What should I do if the refrigeration compressor returns oil
During the operation of the system, the lubricating oil is discharged from the compressor along with the refrigerant, and returns to the compressor after the cycle. Then there will be lubricating oil in and out of the place where the refrigerant enters and exits. There is an essential difference between refrigerant performance and lubricating oil performance. There are two phases in the system cycle of refrigerant, namely liquid refrigerant and vapor refrigerant, while lubricating oil is basically in liquid state. When refrigerant changes from liquid to vapor, the lubricating oil will change from The refrigerant is precipitated. Under the influence of many factors, they are likely to be stored in a certain part or a certain structural point, which will cause the lubricating oil to fail to return to the compressor smoothly, causing the scroll compressor to be short of oil. Failure to solve this will result in insufficient lubrication of the moving parts of the compressor, dry burning and other faults, and greatly accelerate the damage of the scroll compressor.
1. Ensure proper amount of oil
When the compressor discharges refrigerant, it also discharges a trace amount of refrigerating machine oil. Even if the oil rate is only 0.5%, if the oil cannot be circulated back to the compressor through the system, if taking 5HP as an example, the circulation volume is about 330kg/h under ARI conditions, and the compressor can be removed in 50 minutes. All the oil is taken out, and the compressor will burn out in about 2 to 5 hours.
Therefore, in order to ensure that the compressor runs without oil shortage, the following two aspects should be started:
1. Ensure that the refrigerating oil discharged from the compressor returns to the compressor;
2. Reduce the oil rate of the compressor.
2. Ensure that the refrigerating oil discharged from the compressor returns to the compressor
1. The flow rate of the refrigerant in the suction pipe (about 6m/s) should be ensured to allow the oil to return to the compressor, but the z* high flow rate should be less than 15m/s to reduce pressure drop and flow noise. There is a downward slope along the direction of refrigerant flow, about 0.8cm/m.
2. Prevent refrigerating oil from staying in the evaporator.
3. Ensure that the oil return hole of the gas-liquid separator is too large to cause wet compression, and too small will cause insufficient oil return, and stagnant oil will be in the gas-liquid separator.
4. There should be no place where the oil is retained in the system.
5. Ensure that there is enough refrigerating oil in the compressor under the condition of long piping and high drop. Usually, a compressor with an oil mirror is used to confirm that the frequent start of the compressor is not conducive to oil return.
Three, reduce the oil rate of the compressor
1. Make sure that the refrigerant does not dissolve into the refrigerating machine oil during shutdown (use crankshaft heater).
2. To avoid over-humid operation, because it will foam and cause excessive oiling.
3. An oil separator device is installed inside.
4. The oil in the compressor is foamed so that the oil is easily taken out of the compressor.
Four. Long piping height difference
When the piping length is larger than the allowable value, the pressure loss in the piping will increase, reducing the amount of refrigerant in the evaporator, resulting in a decrease in capacity. At the same time, when oil is trapped in the piping, the compressor is short of oil, leading to compressor failure. When the refrigerating machine oil in the compressor is insufficient, the refrigerating machine oil of the same grade as the compressor factory should be added from the high-pressure side.
5. Set the necessary return bend. When the drop exceeds 10m~15m, an oil return elbow should be installed on the air pipe side.
①Necessity: When stopping, avoid the refrigerating oil attached to the piping from returning to the compressor, causing liquid compression. On the other hand, in order to prevent poor oil return in the air pipe, the compressor lacks oil.
②The interval of oil return bend setting; Set one oil return bend every 10m drop.
Sixth, ensure proper refrigerating oil viscosity
The refrigerating machine oil and the refrigerant have mutual solubility. When the machine is stopped, the refrigerant is almost completely dissolved in the refrigerating machine oil, so a crankshaft heater must be installed to prevent dissolution.
1. The refrigerant containing liquid should not be returned to the compressor during operation, that is, to ensure that the compressor suction has superheat.
2. When starting and defrosting, there should be no liquid back phenomenon.
3. Avoid running under excessive heat to avoid oil deterioration.
4. The size of the oil return hole of the gas-liquid separator should be appropriate:
①The hole diameter is too large to inhale liquid refrigerant and cause excessive humidity operation;
②If the hole diameter is too small, the oil return will not be smooth and the oil will stay in the gas-liquid separator.
In terms of equipment:
1. Choice of lubricant
Lubricating oil mainly plays the role of lubrication, sealing, cleaning, heat dissipation, and rust prevention in scroll compressors. Choosing a good lubricating oil will not only improve the reliability of the scroll compressor, but also greatly improve the performance of the air conditioning system.
There are many criteria for the selection of lubricating oil. From the perspective of oil return, the lubricating oil is required to have good flow characteristics at low temperatures. Therefore, it is necessary to choose a low pour point to avoid adhesion at low temperatures and fail to return to the compressor. .
The following table shows the pour points of several commonly used lubricating oils; when the refrigerant is in a vapor state, the lubricating oil flows in a high-pressure and high-speed airflow. When the refrigerant is in a liquid state, the lubricating oil mixes and flows in it. It can flow well in any state without stagnation. When selecting lubricating oil, it is required that the lubricating oil and the refrigerant have good compatibility. The figure below is a typical dissolution curve of lubricating oil and refrigerant. The analysis brings a lot of convenience.
2, the selection of components in the system
Oil separator is generally installed on the exhaust pipe to achieve gasoline separation through rapid pressure drop, and then return to the compressor oil reservoir through the oil return capillary. At present, there are three widely used oil separators:
①The oil separator with a floating ball. If oil accumulates in the oil separator, the floating ball valve set inside will open to return the oil to the compressor;
② Manually return the oil to the oil separator of the compressor. If the oil collects in the oil separator, you need to manually open the oil return valve to return the oil to the compressor;
③ An oil separator without a float valve inside. Although this oil separator has a simple structure, it has very strict requirements on the size of the oil return piping.
Gas-liquid separator. The gas-liquid separator is one of the key parts that affect the oil return. It is generally installed between the air return port and the compressor. The gas-liquid separator has two key indicators, the oil return hole and the balance hole. In the design and selection, the appropriate gas-liquid separator must be selected according to the needs of the own system. In the gas-liquid separator of an oil-deficient system, oil is basically stored. At present, there are many manufacturers of gas-liquid separators. The general air-conditioning manufacturers simply select them instead of designing suitable gas-liquid separators according to their own system requirements, which may easily cause oil collection in the gas-liquid separators. Some companies with research and development capabilities will develop gas-liquid separators suitable for the system according to their own needs when developing distinctive products.
Another key part is the connecting pipe of the internal and external units. At present, many manufacturers have developed multi-line units, but as the length of the oil return pipe increases, the difficulty of oil return gradually increases. How to configure a longer connecting pipe It is a question worth considering.
3, system control system control
mainly involves oil return control and oil equalization control. In the multi-line system, under the condition of partial load operation, oil will be collected in the unoperated load. The more unoperated load and the longer the running time, the more oil is collected outside the compressor, which will return to the inside of the compressor. The less lubricating oil. When the system runs to a certain controlled index (the index can be oil level, running time, temperature, etc.), the oil return system works by adjusting the whole machine load, refrigerant flow, working frequency, motor, system air volume and other controllable factors. Adjust the flow rate and pressure of the refrigerant in the system to increase the flow rate of the refrigerant in the compressor and drive the return of lubricating oil. When the monitoring system detects that the oil volume is sufficient for the compressor to operate, it enters normal load work, and so on.
The oil equalization occurs in the multi-line group. Similarly, detection points such as oil level can be designed in the system. When the system detects that a compressor is lean in oil, it can be removed from the oil-rich compressor system through the oil equalization system Equalize part of the lubricating oil to the lean oil compressor system. If the second compressor also produces lean oil, start the oil equalization again through detection, and so on, until the oil volume of all compressor systems is equalized. Optimized structural design is also conducive to oil return. At present, gasoline balancing technology is commonly used.
In theory, the oil pressure and air pressure in the crankcase of each parallel compressor can be guaranteed, but it is not ideal in practice. Due to the influence of factors such as the design and processing of the balance pipe, the installation of the unit, and the pump oil volume of each compressor, each compressor The oil pressure and air pressure of the crankcase will be different. Therefore, the oil return method must be well controlled from the above aspects, and no more than three compressors should be used.
Another oil return structure uses non-balanced technology, Danfoss' patented structure. The pressure in the system flow path decreases sequentially, so that a pressure gradient is established in the compressor. The lubricating oil s* first flows into the upstream compressor. When the oil level is higher than the bottom of the connecting pipe, it will affect the airflow and pressure difference. Underflow and enter the next compressor. If the oil volume is normal, each compressor can get sufficient lubricating oil.
4. The influence of system speed and pressure on oil return. The change of system working condition has a great influence on the flow rate, pressure and phase state of the refrigerant inside the scroll compressor system.
During the operation of the system, the refrigerant and lubricating oil are almost soluble in each other. The higher the flow rate and pressure of the refrigerant in the pipeline, the more favorable it is for the return of lubricating oil. As mentioned earlier, oil return control generally changes the flow rate of the refrigerant in the unit by controlling the frequency of the unit. When the frequency of the unit increases, in a unit time, the greater the refrigerant flow through the compressor, the more the refrigerant flows in the pipeline. The speed and density are increased, so the speed of lubricating oil return will naturally increase.
During the installation process of the multi-line unit, based on the structural requirements, the internal and external connecting pipes may exceed the size recommended by the manufacturer. As the connecting pipe lengthens, the system pressure loss will increase and the flow rate of the refrigerant in the system will also slow down. The oil return is extremely unfavorable. Lubricating oil will be precipitated in the slow-flowing refrigerant and attached to the inner wall of the pipeline. In some parts that are easy to store oil, it will cause the lubricating oil to accumulate, so that the lubricating oil cannot completely return to the compressor.
Therefore, ① Choose lubricating oil with a lower pour point as much as possible, which is conducive to the flow of lubricating oil in the pipeline;
② The oil separator and gas-liquid separator of the applicable system should be selected. The influence of the length of the connecting pipe on the oil return cannot be ignored. When the connecting pipe is too long, corresponding treatment should be done, such as increasing lubrication;
③ Consider oil return factors as much as possible in the initial design stage, and optimize the system oil return through structural design;
④ Frequency plays a vital role in the flow and velocity of the refrigerant. As the frequency increases, the flow and velocity will increase and the oil return will increase.
