Water-Cooled Screw Chiller Operation and Maintenance

Water-cooled screw chillers are used to provide chilled water for air conditioning systems. They are combined with air handling units such as air handling units and packaged air conditioners to form various large-scale centralized air conditioning systems, which are widely used in hotels, hospitals, shopping malls, office buildings, entertainment venues, and other locations for air conditioning. They can also be used for cooling in industrial production processes.

Main Components of Water-Cooled Screw Chillers

Water-cooled screw chillers primarily consist of semi-hermetic twin-screw compressors, shell-and-tube condensers, flooded evaporators, oil separators, throttling mechanisms, and electrical control systems.

• Evaporator: During operation, the evaporator maintains a low temperature and pressure to allow the evaporated refrigerant gas to absorb heat from the circulating chilled water.
• Condenser: During operation, the condenser maintains a high temperature and pressure to allow the circulating cooling water to absorb heat from the refrigerant.
• Screw compressor: Continuously sends the evaporated refrigerant gas from the evaporator to the condenser, maintaining the high-low pressure difference in the system.
• Oil separator: Separates the refrigeration oil carried by the refrigerant gas and directly returns it to the compressor, ensuring safe and reliable operation of the compressor.
• Electrical control system: Utilizes a PLC or microcontroller-based control system, capable of automatically adjusting the unit’s cooling capacity to meet actual user requirements; controls the water pumps and cooling tower fans on the usage side and heat source side; displays the following parameters: chilled water inlet and outlet temperatures, cooling water inlet and outlet temperatures, evaporation and condensation pressures, and other system parameters; allows for querying of current and historical fault records.

How Does a Water-Cooled Screw Chiller Work?

Water-cooled screw-type chiller units are a type of vapor compression refrigeration system. Its refrigeration principle involves the compressor applying energy to the refrigerant vapor, increasing its pressure and temperature, followed by condensation and throttling processes to reduce it to low-pressure, low-temperature refrigerant liquid.

This liquid then evaporates into vapor within the evaporator, simultaneously absorbing heat from the surrounding environment (the secondary coolant, such as chilled water) to lower its temperature, thereby achieving artificial refrigeration. As can be seen, the vapor compression refrigeration cycle includes four indispensable processes: compression, condensation, throttling, and evaporation. The principles are described as follows:

Compression process: The refrigerant vapor from the evaporator is drawn into the screw compressor. The motor applies energy to the refrigerant vapor through the compressor rotor, increasing its pressure and sending it to the condenser. Simultaneously, the temperature of the refrigerant vapor also increases at the end of the compression process.

Condensation process: The high-pressure, high-temperature refrigerant vapor from the compressor releases heat through the cooling water inside the condenser, causing its temperature to decrease. At the saturation pressure (the condensation pressure corresponding to the condensation temperature), it condenses into a liquid.

At this point, the cooling water, having absorbed heat from the refrigerant vapor, has its temperature increased. The temperature of the cooling water is directly related to the condensation temperature (condensation pressure).

Throttling process: High-temperature, high-pressure refrigerant liquid from the bottom of the condenser undergoes pressure reduction and expansion as it flows through the throttling device, causing both pressure and temperature to decrease. It then enters the evaporator as low-pressure, low-temperature liquid.

Evaporation process: The low-pressure, low-temperature refrigerant liquid absorbs heat from the heat transfer fluid (such as chilled water) within the evaporator, evaporating into a gas while lowering the temperature of the heat transfer fluid, thereby achieving artificial refrigeration. The refrigerant vapor within the evaporator is then drawn into the compressor for compression, repeating the aforementioned compression, condensation, throttling, and evaporation processes. This cycle continues indefinitely to achieve continuous refrigeration.

The cooling capacity is directly proportional to the compressor’s suction flow rate. Screw compressors are equipped with a sliding valve mechanism internally, which is used to control the compressor’s suction flow rate and, consequently, the refrigerant’s evaporation rate, enabling the cooling capacity to be infinitely adjustable within a certain range.

What are The Steps of Water-Cooled Screw Chiller Operation?

After completing the pre-commissioning inspection, the unit can enter the commissioning and operation phase. The specific operational steps are as follows:

1. Power supply preheating: Before starting the unit, power must first be supplied to the unit, but the unit should not be started at this point. The purpose is to allow the compressor heating element to begin operation. This heating process is critical, and the heating time must not be less than 8 hours.This is because adequate preheating effectively reduces the viscosity of the lubricating oil, ensuring proper lubrication of all components during compressor startup, preventing wear caused by cold starts, and extending the compressor’s service life.
2. Water Flow Regulation: Carefully adjust the flow control valve on the refrigeration/cooling water system or the inlet shut-off valve of the unit. During adjustment, utilize monitoring devices such as flow meters within the system to observe real-time changes in water flow values, ensuring the system water flow precisely meets operational requirements.

Insufficient flow may impair the unit’s heat exchange efficiency, resulting in poor cooling performance; excessive flow may increase the operational load on pumps and other equipment, leading to energy waste and equipment wear.

3. Troubleshooting: Access the unit’s fault log query interface via the operation display, and conduct a detailed inspection of all recorded fault information. If faults are detected, refer to the fault code prompts and consult the unit’s technical manual to systematically identify the root cause and completely resolve the issue. After confirming the unit is fault-free, perform a fault log clearance operation to ensure accurate recording of new faults during subsequent operations.
4. Parameter Verification and Startup: Conduct a comprehensive verification of controller parameter settings, including critical parameters such as temperature setpoint range, pressure protection values, and operating modes. Ensure these parameters align with actual usage requirements and equipment technical specifications. Only after confirmation should the power button be pressed to start the unit. Any parameter setting errors may cause abnormal unit operation or even safety incidents.
5. Post-Startup Inspection: After the unit is started, the first step is to quickly verify the rotation direction of the compressor. If reverse rotation is detected, the unit must be immediately shut down, and two phases of the power supply wiring must be adjusted to prevent mechanical damage and performance degradation caused by reverse rotation.

Simultaneously, closely monitor the lubricating oil condition of the compressor, ensuring the oil level in the oil sight glass remains between 1/2 and 2/3 of the visible range. If the oil level is too low, promptly replenish with lubricating oil that meets the unit’s requirements; if the oil level is too high, it may cause the lubricating oil to foam, affecting lubrication effectiveness and normal compressor operation.

6. Trial operation and settings: The unit undergoes a 30-minute trial operation. During this period, set the inlet water temperature based on the user’s load and the actual conditions of the water system. By observing changes in the unit’s operating parameters, such as cooling capacity, power consumption, and inlet/outlet water temperature difference, determine whether the unit is operating normally.

After shutdown, wait 10 minutes before restarting the unit. This is to protect the compressor’s motor windings and control system, preventing frequent starts from causing excessive current surges that could damage the equipment. Finally, carefully check the parameter settings on the operation display again to ensure that all parameters remain unchanged during operation, thereby concluding the trial run.

How to Maintain Water-Cooled Screw Chillers？

To ensure long-term reliable operation of the unit, commissioning should be conducted by professionals or users with adequate HVAC knowledge under professional guidance. Daily operation and maintenance must also be performed by HVAC personnel who have received professional training.

Water-cooled screw chiller units typically operate for approximately 3,000 hours per year. To ensure the unit operates safely and reliably over the long term, extend its operational lifespan, and reduce operating costs, regular scientific maintenance and servicing of the unit are of great importance.

Routine maintenance and servicing during unit operation and repairs are referred to as preventive maintenance and servicing. The customer is responsible for developing annual and monthly maintenance schedules tailored to the unit’s actual operating conditions, and for conducting daily and regular maintenance of the unit.

Daily Startup, Shutdown, and Emergency Operations

Daily unit startup and shutdown include both manual and automatic modes. Automatic startup and shutdown settings should refer to the “Controller Operation Manual” of each manufacturer’s automatic startup and shutdown section. During manual operation, startup and shutdown can be completed using the start/stop button on the controller; the emergency stop switch on the electrical control panel door is primarily used for forced shutdown during unit maintenance, testing, or sudden emergencies and is generally not required during normal operation.

Maintenance Points for Key Components

During unit operation, closely monitor the system’s exhaust and intake pressures at all times. If any abnormalities are detected, immediately investigate the cause and resolve the issue promptly. Additionally, avoid arbitrarily adjusting the setpoints of control and protective components to prevent interference with normal operation and safety protection functions.

Furthermore, electrical connections should be regularly inspected for loosening, and any loose connections should be tightened promptly. The reliability of electrical components should also be tested, and any faulty or unreliable components should be replaced promptly to ensure the stable operation of the electrical system.

Scale Removal Maintenance Strategy

After prolonged operation, the water-side heat transfer surfaces of shell-and-tube heat exchangers may accumulate minerals such as calcium oxide. When these deposits accumulate extensively on the heat transfer surfaces, they severely impair the heat exchanger’s heat transfer performance, leading to significantly increased energy consumption and abnormal rises in exhaust pressure. To address this issue, organic acids such as formic acid, citric acid, or acetic acid can be used to clean the heat exchanger, effectively removing deposits and restoring the unit’s heat transfer efficiency.

Special Operating Procedures for Winter and Spring Seasons

• Winter shutdown procedures: When shutting down the unit during winter, first thoroughly clean the inner and outer surfaces of the unit and dry them completely. Simultaneously, open the drain valve to completely drain all water from the shell-and-tube heat exchanger to prevent freezing at low temperatures, which could damage the equipment.
• Spring startup preparation: After a prolonged shutdown, if the unit is to be restarted, thorough preparatory work must be conducted in advance. This includes a comprehensive inspection and cleaning of the unit; cleaning the water piping system to ensure smooth water flow; checking the operational status of the water pump; tightening all electrical connections to prevent electrical faults; and preheating the unit’s compressor as required to create optimal conditions for smooth startup and operation.

Parts Replacement and Accessory Selection Standards

When replacing unit parts, prioritize the use of original manufacturer accessories provided by the brand manufacturer. Do not arbitrarily use similar accessories as substitutes. Original manufacturer accessories have higher compatibility with the unit in terms of quality, specifications, and performance, ensuring stable operation and extended service life of the unit, and preventing new faults caused by accessory quality issues.

Refrigerant Leak Detection, Recharge, and System Handling

By monitoring suction and discharge pressures in real-time, the refrigerant charge level can be determined. If a refrigerant leak is detected, it must be promptly replenished. After performing refrigerant recharge or replacing components in the refrigeration cycle system, the system must undergo a leak test to ensure there are no leakage points. Depending on the severity of the refrigerant leak, the refrigerant recharge operation can be divided into the following two scenarios:

• Complete refrigerant leakage handling process: If a complete refrigerant leakage occurs, the system must first be thoroughly inspected for leaks using high-pressure nitrogen (15–20 kg pressure) or refrigerant. If welding is required, the system must be completely evacuated of gas before welding can be performed. Before charging refrigerant, the entire refrigeration system must be kept dry and undergo vacuum evacuation. The specific steps are as follows:

1. 1. Ensure that all shut-off valves on the unit are open. Connect the vacuum hose to the refrigerant charging port on the shut-off valves of the compressor suction pipe and condenser discharge pipe.
   2. Use a vacuum pump to perform vacuum evacuation on the system piping.
   3. Once the required vacuum level is achieved, use the refrigerant charging port on the condenser discharge pipe shut-off valve to charge the refrigerant system with refrigerant from the refrigerant cylinder. The required refrigerant charge quantity can be referenced from the unit’s nameplate.

• Refrigerant replenishment procedure: If only refrigerant replenishment is required, follow these steps:

1. 1. Connect the refrigerant charging bottle to the refrigerant charging port on the compressor suction pipe.
   2. Start the refrigeration/cooling water circulation system and activate the unit.
   3. Slowly charge the system with refrigerant while closely monitoring changes in suction and discharge pressures.
   4. Important note: During leak detection and pressure testing, it is strictly prohibited to inject flammable or toxic gases such as oxygen or acetylene into the refrigeration system. Only high-pressure nitrogen or refrigerant should be used to ensure operational safety and prevent accidents.