The compressor is the heart of the refrigeration system. The most common failure is mechanical damage caused by poor lubrication.
There are two main phenomena of poor lubrication: one is that the lubricating oil is not enough, that is, the oil is lacking; the other is that the lubricating oil is not lacking, but it causes poor lubrication.
First, lack of oil
In order to prevent the compressor from being deficient in oil, many compressors are equipped with an oil separator to ensure the quick return of the compressor. We know that Freon refrigerants and lubricants are partially miscible, with approximately 2-3% of the lubricant in the semi-sealed piston and screw compressors, and 0.5-1% in the scroll compressor. Even if the oil is filled, only part of the frozen oil can be separated, and some of the frozen oil will always go to the system. Suppose a 6-cylinder compressor with a displacement of 100m3/hr and a crankcase storage capacity of 5 liters, 3% of the oil means about 0.3-0.8 liters/min of oil, or the compressor has no oil return operation. The time is ten minutes. That is to say, a small single-machine system, even if it does not contain oil, such a small amount of oil, as long as the system is installed, the oil is good, we only need to add a small amount of refrigeration oil, so that the refrigeration oil in the whole system, let the refrigerant The oil that is carried away and the oil that is brought back reach a balanced cycle, and the compressor will not be damaged by lack of oil; large systems, even with oil and oil collectors, need to be replenished with some oil at the beginning of the start-up. The oil in the system reaches a balanced state, but if the system returns to poor oil, the refrigeration oil in the compressor and the oil collector will also go to the system and stay in the system in a short time, resulting in the lack of compressor. oil.
So, how can the frozen oil in the system return to the compressor smoothly?
1 In the pipeline design, the pipe diameter should meet the flow rate of the refrigerant required by the refrigeration design manual, so that the refrigeration oil can flow back to the compressor along the flow of the refrigerant.
2 The construction process of the pipeline shall ensure the smooth flow of the oil. When the liquid pipe branches out of the branch pipe from the main pipe, the branch pipe is taken out from the bottom or side of the main pipe.
3 On the return pipe, all pipes with horizontal upwards should be installed with oil return bends. The oil return bend should be as compact as possible to reduce oil storage. 4 The tracheal branch is inserted into the main pipe from the upper part; when the return main pipe is separated from the branch pipe, the branch pipe should be inserted into the bottom of the main pipe.
5 All refrigeration pipes should be oriented down to about 1% slope along the direction of refrigerant flow. The slope should not be too large to prevent the refrigerant liquid from flowing back to the compressor too quickly.
6 The return line of the system with large load changes must also be careful. When the load is reduced, the return air speed will decrease, and the speed is too low to be good for oil return. In order to ensure oil return under low load, the vertical suction pipe can adopt double riser.
7 Frequent starting of the compressor is not conducive to oil return. Since the continuous operation time is short, the compressor stops, and the return air pipe does not have a stable high-speed air flow, and the lubricating oil can only remain in the pipeline.
Second, it seems that the lubricant is not lacking, but it causes poor lubrication.
Sometimes the compressor seems to be damaged in an instant, the oil pressure difference is not enough to protect, or the oil level in the crankcase of the compressor does not look low, the oil pressure difference does not alarm and the protection stops the compressor, but it happens The lack of oil, what is going on? This phenomenon is usually caused by liquid reflux. If there is a large amount of refrigerant liquid in the crankcase during compressor shutdown, because the Freon volume is greater than the refrigeration oil, it will deposit on the bottom. When the compressor is turned on, the crankcase pressure will decrease rapidly, causing the refrigerant to evaporate quickly. Bring into the cylinder, the lack of oil in an instant, this speed is very fast, because all the protection will have a delay, when there is no protection, the frozen oil will be gone. At this time, the compressor will not only be damaged in the running part, but also the cylinder may be damaged due to the non-compressor characteristics of the liquid. Therefore, when starting up, it is very important to observe the oil level of the crankcase or high-pressure oil and the frosting of the compressor. If the frozen oil is found to be very turbid, it seems to have bubbles, and the inlet of the compressor is frosty. Very dangerous sign. The hazard of returning liquid during normal startup is to dilute the lubricating oil. When the diluted lubricating oil reaches the friction surface, the viscosity is low, and a protective film of a sufficient thickness cannot be formed, which may cause wear over time. When the amount of liquid return is relatively large, the lubricating oil will be very thin, not only can not play a lubricating role, but also dissolve the original oil film, causing the refrigerant to wash.
However, because the refrigerant and the refrigerating oil are liquids, its flow can create a pressure difference, which makes the oil pressure difference or the oil flow switch mistakenly believe that the refrigerating oil is normal, does not issue alarm and protection commands, and eventually causes wear and damage.
So how do you avoid the generation of liquid return?
1 If the evaporator is higher than the compressor, the return pipe of the evaporator should go through a return oil bend and rise above the middle of the evaporator or the air cooler, and then go horizontally. For systems that rely on temperature shutdown, this is the case. more important;
2 The temperature sensor package and balance tube of the expansion valve should be installed in accordance with the specifications. The temperature sensor should be placed between the oil return bend and the evaporator, and it is best to place it in the middle or upper side of the side to allow the expansion valve. The opening degree is consistent with the return air superheat;
3 The closer the distance between the liquid supply solenoid valve and the evaporator, the better. After the solenoid valve is closed, a large amount of refrigerant is still supplied to the evaporator. 4 After the exhaust pipe is discharged from the oil, it is better to make an inverted U-bend. To avoid long-term shutdown, the condensed liquid falls back from the exhaust pipe and enters the compressor.
5 For long-term shutdowns, power up the compressor at least 48 hours in advance when the machine is turned on, allowing the crankcase heater to evaporate the liquid refrigerant that may be present. And when you start the machine, you should observe and record the oil level.