The refrigeration compressor is the core component of a refrigeration system, and its stable operation is critical to the system’s performance and lifespan. In actual operation, liquid hammering is a prominent and highly damaging fault that not only affects compressor operation but can also result in economic losses. Therefore, understanding the causes, hazards, and preventive measures of liquid hammering is of great significance for ensuring the safe operation of the system.
What is Liquid Hammer in Refrigeration Compressors?
Within the scope of refrigeration system operation, liquid hammering is a common yet highly destructive type of failure. Its mechanism of occurrence is as follows: when liquid refrigerant or lubricating oil is drawn into the compressor cylinder along with gas, it directly damages the intake valve plate.
Meanwhile, if these liquids fail to be expelled quickly during the exhaust process, when the piston reaches a position close to the top dead center, the liquids will be compressed, leading to a sudden high-pressure phenomenon, which is defined as liquid hammer.
From the perspective of the action process, the destructive path of liquid hammer has a clear progressive nature. First, valve plate damage occurs during the suction stage, laying the groundwork for subsequent fault escalation; Then, during the compression process, the sudden high pressure caused by the inability to expel the liquid in time exerts a strong impact on the internal structure of the compressor.
This impact is extremely severe and can cause substantial damage to multiple critical components of the compressor, including the exhaust valve plate, valve plate, valve plate gasket, piston top, piston pin, connecting rod, crankshaft, and bearings, within a very short time.
What Causes Liquid Hammer?
Liquid Return Issues
During the operation of the refrigeration system, insufficient evaporation of refrigerant in the evaporator leads to liquid refrigerant flowing back into the compressor, a phenomenon known as liquid return. This is closely related to the selection, calibration, installation, and malfunction status of the expansion valve. Specifically:
- Selection phase: During the design stage of the refrigeration system, the selection of expansion valve specifications is critical. When the expansion valve’s rated flow rate far exceeds the system’s actual requirements, the refrigerant throttling process loses precise control, causing a large amount of liquid refrigerant to enter the suction line without sufficient vaporization.
Especially under load fluctuation conditions, an overly large expansion valve can cause refrigerant flow regulation lag, significantly increasing the risk of liquid hammering. Long-term operation may result in damage to the compressor valve plates or motor burnout.
- Commissioning stage: The superheat setting of the thermostatic expansion valve directly affects evaporation efficiency. If the superheat setting is below the system’s operational requirements (e.g., incorrectly setting the normal 5–8°C superheat to 2–3°C), liquid refrigerant in the evaporator cannot fully vaporize, and unvaporized droplets flow back to the compressor with the gas.
This issue is particularly pronounced during system startup, sudden load changes, or rapid drops in ambient temperature, potentially causing hydraulic shock to the compressor cylinder and posing a serious threat to equipment safety.
- Installation considerations: The temperature sensor, as the core sensing component of the expansion valve, requires extremely high installation precision. If the temperature sensor is not tightly secured to the center of the suction pipe or installed above a horizontal pipe, it may cause temperature detection distortion, leading the expansion valve to misjudge the superheat.
Damage to the insulation layer may cause the suction pipe to exchange heat with the external environment, interfering with the refrigerant vaporization process. Both issues can cause the expansion valve to open or close abnormally, triggering chain reactions such as liquid return or insufficient refrigerant supply.
- Failure factors: Mechanical failures within the expansion valve are a major contributing factor to liquid return hazards. When the valve core becomes blocked by contaminants or seized due to wear, the refrigerant flow regulation function fails, potentially causing excessive liquid refrigerant to enter;
over time, spring elasticity may degrade or the spring may break, disrupting the normal pressure balance for valve core opening and closing, causing the expansion valve to remain at an abnormal opening position. These faults are often accompanied by symptoms such as increased system operating noise and abnormal fluctuations in suction temperature, requiring timely inspection and maintenance.
Excessive Foam During Liquid-Carrying Startup
For suction-cooled compressors, the sudden drop in crankcase pressure during startup disrupts the equilibrium between lubricating oil and refrigerant, triggering liquid-carrying startup. When the compressor is shut down, the refrigerant either dissolves in the lubricating oil or settles below the lubricating oil layer at the bottom of the crankcase.
Upon restarting, the dissolved refrigerant escapes, and the deposited liquid refrigerant boils, causing the lubricating oil to foam violently. The foam enters the cylinder as the compressor operates. Liquid hammer caused by liquid start-up typically occurs within a few seconds to several dozen seconds after startup. If not controlled promptly, it can lead to valve plate fractures, piston damage, and other faults.
Excessive Refrigerant Addition
When refrigerant is overcharged, it can cause multiple faults. From a thermodynamic perspective, liquid refrigerant exceeding the design standard cannot be fully evaporated due to the limited heat exchange capacity of the evaporator. The unevaporated liquid refrigerant enters the compressor with the return gas, forming “liquid hammer.” This causes rigid impact on the compressor’s precision piston components, leading to valve plate deformation, wear, and even severe damage such as piston rod fracture or cylinder body rupture.
Additionally, excessive refrigerant reduces the refrigerant flow velocity in the suction line. During normal operation, gaseous refrigerant must maintain a certain flow rate to carry lubricating oil back to the compressor. However, excessive refrigerant causes a higher liquid-to-gas ratio in the two-phase flow, reducing gas flow velocity. Due to viscous resistance, lubricating oil cannot flow back smoothly. Compressor oil deficiency accelerates component wear, hinders heat dissipation, and may ultimately cause bearing seizure, motor winding burnout, system shutdown, or fire hazards.
Improper Design of Gas-Liquid Separator
The gas-liquid separator is a critical component for preventing compressor oil starvation and liquid hammer, utilizing both gravitational and centrifugal separation principles. During heat pump system mode switching or hot gas defrosting, a large amount of liquid refrigerant flows in the opposite direction. The gas-liquid separator relies on internal structures such as baffle plates to rapidly separate gas and liquid, with liquid stored at the bottom and gas entering the compressor, thereby reducing the risk of liquid hammer.
However, improper volume selection can weaken its protective function. If the volume is too small, it cannot store a large amount of liquid refrigerant, increasing the risk of liquid hammer; if the volume is too large, it increases costs and space requirements, and may also cause poor oil return, exacerbating compressor wear. Additionally, structural defects such as narrow guide channels or poor component layout can reduce separation efficiency, leading to “micro-liquid hammer” and damaging compressor performance.
Refrigerant Migration
After the refrigeration system shuts down, the pressure difference between the evaporator and the compressor causes gaseous refrigerant to flow backward into the compressor along the suction line. During this process, the refrigerant may be adsorbed and dissolved by the lubricating oil or liquefy due to a sudden drop in temperature, forming a mixture with the lubricating oil.
This migration has a time-dependent cumulative effect, particularly pronounced under conditions of large temperature fluctuations, poor system insulation, or prolonged shutdowns. When the compressor restarts, the unevaporated liquid mixture impacts components such as valve plates and pistons during piston movement, causing liquid hammering faults. This leads to abnormal vibration and noise in the equipment, and may even result in valve plate fractures, connecting rod crankshaft damage, reduced equipment lifespan, and increased maintenance costs.
What are the Effects of Liquid Hammer?
Scroll Compressors
Liquid hammer phenomena pose multi-dimensional, cascading destructive effects on refrigeration compressors. When large amounts of liquid refrigerant or lubricating oil enter the compressor cylinder, due to their incompressibility, the instantaneous peak pressure generated by liquid hammer on the scroll plate during the piston compression stroke can reach 5–10 times the normal operating pressure, causing stress concentration and micro-cracks. Prolonged repeated exposure can lead to scroll plate failure.
From the perspective of the lubrication system, liquid refrigerant dilutes the lubricating oil, reducing its viscosity by 30%-50%. When the viscosity drops below the critical lubrication value of 15-20 cSt, an effective oil film of 5-8 μm cannot form on the surfaces of moving components, leading to dry friction and accelerated component wear.
Additionally, when the refrigerant in the lubricating oil comes into contact with high-temperature components, it flashes and boils, creating cavitation, which reduces the lubricating oil’s pumping capacity by 40%-60%, leading to insufficient lubrication. The high-pressure shock waves generated by the collapse of bubbles further exacerbate component fatigue damage.
Reciprocating Compressors
Liquid hammering in reciprocating compressors poses significant risks. When liquid refrigerant or lubricating oil enters the cylinder, the incompressibility of the liquid causes the piston to generate high pressure akin to a “shock wave” during compression, which can damage components within milliseconds.
Valve plates may bend or fracture due to the impact, disrupting valve operation; piston surfaces may wear or even fracture; connecting rods and crankshafts may bear excessive stress, leading to bending or twisting; and piston pins may loosen or fracture due to impact loads. The damage caused by liquid slugging is irreversible, not only leading to shutdowns for repairs but also posing risks of economic losses and production interruptions.
Screw Compressors
Can screw compressors also experience liquid slugging? The answer is yes. Many people mistakenly believe that the positive displacement operating principle of screw compressors, combined with their meshing rotor structure, can prevent liquid slugging. Although screw compressors have stronger resistance to liquid hammer compared to piston-type and scroll-type compressors, thanks to the flexible meshing of the male and female rotors, slide valve regulation, and oil circuit buffering, their tolerance is still limited.
When there is a sudden drop in heat exchange in the evaporator, or abnormalities in the expansion valve or hot gas bypass valve, liquid refrigerant may rush into the compressor. When liquid hammering occurs, the impact of the liquid medium causes the machine body to vibrate violently, resulting in a noise increase of 20-30 dB. Prolonged liquid hammering accelerates bearing wear, causes seal failure and oil emulsification, and may lead to rotor fracture, oil circuit blockage, and in extreme cases, deformation of the discharge end cover and pipe rupture, resulting in equipment damage and safety incidents.
How to Prevent Liquid Hammer in Refrigeration Compressors
Reduce Refrigerant Charge
To effectively avoid liquid hammer damage caused by liquid refrigerant to the compressor, the key is to precisely control the refrigerant charge within the compressor’s rated parameters. If constrained by actual operating conditions, the charge should be scientifically and reasonably reduced while ensuring the performance of the refrigeration system.
It is worth noting that when conditions such as excessively narrow liquid pipe diameters or abnormally low condensing pressure occur, the presence of bubbles in the sight glass may be an important warning signal of imbalanced refrigerant charge. In such cases, immediate investigation and targeted adjustments must be initiated.
Crankcase Heater
The crankcase heater increases the temperature of the refrigeration oil inside the crankcase to a level higher than the compressor suction inlet temperature, thereby effectively suppressing refrigerant migration. In practical applications, this device must maintain continuous operation to ensure stable protective effects.
However, its protective limitations must be clearly understood: the crankcase heater cannot prevent damage to the compressor caused by liquid hammer; additionally, the heater requires a lengthy preheating process before each compressor startup, which may restrict operational flexibility in time-sensitive or frequently started/stopped applications.
Suction Line Gas-Liquid Separator
Installing a gas-liquid separator on the suction pipe is an effective measure to prevent liquid hammer in refrigeration compressors. It uses gravity and velocity differences to store liquid refrigerant at the bottom, while gaseous refrigerant enters the compressor from the top, preventing direct liquid impact on the compressor and ensuring stable refrigerant circulation.
The selection of a gas-liquid separator should consider system cooling capacity, refrigerant type, pressure range, and other parameters. Low-temperature systems require separators with large capacity and high separation efficiency; compact, low-flow models are suitable for small household appliances. Additionally, the installation location should be close to the compressor suction inlet to shorten the flow path of liquid refrigerant and reduce the risk of liquid slugging.
Suction Pipe Superheat Heater
In cold operating conditions, even if the compressor maintains stable operation, there is still a risk of residual liquid refrigerant in the suction line. In such cases, installing an electric heater on the compressor suction pipe section or using hot gas bypass technology can quickly increase the suction superheat. Both solutions are easy to implement and significantly reduce the risk of liquid slugging. If paired with a superheat detection device, real-time monitoring and precise temperature control can be achieved, effectively preventing compressor liquid slugging failures at their source.
Additionally, during the design phase of the refrigeration system, optimizing the throttle valve opening and increasing the evaporator heat exchange area can significantly enhance suction superheat, thereby reducing the risk of liquid slugging during system operation. During refrigerant charging, liquid charging from the low-pressure side must be strictly prohibited to prevent compressor damage caused by liquid-carrying startup.
Conclusion
Liquid hammering in refrigeration compressors is caused by liquid return, liquid-carrying start-up, and excessive refrigerant, which can damage various types of compressors. It can be prevented by controlling the amount of refrigerant charged, installing heating devices, properly installing separation equipment, and standardizing system design and charging processes to ensure the stable operation of the refrigeration system.
