Screw-type refrigeration compressors serve as the core of refrigeration systems, widely utilized in industrial and commercial applications due to their high efficiency and stability. Mastering their core components, operating principles, and troubleshooting methods is crucial for ensuring system reliability, extending equipment lifespan, and reducing operational costs. This article provides practical guidance.
Core Components of Screw Refrigeration Compressors
Screw-type refrigeration compressors are highly efficient positive displacement refrigeration units. Their structure comprises both fundamental core components and specialized assemblies designed for the refrigeration cycle. These parts work in concert to achieve stable refrigeration performance, as detailed below:
Rotors
Comprising two helical rotors with convex blades—a male rotor and a female rotor—these intermesh within the housing to form the working chamber, which is fundamental to compressor operation. Typically larger than the male rotor, the female rotor is driven by the prime mover and rotates the male rotor.
Suction Valve
Positioned at the inlet of the compression chamber, this critical component controls gas flow, capturing and reducing the ingress of dust and harmful particles. It cleans and protects the compressor stage and can be controlled pneumatically or electrically.
Prime Mover (Typically an Electric Motor)
Provides power to the compressor. Its type may vary depending on the design, and the power transmission system may differ among compressors from various manufacturers and models.
Oil System
Comprising an oil tank, oil filter, and oil separator, this system ensures essential lubrication and cooling for compressor operation.
Additionally, screw refrigeration compressors incorporate numerous precision components such as couplings, gears, pistons, housings, and belts. They are equipped with air receivers that buffer and store gas between the compressor and the gas usage system. These receivers are categorized into primary and secondary air receivers.
Working Principle of Screw Refrigeration Compressors
Screw-type refrigeration compressors belong to the category of positive displacement compressors. They utilize a pair of intermeshing male and female rotors rotating within the housing. By periodically altering the volume between each pair of rotor teeth, the compressor completes the processes of suction, compression, and discharge.
Suction Process
As the rotors rotate, the tooth cavity volume gradually expands and connects to the suction inlet. Gas from the evaporator system enters the tooth cavity through the inlet port, initiating the suction process. After the rotors rotate to a certain angle, the inter-tooth volume passes the suction port position and disconnects from it, concluding the suction process.
Compression Process
As the rotors continue rotating, the gas enclosed within the tooth cavity by the housing, suction end cap, and discharge end cap is compressed toward the discharge end. This occurs due to the meshing of the male and female rotors and the interlocking of the teeth, causing the pressure to gradually increase during the compression process.
Discharge Process
When the rotors rotate to align the tooth cavity space with the discharge port on the discharge end cap, the gas is forced out and expelled through the discharge flange, completing the discharge process. Since each tooth cavity space undergoes these three processes per working cycle, multiple tooth cavity pairs simultaneously repeat the intake, compression, and discharge cycles during high-speed compressor operation. This ensures continuous and stable gas delivery.
During operation, screw-type refrigeration compressors discharge large quantities of lubricating oil mixed with refrigerant vapor. Therefore, an oil separator is installed between the compressor and condenser. The oil separator separates lubricating oil carried in the compressor discharge, ensuring the refrigerant entering the condenser is pure. This prevents lubricating oil from entering the condenser and reducing its efficiency. The oil separator also functions as an oil reservoir.
The lubricating oil separated from the high-temperature, high-pressure oil-gas mixture discharged from the compressor is at a relatively high temperature. It cannot be directly sprayed back into the compressor and must first be cooled in an oil cooler to achieve the viscosity and temperature required by the compressor before reuse.
Water-Cooled Oil Cooler
The water-cooled oil cooler is a horizontal shell-and-tube heat exchanger, with oil circulating outside the tubes and water inside. The tube bundle is secured to both end tube sheets. Baffles within the oil cooler shell enhance heat exchange between oil and cooling water. Impurities in the water may cause scaling inside the cooler tubes, reducing heat transfer efficiency. Therefore, regular inspection and cleaning are essential.
During winter shutdowns, ensure the drain plug on the water cap is opened to drain water from the oil cooler, preventing ice damage. The cooling water inlet temperature should be below 32°C. Unit oil temperature should be maintained between 40–65°C. When using WL series specialized lubricating oil, oil temperature can be controlled between 40–70°C.
Thermosiphon Oil Cooler
The thermosiphon oil cooler shares a similar principle to the water-cooled oil cooler, featuring a horizontal shell-and-tube design with oil outside the tubes and refrigerant inside. After condensing in the condenser, the liquid refrigerant flows into the thermosiphon receiver, where a portion is diverted into the thermosiphon oil cooler. Along the way, it absorbs heat from the high-temperature oil outside the tubes and evaporates.
As the refrigerant evaporates, its density gradually decreases. The density of the gas-liquid mixture in the oil cooler suction line becomes lower than that of the liquid in the oil cooler supply line. This imbalance creates a pressure differential that drives the refrigerant flow through the oil cooler. The oil temperature after passing through the thermosiphon oil cooler is typically 10–20°C higher than the condensing temperature.
Liquid Spray Cooling
In units equipped with liquid spray cooling, a line of high-pressure refrigerant liquid drawn from the condenser or receiver of the unit or system passes through a filter, throttle valve, or high-temperature expansion valve before being sprayed into an intermediate port of the compressor.
This serves to absorb compression heat and cool the oil temperature. The opening degree of the high-temperature expansion valve is determined by the discharge temperature. When the discharge temperature is high (above 55°C), the valve opening increases; when the discharge temperature is low (below 50°C), the valve opening decreases.
The compressor features two liquid injection ports: a high-level injection port and a low-level injection port. Liquid refrigerant is injected through the low-level port and discharged through the high-level port. Units equipped with liquid injection systems eliminate the need for an oil cooler, resulting in a more compact and streamlined design.
Common Screw Refrigeration Compressor Faults and Solutions
High Start-up Load
Capacity Control Not at Zero Position
During compressor start-up, if the capacity control device fails to reduce the load to zero, excessive start-up resistance may occur, potentially causing equipment malfunction. In this case, gradually operate the load reduction device to bring the capacity control to zero, enabling the compressor to start smoothly under no-load conditions.
Excessive Misalignment Between Compressor and Motor
Significant misalignment between the compressor and motor causes abnormal vibration and noise during operation. Prolonged operation may damage bearings, shaft seals, and other components. Use professional alignment tools to re-align the components according to installation specifications, ensuring coaxiality meets standards.
Internal Wear and Burn Damage in Compressor
Insufficient lubrication or contamination ingress may cause wear and scorching on internal components like pistons, cylinders, and bearings, impairing compression efficiency and normal operation. Promptly disassemble the compressor for comprehensive inspection of affected parts, repairing or replacing them based on damage severity.
Power Outage or Low Voltage
A power outage halts compressor operation, while low voltage prevents normal startup or causes overload during operation. Inspect power lines, circuit breakers, fuses, etc., to eliminate electrical faults. Ensure stable power supply voltage meets the product’s rated voltage requirements.
Improper adjustment of pressure controller or temperature sensor
Pressure controllers and temperature sensors monitor system pressure and temperature. Improper adjustment causing contacts to remain open prevents the compressor from starting or stopping according to set conditions. Use specialized tools to precisely adjust contact positions based on equipment operating parameters, ensuring proper function.
Pressure Differential Controller or Relay Tripped and Not Reset
The pressure differential controller monitors system pressure differentials, while the relay controls circuit activation. If either remains tripped without resetting, it disrupts the compressor’s control logic. Locate the reset buttons for both the pressure differential controller and relay, press them to restore normal operation.
Motor Winding Burnout or Open Circuit
Motor windings may burn out or develop open circuits due to overload, short circuits, or other issues, preventing normal motor operation. Use testing tools like a multimeter to accurately identify the fault point in the windings. Rewind or replace any burnt-out or open-circuit windings.
Burnt Contactor/Intermediate Relay Coils or Poor Contact
Contactors and intermediate relays play critical roles in control circuits. Burnt coils prevent engagement, while poor contact leads to unstable circuit operation. Disassemble the contactor and intermediate relay to inspect coils and contacts. Repair or replace damaged components.
Improperly Adjusted or Faulty Temperature Controller
Inaccurate setpoints or controller malfunctions prevent the compressor from effectively regulating based on actual temperatures. Adjust the controller’s setpoint according to process requirements. Replace the controller promptly if faulty.
Control Circuit Failure
Issues such as aged wiring, loose connections, or damaged components within the control circuit can disrupt normal compressor operation. Conduct a comprehensive inspection of the entire control circuit, including wiring connections and component functionality, to identify and rectify faults.
Excessive Unit Vibration
Unit Footing Bolts Loose
- Symptoms: Noticeable swaying during operation accompanied by abnormal vibration noise. Prolonged neglect may worsen bolt loosening, cause equipment displacement, compromise overall stability, or damage connected components.
- Remedy: Clean debris from anchor bolt holes and washer surfaces. Use appropriately sized shims to fill gaps, ensuring unit levelness meets specifications. Tighten anchor bolts in a diagonal cross pattern using a torque wrench in 2-3 stages to achieve specified torque. Re-check unit levelness and stability after tightening.
Excessive Misalignment between Compressor and Motor
- Symptoms: Periodic abnormal noise during operation, abnormal temperature rise at motor-compressor connection, accelerated shaft seal wear/leakage, reduced transmission efficiency. Severe cases may cause coupling damage or motor overload burnout.
- Remedy: Remove the coupling guard. Measure coaxiality deviation using a dial indicator or laser alignment tool. Loosen the foundation bolts securing the compressor and motor bases. Re-align by adding/removing shims or adjusting equipment position, ensuring radial deviation ≤0.05mm and axial deviation ≤0.03mm. After realignment, tighten the anchor bolts to standard torque, reinstall the coupling, and recheck coaxiality.
Resonance Due to Similar Natural Frequencies between the Unit and Piping
- Symptoms: Severe vibration in the unit and piping accompanied by high-pitched, shrill noise. Pipe supports and hangers may loosen, welds may crack, significantly impacting equipment lifespan and production safety, potentially causing medium leakage.
- Remedial Action: Use a vibration tester to measure the vibration frequencies of the pipeline and unit, analyzing the cause of resonance. Alter the pipeline’s natural frequency to avoid the resonance range by adding or adjusting support points (e.g., installing spring vibration-damping supports, changing rigid bracket spacing) or wrapping damping material to increase pipeline damping. Monitor vibration and noise changes after adjustments.
Excessive Liquid Refrigerant Ingestion
- Symptoms: Dull compressor operation sound, liquid hammering, sudden drop in suction temperature, reduced discharge temperature, frosting on compressor casing surface. Prolonged operation may damage compressor valve plates, deform piston connecting rods, or even cause compressor seizure.
- Remedy: Inspect the expansion valve operation. For thermostatic expansion valves(TEVs or TXVs), gradually adjust the rod to reduce valve opening and limit liquid supply. For electronic expansion valves (EEVs or ExVs), change PID parameters of controller (r06 is PIDproportional gain, suggest from 5 to 20), r07 is PID integral time, r08 is PID derivative time in EVDPro or EVDCool ) to restrict flow. During adjustment, closely monitor suction pressure, temperature, and compressor noise. Proceed incrementally to prevent excessive liquid supply from impairing refrigeration performance.
Abnormal Noise During Compressor Operation
Loose Coupling Key
A loose key causes unstable power transmission, leading to abnormal vibration and noise. Immediately tighten the bolts to the specified torque using a torque wrench. If the key shows wear, deformation, or other damage, replace it promptly. After replacement, check the coupling’s concentricity and axial deviation to ensure a secure connection and efficient power transmission.
Misalignment between Compressor and Motor
When misalignment between the compressor and motor exceeds limits, it accelerates bearing and seal wear and may cause severe equipment vibration. Use professional tools like a laser alignment tool or dial indicator to gradually correct horizontal and vertical deviations by adjusting the thickness and position of shims on the compressor or motor base, achieving coaxiality requirements. After adjustment, retighten the coupling bolts and conduct a test run to monitor vibration levels.
Excessive Liquid Refrigerant Ingestion
Liquid refrigerant entering the compressor may cause “liquid hammer,” damaging components like valve plates and pistons. First, adjust the expansion valve opening to control refrigerant flow while inspecting the liquid supply line for blockages or solenoid valve malfunctions. If liquid hammer has occurred, shut down the compressor to inspect internal components. Only restart after confirming no damage, and establish comprehensive liquid level monitoring and automatic protection mechanisms.
Foreign Objects Inside the Compressor
Foreign objects entering the compressor may scratch the cylinder wall, damage valve plates, or severely cause shaft seizure. During maintenance, thoroughly disassemble the compressor. Use specialized tools to remove residual metal debris, welding slag, and other contaminants. Inspect the suction filter’s efficiency; replace it immediately if damaged. After cleaning, flush and dry the compressor interior, then replace the lubricating oil to prevent residual contaminants from affecting operation.
Excessive Bearing Wear or Damage
Bearing wear increases operational noise and vibration, shortening the compressor’s lifespan. Before replacement, inspect the journal surface for wear or scratches; minor damage may be polished out. When installing new bearings, use specialized tools to ensure proper seating without hammering damage. After replacement, apply the appropriate amount of compatible grease and monitor bearing temperature and vibration to confirm normal compressor operation.
Conclusion
In summary, the stable operation of screw refrigeration compressors relies on the coordinated functioning of core components. Through the processes of suction, compression, and discharge, supplemented by oil cooling and liquid injection cooling, these units maintain efficiency.
During actual operation, common issues such as high startup loads, abnormal vibrations, and unusual noises may occur, requiring precise troubleshooting and resolution. Mastering this knowledge enables practitioners to maintain equipment effectively and ensure the efficient operation of refrigeration systems.
