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In general, the reason for the poor oil return of the water-cooled screw chiller is mainly due to the phenomenon of gas mixture of lubricating oil and refrigerant during operation, specifically in the operation of the refrigeration system, encountering refrigerant and freezing. The mutual solubility of the lubricating oil will cause the lubricating oil in the refrigeration compressor to operate and the refrigerant to be discharged into the condenser in the form of gas mist and droplets. If the oil separator is not effective or the system design is not good, the separation effect will be poor and the system will not return smoothly.
01
What is the impact of water-cooled screw chillers on poor oil return?
The screw chiller has poor oil return, which will cause a large amount of lubricating oil to stay in the evaporator pipe. When the oil film is increased by 0.1mm, it will directly affect the system cooling.
In addition, the machine can't run out of oil, and it needs to constantly replenish the lubricating oil, which will cause the lubricating oil in the system to accumulate more, resulting in vicious cycle, increasing operating costs and reducing operational reliability. In general, it is permissible to circulate the oil and gas mixture in the system with a refrigerant gas flow rate of less than 1%.
02
How to solve the problem of poor oil return of water-cooled screw chillers?
There are two ways to return oil to the compressor. One is to return the oil to the oil separator, and the other is to return the oil to the return line.
The oil separator is installed on the exhaust pipe of the compressor, generally can separate 50-95% of the oil, the oil return effect is good, the speed is fast, and the amount of oil entering the system pipeline is greatly reduced, thereby effectively prolonging the oil-free operation. time.
It is not unusual to have a particularly long refrigeration system, a full-liquid ice-making system, and a lyophilized equipment with a very low temperature. It is not unusual to return oil or return a very small amount of oil after ten minutes or even tens of minutes after starting the machine. The system will have the problem that the compressor oil pressure is too low and the machine is shut down. The installation of the high-efficiency oil separator in this refrigeration system can greatly extend the non-return operation time of the compressor, and make the compressor safety pass the crisis stage without oil return after starting the machine.
Lubricating oil that has not been separated will enter the system and flow with the refrigerant in the tube to form an oil circulation. After the lubricating oil enters the evaporator, on the one hand, the solubility is low due to low temperature, and a part of the lubricating oil is separated from the refrigerant; on the other hand, the temperature is low and the viscosity is large, and the separated lubricating oil is likely to adhere to the inner wall of the pipe, and the flow is difficult. The lower the evaporation temperature, the more difficult it is to return oil. This requires that the design of the evaporative piping and the design and construction of the return line must be beneficial for oil return. A common practice is to use a downcomer design and ensure a large air flow rate.
For refrigeration systems with extremely low temperatures, in addition to high-efficiency oil separators, special solvents are usually added to prevent the oil from plugging the capillary and expansion valve and helping to return oil. At the same time, some people use the built-in oil of the air conditioner to replace the external oil, which saves the cost on the surface, but in terms of the long-term use cost of the system, it will only greatly increase the operating cost, and the efficiency of the system will become worse.
In practical applications, oil return problems caused by improper design of evaporators and return lines are not uncommon. For the R22 and R404A systems, the oil return of the flooded evaporator is very difficult and the system return line design must be very careful. For such a system, the use of high-efficiency oil can greatly reduce the amount of oil entering the system piping, effectively extending the return time of the return air pipe after starting.
When the compressor is positioned higher than the evaporator, the oil return bend on the vertical return line is essential. The oil return bend should be as compact as possible to reduce oil deposits. The spacing between the oil return bends should be appropriate. When the number of oil return bends is relatively large, some lubricants should be added. The return line of the variable load system 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.
Moreover, the 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. If the oil is less than the oil, the compressor will be short of oil. The shorter the running time, the longer the pipeline, the more complicated the system, and the more serious the oil return problem. Therefore, in general, do not start the compressor frequently.
In short, lack of oil can cause serious lubrication shortage. The root cause of lack of oil is not how much and how fast the screw chiller runs, but the system does not return oil well. The installation of a high-efficiency oil separator can quickly return oil and extend the compressor's return-free operation time. The design of the evaporator and return line must take into account the return oil. Avoid frequent starts, timed defrosting, timely replenishment of refrigerants, and timely replacement of consumables (eg, bearings) and other maintenance measures can also help return oil.
As a process refrigeration expert, the water-cooled screw chiller developed by Shanghai SUNNY-HVAC has been designed with rigorous, professional and efficient quality, which effectively avoids the problem of poor oil return due to product system problems, and provides customers with high quality and quality. Guarantee!