Reference for common faults in the operation and maintenance of refrigeration units

This article mainly explains issues related to liquid return, liquid-carrying start-up, oil return, the influence of evaporation and gas return parameters, exhaust-related parameters and liquid slugging prevention, as well as precautions for adding refrigerant, for your reference.

1. Liquid Refrigerant Return Issues

Definition: Liquid refrigerant return refers to incompletely evaporated liquid refrigerant entering the compressor with the return gas, potentially causing liquid slugging, poor lubrication, or even mechanical damage.

1. Expansion Valve Factors

• Oversized Expansion Valve:
  The expansion valve capacity far exceeds system requirements, leading to excessive liquid supply.
• Inappropriate Superheat Setting:
  Too low a superheat (e.g., close to 0°C) indicates incomplete evaporation, making liquid refrigerant prone to backflow.
• Incorrect Temperature Sensor Installation or Insulation Damage:
  If the temperature sensor is not in close contact with the return gas pipe or the insulation is ineffective, it will misjudge the return gas temperature, causing the expansion valve to open too wide.
• Expansion Valve Malfunction:
  Issues such as valve core jamming or diaphragm aging can prevent proper throttling.

2. Refrigerant Charge Control (For Capillary Tube Systems)

• The capillary tube system lacks adjustment capabilities, making the system extremely sensitive to the charge amount.
• Overcharging Refrigerant:
  → Evaporator cannot completely vaporize → Liquid refrigerant returns to the compressor → Liquid refrigerant return.

3. Evaporator and Fan Issues

• Severe frost buildup on the evaporator
  Frost hinders heat exchange, preventing refrigerant from absorbing heat and evaporating.
• Fan malfunction/Insufficient airflow
  Poor airflow reduces heat transfer efficiency → Incomplete evaporation → Refrigerant return.

4. Temperature Fluctuations (Especially in Cold Storage)

• Frequent start-ups and shutdowns or significant temperature fluctuations → Frequent expansion valve adjustments → Control lag or malfunction → Uncontrolled refrigerant supply → Refrigerant return.

 

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2. Liquid Start-up Problem

Phenomenon Description

• Upon startup of a return-gas cooled compressor, the lubricating oil in the crankcase foams violently, sometimes even spraying out from the exhaust port.

Root Cause

• During shutdown, refrigerant migration occurs: Due to pressure and temperature differences, refrigerant vapor in the evaporator slowly migrates to the cooler compressor crankcase and dissolves in the lubricating oil.
• During startup, the crankcase pressure drops sharply → the dissolved refrigerant rapidly boils (flash evaporation) → the lubricating oil foams → the oil pump draws in air or foam is carried into the cylinder → lubrication failure and risk of liquid slugging.

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3. Oil Return Issues

Lubricating oil must circulate back to the compressor with the refrigerant; otherwise, it will lead to oil shortage and wear in the compressor.

1. Oil Return Bend Design

• When the compressor is higher than the evaporator (e.g., in rooftop units), an oil return bend must be installed on the vertical return pipe.
• The oil return bends should be compact and reasonably spaced (usually one every 6 meters) to avoid forming an “oil pool” that traps lubricating oil.

2. Low Load

• At low loads, the refrigerant flow rate decreases → oil carrying capacity decreases → oil accumulates in the pipes or evaporator.
• Solution: Use a double riser; at low loads, only the smaller riser should be used to maintain sufficient flow rate for oil return.

3. Compressor Start-Stop Frequency

• Frequent start-stops
  → Oil does not return in time before shutdown → oil accumulation → long-term oil shortage.
• A minimum operating time should be set, short-cycle start-stops should be avoided, and an oil separator or crankcase heater can be added as an auxiliary measure.

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4. Relationship between Evaporation Temperature, Return Gas Temperature, and Return Gas Pressure

Effect of Evaporation Temperature:

• Increasing the evaporation temperature
  → Increases refrigerant density → Increases mass flow rate → Increases compressor load and motor power.
• Decreasing the evaporation temperature
  → Decreases mass flow rate → Reduces motor load, but significantly decreases COP, and causes a sharp increase in discharge temperature.

Return Gas Temperature:

• Should be slightly higher than the evaporation temperature (usually maintained at 5~8℃ superheat).
• Too low → Risk of liquid return; Too high → Increased suction specific volume → Decreased cooling capacity and increased discharge temperature.

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5. Exhaust Temperature, Exhaust Pressure, and Exhaust Volume

Reasons for Excessive Exhaust Temperature

1. High Return Gas Temperature
   → High temperature of the intake gas itself.
2. Motor Overheating
   In return gas cooling compressors, the refrigerant carries away heat as it flows through the motor windings; if there is insufficient return gas, the motor will overheat.
3. High Compression Ratio
   Condensing Pressure / Evaporating Pressure → Large temperature rise during compression.
4. High Condensing Pressure
   → Poor heat dissipation, excessive refrigerant, air mixed into the system, etc.
5. High Refrigerant Adiabatic Index
   (e.g., R22 > R134a) → Even greater temperature rise during compression.

 

• Refrigerant does not pass through the motor windings → No “motor heating” effect, but the exhaust temperature is still affected by the compression ratio, condensing conditions, etc.

Excessive Exhaust Pressure

• Mainly caused by excessively high condensing pressure:
  • Clogged condenser, fan malfunction, high ambient temperature;
  • Excessive refrigerant charge;
  • Presence of non-condensable gases (e.g., air) in the system.
• Insufficient exhaust volume is closely related to intake resistance:
  • Excessively long intake pipe or too small pipe diameter → large pressure drop → reduced intake pressure → reduced actual intake volume → decreased exhaust volume.

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6. Liquid Slugging

• Core Measures: Ensure
  the intake gas has a certain degree of superheat (usually 5~10℃).
• Adjustment Method:
  Control the liquid supply by adjusting the expansion valve opening to ensure the evaporator outlet gas is superheated.
• Balance Point:
  The superheat should not be too high (reducing efficiency) nor too low (risk of liquid backflow).

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7. Precautions for Adding Refrigerant

• Insufficient Refrigerant:
  Premature frosting on the expansion valve, filter, and evaporator inlet (due to excessively low pressure and a sudden drop in temperature after throttling).
  Abnormally high superheat in the return gas.

• Excessive Refrigerant:
  Frosting (or even freezing) on ​​the compressor return pipe → Indicates that liquid refrigerant has flowed back to the vicinity of the compressor.
  No superheat at the evaporator outlet, or even liquid carryover.

 

This article analyzes the causes and countermeasures for common problems in refrigeration systems, such as liquid return, liquid-carrying start-up, poor oil return, and abnormal exhaust, from the perspectives of operation control and maintenance, for reference.