Refrigeration Repair Techniques – An Overview of Compressor Classification and Fault Analysis

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Refrigeration repair technology – An overview of compressor classifications and fault analysis. The compressor is the core component of a refrigeration system; it is through this component that electrical energy is converted into mechanical power, allowing the low-temperature, low-pressure gaseous refrigerant to be compressed into high-temperature, high-pressure gas, thereby ensuring the proper operation of the refrigeration cycle. I. Positive-displacement type: Compresses a fixed amount of gas drawn in periodically by changing the volume of the working chamber. 1. Reciprocating piston type: The working volume of the cylinder is changed by the reciprocating motion of the piston. Based on the external structure, they are classified as: ① Fully enclosed: With a cooling capacity of less than 60 KW, these are commonly used in air conditioners and small refrigeration devices. The drive motor and moving components are enclosed in the same space, resulting in a compact structure, good sealing performance, and low noise. But it has low power and is difficult to repair. Common brands include: Fa Mei Brazil, Taikang; Fa Mei Youle; Mei Gu Lun, Bristol; Danish Danfoss; Italian Embraer, Electrolux; Japanese Hitachi, Panasonic, Toshiba, Sanyo, Mitsubishi, DAKIN; Korean LG; Chinese Chunlan, and others. ② Semi-hermetic: Cooling capacity of 60–600 KW, suitable for use in various air conditioning and refrigeration systems. It forms a sealed space consisting of the crankcase and the motor housing; it operates stably with a long service life, has a high cooling capacity, can be used in various operating conditions, is repairable, but produces slightly higher noise levels. They are divided into single-stage compression types (conventional type, disc valve type, unloading type, connected type) and two-stage compression types. Common brands include Meide, Gulong, Debizer, Bok, Grasso; in Italy, Fujihao and Leifukang; in Japan, Mitsubishi, Hitachi, and Sanyo; in China, Taizhou Xuemei, Dalian Bing Shan, Nanjing Wuzhou, and others. ③ In the open-type design, the compressor and the motor are two separate devices that are connected externally; this results in a complex and bulky structure as well as unstable operation, and such designs are now nearly obsolete. 2. Rotary type: The working volume of the cylinder is changed by the rotational motion of a rotating body. Classified by internal structure: ① Rotary rotor type: Cooling capacity of 8–12 KW, commonly used in small air conditioners and cooling equipment. It is fully enclosed, with a compact structure, good sealing performance, and low noise levels. But it has low power and is difficult to repair. Common brands: Japanese Mitsubishi, Hitachi, Panasonic, Sanyo, Toshiba; Chinese brands such as Qingan and Huangshi Dongbei. ② Vortex type: Cooling capacity of 8–150 KW, suitable for use in various air conditioning and refrigeration systems. It is fully enclosed, features a simple and compact structure, high operational performance, excellent sealing, and low noise levels, making it the dominant model in the future. Common brands: Midea, Gulliver; France’s Meuleux; Japan’s Hitachi, Panasonic, Daikin, Sanyo; China’s Chunlan, etc. ③ Screw type: Cooling capacity of 100–1200 KW, suitable for large and medium-sized air conditioning and refrigeration systems. It is semi-hermetic, compact in structure, features high operational performance and large cooling capacity with stepless adjustment possible; however, its lubrication system is complex and it generates relatively high noise. It is divided into single-screw and twin-screw types. Common brands include: German Beitzel, Grasshoff; Italian Fuji Hao, Lefkon, Doori; Japanese Hitachi, Daikin, Mitsubishi Heavy Industries, Shinko; Korean brands; foreign brands; Taiwanese brands such as Fusheng and Hanbell; as well as Chinese brands like Chongqing Jialing and Dalian Bingshan. II. Centrifugal type: The inhaled gas is continuously compressed by the force of centrifugal force. The cooling capacity can reach up to 30,000 KW, and it is used in large-scale air conditioning and refrigeration systems. Stable operation, high performance and long lifespan, large cooling capacity, with stepless adjustment possible. Common brands include TRANE and CARRIER from the United States, YORK and MCQUAY from the United States as well, MITSUBISHI, HITACHI, and EBARA from Japan, SULZER from Switzerland, as well as manufacturers from South Korea and China. Analysis of Common Compressor Failures (1) – Motor Burnout. The failures of motor-compressors (hereinafter referred to as compressors) can be divided into motor-related failures and mechanical failures (including the crankshaft, connecting rods, pistons, valve plates, cylinder head gaskets, etc.). Mechanical failures often cause motors to operate under overload or even stall, which is one of the main reasons for motor damage. Damage to motors is mainly manifested as damage to the insulation layer of the stator windings (short circuits) and open circuits. Damage to the stator windings is difficult to detect in a timely manner, which can ultimately lead to the windings burning out. After the winding burns out, it masks some of the phenomena or direct causes that led to the burnout, making post-event analysis and root cause investigation more difficult. However, the operation of a motor relies on a proper power supply, a reasonable motor load, good heat dissipation, and protection for the insulation layer of the winding enamel wire. By examining these aspects, it is not difficult to see that the reasons for winding burnout are essentially one of the following six: (1) abnormal load and stall ； (2) Winding short circuit caused by metal shavings ； (3) Contactor issues ； (4) Power supply phase loss and voltage abnormalities ； (5) Insufficient cooling ； (6) Evacuate using a compressor. In fact, motor damage caused by a combination of various factors is more common. 1. Abnormal load and stall: The motor load includes the load required to compress gas as well as the load needed to overcome mechanical friction. An excessively high pressure ratio, or an excessive pressure difference, will make the compression process more difficult ； The increased frictional resistance caused by lubrication failure, along with motor stalling under extreme conditions, will **increase the motor load**. Lubrication failure and increased frictional resistance are the primary causes of abnormal loads. Returning liquid to dilute the lubricating oil, overheating of the lubricating oil, coking and deterioration of the lubricating oil, as well as oil shortages, can all disrupt proper lubrication and lead to lubrication failure. The returning liquid dilutes the lubricating oil, affecting the formation of a proper oil film on the friction surfaces; it may even wash away the existing oil film, thereby increasing friction and wear. Compressor overheating can cause the lubricating oil to become thin or even coker at high temperatures, affecting the formation of a proper oil film. The system’s oil return is poor, resulting in a lack of oil in the compressor, which naturally prevents proper lubrication. As the crankshaft rotates at high speed and components such as connecting rods and pistons move rapidly, the friction surfaces that lack oil film protection heat up quickly. The high local temperatures cause the lubricating oil to evaporate or coker rapidly, making it even more difficult to lubricate those areas; severe local wear can occur within just a few seconds. Lubrication failure and localized wear require more torque to rotate the crankshaft. Low-power compressors (such as those in refrigerators and household air conditioners), due to their low motor torque, often experience stall conditions when lubrication fails; this leads to a vicious cycle of stall – thermal protection – stall, and engine burnout is only a matter of time. High-power semi-hermetical compressor motors have high torque, and local wear does not cause stalling. The motor power increases with the load within a certain range, which leads to more severe wear, and may even result in serious damages such as piston seizure (the piston getting stuck inside the cylinder) or broken connecting rods. The current during stall (stall current) is approximately 4–8 times the current under normal operation. At the moment the motor starts, the peak value of the current can approach or reach the stall current. Since the heat generated by resistance is proportional to the square of the current, the currents during startup and stall cause the windings to heat up rapidly. Thermal protection can safeguard the electrodes during stall, but it generally does not respond quickly and cannot prevent temperature changes in the windings caused by frequent starting. Frequent starting and abnormal loads subject the windings to high temperatures, which reduces the insulation performance of the enameled wire. Furthermore, the load required to compress gas also increases as the compression ratio and pressure difference increase. Therefore, using a high-temperature compressor in a low-temperature application, or a low-temperature compressor in a high-temperature application, affects the motor load and heat dissipation; it is inappropriate and will shorten the service life of the electrodes. Once the insulation performance of the windings deteriorates, other factors (such as metal shavings forming a conductive path, acidic lubricants, etc.) can easily lead to short circuits and damage. 2. Short circuits caused by metal shavings: Metal shavings mixed within the windings are the main cause of short circuits and low insulation resistance. The normal vibration of the compressor during operation, as well as the twisting of the windings due to electromagnetic forces each time it starts up, both cause relative movement and friction between the metal shavings trapped among the windings and the enameled wire of those windings. Sharp metal shavings can scratch the insulation layer of enameled wire, causing a short circuit. Sources of metal shavings include copper pipe shavings left over from construction, welding slag, and metal shavings that result from wear inside compressors and damage to components (such as broken valve plates). For hermetically sealed compressors (including hermetically sealed scroll compressors), these metal shavings or fragments will fall on the windings. In semi-hermetical compressors, some particles flow through the system along with the gas and lubricating oil, and eventually accumulate in the windings due to their magnetic properties ； And some metal shavings (such as those generated by bearing wear and wear of the motor rotor and stator, known as slot cleaning) fall directly onto the windings. Once metal shavings accumulate in the windings, a short circuit is only a matter of time. Special attention should be paid to the two-stage compressor. In a two-stage compressor, the return gas and normal oil return flow directly into the cylinders of the first stage (low-pressure stage). After compression, they pass through medium-pressure pipes to reach the motor chamber where they cool the windings; thereafter, just like in a conventional single-stage compressor, they proceed to the second stage (high-pressure stage cylinders). The return gas contains lubricating oil, which already makes the compression process extremely risky; if liquid also returns, the valve plates in the first-stage cylinder can easily be damaged. The broken valve pieces can enter the winding through the medium-pressure pipe. Therefore, two-stage compressors are more prone to motor short circuits caused by metal shavings than single-stage compressors. Misfortunes often come together; when analyzing a compressor that is having problems, one often detects a burnt smell of lubricant. The temperature is very high when the metal surface is severely worn, and lubricating oil begins to coker at temperatures above 175ºC. If there is excessive moisture in the system (due to inadequate vacuum extraction, high water content in the lubricating oil and refrigerant, or air entering after a rupture in the negative pressure return pipe), the lubricating oil may become acidic. Acidic lubricants can corrode copper pipes and the insulation layers of windings; on one hand, it causes copper plating ； On the other hand, this acidic lubricant containing copper atoms has poor insulating properties, which creates conditions for winding short circuits. 3. Contactor issues: The contactor is one of the important components in a motor control circuit; an inappropriate selection can damage even the best compressors. It is extremely important to select the contactor appropriately based on the load. 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 chosen must prevent welding under conditions of high current such as startup or stall. For safety and reliability, the compressor contactor must disconnect all three phases of the circuit at the same time. Takumi Corporation does not recommend the method of disconnecting two-phase circuits. In the United States, contactors approved by Goulds must meet the following four requirements: · The contactors must comply with the operating and testing criteria specified in ARI Standard 780-78, \"Standard for Special Purpose Contactors\". · The manufacturer must ensure that the contactor can close at room temperature at 80% of the minimum voltage specified on the nameplate. · When using a single contactor, its rated current must be greater than the motor’s nameplate current rating (RLA). Additionally, the contactor must be able to handle the motor’s locked-rotor current. If there are other loads downstream of the contactor, such as motor fans, they must also be taken into consideration. · When two contactors are used, the stall rating of the separately wound windings of each contactor must be equal to or greater than the stall rating of the half-wound windings of the compressor. The rated current of the contactor must not be lower than the rated current specified on the compressor’s nameplate. Contactors of small size or poor quality cannot withstand the high current surges that occur during compressor startup, stall conditions, and low voltage; as a result, they are prone to single-phase or multi-phase contact oscillation, welding, or even detachment, which can lead to motor damage. Contactors with vibrating contacts frequently start and stop the motor. Frequent starting of the motor, along with the high starting current and resulting heat generation, accelerates the aging of the winding insulation layer. Each time it is started, the magnetic torque causes slight movement and friction between the motor windings. If other factors are present (such as metal shavings, lubricants with poor insulation, etc.), it is easy to cause short circuits between the windings. The thermal protection system is not designed to prevent such damage. Furthermore, the vibrating contactor coil is prone to failure. If the contact coil is damaged, a single-phase condition is likely to occur. If the contactor is selected to be too small, its contacts cannot withstand the arc as well as the high temperatures generated by frequent switching cycles or unstable control circuit voltages, which may cause them to weld together or fall off the contact holder. The welded contacts will create a permanent single-phase condition, causing the overload protector to continuously cycle on and off. It should be emphasized that once the contacts of the contactor are welded together, all controls that rely on the contactor to disconnect the compressor’s power supply circuit – such as high/low pressure control, oil pressure control, defrost control, etc. – will cease to function, leaving the compressor without any protection. Therefore, checking the contactor is an essential step after the motor burns out. Contactors are an often-overlooked but important cause of motor damage. 4. Power supply phase loss and voltage abnormalities. Abnormal voltages and phase loss can easily destroy any motor. The range of variation in supply voltage must not exceed ±10% of the rated voltage. The voltage imbalance between the three phases must not exceed 5%. High-power motors must be powered independently to prevent low voltage caused by the startup and operation of other high-power devices on the same circuit. The motor power cable must be able to carry the motor’s rated current. If a phase loss occurs while the compressor is running, it will continue to operate but with a high load current. The motor windings will overheat quickly, and under normal conditions the compressor will be protected by a thermal switch. When the motor windings cool down to the set temperature, the contactor closes, but the compressor fails to start and experiences stall, entering a dead loop of \"stall – thermal protection – stall\". The differences in modern motor windings are very small; when the power supply is three-phase balanced, the differences in phase currents can be ignored. Under ideal conditions, the phase voltages remain equal at all times; installing a protector on any one of the phases is sufficient to prevent damage caused by overcurrent. In fact, it is difficult to ensure the balance of phase voltages. The percentage of voltage imbalance is calculated as the ratio of the maximum deviation of the phase voltages from the average value of the three-phase voltages to that average value. For example, in a three-phase power supply with a nominal voltage of 380V, the voltages measured at the compressor terminals are 380V, 366V, and 400V respectively. The average value of the three-phase voltages is 382V, and the maximum deviation is 20V; therefore, the percentage of voltage imbalance is 5.2%. As a result of voltage imbalance, the imbalance in load current under normal operation is 4 to 10 times the percentage of voltage imbalance. In previous cases, a 5.2% unbalanced voltage can cause a 50% current imbalance. The NEMA Motor and Generator Standards publication of the United States’ Electrical Manufacturers Association states that the percentage increase in temperature of the phase windings caused by unbalanced voltage is approximately twice the square of the percentage of voltage imbalance. In the previous case, the number of voltage imbalance points was 5.2, and the percentage increase in winding temperature was 54%. As a result, one winding overheated while the temperatures of the other two windings remained normal. A survey conducted by U.L. (Underwriters Laboratories, USA) showed that 43% of power companies allow a voltage imbalance of 3%, while another 30% of power companies permit a voltage imbalance of 5%. 5. Insufficient cooling: Compressors with high power are generally of the return-air cooling type. The lower the evaporation temperature, the smaller the system mass flow tends to be. When the evaporation temperature is very low (above the manufacturer’s specifications), the flow rate is insufficient to cool the motor, causing it to operate at a higher temperature. Air-cooled compressors (generally up to 10HP) rely less on return air, but they have specific requirements regarding the compressor’s operating temperature and the volume of cooling air. A large leak of refrigerant can also lead to a decrease in the system’s mass flow, which in turn affects the cooling of the motor. In some unmonitored cold storage facilities, it is often only when the cooling performance becomes very poor that a large leakage of refrigerant is discovered. When the motor overheats, it will trigger frequent protection mechanisms. Some users do not investigate the reasons behind this in depth; they even short-circuit the thermal protector, which is a very bad thing to do. It won’t be long before the motor burns out. Compressors all have a safe operating range. The main considerations for safe operation are the load and cooling of the compressor and motor. Due to the different prices of compressors in various temperature ranges, it was common in China’s refrigeration industry in the past for compressors to be used beyond their designated scope. As professional knowledge has increased and economic conditions have improved, the situation has clearly improved. 6. Vacuuming with a compressor – Open-type refrigeration compressors have been forgotten by most people, but some field technicians in the refrigeration industry still adhere to the old practice of using compressors to create a vacuum. This is very dangerous. Air acts as an insulating medium. After evacuating a sealed container, discharge phenomena between the electrodes inside it occur easily. Therefore, as the vacuum level inside the compressor housing increases, the insulating medium is lost between the exposed terminals inside the housing or between windings that have minor damage to their insulation layers; once electricity is applied, the motor may short-circuit and burn out in an instant. If the housing leaks electricity, it can also cause electric shock to people. Therefore, vacuuming using a compressor is prohibited, and it is strictly forbidden to power on the compressor when the system and the compressor are in a vacuum state (after vacuuming but before refrigerant has been added). In summary, the burnout of the motor concealed the damage to the windings, posing certain difficulties for fault analysis. However, the root cause of damage to the compressor motor does not disappear. Abnormal loads or even stall caused by poor or failed lubrication, along with insufficient heat dissipation, can all shorten the lifespan of the windings ； The presence of metal shavings in the windings further facilitates short circuits ； Contactor welding will prevent the compressor from being protected ； An abnormality in the power supply on which the motor relies for operation will completely destroy any motor ； Creating a vacuum with a compressor may cause discharge at the internal terminals. Unfortunately, the aforementioned adverse factors can also trigger each other: the high currents during abnormal loads and stall conditions may cause the contactors to weld together ； Arcing or even welding at a single contact can cause phase imbalance or single-phase operation ； Phase imbalance can cause heat dissipation problems ； Insufficient heat dissipation can cause wear ； Wear and tear generates metal shavings… Therefore, proper installation and use of the compressor, along with adequate routine maintenance, can prevent such adverse effects; this is the fundamental way to avoid damage to the compressor motor. Analysis of Common Compressor Failures (2) – Liquid Slugging 1. Introduction The phenomenon in which liquid refrigerant and/or lubricating oil, drawn into the compressor cylinder along with gas, damages the suction valve plates, as well as the situation where this liquid does not exit quickly during the exhaust process and is compressed when the piston approaches the top dead center, resulting in a sudden increase in hydraulic pressure, is commonly referred to as liquid slugging. Water hammer can cause damage to components subjected to compressive forces (such as valve plates, pistons, connecting rods, crankshafts, piston pins, etc.) in a very short time, and it is a deadly threat to reciprocating compressors. Reducing or preventing liquid from entering the cylinder can prevent liquid hammer, so liquid hammer can be completely avoided. Generally, the liquid hammer phenomenon can be divided into two parts or processes. Firstly, when a large amount of liquid refrigerant, lubricating oil, or a mixture of the two enters the compressor cylinder at high speed along with the intake gas, the impact of the liquid and its incompressibility can cause the intake valve leaf to bend excessively or break ； Secondly, when the liquid that has not evaporated or been expelled from the cylinder is compressed by the piston, the extremely high pressure that arises in an instant causes deformation and damage to the components under stress. These stress-bearing components include intake and exhaust valve plates, valve decks, valve deck gaskets, pistons (top), piston pins, connecting rods, crankshafts, bearing shells, etc. 2. Processes and Phenomena (1) Breakage of the intake valve plate. A compressor is a machine used to compress gases. Typically, the piston compresses the gas 1,450 times per minute (for semi-hermetical compressors) or 2,900 times per minute (for fully hermetical compressors); thus, the time required to complete one suction or exhaust cycle is 0.02 seconds or even less. The size of the intake and exhaust holes on the valve plate, as well as the elasticity and strength of the intake and exhaust valve elements, are all designed in consideration of gas flow. From the perspective of the force on the valve disc, the impact force generated by gas flow is relatively uniform. The density of liquids is dozens or even hundreds of times that of gases; therefore, the momentum of liquids in motion is much greater than that of gases, resulting in a much stronger impact force. The flow of air containing many liquid droplets entering the cylinder is a two-phase flow. The impact generated by two-phase flow on the suction valve plates is not only intense but also occurs at a high frequency; it’s like a typhoon carrying pebbles striking a window, and its destructive power is self-evident. The breakage of the suction valve disc is one of the typical characteristics and processes of water hammer. (2) Rod fracture: The compression stroke takes about 0.02 seconds, while the exhaust process is even shorter. The droplets or liquid in the cylinder must be expelled from the exhaust port in such a short time, resulting in very high speeds and momentum. The exhaust valve disc is similar to the intake valve disc; the difference is that the exhaust valve disc is supported by a limit plate and a spring plate, making it less likely to break. In severe impacts, the limit plate can also deform and warp. If the liquid does not evaporate and drain from the cylinder in time, the piston will compress the liquid as it approaches the top dead center. Due to the short duration of this compression, it resembles a collision, and a metallic knocking sound can be heard from the cylinder head. Compressed liquid is another aspect or process of the water hammer phenomenon. The high pressure generated instantly by water hammer is highly destructive; in addition to the bending or even breaking of connecting rods, which is well-known, other components subjected to compressive forces such as valve plates, valve plate gaskets, crankshafts, pistons, piston pins, etc., can also become deformed or damaged. However, this is often overlooked, or confused with excessive exhaust pressure. When servicing a compressor, it is easy to detect bent or broken connecting rods and replace them, but people forget to check whether other parts are deformed or damaged, thus laying the groundwork for future failures. Rod breakage caused by water hammer is different from bearing seizure and piston scoring, and it can be distinguished. Firstly, liquid hammer causing the rod to bend or break occurs in a short period of time; the pistons at both ends of the rod as well as the crankshaft move freely, and generally there is no shaft seizure or cylinder scoring resulting from severe wear. Although the valve plate fragments can occasionally cause severe scratches on the piston and cylinder surfaces after the intake valve plate breaks, these surface scratches are different from the wear caused by lubrication failure. Secondly, the rod fracture caused by water hammer is resulting from pressure, and the rod and the fractured end exhibit compression characteristics. Although a break in the connecting rod after piston seizure can also involve compression, this is only possible if the piston is stuck inside the cylinder. It’s even different in the case of a broken connecting rod after it has been clamped to the crankshaft; the large end of the connecting rod suffers severe wear against the crankshaft, and the force that causes the break is shear force, with a different shape to the fracture surface as well. Finally, before the shaft breaks or the cylinder is damaged, the motor will operate under overload conditions, resulting in severe overheating; as a result, the thermal protector will activate. 3. Cause analysis: Obviously, the liquids that can cause liquid slugging in the compressor can originate from the following sources: 1) Return fluid, that is, the liquid refrigerant or lubricating oil that flows back from the evaporator to the compressor ； 2) Foam during start-up with liquid present ； 3) There is too much lubricating oil in the compressor. This article will analyze these reasons one by one. (1) Return flow: Generally, return flow refers to the phenomenon or process by which the liquid refrigerant in the evaporator returns to the compressor through the suction line while the compressor is operating. For refrigeration systems that use expansion valves, liquid return is closely related to the improper selection and use of the expansion valve. Choosing an expansion valve that is too large, setting the superheat value too low, incorrect installation of the temperature sensing element or damage to the insulating covering, or malfunction of the expansion valve can all lead to liquid return. For small refrigeration systems that use capillaries, an excessive amount of liquid added can cause backflow. Systems that use hot air for defrosting are prone to liquid return. Whether the heat pump mode is used with a four-way valve or the cooling mode is used with a hot gas bypass valve, the melting of frost in the hot gas results in the formation of a large amount of liquid in the evaporator; this liquid may return to the compressor at the start of the subsequent cooling operation. Furthermore, severe frosting on the evaporator or fan failure leads to poor heat transfer, and the unevaporated liquid can cause backflow. Frequent fluctuations in the temperature of the cold storage can also cause the expansion valve to malfunction, resulting in liquid return. Liquid hammer accidents caused by backflow mostly occur in air-cooled (abbreviated as air-cooled) semi-hermetical compressors and single-unit two-stage compressors, as the cylinders of these compressors are directly connected to the return pipe, making it easy for liquid hammer to occur once backflow takes place. Even if liquid slugging does not occur, the return of liquid into the cylinder will dilute or wash away the lubricating oil on the piston and cylinder walls, exacerbating piston wear. In semi-hermetic and hermetic compressors cooled by return gas (refrigerant vapor), liquid return rarely causes liquid slugging. But it will dilute the lubricating oil in the crankcase. Lubricating oils containing large amounts of liquid refrigerant have low viscosity, and cannot form a sufficient oil film on the friction surfaces, resulting in rapid wear of the moving parts. Furthermore, the refrigerant in the lubricating oil boils when exposed to heat during transportation, which affects the proper delivery of the lubricating oil. The further away from the oil pump, the more apparent and severe the problem becomes. If the bearings at the motor end suffer severe wear, the crankshaft may sink to one side, which can easily lead to the stator scraping against the frame and the motor burning out. Clearly, backflow not only causes liquid slugging but also dilutes the lubricating oil, leading to wear. When worn out, the load and current on the motor **increase**, which over time can lead to motor failure. For refrigeration systems where liquid return is difficult to avoid, installing a gas-liquid separator and using evacuation shutdown control can effectively prevent or reduce the hazards caused by liquid return. (2) Wet start: The phenomenon of intense foaming of the lubricating oil in the crankcase during the startup of a return-air cooling type compressor is called wet start. The foaming phenomenon during start-up with liquid can be clearly observed on the oil sight glass. The fundamental reason for start-up with liquid present is the large amount of refrigerant dissolved in the lubricating oil as well as that settled at its bottom, which suddenly boils when pressure drops, causing foaming in the lubricating oil. This phenomenon is very similar to the bubbling that occurs when people suddenly open a Coke bottle in everyday life. The duration of foaming is related to the amount of refrigerant, usually lasting a few minutes or a little over ten minutes. A large amount of foam floated on the surface of the oil, and it even filled the crankcase. Once drawn into the cylinder through the intake, the foam is reduced to a liquid (a mixture of lubricant and refrigerant), which can easily cause liquid slugging. Obviously, the water hammer caused by starting with liquid occurs only during the startup process. Unlike return fluid, the refrigerant that causes start-up with liquid present enters the crankcase through \"refrigerant migration\". Refrigerant migration refers to the process or phenomenon in which, when the compressor stops operating, the refrigerant in the evaporator enters the compressor via the return pipeline in gaseous form and is absorbed by the lubricating oil, or it condenses inside the compressor and mixes with the lubricating oil. After the compressor stops, the temperature drops while the pressure rises. Due to the low vapor pressure of the refrigerant in the lubricating oil, it absorbs the refrigerant vapor on the surface of the oil, resulting in the crankcase pressure being lower than the evaporator pressure. The lower the oil temperature, the lower the steam pressure, and the greater the absorption capacity for refrigerant vapor. The steam in the evaporator will gradually \"migrate\" toward the crankcase. Furthermore, if the compressor is located outside, in cold weather or at night, its temperature is usually lower than that of the indoor evaporator; as a result, the pressure inside the crankcase is also lower. When the refrigerant moves to the compressor, it tends to condense and mix into the lubricating oil. Refrigerant migration is a very slow process. The longer the compressor is shut down, the more refrigerant will migrate into the lubricating oil. This process will continue as long as there is liquid refrigerant in the evaporator. Since the lubricating oil in which the refrigerant is dissolved is heavier, it settles at the bottom of the crankcase, while the lubricating oil on top can absorb more refrigerant. In addition to easily causing liquid slugging, refrigerant migration also dilutes the lubricating oil. When very thin lubricating oil is pumped to the various friction surfaces, it may wash away the existing oil film, causing severe wear (this phenomenon is often referred to as refrigerant erosion). Transition wear increases the clearance between the mating parts, leading to oil leakage and thus affecting the lubrication in more distant areas; in severe cases, this can cause the oil pressure protector to activate. Due to structural reasons, the drop in crankcase pressure when an air-cooled compressor starts is much slower; the foaming phenomenon is not severe, and it is difficult for foam to enter the cylinders. Therefore, air-cooled compressors do not suffer from the problem of liquid-induced surging during startup. Theoretically, installing a crankcase heater (electric heater) in the compressor can effectively prevent refrigerant migration. After a short shutdown (such as at night), keeping the crankcase heater powered on can keep the lubricant temperature slightly higher than that in other parts of the system, preventing refrigerant migration. After a long period of inactivity (such as over a winter), heating the lubricating oil for several hours or even a dozen hours before starting up can evaporate most of the refrigerant present in the oil. This not only **reduces the risk of liquid slugging when starting up with liquid present, but also minimizes the damage caused by the impact of refrigerant. However, in practical applications, it is difficult to maintain power supply to the heater after shutdown or to supply power to the heater several hours before startup. Therefore, the actual effectiveness of the crankcase heater is greatly reduced. For larger systems, allowing the compressor to evacuate the liquid refrigerant from the evaporator before shutting down (a process known as evacuation shutdown) can fundamentally prevent refrigerant migration. Installing a gas-liquid separator on the return pipeline can increase the resistance to refrigerant migration and reduce the amount of migration. Of course, by improving the compressor structure, refrigerant migration can be prevented and the foaming of lubricating oil can be reduced. By improving the oil return path in the backflow-cooled compressor and adding barriers (such as oil return pumps) to the passages that lead from the motor chamber to the crankcase, it is possible to cut off these pathways after shutdown, preventing refrigerant from entering the crankcase ； Reducing the cross-sectional area of the passage between the intake tract and the crankcase can slow down the rate of pressure drop in the crankcase during startup, thereby controlling the degree of foaming and the amount of foam that enters the cylinders. 3) Too much lubricating oil: Semi-hermetical compressors usually come equipped with an oil sight glass to monitor the oil level. If the oil level is above the range indicated by the oil sight glass, it means there is too much oil. If the oil level is too high, the rapidly rotating crankshaft and connecting rod caps may frequently strike the oil surface, causing a large amount of lubricant to splash. If the splashing lubricating oil enters the intake tract and then reaches the cylinders, it can cause liquid slugging. When installing and commissioning large refrigeration systems, it is often necessary to add lubricant appropriately. However, for systems with poor oil return, it is necessary to carefully identify the root cause of this issue; simply adding more lubricant is dangerous. Even if the oil level is not high at the moment, be aware of the potential dangers that can arise when lubricant returns in large quantities suddenly (for example, after defrosting). Liquid slugging caused by lubricating oil is not uncommon. 4. Conclusion: Liquid impact is a common fault in compressors. The occurrence of liquid hammer indicates that there must be some problem in the system or during maintenance, which needs to be corrected. By carefully observing and analyzing the design, construction, and maintenance of the system, it is not difficult to identify the root cause of water hammer. Failing to prevent liquid slugging at its source and simply repairing the faulty compressor or replacing it with a new one will only lead to the recurrence of liquid slugging. Compressor Failure Analysis (3) – Oil Deficiency and Insufficient Lubrication 1. Introduction A compressor is a complex machine that operates at high speeds; ensuring adequate lubrication of its moving parts such as the crankshaft, bearings, connecting rods, and pistons is a fundamental requirement for the proper operation of the machine. To this end, compressor manufacturers require the use of lubricants of specified grades, and insist on regular checks of the lubricant level and color. However, due to negligence in the design, installation, and maintenance of refrigeration systems, insufficient lubrication of moving parts is a common issue caused by factors such as oil deficiency in the compressor, oil coking and degradation, liquid backflow leading to dilution, coolant erosion, and the use of low-quality lubricants. Insufficient lubrication can cause wear or scratches on the bearing surfaces; in severe cases, it may lead to the shaft seizing, the piston getting stuck inside the cylinder, and subsequent bending or breaking of the connecting rod. 2. Lack of oil – Lack of oil is one of the compressor failures that is easy to identify; when a compressor lacks oil, there is very little or no lubricating oil in the crankcase. A compressor is a special type of air pump; as a large amount of refrigerant gas is discharged, it also carries away a small amount of lubricating oil (known as oil leakage). Oil leakage from the compressor is inevitable; it’s just that the speed of such leakage varies. Approximately 2-3% of the lubricating oil is present in the exhaust gas of semi-screw piston compressors, whereas it is 0.5-1% in scroll compressors. For a 6-cylinder compressor with a capacity of 100 m3/hr and a crankcase oil volume of 6 liters, 3% oil leakage means an oil loss rate of approximately 0.3–0.8 liters per minute, or the compressor can operate without oil return for around ten minutes. If the lubricating oil from the compressor is not returned, the compressor will run out of oil. There are two ways for the compressor to return oil: one is through the oil separator, and the other is through the return air pipe. The oil separator is installed on the compressor exhaust line and is generally capable of separating 50-95% of the oil mist. It provides an effective oil return with fast speed, **reducing the amount of oil that enters the system pipes, thereby effectively extending the operating time without oil return. In cold storage refrigeration systems with particularly long pipelines, flooded ice-making systems, and freeze-drying equipment operating at very low temperatures, it is not uncommon for no oil to return to the compressor or only a very small amount of oil to return after the system is started, over ten minutes or even several dozen minutes. Systems that are not properly designed may experience problems such as excessively low compressor oil pressure, which leads to shutdown of the compressor. Installing an efficient oil separator in such a refrigeration system can **extend the time during which the compressor can operate without oil return, allowing it to get through the critical phase right after startup when there is no oil return. The lubricating oil that has not been separated will enter the system and flow along with the refrigerant within the pipes, thus forming an oil circulation. After the lubricating oil enters the evaporator, due to the low temperature which results in low solubility, a portion of the lubricating oil separates from the refrigerant ； On the other hand, at low temperatures the viscosity is high, and the separated lubricating oil tends to adhere to the inner wall of the pipe, making flow more difficult. The lower the evaporation temperature, the more difficult it is to return the oil. This requires that the design, construction of the evaporation pipeline and the return air pipeline must facilitate oil return; a common approach is to use a downward-flowing pipeline design and ensure a high airflow velocity. For refrigeration systems operating at extremely low temperatures, such as medical cryostats at –85°C and –150°C, in addition to using efficient oil separators, special solvents are often added to prevent the lubricating oil from clogging the capillaries and expansion valves, and to assist in the return of oil. In practical applications, oil return problems caused by improper design of the evaporator and return air circuits are not uncommon. For R22 and R404A systems, it is very difficult to return oil to a full-liquid evaporator; therefore, great care must be taken in the design of the system’s oil return piping. For such systems, using high-efficiency oil can **reduce the amount of oil entering the system pipelines, thereby effectively extending the period during which no oil returns in the return pipe after the system is started up. When the compressor is located higher than the evaporator, an oil return bend on the vertical return pipe is necessary. The return oil bend should be as compact as possible to reduce oil accumulation. The spacing between the return oil bends should be appropriate; when there are a large number of such bends, some lubricating oil should be added. Care must also be taken with the return oil lines in variable load systems. When the load decreases, the return air velocity drops, and a too low velocity is not conducive to oil return. To ensure oil return under light load conditions, a vertical suction pipe can employ double standpipes. Frequent starting of the compressor is not conducive to oil return. Due to the short continuous operation time, the compressor stops, and there is not enough time for a stable high-speed airflow to form in the return pipe, so the lubricating oil remains inside the pipeline. If the return oil is less than the oil flowing out, the compressor will run out of oil. The shorter the operating time, the longer the pipelines, and the more complex the system, the more prominent the oil return issue becomes. For fully hermetically sealed compressors (including scroll compressors and rotary vane compressors) that lack a hydraulic pressure safety switch, as well as some semi-hermetically sealed compressors, damage caused by frequent starting is relatively common. Compressor maintenance is equally important. During defrosting, the evaporator temperature rises, the viscosity of the lubricating oil decreases, allowing it to flow more easily. After the defrost cycle, the refrigerant flow rate is high, and the retained lubricating oil returns to the compressor in large quantities. Therefore, the frequency of the defrost cycle and its duration per cycle also need to be carefully set to avoid significant fluctuations in oil level or even oil slugging. When there is a significant leak of refrigerant, the return gas velocity decreases. If this velocity is too low, the lubricating oil remains in the return gas line and cannot return to the compressor quickly. The return of lubricating oil to the compressor housing does not mean it returns to the crankcase. In compressors that use the principle of negative pressure in the crankcase for oil return, if leakage occurs in the pistons due to wear or other reasons, the pressure in the crankcase rises. The oil return check valve closes automatically as a result of this pressure difference, and the lubricating oil returning through the oil return pipe gets trapped in the motor chamber and cannot enter the crankcase. This is the issue of internal oil return, and it can also lead to a lack of oil. Such accidents occur not only in worn-out old machines; liquid entrapment resulting from refrigerant migration can also cause difficulties in oil return, although this usually lasts for a short period of time, at most a few dozen minutes. When an internal oil return issue occurs, it can be observed that the compressor oil level keeps dropping until the oil pressure safety device activates. After the compressor stops, the oil level in the crankcase returns to normal quickly. The root cause of the internal oil return issue is cylinder leakage; the worn piston assembly should be replaced promptly. The hydraulic safety device will automatically shut down in the event of a lack of oil, protecting the compressor from damage. Hermetically sealed compressors (including rotary and scroll compressors) that lack an oil sight glass and oil pressure safety device, as well as air-cooled compressors, show no obvious symptoms when low on oil and do not shut down; as a result, the compressors wear out and get damaged unnoticed. Compressor noise, vibration, or excessive current may be related to a lack of oil; therefore, it is very important to accurately assess the operating condition of both the compressor and the system. Excessively low ambient temperatures may cause some oil pressure safety devices to fail, leading to compressor wear. The wear caused by a lack of oil in the compressor is generally quite uniform. If there is little or no lubricating oil, severe friction occurs on the bearing surfaces, and the temperature rises rapidly within a few seconds. If the motor has sufficient power, the crankshaft will continue to rotate; otherwise, the surfaces of the crankshaft and bearings will get worn or scratched. In that case, the crankshaft will be locked by the bearings and stop rotating. The back-and-forth movement of the piston within the cylinder is the same; a lack of oil can lead to wear or scratches, and in severe cases the piston may get stuck inside the cylinder and be unable to move. 3. Insufficient lubrication: The direct cause of wear is insufficient lubrication. A lack of oil will certainly lead to insufficient lubrication, but insufficient lubrication is not necessarily caused by a lack of oil. The following three reasons can also lead to insufficient lubrication: the lubricant fails to reach the bearing surface ； Although the lubricant has reached the bearing surface, its viscosity is too low to form an oil film of sufficient thickness ； Although the lubricant has reached the bearing surface, it has decomposed due to overheating and cannot perform its lubricating function. Issues such as clogged oil suction screens or oil supply lines, as well as pump failures, can affect the delivery of lubricating oil, preventing it from reaching the friction surfaces that are far away from the pump. The oil suction screen and oil pump are in good condition, but issues such as bearing wear and excessive clearance can lead to oil leakage and low oil pressure; as a result, the friction surfaces far from the oil pump do not receive lubrication, causing wear and scratches. Liquid return is a common system issue, and one of the major hazards of liquid return is the dilution of lubricating oil. When the diluted lubricating oil reaches the friction surfaces, its low viscosity prevents the formation of a protective oil film of sufficient thickness, which over time leads to wear. When the amount of returning liquid is high, the lubricating oil becomes very thin; it fails to provide lubrication and may even dissolve and wash away the existing oil film, leading to refrigerant erosion. For various reasons (including during the compressor startup phase), the temperature of the friction surfaces where the lubricating oil is present can rise rapidly; once it exceeds 175°C, the lubricating oil begins to decompose. “\"Insufficient lubrication – friction – high surface temperature – oil decomposition\" is a typical vicious cycle, and many serious accidents, including rod seizure and piston sticking in the cylinder, are related to this cycle. Insufficient lubrication and oil deficiency can be seen in the disassembled compressor. Oil deficiency generally manifests as widespread, relatively uniform surface damage and high temperatures, whereas insufficient lubrication results in wear, scratches, and high temperatures in specific areas, such as the bearing surfaces far from the oil pump. As the piston moves up and down, the load on the piston pin alternates between the upper and lower surfaces of the bearing, which allows the lubricating oil to evenly coat the piston pin and provide sufficient lubrication. If the exhaust valve leaf is bent or broken, or if the compressor operates at a high pressure ratio for an extended period, it will result in insufficient lubrication on one side of the piston pin, leading to wear and an increase in pore size. If there is play in the piston pin, the piston will be thrown to the top dead center and strike the valve plates, resulting in a knocking sound. Therefore, when replacing the valve disc, the wear of the piston pin should be checked. 4. Conclusions and Recommendations A lack of oil can lead to severe lubrication deficiencies. The root cause of this issue is not related to the amount or speed at which oil is discharged from the compressor, but rather to poor oil return within the system. Installing an oil separator allows for rapid oil return, extending the time during which the compressor can operate without oil return. The design of the evaporator and return air circuits must take oil return into account. Maintenance measures such as avoiding frequent startups, scheduling defrosting, replenishing refrigerant in a timely manner, and replacing worn piston components promptly also help with oil return. Backflow and refrigerant migration dilute the lubricating oil, which hinders the formation of an oil film ； Oil pump failures and clogged oil circuits can affect the oil supply volume and oil pressure, resulting in a lack of oil at the friction surfaces ； High temperatures on the friction surface cause the lubricant to decompose, resulting in it losing its lubricating properties. Lack of lubrication caused by these three issues often also leads to compressor damage. The root cause of the oil shortage lies in the system. Therefore, simply replacing the compressor or some components cannot fundamentally solve the oil shortage problem. Compressor Fault Analysis (4) – Overheating 1. Introduction The heat generated during the normal operation of a compressor should not cause overheating. Normal motor heating, compression heat, and friction heat are all carefully taken into account during the design of compressors, with corresponding cooling measures in place. However, in practical use, overheating phenomena such as high motor temperatures, excessive exhaust temperatures, and burnt lubricating oil occur quite frequently due to issues like misuse, abnormal power supply, motor overload, refrigerant leakage, and excessively high condensing pressure; these have become one of the common faults in compressors. The cylinder exhaust temperature is one of the important indicators for determining whether a compressor is overheating. Due to measurement difficulties, in practical applications, it is the temperature of the exhaust pipe surface (i.e., the exhaust pipe temperature) that is measured to determine whether it is overheated. Since lubricating oil becomes very thin at 150°C and begins to decompose and deteriorate around 175°C, the exhaust temperature in the cylinder should be kept below 150°C, while the temperature in the exhaust pipe is usually 10–40°C lower than the exhaust temperature. Therefore, if the exhaust pipe temperature exceeds 135°C, it is generally considered that the compressor is in a severe overheating condition ； And if the exhaust temperature is below 120°C, the compressor temperature is normal. The exhaust temperature of air-conditioning compressors and refrigerator compressors is usually even lower. 2. Hazards: High temperatures pose a significant threat to the compressor motor and lubricating oil. Prolonged overheating not only reduces the insulation properties and reliability of the motor as well as shortening its lifespan, but it also diminishes the lubricating capacity of the oil, and may even cause carbonization and acid degradation of the oil. After the lubricating oil carbonizes, its lubricating capacity **decreases**, which leads to severe wear on components such as the crankshaft, connecting rods, pistons, and piston rings. It can even result in situations like shaft seizure or cylinder jamming, as well as broken connecting rods caused by these jams. Kerosene carbide can also cause carbon buildup on the valve plates and valves, leading to valve leakage and valve failure. Acidic substances in lubricating oil can corrode the enameled wire of the windings and reduce their insulation properties. Acidified lubricating oil can also cause copper plating. In practice, the carbonization of lubricating oil is always accompanied by acid cleavage; therefore, wear and corrosion always occur together. The fine metal particles resulting from wear become mixed into the lubricating oil, which weakens the lubricating effect of the oil on one hand ； On the other hand, tiny metal shavings gather in the motor windings due to their magnetism, forming conductive circuits. When the insulation layer of enameled wire is corroded, some tiny exposed areas may appear, which can easily lead to partial discharge. If metal particles form a conductive circuit, a short circuit or breakdown will occur immediately, destroying the motor. Worn piston rings and pistons can also cause difficulties in oil return and trigger the operation of the oil pressure protector. Many semi-hermetical compressors rely on negative pressure to return oil; that is, when the pressure in the crankcase is lower than that in the motor chamber, the oil return check valve opens, allowing the lubricating oil to return to the crankcase. When the piston and piston rings wear out, high-pressure gas leaks into the crankcase, disrupting the negative pressure in the crankcase and making it difficult for oil to return. This issue often manifests as a continuous drop in the compressor oil level; eventually, the oil pressure protector activates and the compressor stops running. After it stops, the oil level gradually returns to normal. After restarting the compressor, everything was normal, but after some time the aforementioned issue reoccurred. Furthermore, fine iron particles mixed in the lubricating oil can also accumulate outside the oil strainer in the oil suction pipe of the oil pump due to suction, causing the oil strainer to become clogged. 3. Motor overheating: Motor overheating is relative to the motor’s normal operating temperature. The normal operating temperature of the motor must not exceed the maximum allowable temperature corresponding to its insulation class (see table below). The refrigeration compressor itself does not have specified heat-resistant insulation ratings, whereas the motor does (see table below). However, this insulation class can only serve as a reference for compressor motors, as the operating conditions of these motors differ significantly from those of ordinary motors. Thermal aging of insulation is an inevitable phenomenon in electrical equipment. The empirical relationship between insulation life and temperature, known as the “10 rule,” states that for every 10°C increase in temperature, the insulation life is halved (see table below). Insulation class table: Insulation classes A, E, B, F, H, C. Allowable operating temperature (°C): 105, 120, 130, 155, 180, 220. Clearly, high temperatures in motors are very harmful. The compressor is designed with motor cooling in mind; under normal operation, it should not experience high temperatures, let alone shut down due to thermal protection. The two necessary conditions for a thermal protection shutdown are that the temperature exceeds the set safety limit and that the high temperature persists for longer than the response time of the thermal protection system (usually within 5 minutes). The reasons for an increase in motor temperature are either excessive heat generation, insufficient cooling, or both. (1) High motor heating: Abnormal power supply can cause the motor to generate more heat. Unstable voltage, voltage that is too low or too high, voltage imbalance, and missing phases are all examples of abnormal power supply conditions. The starting current and stall current are 4–8 times the normal current; therefore, issues such as frequent compressor startups, rod seizure, piston seizure in the cylinder, insufficient lubrication, or lack of oil will all **increase heat generation**. Furthermore, the improper use of compressors can easily lead to motor overheating and damage, a problem that occurs frequently in the refrigeration industry. For every 10°C increase in evaporation temperature, the motor load can increase by 30% or even more, resulting in overloading the motor. Therefore, in medium- to high-temperature systems where low-temperature compressors are used, the cooling process in cold storage lasts for an extended period of time, causing the compressor to operate under overload conditions for long periods. This causes significant damage to the motor, **reducing its reliability and making it prone to burning out in the event of voltage fluctuations, surges, or other sudden disturbances. (2) Insufficient motor cooling: The lower the evaporation temperature, the smaller the mass flow rate of the refrigerant, and thus the lower the motor power required. Therefore, when air-conditioning compressors and medium-to-high temperature refrigeration compressors are used at low temperatures, although the actual power consumption of the motor is significantly lower than its nominal power, it remains too high compared to the actual power requirements and cooling conditions at those low temperatures, which makes it easy for problems to arise with motor cooling. Furthermore, when the amount of refrigerant leakage is high, the cooling of the return-air-cooled motor cannot be ensured either. Cooling for air-cooled compressors is also a problem in high-temperature environments or when the cooling fan fails. If the compressor is equipped with additional cooling (such as a liquid spray cooling system), the normal operation of this additional cooling must be maintained. To prevent the motor from being damaged by high temperatures, compressor motors are equipped with thermal protectors. The thermal protection trip temperatures vary among different motors, generally ranging from 100 to 135°C. Clearly, thermal protection is the last line of defense for motor safety; a shutdown due to thermal protection indicates that the motor has overheated severely. 4. Excessively high exhaust temperature. The main reasons for an excessively high exhaust temperature are as follows: high return air temperature, high heating capacity of the motor, high compression ratio, high condensing pressure, and improper selection of refrigerant. (1) High return air temperature. The level of the return air temperature is relative to the evaporation temperature. To prevent backflow, the return air circuit generally requires a return air superheat of 20°C. If the return air pipeline is not properly insulated, the superheat will far exceed 20°C. The higher the return air temperature, the higher the intake and exhaust temperatures in the cylinder. For every 1°C increase in the return air temperature, the exhaust air temperature will rise by 1–1.3°C. (2) Motor heating: In recirculating air-cooled compressors, the refrigerant vapor is heated by the motor as it passes through the motor compartment, causing the suction temperature in the cylinder to rise again. The heat generated by a motor is influenced by power and efficiency, while the power consumption is closely related to displacement, volumetric efficiency, operating conditions, frictional losses, and other factors. To make the temperature rise caused by motor heat more apparent, this paper derives the following relationship: C1 (1-h) Dh 1 Cp COP DT1 = (1) Here, DT1 and Cp represent the temperature rise of the refrigerant vapor flowing through the motor chamber and its specific heat, respectively ； h is the motor efficiency, and C1 represents the proportion of motor heat absorbed by the return air. The higher the ambient temperature, the worse the air cooling, and the closer C1 gets to 100%. The enthalpy difference Dh represents the cooling capacity per kilogram of refrigerant ； COP is the coefficient of performance. Equation (1) clearly shows the relationship between temperature rise and COP: the lower the COP, the greater the gas temperature rise. For R22 compressors, when the evaporation temperature is reduced from –5°C to –40°C, the COP generally decreases by a factor of 4, while other parameters remain largely unchanged; the temperature rise of the gas in the motor chamber increases by three to four times. As the intake temperature of the cylinder rises by 1°C, the exhaust temperature can increase by 1–1.3°C. Therefore, as the evaporation temperature drops from –5°C to –40°C, the exhaust steam temperature will rise by about 30–40°C. For the return-air cooling type semi-hermetical compressor, the temperature rise of the refrigerant in the motor compartment is generally between 15 and 45°C. In air-cooled compressors, the refrigerant does not pass through the windings, so there is no issue of motor heating. (3) Excessively high compression ratio: The exhaust temperature is greatly influenced by the compression ratio; the higher the compression ratio, the higher the exhaust temperature. Reducing the compression ratio can significantly lower the exhaust temperature; methods to achieve this include increasing the intake pressure and reducing the exhaust pressure. The suction pressure is determined by the evaporation pressure and the resistance in the suction line. Increasing the evaporation temperature can effectively raise the suction pressure, rapidly reduce the compression ratio, and thereby lower the exhaust temperature. Some users mistakenly believe that the lower the evaporation temperature, the faster the cooling rate; this idea actually has many problems. Although lowering the evaporation temperature can increase the freezing temperature difference, the cooling capacity of the compressor decreases, so the freezing speed is not necessarily faster. Moreover, the lower the evaporation temperature, the lower the coefficient of performance, while the load increases, the operating time lengthens, and power consumption rises. Reducing the resistance in the return air circuit can also increase the return air pressure; methods to achieve this include replacing the dirty or clogged return air filter in a timely manner, and minimizing the length of the evaporation tubes and return air circuits as much as possible. Furthermore, a lack of refrigerant is also a factor contributing to low suction pressure. The refrigerant should be replenished promptly in case of leakage. Practice has shown that reducing the exhaust temperature by increasing the intake pressure is simpler and more effective than other methods. The main reason for excessive exhaust pressure is too high condensing pressure. Insufficient cooling surface area of the condenser, fouling, inadequate volume of cooling air or water, and excessively high temperatures of the cooling water or air can all lead to high condensation pressure. It is very important to select an appropriate condensation area and maintain a sufficient flow rate of the cooling medium. At high temperatures, the operating compression ratio of air-conditioning compressors is relatively low; after cooling, this compression ratio increases significantly, resulting in very high exhaust temperatures. The cooling system is not able to keep up, which leads to overheating. It is necessary to avoid overusing the compressor and to operate it at the lowest possible pressure ratio. In some low-temperature systems, superheating is the primary cause of compressor failures. (4) Anti-expansion and gas mixing: After the intake stroke begins, the high-pressure gas remaining in the cylinder clearance undergoes an anti-expansion process. After counter-expansion, the gas pressure returns to the intake pressure, and the energy consumed to compress this portion of gas is lost during the counter-expansion. The smaller the clearance, the lower the power consumption caused by back expansion on one hand, and the greater the volume of air drawn in, which in turn **increases** the compressor’s efficiency ratio. During the reverse expansion process, the gas absorbs heat by coming into contact with the high-temperature surfaces of the valve plate, the top of the piston, and the top of the cylinder; as a result, the gas temperature does not drop to the intake temperature at the end of the reverse expansion. Only after the anti-expansion is complete does the actual inhalation process begin. Once the gas enters the cylinder, it mixes with the counter-expanding gas, causing the temperature to rise ； On the other hand, the mixed gas absorbs heat from the wall surface and heats up. Therefore, the gas temperature at the start of the compression process is higher than the intake temperature. However, since the anti-expansion process and the suction process are very brief, the actual temperature rise is extremely limited, generally less than 5°C. Anti-expansion is caused by the clearance in the cylinder, and it is a drawback that traditional piston compressors cannot avoid. If the gas in the valve plate’s exhaust holes cannot be discharged, reverse expansion will occur. The patented disc-type valve plate of Tanaka Company’s exhaust valves is highly specialized; it eliminates gaps in the exhaust ports as well as gas retention, thereby effectively controlling back expansion. Since its invention, disc valve compressors have maintained the record for highest efficiency. 5) Compression temperature rise and type of refrigerant: Different refrigerants have different thermophysical properties, resulting in varying increases in exhaust temperature after undergoing the same compression process. Therefore, different refrigerants should be selected for different cooling temperatures. Figures 1-3 show the temperature rise caused by adiabatic compression of different refrigerants at a condensation temperature of 50°C and a return gas superheat of 20°C. The adiabatic compression temperature rise of R404A and R502 is about 20–40°C lower than that of R22 and R410A. As can be seen from the graph, when the evaporation temperature falls below –30°C, the compression temperature rise for R22 and R410A exceeds 130°C. Taking into account a return gas superheat of 20°C and motor heating of 30°C, the theoretical exhaust temperature will exceed 150°C, requiring additional cooling. For systems with an evaporation temperature above 0°C (such as air conditioners), the exhaust temperature should not exceed 110°C, so there is no issue of overheating. 5. Conclusions and Recommendations: When operating within its specified range, a compressor should not experience overheating issues such as high motor temperature or excessive exhaust gas temperature. Compressor overheating is an important fault indicator, indicating serious problems with the refrigeration system or improper use and maintenance of the compressor. If the root cause of compressor overheating lies in the refrigeration system, the problem can only be resolved by improving the design and maintenance of the refrigeration system. Replacing it with a new compressor cannot fundamentally resolve the overheating issue. CPI Lubricants in the United States: www.cpihualai.com. Engineer Wang: 13926549484