How to Solve the High Pressure Fault and Oil Return Failure of Chiller?

In the field of air conditioning and refrigeration, cooling equipment is primarily categorized into air-cooled and water-cooled units. Air-cooled units rely on fresh air heat exchange for cooling and heating, while water-cooled units utilize thermal carrier water in conjunction with fan coil units to provide cooling and heating.

Chillers are widely used in industrial and commercial settings, also known as refrigeration units. Requirements vary based on specific application needs. They consist of four core components—compressor, evaporator, condenser, and expansion valve—which work together to achieve cooling and heating.

During chiller operation, high-pressure faults are relatively common. This fault indicates excessively high discharge pressure from the compressor, triggering high-pressure protection. Discharge pressure reflects condensing pressure, with a normal range of 1.4–1.8 MPa. The protection threshold should not exceed 2.0 MPa. Sustained high pressure causes excessive compressor current, leading to motor burnout and equipment damage.

What is a High-Pressure Fault?

A high-pressure fault in a chiller occurs when compressor discharge pressure exceeds 2.0 MPa, triggering the high-pressure protection relay to activate self-protection. During normal operation, compressor discharge pressure should remain between 1.4–1.8 MPa. Sustained overpressure causes a sharp increase in operating current, leading to motor winding overheating, insulation degradation, short circuits, and burnout. This further results in mechanical damage such as compressor bearing wear and piston seizure, potentially causing complete refrigeration system failure.

Primary Causes and Solutions for High-Pressure Faults

Primary Cause

Excessive Refrigerant Charge

Overcharging is a common cause. This typically occurs after equipment maintenance, manifesting as elevated suction, discharge, and equilibrium pressures alongside high compressor operating current. Solution: Under rated conditions, discharge refrigerant based on suction/discharge pressures, equilibrium pressure, and operating current until all parameters normalize.

Excessively High Cooling Water Temperature

Elevated cooling water temperatures impair condensation efficiency, triggering high-pressure faults in chillers. The original text addresses both causes and solutions; abbreviations may simplify the description while retaining critical information.

Cooling water temperatures exceeding the rated range (30–35°C) cause poor heat dissipation, raising condensing pressure—particularly pronounced during hot seasons. Troubleshooting involves inspecting cooling tower operation, including fan activation/deactivation, rotation direction, and water distributor rotation. For issues like high ambient temperatures, short water circuits, or low circulation volume, installing a storage tank can lower water temperatures.

Insufficient Cooling Water Flow

Insufficient cooling water flow is another significant factor. This manifests primarily as a reduced pressure differential between the unit’s inlet and outlet water (compared to the initial system pressure differential) and an increased temperature differential. Solutions include inspecting pipeline filters for blockages or overly fine mesh that restricts water flow capacity. Appropriate filters should be selected and screens cleaned regularly. Additionally, verify that the pump size is adequate and properly matched to the system.

Condenser Scaling or Blockage

Condenser scaling and blockage are also critical issues. Since tap water is commonly used for condensation, scaling occurs easily above 30°C. Open cooling towers allow dust and debris to enter the cooling water system, causing condenser fouling. This reduces heat exchange area, lowers efficiency, and affects water flow. Symptoms include increased pressure differential and temperature difference between inlet and outlet water, elevated temperatures at both ends of the condenser, and hot copper outlet pipes. The solution involves regular backwashing and chemical cleaning for descaling when necessary.

False Alarms Caused by Electrical Faults

False alarms triggered by electrical faults often result from moisture damage to high-pressure protection relays or unit circuit boards, as well as communication failures. Symptoms include: circuit board fault indicator lights failing to illuminate or glowing dimly, ineffective manual reset of high-pressure protection relays, and normal compressor operating current alongside standard suction and discharge pressures. The remedy is to inspect and replace damaged components.

Non-Condensable Gases in Refrigerant

The presence of non-condensable gases like air or nitrogen in the refrigerant is another cause. When air is present in the refrigeration system, the high-pressure gauge needle often exhibits severe oscillation. This typically occurs after servicing when vacuum evacuation is incomplete. The solution is to shut down the system, vent from the highest point of the condenser, or perform a new vacuum evacuation and refrigerant charge.

How to Address

• Refrigerant Overcharge: Excess refrigerant increases compressor load and reduces cooling efficiency. Under rated operating conditions, monitor parameters like pressure and current using professional instruments. Slowly discharge refrigerant until readings return to normal. Connect recovery equipment to the discharge port during discharge to prevent refrigerant leakage and environmental contamination.
• Excessively high cooling water temperature: Elevated cooling water temperature impairs system heat dissipation. Verify cooling tower fan rotation direction; reverse rotation requires adjusting power phase sequence. Inspect water distributor for blockages or jamming and clean promptly. If caused by external high temperatures, short water circuits, or insufficient circulation volume, increase storage tank capacity or install auxiliary cooling coils for heat dissipation.
• Insufficient Cooling Water Flow: Low flow causes inadequate heat dissipation and potential failures. Investigate three areas: Inspect pipeline filters—replace with compatible models and clean regularly if screens are clogged or pipe diameters are too narrow; verify pump specifications—replace pumps with mismatched flow rates or head; check for pipe blockages or kinks and repair/clear promptly.
• Condenser scaling or blockage: Scaling or blockages in the condenser impair heat exchange. Implement a regular backflushing regimen using high-pressure water flow to reverse-flush and remove scale. For stubborn blockages, employ chemical cleaning with suitable descaling agents circulated through the system. Conduct thorough inspections, protective measures, and rinsing before and after cleaning to prevent equipment corrosion and chemical residue buildup.
• False alarms caused by electrical faults: Moisture ingress or damage to electrical components can trigger false high-pressure fault alarms. Troubleshooting involves inspecting terminal connections for tightness and moisture, as well as checking circuit boards for short circuits and component damage. Moisture-affected parts can be dried; damaged components should be replaced with identical specifications.After replacement,conduct comprehensive system testing to confirm fault resolution.
• Non-condensable gases mixed with refrigerant: Non-condensable gases in the refrigerant can cause elevated system pressure and reduced cooling efficiency. Resolution: After shutting down and allowing system pressure to stabilize, open the vent valve at the highest point of the condenser to purge air. If ineffective, use a high-precision vacuum pump to evacuate the system to the specified vacuum level. Then recharge refrigerant according to standard procedures to ensure refrigerant purity and efficient system operation.

Understanding Oil Return Failures in Chillers

Oil return failure in water-cooled units refers to the inability of lubricating oil to return to the compressor normally within the refrigeration system. During normal operation, lubricating oil circulates with the refrigerant to lubricate and cool the compressor before returning. When failure occurs, oil accumulates in the evaporator or piping, causing insufficient compressor lubrication. This leads to component wear, reduced heat dissipation, and increased temperatures. Severe cases may result in compressor seizure or motor burnout, impacting the unit’s cooling efficiency and lifespan.

Common Causes and Solutions for Oil Return Failure

Common Causes

The primary causes of oil return failure in water-cooled units can be analyzed from system design, operating conditions, and component abnormalities, specifically:

Obstructed Oil Return Path

Improper refrigerant piping design is the main cause of poor oil return. Factors such as pipe diameter, elbows, horizontal sections, and slope all affect oil return:

1. Pipe diameters below design standards significantly increase mixed-fluid resistance. In high-pressure systems like R410A, a 10% reduction in pipe diameter increases oil return pressure loss by 20%-30%.
2. Excessive 90-degree elbows create vortices, causing oil droplets to adhere to pipe walls.
3. Horizontal runs exceeding 6 meters without a slope of at least 1/100 readily form “oil plugs.”
4. Insufficient slope causes oil accumulation after shutdown, obstructing return flow.

Abnormal Refrigerant Circulation

Refrigerant charge volume exhibits a nonlinear relationship with oil return efficiency. When charge falls below 80% of rated value, refrigerant oil-carrying capacity significantly decreases. At low temperatures, oil viscosity increases and fluidity worsens, exacerbating oil return difficulties.

Exceeding 110% of rated value may cause liquid refrigerant to form a “liquid seal” in the suction line, potentially flooding the oil separator in a flooded evaporator and causing oil return failure. Additionally, system leaks and abnormal throttling devices disrupting refrigerant circulation indirectly impair oil return efficiency.

Compressor Operating Conditions

When compressor load falls below 30% of rated capacity and suction velocity drops below 0.8 m/s, effective oil return becomes impossible. In multi-head parallel systems, low-load compressors with reduced suction pressure cause lubricating oil to flow toward high-load compressors. Frequent start-stop cycles (intervals <5 minutes) in screw compressors prevent stable oil circulation within 3-5 minutes, leading to oil accumulation in evaporators and piping and preventing the establishment of a stable oil return pressure differential.

Component Failure or Blockage

Failures in components like oil separators, filters, dryers, or evaporators severely impair oil return in water-cooled units. Damaged oil separator blades reduce separation efficiency from 98% to below 60%; Filter blockage causes differential pressure exceeding 0.15MPa; molecular sieve powdering in dryers forms sludge that clogs throttle valves; evaporator frost exceeding 15mm reduces heat exchange efficiency by 30%, and each 0.0003m²・K/W increase in fouling thermal resistance decreases oil return efficiency by 5%. Abnormal Lubricant Characteristics

Lubricants must precisely match refrigerants and operating conditions. When using POE lubricants in R32 systems, misusing ISO VG32 instead of VG68 causes low-temperature start-up viscosity to surge over 200%, resulting in loss of oil fluidity. Oxidized lubricant (acid value > 0.2 mgKOH/g) generates sludge that clogs capillary tubes, while additive particle precipitation accelerates wear. Lubricants with excessive moisture content (>50 ppm) form “ice sludge” at low temperatures, severely impeding oil return.

These factors, acting individually or in combination, prevent normal oil return and cause unit failure.

How to Address

• A

When oil is trapped in the evaporator, closely monitor chilled water temperature. First, moderately reduce flow to lower outlet water temperature, then rapidly increase flow to sharply raise evaporator outlet temperature. This will cause water temperature to significantly exceed refrigerant saturation temperature, inducing violent refrigerant boiling. Chilled oil will then be drawn into the compressor along with churning refrigerant foam.

The discharge temperature should decrease slightly but remain well above the separation temperature of oil and refrigerant. This carries the oil back to the oil separator for separation. After repeating this process several times, all oil will be recovered into the oil separator. Note that the timing intervals between each water flow adjustment are crucial; ideally, the discharge temperature should return to its optimal pre-adjustment state after each adjustment.

• B

When oil is present in the condenser, water temperature becomes less critical—as long as the unit operates normally, significant issues are unlikely. Gradually open the expansion valve to its maximum position (avoid rapid opening to prevent excessive liquid flow damaging evaporator copper tubing). Wait several minutes until high and low pressures equalize.

At this point, observe substantial white foam through the evaporator sight glass—indicating oil has migrated from the condenser into the evaporator. Simply repeat the steps in A to recover the oil quickly.

How to Prevent High Pressure Faults and Oil Return Failure ?

Preventing high-pressure faults and oil return issues requires integrating the unit’s operating principles with common causes. Targeted measures should be developed through routine maintenance, operational monitoring, and system inspections, as detailed below:

Measures to Prevent High Pressure Faults

• Ensure efficient operation of the condensing system

1. Clean the condenser: For water-cooled units, regularly remove scale and impurities from copper tubes (via chemical or mechanical cleaning) and ensure cooling water flow meets specifications. For air-cooled units, clear dust and debris from fins to guarantee fan operation and heat dissipation.
2. Monitor temperatures: Maintain cooling water inlet temperature between 30-35°C. Investigate issues related to the environment, cooling tower, or water circuit. For air-cooled units, prevent condenser obstruction and ensure adequate ventilation.

• Maintain Normal Refrigerant System Operation​

1. Strictly control refrigerant charge: Avoid overcharging (which can cause sudden condensing pressure spikes). Regularly inspect refrigerant levels via pressure gauges and sight glasses. Repair leaks promptly and replenish to standard charge.
2. Ensure proper function of exhaust valves, safety valves, etc.: Regularly verify compressor exhaust valves are fully open and safety valves reliably trip at specified pressure (e.g., 2.0MPa) to prevent abnormal exhaust pressure increases due to valve sticking.

• Optimize compressor operating conditions

1. Avoid overloading: Adjust operating parameters based on the unit’s rated capacity to prevent prolonged full-load or overloaded operation that increases discharge pressure. Implement reasonable unloading protection logic to automatically reduce load when pressure approaches protection thresholds.
2. Regularly inspect compressor components: Test motor winding insulation and bearing condition to prevent increased operational resistance from mechanical failures, which indirectly causes discharge pressure to rise.

Measures to Prevent Oil Return Failures

• Enhance Oil Return Path Stability

1. Regularly inspect piping conditions: Ensure suction lines are free of deformation or blockages, maintain design-specified slope (≥1%), and prevent liquid accumulation at elbows or valves. Manually drain accumulated lubricant from long lines or oil-prone areas periodically.
2. Maintain auxiliary oil return devices: Regularly clean oil separators and return bends. Verify responsiveness of oil level controllers, solenoid valves, and other components to ensure separated lubricating oil promptly returns to the compressor.

• Maintain refrigerant-lubricant compatibility

1. Strictly use compliant oils: Charge only specified grades and viscosities per unit manuals; avoid mixing different oil types. Regularly test oil quality (e.g., viscosity, moisture content) and replace immediately upon detecting degradation.
2. Stabilize refrigerant circulation: Monitor refrigerant flow through the sight glass to ensure no bubbles or interruptions, preventing oil-carrying capacity issues caused by insufficient or excessive refrigerant. Thoroughly evacuate the system after maintenance to prevent moisture or air ingress that could degrade oil performance.

• Standardize Operation and Maintenance Procedures​

1. Optimize operating strategies: Set minimum compressor load thresholds to avoid prolonged low-load operation. If low-load operation is necessary, switch to full load for 10-15 minutes every 2 hours to assist oil return. Maintain a minimum 10-minute interval between start/stop cycles to ensure adequate oil return time.​
2. Implement Regular Inspections:Maintain oil level at 1/2 to 2/3 capacity monthly via oil sight glass;Clean filters and dryer cartridges quarterly; Perform annual deep cleaning of evaporators and condensers to prevent impurity blockages and heat exchange surface scaling that impede oil circulation.

Through these measures, the probability of high-pressure and oil return failures can be reduced at the source, extending unit lifespan and ensuring stable, efficient operation.

Conclusion

In summary, while high-pressure and oil return failures in chillers involve complex causes, understanding their origins, implementing proper handling procedures, and adopting effective preventive measures can significantly minimize their impact. Equipment managers should prioritize routine maintenance and regular inspections to eliminate potential faults at their earliest stages. This ensures chillers deliver continuous, stable service for production and daily operations, thereby creating greater value.