Design, Fault Analysis, and Common Knowledge of Refrigeration Systems

May 22, 2024

Design, Fault Analysis, and Common Knowledge of Refrigeration Systems



The importance of pipeline design


1. When the system is running normally, a small amount of oil will continue to leave the compressor with the exhaust. When the system's circuit design is good, this oil will return to the compressor;

2 If there is too much oil in the system, it will have a negative impact on the efficiency of the condenser and evaporator;

3. The oil returning to the compressor is less than the oil leaving the compressor, ultimately causing damage to the compressor;

4. Refueling the compressor can only maintain the oil level for a short period of time;

5. Only with correct pipeline design can the system maintain good oil balance.


Design of suction pipeline


1. The horizontal suction pipeline should have a slope of more than 0.5% along the direction of cooling air flow;

2. The cross-section of the horizontal suction pipeline must maintain a gas flow rate of not less than 3.6m/s;

3. In a vertical suction pipeline, it is necessary to ensure that the gas flow rate is not less than 7.6-12m/s;

4. A gas flow rate greater than 12m/s cannot significantly improve the return oil, resulting in high noise and a higher pressure drop in the suction pipeline;

5. At the bottom of each vertical suction pipeline, a U-shaped oil return bend must be installed;

6. If the height of the vertical suction pipeline exceeds 5m, a U-shaped oil return bend must be set up for every additional 5m;

7. The length of the U-shaped oil return bend should be as short as possible to avoid excessive oil accumulation.


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Design of evaporator suction pipeline


1. When the system does not use a vacuum cycle; At the outlet of each evaporator, a U-shaped interception bend should be installed. To prevent liquid refrigerant from flowing into the compressor under the action of gravity during shutdown;

2. When the suction rising pipe is connected to the evaporator, a horizontal pipe and a cut-off bend should be left in the middle for installing the temperature sensing bulb; Prevent false operation of the expansion valve.

Exhaust pipeline design

When the condenser is installed higher than the compressor, a U-shaped bend is required at the inlet pipe of the condenser to prevent oil from returning to the exhaust side of the compressor during shutdown, and also to prevent liquid refrigerant from flowing back from the condenser to the compressor;


Liquid pipeline design


1. Liquid pipelines usually have no special restrictions on the flow rate of refrigerant. When using solenoid valves, the refrigerant flow rate should be less than 1.5m/s;

2. How to ensure that the refrigerant entering the expansion valve is an undercooled liquid;

3. When the pressure of the liquid refrigerant drops to its saturation pressure, a portion of the refrigerant will flash into gas.


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The hazards of refrigerant flash gas


1. Reduce the cooling capacity of the expansion valve;

2. Will corrode the valve needle and valve seat of the expansion valve, causing noise;

3. Causing abnormal liquid supply from the expansion valve to the evaporator.


Refueling volume and oil separator


1. In most refrigeration systems, the amount of oil added to the compressor is already sufficient;

2. When the pipeline exceeds 20m, or there are many oil wells in the pipeline, or when an oil separator is installed in the system, additional refrigeration oil needs to be added;

3. In some refrigeration systems, there is a risk of slow oil return. When there are multiple evaporators or multiple condensers in parallel, it is recommended to install an oil separator.


Expansion valve/drying filter


1. Expansion valve or drying filter, selected according to the refrigerant used;

2. When selecting a drying filter, it is important to consider its water absorption capacity, system cooling capacity, and refrigerant filling capacity.


Working voltage and number of starts


1. The working voltage should be within the specified range;

2. The number of starts cannot exceed 10-12 times per hour;

3. The running time after each startup should not be less than 5 minutes to ensure correct oil return and motor cooling. The system design must ensure the minimum compressor running time.




1. The selection of evaporator should match the load of the system and the refrigeration capacity of the compressor;

2. The heat exchange area is too large, the return air temperature is high, and the evaporation temperature cannot be reduced;

3. The heat exchange area is too small, and the refrigerant cannot fully evaporate, resulting in a return liquid.




1. The selection of condenser should match the load and cooling capacity of the compressor;

2. To refer to the technical information of the manufacturer;

3. The heat exchange area is too small, and the refrigerant gas cannot be fully condensed, resulting in an increase in exhaust temperature and pressure.


Migration of liquid refrigerant during shutdown


1. After system shutdown and pressure balancing, the refrigerant condenses in the coldest part of the system;

2. The refrigerant in the system will condense into the compressor crankcase;

3. The refrigerant will dissolve in the compressor oil until the refrigerant in the oil is completely saturated;

4. When the compressor starts, the pressure decreases, and the refrigerant evaporates violently, forming oil foam;

5. Causing liquid or oil shock, damaging valve discs and plates;

6. The oil is diluted by refrigerant, resulting in a significant decrease in lubrication capacity.


Prevent liquid refrigerant migration during shutdown


1. Using a return gas pipeline gas-liquid separator;

2. Install the solenoid valve for the liquid supply pipeline;

3. Using a crankcase heater;

4. 4 hours before starting the compressor, turn on the heater.


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System cleaning


1. Unclean systems are one of the main factors affecting the lifespan of compressors;

2. It is crucial to ensure the cleanliness of the refrigeration system when configuring it;

3. Factors that cause system pollution: oxides from brazing and welding, burrs and bumps in pipelines, brazing flux, moisture, etc;

4. Never drill holes in the pipeline after installation is completed.


System pressure test


1. Suggest conducting a pressure test with pure dry nitrogen gas;

2. The high-pressure and low-pressure sides cannot exceed the maximum allowable pressure;


System leak detection


1. It is necessary to use pure dry nitrogen and the refrigerant used for leak detection;

2. Do not use other gases such as oxygen, dry air, or acetylene gas;

3. The pressure for leak detection should not exceed the test pressure.


The system vacuums out water


1. The air and moisture in the system generate high exhaust temperatures, leading to an increase in condensation pressure;

2. Causing mechanical and electrical faults in the compressor;

3. A vacuum pump must be used to evacuate both high and low pressure ends of the system simultaneously;

4. To start evacuating, close the suction and exhaust valves of the compressor;

5. Firstly, use a vacuum pump to extract 1500 micrometers of mercury, and then add refrigerant to the system through a drying device to break the vacuum state;

6. Repeat the above steps again;

7. Open the suction and exhaust valves of the compressor, and vacuum the system to 500 micrometers of mercury;

8. Fill in refrigerant and turn off the vacuum pump.


Precautions for System Evacuation


1. Never use a compressor as a vacuum pump to vacuum the system;

2. When the system is in a vacuum state, do not start or install the compressor under any circumstances, otherwise it may cause the compressor to burn out.


System check and startup


1. Check the electrical wiring to ensure that it is securely fastened and error free;

2. Observe the oil level of the compressor, and the oil level should be slightly above the middle of the sight glass;

3. Remove or loosen the transportation bracket under the compressor;

4. Check the high-pressure and low-pressure pressure controllers, compressor suction and exhaust valves, oil pressure safety controllers, and other safety control devices;

5. Check if the temperature controller is functioning properly;

6. Mark and indicate the refrigerant used in the system;

7. Read the instructions and wiring diagrams, and keep them properly for future reference.


System check and startup


1. Liquid refrigerant must be injected from the high-pressure side of the compressor;

2. Pay attention to the pressure of the system before and during the filling process;

3. Continue to charge the system until there is sufficient refrigerant, do not overcharge;

4. Do not leave the refrigeration device unattended when the system is not in normal operation and the oil level is not maintained in the center of the sight glass.


System operation inspection


1. Check the suction and discharge pressures of the compressor;

2. Check the operation of the sight glass and expansion valve;

3. Observe the oil level in the compressor crankcase;

4. Check the heat setting value of the thermal expansion valve and the position of the temperature sensing bulb;

5. Check the voltage and current values at the compressor terminals;

6. Check if the operation direction of the evaporator and condenser is correct;

7. Check if the crankcase heater is functioning properly;

8. Other items that need to be checked




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