over mixture of oil and liquid refrigerant doesn’t have lubricating properties and can cause severe damage to the Compressor.

Q.Does Compressor Flood back effect its efficiency?

Ans: When the liquid refrigerant enters the Compressor, it readily mixes with the lube oil and dilutes it, causing inadequate lubrication. Certain features may overheat and fail.

Since liquid refrigerant is non-compressible, high hydraulic pressure is required inside the cylinder, resulting in excessive stress generation. More than average crankcase pressure is required to pump the refrigerant through the cylinder, resulting in lower system efficiency.

Sangeeta Das

I am Sangeeta Das. I have completed my Masters in Mechanical Engineering with specialization in I.C Engine and Automobiles. I have around ten years of experience encompassing industry and academia. My area of interest includes I.C. Engines, Aerodynamics and Fluid Mechanics. You can reach me at

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