At present, many units on the market mix refrigerated oil with refrigerant, and use an oil separator to separate the refrigerant from the refrigerant, and then return the refrigerant to the compressor through the flooded evaporator. This model has a good cooling effect, but there is a problem in its operation, that is, it is easy to "run out of oil". That is, when the cooling water temperature is low, the exhaust superheat is not high, so that the oil and the refrigerant are not completely separated, and the mixture enters the heat exchanger, resulting in low pressure of the unit, and even worse, it will lead to the blockage of the expansion valve and cause the unit to fail to start.
What is the reason for the phenomenon of oil running out of refrigeration units? How much of an impact will it have? And how do we deal with it? Today, I will take you to find out.
01
What is the phenomenon of running oil?
1. There is no trace of oil in the oil separator, the oil is not collected in the condenser at this moment, but enters the evaporator through the expansion valve, and the oil will stick to the evaporator heat exchange copper pipe, so that the evaporation effect of the evaporator is not good, resulting in low pressure in the evaporator, and at the same time, the suction superheat of the compressor is very low and even directly inhales the refrigerant liquid, the compressor exhaust temperature is very low, and the oil and the refrigerant are still unable to separate, so a vicious circle will make all the oil collect in the evaporator. The compressor made a loud noise due to insufficient oil supply, and even the oil level alarm of the oil separator of the unit stopped the machine.
2. There is no trace of oil in the oil separator, all the oil comes to the condenser with the exhaust, if the water temperature continues to be low, then all the oil will be collected into the condenser, and the expansion valve is blocked, and the pressure in the evaporator begins to decrease due to insufficient liquid supply, until the low pressure alarm stops.
02
What are the effects of poor oil return?
When the oil film increases by 0.1mm, it will directly affect the cooling of the system, on the other hand, the machine can not run due to lack of oil, and it needs to be continuously replenished with lubricating oil, which will lead to the accumulation of more and more lubricating oil in the system, resulting in a vicious circle, increasing operating costs and reducing operational reliability. In general, it is permissible for the oil-gas mixture to circulate in the system with a refrigerant gas flow rate of less than 1%.
03
The way the compressor returns the oil
There are two ways to return oil to the compressor: oil return from the oil separator and oil return from the return pipe.
It is not uncommon for cold storage refrigeration systems with particularly long pipelines, flooded ice-making systems and freeze-drying equipment with very low temperatures, etc., to not return oil for more than ten minutes or even dozens of minutes after start-up, or the amount of oil return is very small, and the problem of compressor oil pressure is too low and shut down in a poorly designed system. The installation of a high-efficiency oil separator can greatly extend the operating time of the compressor without oil return, so that the compressor can safely pass the crisis stage of no oil return after start-up.
The refrigeration compressor starts frequently, which is not conducive to oil return. Due to the short continuous operation time, the compressor stopped, and the return air line did not have time to form a stable high-speed air flow, so the lubricating oil had to remain in the pipeline. If the return oil is less than the incoming oil, the compressor will run out of oil. The shorter the run time, the longer the pipeline, and the more complex the system, the more prominent the oil return problem becomes.
In addition, if the temperature of the chilled water is low and the exhaust temperature is low, it will also cause difficulties in oil return, in which case the cold refrigerated oil is generally in the evaporator. Look inside the evaporator sight glass to see if there is a large amount of white foam tumbling, if so, the oil is in the evaporator, and vice versa, it may be in the condenser.
04
The treatment method of oil running in the refrigeration unit
Oil inside the evaporator:
At this time, pay close attention to the temperature of the chilled water, you can first reduce its flow rate, let the water temperature decrease, and then quickly increase the flow rate, so that the evaporator water temperature rises rapidly, the water temperature will exceed the saturation temperature of the refrigerant a lot, so that the refrigerant boils violently, and the refrigerant oil will be sucked into the compressor with the tumbling refrigerant foam, at this time the exhaust temperature should drop some, but it should also far exceed the separation temperature of oil and refrigerant, so as to bring the oil back to the oil separator and separate them. After repeating this several times, the oil will all be retracted into the oil separator. Here pay attention to the time interval of each adjustment of the water volume, it is best to make the exhaust temperature rise to the best before the adjustment after each adjustment.
Oil inside the condenser:
At this time, there is no need to pay too much attention to the water temperature, as long as the unit can operate normally, there will be no major problems. At this time, the expansion valve should be gradually opened to the maximum (the opening process is not too fast to prevent the amount of liquid supplied from damaging the copper pipe in the evaporator), wait for a few minutes, until the high and low pressure of the unit is balanced, then you can see a large amount of white foam in the sight glass of the evaporator, which means that the oil has entered the evaporator from the condenser. Just repeat step A and the oil will be quickly recovered.
In addition, we also need to maintain the oil circuit system regularly, and maintenance measures such as cleaning the oil filter in time and replacing wearing parts in time will also help to return the oil.