In Refrigeration Repair, Analyzing and Handling Three Major Series of Malfunctions - "Blockage", "High Pressure" and "High Discharge Temperature" are Essential!

Doing refrigeration debugging and maintenance is not actually very difficult. By mastering some common methods of fault analysis and being able to draw inferences about other cases from one instance, it is easy to troubleshoot other faults. When it comes to maintenance, always remember not to complicate simple problems. Many faults are caused by us overcomplicating the issues in our minds. In fact, the more incredible the problem seems, the simpler the cause often is.

When encountering such problems, by using conventional means and checking one cause after another, we can ultimately find the cause of the fault and eliminate the fault in the refrigeration system. Then, the following common faults must be mastered!
I. "Blockage" Series of Faults
1. Oil Blockage Fault
The main cause of oil blockage: It is caused by severe wear of the compressor cylinder body or an excessively large clearance between the cylinders. The oil discharged with the compressor gas is sent into the condenser, and then enters the dryer filter together with the refrigerant. Due to the high viscosity of the oil, it is blocked by the desiccant in the filter. When there is too much oil, a blockage is formed at the inlet of the filter, preventing the refrigerant from circulating normally.
There is an excessive amount of refrigeration oil remaining in the refrigeration system, which affects the refrigeration effect and may even prevent refrigeration. Therefore, the refrigeration oil in the system must be completely removed.
Treatment method for oil blockage: When the filter is blocked by oil, a new filter should be replaced. At the same time, use high-pressure nitrogen to blow out part of the refrigeration oil accumulated in the condenser. It is best to use a hair dryer to heat the condenser while introducing nitrogen.
The main reason for the generation of the oil film is that the lubricating oil that has not been separated by the oil separator will enter the system and flow in the pipeline along with the refrigerant, forming an oil cycle. There is an essential difference between the oil film and the oil blockage.
Harm of the oil film:
If an oil film adheres to the surface of the heat exchanger, it will increase the condensing temperature and decrease the evaporation temperature, leading to an increase in energy consumption;
When an oil film of 0.1mm adheres to the surface of the condenser, the cooling capacity of the refrigeration compressor decreases by 16%, and the power consumption increases by 12.4%;
When the oil film in the evaporator reaches 0.1mm, the evaporation temperature will decrease by 2.5°C, and the power consumption will increase by 11%.
Treatment method for the oil film:
Using an efficient oil separator can greatly reduce the amount of oil entering the system pipeline;
If there is already an oil film in the system, nitrogen can be used for multiple flushes until there is no misty gas.
2. Ice Blockage Fault
The occurrence of the ice blockage fault is mainly due to the excessive moisture content in the refrigeration system. With the continuous circulation of the refrigerant, the moisture in the refrigeration system gradually concentrates at the outlet of the throttle valve. Since the temperature at the outlet of the throttle valve is the lowest, the water freezes and gradually increases. To a certain extent, the capillary tube is completely blocked, and the refrigerant cannot circulate.
Main sources of moisture:
Moisture remaining in the components and connecting pipes of the refrigeration system due to insufficient drying;
The refrigeration oil and refrigerant contain more moisture than the allowable amount;
Moisture enters due to failure to evacuate the system or improper installation during construction;
The motor insulation paper in the compressor contains moisture.
Manifestations of ice blockage:
The airflow gradually weakens and is intermittent;
When the blockage is severe, the airflow sound disappears, the refrigerant circulation is interrupted, and the condenser gradually cools down;
Due to the blockage, the exhaust pressure increases, and the running sound of the machine becomes louder;
There is no refrigerant flowing into the evaporator, the frost-covered area gradually becomes smaller, and the refrigeration effect deteriorates;
After the machine is shut down for a period of time, the refrigerant starts to cool again (the ice blocks start to melt).

Ice blockage forms a periodic repetition of being blocked and then unblocked, blocked again after being unblocked, and unblocked again after being blocked.
Treatment of ice blockage:
The ice blockage fault in the refrigeration system occurs because there is excessive moisture in the system. Therefore, the entire refrigeration system must be dried. The treatment methods are as follows:
Evacuate the system, replace the dryer filter, and wait until the moisture indicator in the sight glass of the refrigeration system turns green as a sign of passing the test;
If a large amount of water enters the system, use nitrogen to flush the dirt section by section, replace the filter, replace the refrigeration oil, replace the refrigerant, and evacuate the system until the moisture indicator in the sight glass turns green.
3. Dirt Blockage Fault
After a dirt blockage occurs in the refrigeration system, since the refrigerant cannot circulate, the compressor runs continuously. The evaporator is not cold, the condenser is not hot, the compressor casing is not hot, and there is no airflow sound in the evaporator. If there are many impurities in the system, the dryer filter will gradually be blocked, and the filter screen of the throttling mechanism will be blocked.
Main causes of dirt blockage:
Dust and metal shavings during construction and installation, and the oxide layer on the inner wall surface falling off during pipeline welding;
During the processing of various components, the inner and outer surfaces are not cleaned thoroughly, and dust enters the pipeline due to poor pipeline sealing;
Impurities in the refrigeration oil and refrigerant, and low-quality desiccant powder in the dryer filter;
The close distance between the capillary tube and the filter screen in the dryer filter is also likely to cause dirt blockage faults.
Manifestations after dirt blockage:
When it is partially blocked, the evaporator has a cool or icy feeling but does not frost;
When touching the outer surfaces of the dryer filter and the throttle valve, the feeling is very cold, and there is frost formation, and even a layer of white frost may form;
The evaporator is not cold, the condenser is not hot, and the compressor casing is not hot.
Treatment of dirt blockage faults: Dirt blockage generally occurs in the dryer filter, the filter screen of the throttling mechanism, the suction filter screen, etc. The filter screen of the throttling mechanism and the suction filter screen can be removed and cleaned, and the dryer filter is generally replaced.

After the replacement is completed, the refrigeration system needs to be leak-tested and evacuated.
II. "High Pressure" Series of Faults
The condensing pressure is what we usually call "high pressure". There are usually three types of faults related to the condensing pressure:
01. The condensing pressure is too high;
02. The condensing pressure is too low;
03. Pressure vibration;
2.1 The refrigerant pressure is too high
The reasons for the too high condensing pressure usually include the following:
a) Air or other non-compressible gases in the refrigeration system;
b) The surface area of the condenser is too small;
c) Overcharging of the refrigeration system (liquid recovery in the condenser);
d) The condensing pressure adjustment is set to a too high pressure;
e) Dirt on the surface of the condenser.
The above are the common fault causes for both air-cooled and water-cooled systems:
Elimination methods:
a) Clear the condenser by starting and running the system using the recovery system until the system reaches the operating temperature. If necessary, clear it again;
b) Replace it with a larger condenser;
c) Recover the refrigerant until the condensing pressure is normal. The sight glass must remain full;
d) Set the correct pressure;
e) Clean the condenser.
Regarding the air condenser (finned heat exchanger), for high-pressure alarms, the following aspects usually need to be analyzed:
1) The fan motor or blades are defective or too small.
2) The air flow to the condenser is restricted.
3) The ambient temperature is too high.

4) The air flow through the condenser is in the wrong direction.
5) There is a short circuit between the air side pressure and the suction side of the condenser fan.
Troubleshooting methods:
1) Replace the motor or blades, or replace both the motor and blades.
2) Remove obstacles from the air inlet or move the condenser.
3) Create a clean air inlet or move the condenser.
4) Change the rotation direction of the fan motor. The air in the condensing unit must flow through the condenser and then into the compressor.
5) Install ducts that may be suitable for outdoor air.

Regarding the water-cooled condenser (finned heat exchanger), for high-pressure alarms, the following aspects usually need to be analyzed:
1) The cooling water temperature is too high.
2) The water volume is too small.
3) There is sediment (such as dirt) inside the water pipe.
4) The cooling water pump is defective or has stopped working.
Troubleshooting methods:
1) Ensure a lower water temperature.
2) Increase the water volume, and it may be necessary to use an automatic water valve to increase the water volume.
3) Clean the condenser water pipe, and it may be necessary to use deoxidation for cleaning.
4) Investigate the cause and replace or repair the cooling water pump (if installed).

2.2 Low Condensing Pressure
The common causes are as follows:
a) The condenser surface is too large.
b) The load on the evaporator is small.
c) The suction pressure is too low, for example, there is insufficient liquid in the evaporator.
d) The compressor suction valve or discharge valve may be leaking.
e) The pressure setting of the condensing pressure regulating valve is too low.
f) The uninsulated receiver placed is too cold compared with the condenser (the receiver is used as a condenser).
g) The cooling air temperature is too low. (For air cooling)
h) The air volume of the condenser is too large. (For air cooling)
j) The water volume is too large. (For water cooling)
k) The water temperature is too low. (For water cooling)
How to troubleshoot?
According to the above faults, we will troubleshoot them one by one in the following ways:
a) Adjust the condensing pressure or replace the condenser.
b) Adjust the condensing pressure.
c) Check for faults in the pipeline between the condenser and the thermostatic expansion valve (see "Low Suction Pressure").
d) Replace the compressor valve plate.
e) Set the correct pressure for the condensing pressure regulating valve.
f) Move the receiver or install a suitable thermal insulation cover for the receiver.
g) Adjust the condensing pressure.
h) Replace the fan with a smaller one or adjust the motor speed.
j) Reduce the cooling water flow rate.
k) Increase the cooling water temperature.

2.3 Condensing Pressure Oscillation
Cause analysis:
a) There is a difference when starting/stopping the pressure controller because the condenser fan is too large. Because the refrigerant recovery in the condenser may form steam in the liquid pipeline after the condenser fan has been running for a while.
b) The thermostatic expansion valve vibrates.
c) Faults in the KVR/KVD condensing pressure regulating valve (the orifice is too large).
d) The result of suction pressure vibration.
e) The check valve is of the wrong size or is installed in the wrong position in the condenser pipeline.
How to troubleshoot?
According to the above faults, we will troubleshoot them one by one in the following ways:
a) Set a lower difference value or use valve regulation (KVD+KVR) or use fan motor speed regulation.
b) Set a higher superheat degree for the thermostatic expansion valve or replace it with a smaller orifice.
c) Replace the valve with a smaller one.
d) See "Suction Pressure Vibration".
e) Check the size. Install the check valve near the receiver inlet under the condenser.

III. "Discharge Temperature" Related Faults
3.1 High Discharge Temperature
The theoretical calculation formula for discharge temperature:
T2 = T1(P2÷P1)^[(k - 1)÷k]
Where:
T2: Discharge temperature;
T1: Suction temperature;
P2: Discharge pressure;
P1: Suction pressure
K: The adiabatic index of the gas (K = 1.4 for air).
This formula reflects the importance of the suction temperature (T1) and the pressure ratio (P2÷P1).
These two data are directly related to the operating temperature and quality of the air compressor.
Because the higher the suction temperature and the compression ratio, the higher the discharge temperature will be in multiples!
According to the above formula, we can draw the following conclusions:
Factors affecting the abnormal discharge temperature: adiabatic index, compression ratio, suction temperature
The compressor discharge temperature can be read from the thermometer on the exhaust pipe. It is related to the adiabatic index of the refrigerant, the compression ratio (condensing pressure/evaporation pressure) and the suction temperature.
The higher the suction temperature and the compression ratio, the higher the discharge temperature, and vice versa.
When the suction pressure remains unchanged and the discharge pressure rises, the discharge temperature rises.
If the discharge pressure remains unchanged and the suction pressure drops, the discharge temperature will also rise.
Both of these situations are caused by an increase in the compression ratio.
Both too high condensing temperature and discharge temperature are harmful to the operation of the compressor and should be prevented.
Too high discharge temperature will thin the lubricating oil and even cause carbonization and coking, thus deteriorating the lubrication conditions of the compressor.
The discharge temperature is directly proportional to the compression ratio (condensing pressure/evaporation pressure) and the suction temperature.
If the suction superheat temperature is high and the compression ratio is large, the discharge temperature will also be high.
If the suction pressure and temperature remain unchanged, when the discharge pressure rises, the discharge temperature also rises.
The reasons for the increase in discharge temperature are as follows:
(1) The suction temperature is high, and the discharge temperature of the refrigerant vapor after compression is also high.
(2) The condensing temperature rises, and the condensing pressure is high, resulting in an increase in the discharge temperature.
(3) The discharge valve plate is broken, and the high-pressure steam is repeatedly compressed and the temperature rises. The cylinder and cylinder head are hot to the touch, and the thermometer reading on the exhaust pipe also rises.
The most practical factors are as follows:
The intermediate cooling efficiency is low, or there is too much scale in the intercooler, which affects the heat transfer, then the suction temperature of the subsequent stage will inevitably be on the high side, and the discharge temperature will also rise.
The air valve leaks and the piston ring leaks.
This will not only cause the discharge temperature to rise but also change the interstage pressure.
As long as the compression ratio is higher than the normal value, the discharge temperature will rise.
In addition, for water-cooled machines, water shortage or insufficient water volume will cause the discharge temperature to rise.
The condensing pressure is abnormal and the discharge pressure is reduced.