Refrigeration systems use refrigerants as working fluids, and the refrigerants generally have two forms: liquid and gas. Today we will talk about the relevant knowledge about liquid refrigerants.
1. Is the refrigerant liquid or gas?
Refrigerants can be divided into 3 categories: single refrigerant refrigerants, non-azeotropic mixed refrigerants, and azeotropic mixed refrigerants.
The composition of the single working substance refrigerant will not change whether it is gaseous or liquid, so the gaseous state can be charged when charging the refrigerant.
Although the composition of the azeotropic refrigerant is different, because the boiling point is the same, the composition of the gas and liquid is also the same, so the gas can be charged;
Due to the different boiling points of non-azeotropic refrigerants, liquid refrigerants and gaseous refrigerants are actually different in composition. If gaseous refrigerants are added at this time, the composition of the added refrigerants will be different. For example, only a certain gaseous refrigerant is added. Refrigerant, so only liquid can be added.
That is to say, non-azeotropic refrigerants must be added with liquid, and non-azeotropic refrigerants all start with R4. This kind of liquid is added. Common non-azeotropic refrigerants are: R40, R401A, R403B, R404A, R406A , R407A, R407B, R407C, R408A, R409A, R410A, R41A.
As for other common refrigerants, such as: R134a, R22, R23, R290, R32, R500, R600a, the composition of the refrigerant will not be affected by the addition of gas or liquid, so it is convenient.
When adding refrigerant, we should pay attention to the following:
(1) Observe the bubbles in the sight glass;
(2) Measure high and low pressure;
(3) Measure the compressor current;
(4) Weigh the injection.
In addition, it should be noted and emphasized that:
Non-azeotropic refrigerants must be added in a liquid state. For example, R410A refrigerant, its composition is as follows:
R32 (difluoromethane): 50%;
R125 (pentafluoroethane): 50%;
Because the boiling points of R32 and R125 are different, when the R410A refrigerant cylinder is left standing, the boiling point of R32 and R125 is different, which will inevitably lead to the vaporized gaseous refrigerant in the upper part of the refrigerant cylinder, and the composition is not 50% R32+ 50% R125, because the boiling point of R32 is low, it is very likely that the upper part of the refrigerant is a component of R32.
Therefore, if a gaseous refrigerant is added, the refrigerant added is not R410A, but R32.
Second, the common problems of liquid refrigerants
1. Liquid refrigerant migration
Refrigerant migration refers to the accumulation of liquid refrigerant in the compressor crankcase when the compressor is shut down. As long as the temperature inside the compressor is cooler than the temperature inside the evaporator, the pressure difference between the compressor and the evaporator will drive the refrigerant to a cooler location. This phenomenon is most likely to occur in cold winters. However, for air conditioners and heat pumps, when the condensing unit is far away from the compressor, migration may occur even if the temperature is high.
Once the system is shut down, if it is not turned on within a few hours, even if there is no pressure difference, the migration phenomenon may occur due to the attraction of the refrigerant in the crankcase to the refrigerant.
If the excess liquid refrigerant migrates into the crankcase of the compressor, a severe liquid slam phenomenon will occur when the compressor is started, resulting in various compressor failures, such as valve plate rupture, piston damage, bearing failure and bearing erosion (The refrigerant flushes the oil from the bearings).
2. Liquid refrigerant overflow
When the expansion valve fails, or the evaporator fan fails or is blocked by the air filter, the liquid refrigerant will overflow in the evaporator and enter the compressor through the suction pipe in the form of liquid rather than vapor. When the unit is running, due to the liquid overflow diluting the refrigeration oil, the moving parts of the compressor are worn, and the oil pressure decreases, causing the oil pressure safety device to act, thereby causing the crankcase to lose oil. In this case, if the machine is shut down, the phenomenon of refrigerant migration will occur rapidly, resulting in liquid hammer on restart.
3. Liquid strike
When the liquid hammer occurs, the metal slamming sound from the inside of the compressor can be heard, and it may be accompanied by the violent vibration of the compressor. Liquid slam can cause valve rupture, compressor head gasket damage, connecting rod breakage, crankshaft breakage, and damage to other types of compressors. Liquid hammer occurs when the liquid refrigerant migrates into the crankcase and restarts. In some units, due to the piping structure or the location of components, liquid refrigerant will accumulate in the suction pipe or evaporator during shutdown of the unit and enter the compressor as pure liquid and at a particularly high speed when the unit is turned on. . The speed and inertia of the liquid slam is sufficient to defeat any built-in compressor protection against liquid slam.
4. Action of hydraulic safety control device
In a set of low temperature units, after the defrost period, the oil pressure safety control device is often caused to act due to the overflow of liquid refrigerant. Many systems are designed to allow refrigerant to condense in the evaporator and suction line during defrost, and then flow into the compressor crankcase at startup causing a drop in oil pressure, causing the oil pressure safety device to operate.
Occasionally one or two actions of the oil pressure safety control device will not have a serious impact on the compressor, but repeated many times without good lubrication conditions will cause the compressor to fail. The oil pressure safety control device is often regarded as a minor fault by the operator, but it is a warning that the compressor has been running for more than two minutes without lubrication, and remedial measures need to be implemented in time.
3. Solutions to the problem of liquid refrigerants
A well-designed, efficient compressor for refrigeration, air conditioning, and heat pumps is essentially a vapor pump that can only handle a certain amount of liquid refrigerant and refrigeration oil. In order to design a compressor that can handle more liquid refrigerants and refrigeration oil, a combination of size, weight, cooling capacity, efficiency, noise and cost must be considered. Aside from design factors, the amount of liquid refrigerant that a compressor can handle is fixed, and its handling capacity depends on the following factors: crankcase volume, refrigerant oil charge, type of system and controls, and normal operating conditions.
When the refrigerant charge increases, it will increase the potential danger of the compressor. The reasons for the damage can generally be attributed to the following points:
(1) Excessive refrigerant charge.
(2) The evaporator is frosted.
(3) The evaporator filter is dirty and blocked.
(4) The evaporator fan or fan motor fails.
(5) Incorrect capillary selection.
(6) The selection or adjustment of the expansion valve is incorrect.
(7) Refrigerant migration.
1. Liquid refrigerant migration
Refrigerant migration refers to the accumulation of liquid refrigerant in the compressor crankcase when the compressor is shut down. As long as the temperature inside the compressor is cooler than the temperature inside the evaporator, the pressure difference between the compressor and the evaporator will drive the refrigerant to a cooler location. This phenomenon is most likely to occur in cold winters. However, for air conditioners and heat pumps, when the condensing unit is far away from the compressor, migration may occur even if the temperature is high.
Once the system is shut down, if it is not turned on within a few hours, even if there is no pressure difference, the migration phenomenon may occur due to the attraction of the refrigerant in the crankcase to the refrigerant.
If the excess liquid refrigerant migrates into the crankcase of the compressor, a severe liquid slam phenomenon will occur when the compressor is started, resulting in various compressor failures, such as valve plate rupture, piston damage, bearing failure and bearing erosion (The refrigerant flushes the oil from the bearings).
2. Liquid refrigerant overflow
When the expansion valve fails, or the evaporator fan fails or is blocked by the air filter, the liquid refrigerant will overflow in the evaporator and enter the compressor through the suction pipe in the form of liquid rather than vapor. When the unit is running, due to the liquid overflow diluting the refrigeration oil, the moving parts of the compressor are worn, and the oil pressure decreases, causing the oil pressure safety device to act, thereby causing the crankcase to lose oil. In this case, if the machine is shut down, the phenomenon of refrigerant migration will occur rapidly, resulting in liquid hammer on restart.
3. Liquid strike
When the liquid hammer occurs, the metal slamming sound from the inside of the compressor can be heard, and it may be accompanied by the violent vibration of the compressor. Liquid slam can cause valve rupture, compressor head gasket damage, connecting rod breakage, crankshaft breakage, and damage to other types of compressors. Liquid hammer occurs when the liquid refrigerant migrates into the crankcase and restarts. In some units, due to the piping structure or the location of components, liquid refrigerant will accumulate in the suction pipe or evaporator during shutdown of the unit and enter the compressor as pure liquid and at a particularly high speed when the unit is turned on. . The speed and inertia of the liquid slam is sufficient to defeat any built-in compressor protection against liquid slam.
4. Action of hydraulic safety control device
In a set of low temperature units, after the defrost period, the oil pressure safety control device is often caused to act due to the overflow of liquid refrigerant. Many systems are designed to allow refrigerant to condense in the evaporator and suction line during defrost, and then flow into the compressor crankcase at startup causing a drop in oil pressure, causing the oil pressure safety device to operate.
Occasionally one or two actions of the oil pressure safety control device will not have a serious impact on the compressor, but repeated many times without good lubrication conditions will cause the compressor to fail. The oil pressure safety control device is often regarded as a minor fault by the operator, but it is a warning that the compressor has been running for more than two minutes without lubrication, and remedial measures need to be implemented in time.
3. Solutions to the problem of liquid refrigerants
A well-designed, efficient compressor for refrigeration, air conditioning, and heat pumps is essentially a vapor pump that can only handle a certain amount of liquid refrigerant and refrigeration oil. In order to design a compressor that can handle more liquid refrigerants and refrigeration oil, a combination of size, weight, cooling capacity, efficiency, noise and cost must be considered. Aside from design factors, the amount of liquid refrigerant that a compressor can handle is fixed, and its handling capacity depends on the following factors: crankcase volume, refrigerant oil charge, type of system and controls, and normal operating conditions.
When the refrigerant charge increases, it will increase the potential danger of the compressor. The reasons for the damage can generally be attributed to the following points:
(1) Excessive refrigerant charge.
(2) The evaporator is frosted.
(3) The evaporator filter is dirty and blocked.
(4) The evaporator fan or fan motor fails.
(5) Incorrect capillary selection.
(6) The selection or adjustment of the expansion valve is incorrect.
(7) Refrigerant migration.
Post time: May-31-2022