What is the cause of poor oil return of screw chiller?

What is the cause of poor oil return of screw chiller?

If the screw chiller oil return is not smooth, it is a very headache. Generally speaking, the screw chiller oil return is not smooth mainly because of the gas mixture of lubricating oil and refrigerant in the process of operation. Specifically, in the operation of the refrigeration system, the lubricant in the refrigeration compressor will be discharged into the condenser with the operation of the machine and the refrigerant in the form of gas mist and droplets. If the oil separator is not effective or the system is not well designed, the separation effect will be poor and the system oil return is not smooth. Of course, you didn't use fake oil.

What are the effects of poor oil return of screw chiller?

Screw chillers in poor oil return, can lead to appear a large number of lubricating oil retention in the evaporator pipes, when the oil film increased by 0.1 mm will directly affect the cooling system, on the other hand machine oil shortage cannot run, need to constantly add lubricating oil, and this leads to the oil in the system, create a vicious cycle, increase the operation cost, lower reliability. Under normal circumstances, the refrigerant gas flow rate is less than 1% of the oil and gas mixture circulation in the system is allowed.

How to solve the problem of poor oil return of screw chiller?

Compressor oil return has two ways, one is the oil separator oil return, the other is the return pipe oil return. The oil separator is installed on the exhaust pipe of the compressor, and can generally separate 50-95% of the running oil. The oil return effect is good, the speed is fast, and the amount of oil entering the system pipeline is greatly reduced, so as to effectively extend the operation time without returning oil. It is not uncommon for the cold storage refrigeration system with particularly long pipelines, the full liquid ice system and the freeze-drying equipment with very low temperature to return oil or return oil for more than ten minutes or even dozens of minutes after the startup. The bad design of the system will appear the problem of compressor oil pressure being too low and shutdown. The high efficiency oil separator installed in the refrigeration system can greatly prolong the operation time of the compressor without oil return, so that the compressor can safely pass the crisis stage without oil return after starting up. The unseparated lubricating oil will enter the system and flow with the refrigerant in the tube, forming an oil cycle. After the lubricating oil enters the evaporator, on the one hand, due to low temperature and small solubility, part of the lubricating oil is separated from the refrigerant; On the other hand, the temperature is low and the viscosity is large, so the separated lubricating oil is easy to adhere to the inner wall and the flow is difficult. The lower the evaporation temperature, the more difficult the oil return. This requires evaporation pipeline design and return pipe design and construction must be conducive to oil return, the common approach is to use descending pipeline design, and ensure a large airflow speed. For especially low temperature refrigeration system, in addition to the selection of efficient oil separator, usually also add special solvent, prevent lubricating oil blocking capillary and expansion valve, and help oil return. At the same time, someone uses the built-in oil of air conditioner to replace the external oil, which saves costs on the surface, but will only greatly increase operating costs from the long-term use of the system. The system becomes less and less efficient.

In practical application, the oil return problem caused by improper design of evaporator and return pipe is not rare. For R22 and R404A systems, full evaporator oil return is very difficult, system return tubing design must be very careful. For such a system, the use of high efficiency oil can greatly reduce the amount of oil into the system pipeline, effectively extend the return pipe no oil return time after starting.

When the compressor is higher than the evaporator, a vertical return bend on the return pipe is required. Oil return bend should be as compact as possible to reduce oil storage. The spacing between oil return turns should be appropriate. If there are many oil return turns, some lubricating oil should be added. The return line of the variable load system must also be careful. When the load reduction is small, the gas return speed will decrease, too low speed is not conducive to oil return. To ensure oil return under low load, vertical suction lines can be used with double risers.

And the frequent start of the compressor is not conducive to oil return. Because the continuous operation time is very short, the compressor stops, and there is no time to form stable high-speed air flow in the return pipe, so the lubricating oil can only stay in the pipeline. If the return oil is less than the rush oil, the compressor will be short of oil. The shorter the operation time, the longer the pipeline, and the more complex the system, the greater the oil return problem. Therefore, in general, do not frequently start the compressor.

In the end, lack of oil will cause serious insufficient lubrication, the root cause of lack of oil is not the screw chiller running oil and speed, but the system oil return is not good. The installation of oil separator can quickly return oil and prolong the operation time of compressor without oil return. The evaporator and return pipe must be designed with oil return in mind. Maintenance measures such as avoiding frequent startup, scheduled defrosting, timely replenishment of refrigerant, and timely replacement of wearing parts (such as bearings) are also conducive to oil recovery.