When the compressor discharges the refrigerant, it will also discharge a small amount of refrigerant oil. Even if the oil filling rate is only 0.5%, if the oil cannot be circulated back to the compressor through the system. Taking 5HP as an example, if the circulation volume is about 330kg / h under ARI working condition, all the oil in the compressor can be brought out in 50 minutes, and the compressor will burn out in about 2 ~ 5 hours. Therefore, in order to ensure that the compressor does not run out of oil, it should start from the following two aspects:

1. Ensure that the refrigerant oil discharged from the compressor returns to the compressor.

2. Reduce the oil rate of the compressor. (frequent compressor startup is not conducive to oil return.)

2、 How to ensure that the discharged refrigerant oil returns to the compressor

1. Ensure the flow rate of refrigerant in the suction pipe (about 6m / s) to make the oil return to the compressor, but the maximum flow rate shall be less than 15m / s to reduce pressure drop and flow noise. For the horizontal pipe, there shall also be a downward slope along the refrigerant flow direction, about 0.8cm/m.

2. Prevent refrigerant oil from staying in the evaporator.

3. Ensure the proper oil return hole of the gas-liquid separator. If it is too small, it will cause wet compression. If it is too small, there will be insufficient oil return, and the stagnant oil will be in the gas-liquid separator.

4. There shall be no part in the system that can make the oil stay.

5. Ensure that there is enough refrigerant oil in the compressor under the condition of high drop of long piping, which is usually confirmed by the compressor with oil level mirror.

3、 How to reduce the oil rate of compressor

1. During shutdown, ensure that the refrigerant does not dissolve into the refrigerant oil (use the crankshaft heater).

2. Avoid over wet operation because it will cause too much oil due to foaming.

3. An oil separator device is set inside.

4. Oil bubbles inside the compressor make it easy for oil to be taken out of the compressor.

4、 Height drop of long piping

When the allowable value of pipe length ratio is large, the pressure loss in the pipe will increase, reducing the amount of refrigerant in the evaporator and reducing the capacity. At the same time, when there is oil retention in the piping, the compressor is short of oil, resulting in compressor failure. When the refrigerant oil in the compressor is insufficient, the refrigerant oil of the same brand as that of the compressor shall be added from the high-pressure side.

5、 Necessity of setting oil return bend

When the drop exceeds 10m ~ 15m, an oil return elbow shall be set at the gas pipe side.

Necessity: during shutdown, avoid returning the refrigerant oil attached to the piping to the compressor, causing liquid compression. On the other hand, in order to prevent poor oil return of air pipe from causing oil shortage of compressor.

Setting interval of oil return bend: one oil return bend is set every 10m drop.

6、 How to ensure proper refrigerant oil viscosity

1. Refrigeration oil and refrigerant are miscible. When shutdown, almost all the refrigerant is dissolved in refrigeration oil. Therefore, crankshaft heater needs to be installed to prevent dissolution.

2. During operation, the refrigerant containing liquid shall not return to the compressor, that is, ensure that the suction of the compressor has superheat.

3. There shall be no liquid return during starting and defrosting.

4. Avoid operating under excessive overheating and oil deterioration.

5. The size of oil return hole of gas-liquid separator shall be appropriate:

① Too large aperture will suck in liquid refrigerant and cause too wet operation

② If the hole diameter is too small, the oil return will not be smooth and the oil will remain in the gas-liquid separator

7、 Main causes of compressor motor damage

1. Abnormal load and locked rotor.

2. Short circuit of winding caused by metal chips.

3. Contactor problem.

4. Lack of phase and abnormal voltage of power supply.

5. Insufficient cooling.

6. Vacuum with compressor.

8、 Main causes of abnormal load or locked rotor

If the pressure ratio is too large or the pressure difference is too large, the compression process will be more difficult; The increase of friction resistance caused by lubrication failure and motor locked rotor in extreme cases will greatly increase the motor load. If the load increases to the thermal protection action and the protection is automatically reset, it will enter the dead cycle of "locked rotor thermal protection locked rotor", frequent startup and abnormal load will make the winding withstand the high temperature test, which will reduce the insulation performance of enameled wire. After the insulation performance of the winding becomes poor, if there are other factors (such as metal chips forming a conductive circuit, acid lubricating oil, etc.) to cooperate, it is easy to cause short circuit and damage.

1. Winding short circuit caused by metal chips

The sources of metal scraps include copper pipe scraps left during construction, welding slag, metal scraps dropped when the compressor is worn and parts are damaged, etc. During operation, driven by the air flow, these metal chips or fragments will fall on the winding. The normal vibration during the operation of the compressor and the twisting of the winding under the action of electromagnetic force during each startup will promote the relative movement and friction between the metal chips mixed between the windings and the winding enamelled wire. Sharp metal chips will scratch the insulating layer of enameled wire, cause short circuit and burn the motor.

2. Contactor problem

For safety and reliability, the compressor contactor shall disconnect the three-phase circuit at the same time. Contactors must be able to meet harsh conditions such as rapid cycling, continuous overload and low voltage. They must have a large enough area to dissipate the heat generated by the load current, and the contact material must be selected to prevent welding under high current conditions such as startup or locked rotor. Otherwise, after the contactor contacts are welded, all controls that rely on the contactor to disconnect the compressor power circuit (such as high and low voltage control, temperature control, defrosting control, etc.) will fail, and the compressor will be in an unprotected state. Therefore, when the motor is burned out, checking the contactor is an essential process.

3. Lack of phase and abnormal voltage of power supply

The variation range of power supply voltage shall not exceed ± 10% of rated voltage. The voltage imbalance between three phases shall not exceed 3%. If the compressor is running when phase loss occurs, it will continue to run, but there will be a large load current. The motor winding will overheat quickly, and the compressor is thermally protected under normal conditions. When the motor winding cools to the set temperature, the contactor will close, but the compressor cannot start, locked rotor occurs, and enters the dead cycle of "locked rotor thermal protection locked rotor". In case of phase loss, the compressor will not start up and locked rotor will enter the "locked rotor thermal protection locked rotor" dead cycle. The calculation method of voltage unbalance percentage is the ratio of the maximum deviation between the average value of phase voltage and three-phase voltage to the average value of three-phase voltage. As a result of voltage imbalance, the imbalance of load current is 4-10 times of the percentage of voltage imbalance in normal operation.

4. Insufficient cooling of compressor motor

A large amount of refrigerant leakage or low evaporation pressure will reduce the mass flow of the system, so that the motor can not be well cooled, and frequent protection will occur after the motor is overheated.

9、 The main cause of compressor liquid hammer damage -- liquid return

Liquid return can easily lead to liquid hammer accidents. Even if no liquid hammer is caused, the return liquid of the high-pressure chamber structure will dilute or wash away the lubricating oil on the sliding surface and aggravate wear. The return liquid of the low-pressure chamber structure will dilute the lubricating oil in the oil pool. Lubricating oil containing a large amount of liquid refrigerant has low viscosity and can not form enough oil film on the friction surface, resulting in rapid wear of moving parts. In addition, the refrigerant in the lubricating oil will boil when it meets heat during transportation, affecting the normal transportation of lubricating oil. The farther away from the oil pump, the more obvious and serious the problem is. If the bearing at the motor end is seriously worn, the crankshaft may settle to one side, which is easy to cause stator chamber sweeping and motor burning.

For the refrigeration system where liquid return is difficult to avoid, the installation of gas-liquid separator and evacuation shutdown control can effectively prevent or reduce the harm of liquid return.

10、 The main cause of compressor liquid hammer damage -- starting with liquid

Bubbles can be clearly observed on the oil sight glass when starting with liquid. The root cause of liquid start-up is that a large amount of refrigerant dissolved in the lubricating oil and sunk under the lubricating oil suddenly boils when the pressure suddenly decreases, and causes the foaming of the lubricating oil. The refrigerant with liquid start-up enters the crankcase in the way of "refrigerant migration". Because the refrigerant vapor partial pressure in the lubricating oil is low, it will absorb the refrigerant vapor on the oil surface, resulting in the phenomenon that the air pressure in the oil pool is lower than that of the evaporator. The lower the oil temperature and the lower the steam pressure, the greater the absorption of refrigerant steam. The steam in the system will slowly "migrate" to the compressor. The longer the shutdown time, the more refrigerant will migrate into the lubricating oil. Refrigerant migration will dilute the lubricating oil. It is also easy to cause liquid hammer to the low-pressure chamber. Liquid refrigerant or the mixture of oil and refrigerant is not a good lubricant, which will cause wear and even jamming. At this time, because the motor is immersed in liquid, the overload protector on the motor will not act. Installation of crankcase heater, gas-liquid separator and evacuation shutdown control can effectively prevent or reduce refrigerant migration.

11、 The main cause of compressor liquid hammer damage -- too much lubricating oil

For low-pressure chamber compressor, high-speed rotating parts such as rotor will frequently impact the oil level. If the oil level is too high, a large amount of lubricating oil will splash. Once the splashed lubricating oil flows into the air inlet and into the cylinder, it may cause liquid hammer.

12、 Main causes of compressor high temperature damage

Overheating phenomena such as high temperature of motor, high exhaust temperature and burnt lubricating oil caused by out of range use, abnormal power supply, motor overload, refrigerant leakage and too high condensation pressure. Compressor surface temperature is one of the important indexes to judge whether the compressor is overheated. If the surface temperature exceeds 135 ° C, it is generally considered that the compressor has been seriously overheated; If the surface temperature is lower than 120 ° C, the compressor temperature is normal. The heating capacity of the motor is large, and the abnormal power supply will cause the heating capacity of the motor to increase. For example, unstable voltage, too low or too high voltage, unbalanced voltage and lack of power are all abnormal power supply.

Frequent start-up of the compressor, shaft holding of the connecting rod, cylinder biting of the piston, insufficient lubrication or lack of oil will greatly increase the calorific value. Using the compressor beyond the range is easy to cause overheating and damage of the motor and insufficient cooling of the motor. Low evaporation temperature and low refrigerant mass flow lead to insufficient cooling of the motor. When the refrigerant leakage is large, the refrigerant mass flow will also be small, resulting in insufficient cooling of the motor.

13、 Main reasons for excessive exhaust temperature

The main reasons for overheating of exhaust temperature are as follows: high return air temperature, large heating capacity of motor, high compression ratio, high condensation pressure and improper selection of refrigerant.

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