The core problem of compressor oper
Common oil shortage failures of scroll refrigeration compressors
Common oil shortage failures of scroll compressors
The compressor is short of oil for a long time-the mechanism and the friction pairs are overheated, causing the bearings to sinter and hold the shaft.
The compressor is short of oil for a short time-the mechanism and the friction pairs are abnormally worn, causing vibration and noise.
How to ensure the proper amount of oil
When the compressor discharges refrigerant, it also discharges a trace amount of refrigerating machine oil. Even if the oil rate is only 0.5%, if the oil cannot circulate back to the compressor through the system. Taking 5HP as an example, the circulation volume is about 330kg/h under ARI conditions, and all the oil in the compressor can be taken out in 50 minutes, and the compressor will burn out in about 2 to 5 hours. Therefore, in order to ensure that the compressor runs without oil shortage, the following two aspects should be started:
1. Ensure that the refrigerating oil discharged from the compressor returns to the compressor.
2.Reduce the oil rate of the compressor. (Frequent starting of the compressor is not conducive to oil return.)
How to ensure that the discharged refrigerating oil returns to the compressor
1. The flow rate of the refrigerant in the suction pipe (about 6m/s) should be ensured to allow the oil to return to the compressor, but the z*high flow rate should be less than 15m/s to reduce pressure drop and flow noise. There is a downward slope along the direction of refrigerant flow, about 0.8cm/m.
2. Prevent refrigerating machine oil from staying in the evaporator.
3. Ensure that the oil return hole of the gas-liquid separator is too large to cause wet compression, and too small will cause insufficient oil return, and stagnant oil in the gas-liquid separator.
4. There should be no parts in the system where oil will stay.
5. Ensure that there is enough refrigerating oil in the compressor under the condition of long piping and high drop, usually confirm with a compressor with an oil mirror.
How to reduce the oil rate of the compressor
1. Make sure that the refrigerant does not dissolve into the refrigerating machine oil when shutting down (use crankshaft heater).
2, excessive wet operation should be avoided, because excessive oiling caused by foaming.
3. An oil separator device is installed inside.
4. The oil inside the compressor foams so that the oil is easily taken out of the compressor.
Long piping height drop
When the piping length is larger than the allowable value, the pressure loss in the piping will increase, reducing the amount of refrigerant in the evaporator, resulting in a decrease in capacity. At the same time, when oil is trapped in the piping, the compressor is short of oil, leading to compressor failure. When the refrigerating machine oil in the compressor is insufficient, the refrigerating machine oil of the same grade as the compressor factory should be added from the high-pressure side.
The necessity of setting the return bend
When the drop is more than 10m~15m, an oil return elbow should be installed on the air pipe side.
Necessity: When stopping, avoid the refrigerating machine oil adhering to the piping from returning to the compressor, causing liquid compression. On the other hand, in order to prevent poor oil return in the air pipe, the compressor lacks oil.
Oil return curve setting interval: set one oil return curve every 10m drop.
How to ensure proper refrigerator oil viscosity
1. The refrigerating machine oil and the refrigerant are mutually soluble. When the machine stops, the refrigerant is almost completely dissolved in the refrigerating machine oil. Therefore, a crankshaft heater must be installed to prevent dissolution.
2. The refrigerant containing liquid should not be returned to the compressor during operation, that is, to ensure that the compressor suction has superheat.
3. When starting and defrosting, there should be no liquid back phenomenon.
4. Avoid running under excessive heat to avoid oil deterioration.
5. The size of the oil return hole of the gas-liquid separator should be appropriate:
①The hole diameter is too large to suck in liquid refrigerant and cause excessive humidity operation
②If the hole diameter is too small, the oil return will not be smooth and the oil will stay in the gas-liquid separator
The main reason for compressor motor damage
1. Abnormal load and locked rotor.
2. Short circuit of winding caused by metal shavings.
3. Contactor problem.
4. The power supply lacks phase and voltage is abnormal.
5. Insufficient cooling.
6. Vacuum with a compressor.
The main cause of abnormal load or stall
Excessive pressure ratio or excessive pressure difference will make the compression process more difficult; and the increase in friction resistance caused by lubrication failure and the motor blockage in extreme cases will greatly increase the motor load. If the load increases until the thermal protection is activated and the protection is automatically reset, it will enter the infinite loop of "blocked rotor-thermal protection-blocked rotor". Frequent starts and abnormal loads will cause the winding to withstand high temperatures and reduce the enameled wire Insulation performance. After the winding insulation performance deteriorates, if there are other factors (such as metal chips forming a conductive loop, acid lubricating oil, etc.), it is easy to cause a short circuit and damage.
winding short circuit caused by metal shavings
The sources of metal shavings include copper pipe shavings left during construction, welding slag, internal wear of the compressor, and metal shavings dropped when parts are damaged. When working, driven by the airflow, these metal shavings or particles will fall on the winding. The normal vibration when the compressor is running, and the twisting of the windings by electromagnetic force every time it starts, will promote the relative movement and friction between the metal shavings contained in the windings and the winding enameled wire. Sharp metal shavings can scratch the enameled wire insulation layer, cause a short circuit, and cause the motor to burn out.
Contactor problem
In order to be safe and reliable, the compressor contactor should disconnect the three-phase circuit at the same time. The contactor must be able to meet harsh conditions such as fast 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 starting or stalling. Otherwise, after the contactor contacts are welded, all controls (such as high and low pressure control, temperature control, defrost control, etc.) that rely on the contactor to disconnect the compressor power circuit will all fail, and the compressor will be in an unprotected state. Therefore, when the motor is burned out, checking the contactor is an essential process.
Power phase loss and abnormal voltage
The power supply voltage variation range cannot exceed ±10% of the rated voltage. The voltage unbalance between the three phases cannot exceed 3%. If the compressor is running when a phase loss occurs, it will continue to run but there will be a large load current. The motor windings will overheat very quickly, and the compressor will be thermally protected under normal circumstances. When the motor windings are cooled to the set temperature, the contactor will be closed, but the compressor will not start up, a locked rotor will appear, and a "locked rotor-thermal protection-locked rotor" endless loop will enter. If the compressor is started due to lack of phase, the compressor will not start, and it will be locked, and enter a "locked rotor-thermal protection-locked rotor" endless loop. The voltage unbalance percentage calculation method is the ratio of the z* maximum deviation value of the phase voltage and the average value of the three-phase voltage to the average value of the three-phase voltage. As a result of the voltage unbalance, the unbalance of the load current during normal operation is the voltage unbalance 4-10 times the percentage point.
Insufficient cooling of compressor motor
A large amount of refrigerant leakage or low evaporation pressure will cause the system mass flow to decrease, making the motor unable to get good cooling, and frequent protection will occur after the motor is overheated.
The main reason for the damage of compressor liquid hammer--liquid return
Liquid return can easily cause a liquid shock accident. Even if there is no liquid shock, the liquid return of the high-pressure chamber structure will dilute or wash away the lubricating oil on the sliding surface, and aggravate the wear. The liquid return 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 a low viscosity and cannot form a sufficient oil film on the friction surface, resulting in rapid wear of moving parts. In addition, the refrigerant in the lubricating oil will boil when heated during transportation, which affects the normal delivery of lubricating oil. The farther away from the oil pump, the more obvious and serious the problem. If the bearing at the motor end is severely worn, the crankshaft may sink to one side, which may easily cause the stator bore and the motor to burn out.
For refrigeration systems where liquid return is difficult to avoid, the installation of a gas-liquid separator and the use of pump-down shutdown control can effectively prevent or reduce the harm of liquid return.
The main cause of compressor damage due to liquid shock--starting with liquid
Bubbling can be clearly observed on the oil sight glass when starting with liquid. The root cause of starting with liquid is that a large amount of refrigerant dissolved in the lubricating oil and sinking under the lubricating oil suddenly boils when the pressure drops suddenly and causes the lubricating oil to bubble. The refrigerant that starts with liquid enters the crankcase by means of "refrigerant migration". Because the partial pressure of refrigerant vapor in the lubricating oil is low, it will absorb the refrigerant vapor on the oil surface, causing the phenomenon that the air pressure in the oil pool is lower than the air pressure of the evaporator. The lower the oil temperature and the lower the vapor pressure, the greater the absorption power of refrigerant vapor. The steam in the system will slowly "migrate" to the compressor. The longer the downtime, the more refrigerant will migrate into the lubricant. The migration of refrigerant will dilute the lubricating oil. It is also easy to cause liquid shock to the low pressure chamber. Liquid refrigerant or the mixture of oil and refrigerant is not a good lubricant, and will cause wear and even seizure. At this time, because the motor is immersed in liquid, the overload protector on the motor will not act. The installation of crankcase heater, gas-liquid separator and the use of pump-down shutdown control can effectively prevent or reduce refrigerant migration.
The main cause of compressor damage due to hydraulic shock--too much lubricating oil
For low-pressure cavity compressors, high-speed rotating parts such as rotors will frequently hit the oil surface. If the oil surface is too high, a large amount of lubricating oil will splash. Once the splashed lubricating oil enters the intake duct and is brought into the cylinder, it may cause liquid shock.
The main reason for high temperature damage of compressor
The overheating phenomenon of the motor, the exhaust temperature is too high, and the lubricating oil burnt due to problems such as over-range use, abnormal power supply, motor overload, refrigerant leakage, and too high condensation pressure. The surface temperature of the compressor is one of the important indicators for judging whether the compressor is overheated. If the surface temperature exceeds 135°C, it is generally considered that the compressor has been seriously overheated; and if the surface temperature is lower than 120°C, the compressor temperature is normal. The motor generates a large amount of heat. Abnormal power supply will cause the motor to generate more heat. For example, the voltage is unstable, the voltage is too low or too high, the voltage is unbalanced, and the lack of equivalence all belong to the abnormal power supply.
Problems such as frequent compressor starting, connecting rod holding shaft, piston biting cylinder, insufficient lubrication or lack of oil will greatly increase the heat generation. Excessive use of the compressor can easily cause overheating and damage to the motor, and insufficient motor cooling. Low evaporating temperature and low refrigerant mass flow result in insufficient motor cooling. When the refrigerant leakage is relatively large, the refrigerant mass flow will also be small, resulting in insufficient motor cooling.
The main reason for excessive exhaust temperature
The main reasons for the overheating of the exhaust temperature are as follows: high return air temperature, large heating capacity of the motor, high compression ratio, high condensation pressure, and improper refrigerant selection.
