Water-cooled Screw Chiller: Operation and Maintenance

I. Unit Structure
Screw water-cooled chiller unit is mainly composed of semi-hermetic twin-screw compressor, shell and tube condenser and flooded evaporator, oil separator, throttling mechanism, electrical control system, etc.
(1) Evaporator:
During the operation of the unit, the evaporator always maintains a low temperature and pressure so that the evaporated refrigerant gas can take away the heat of the chilled water flowing through it.
(2) Condenser:
During the operation of the unit, the condenser always maintains a high temperature and pressure so that the cooling water flowing through the condenser can take away the heat in the refrigerant.
(3) Screw compressor:
Continuously send the evaporated refrigerant gas in the evaporator to the condenser to maintain the high and low pressure difference of the system.
(4) Oil separator:
Separate the refrigeration oil discharged with the refrigerant gas and send it directly back to the compressor to ensure the safe and reliable operation of the compressor.
(5) Electric control system:
Adopting PLC or single-chip microcomputer control system, it can automatically adjust the unit's output cooling capacity to meet the actual needs of users; it can control the water pumps on the user side and heat source side and the cooling tower fan; display the following parameters: chilled water inlet and outlet temperatures, cooling water inlet and outlet temperatures, evaporation, condensation pressure and other system parameters; it can query current faults and historical fault records.

II. Introduction to Refrigeration System
Water-cooled screw chiller unit is a kind of vapor compression refrigeration unit. Its refrigeration principle is to apply energy to the refrigerant vapor through the compressor to increase its pressure and temperature. Then through the condensation and throttling processes, it becomes low-pressure and low-temperature refrigerant liquid. The refrigerant evaporates into vapor in the evaporator. At the same time, it takes heat from the surrounding environment (such as chilled water in the secondary refrigerant) to reduce the temperature of the secondary refrigerant, so as to achieve the purpose of artificial refrigeration. Thus, the vapor compression refrigeration cycle includes four essential processes: compression, condensation, throttling, and evaporation. The principles are described as follows:
Compression process:
After the refrigerant vapor in the evaporator is sucked in by the screw compressor, the motor applies energy to it through the compressor rotor to increase the pressure of the refrigerant vapor and enter the condenser; at the same time, the temperature of the refrigerant vapor also increases correspondingly at the end of compression.
Condensation process:
The high-pressure and high-temperature refrigerant vapor from the compressor releases heat through the cooling water in the tube in the condenser, and the temperature drops. At the same time, under the saturated pressure (the condensation pressure corresponding to the condensation temperature), it condenses into a liquid. At this time, the temperature of the cooling water will increase because it has taken heat from the refrigerant vapor. The temperature of the cooling water is directly related to the condensation temperature (condensation pressure).
Throttling process:
The high-temperature and high-pressure refrigerant liquid from the bottom of the condenser undergoes decompression and expansion when flowing through the throttling device. The pressure and temperature both decrease and become low-pressure and low-temperature liquid and enter the evaporator.

Evaporation process:
The low-pressure and low-temperature refrigerant liquid takes heat from the secondary refrigerant (such as chilled water) in the evaporator and then evaporates into gas. At the same time, the temperature of the secondary refrigerant is reduced, thus realizing artificial refrigeration. The refrigerant vapor in the evaporator is sucked in by the compressor again for compression, and the above compression, condensation, throttling, and evaporation processes are repeated. In this way, it cycles continuously to achieve the purpose of continuous refrigeration.
The cooling capacity is proportional to the suction flow rate of the compressor. The screw compressor is equipped with a slide valve mechanism inside it. It is used to control the suction flow rate of the compressor and also the evaporation amount of the refrigerant, so that the cooling capacity can be adjusted steplessly within a certain range.
Oil circuit circulation system:
Inside the compressor, the refrigeration oil relies on the high and low pressure difference of the system. Through the built-in oil circuit, it provides lubrication and cooling to the bearings and rotors respectively. During the continuous exhaust process of the compressor, the refrigeration oil will be discharged from the compressor together with the refrigerant gas. If the discharged refrigeration oil cannot return to the compressor, it may cause the compressor to lose oil and seriously damage the compressor. The oil return system ensures that the refrigeration oil discharged with the refrigerant gas smoothly returns to the compressor to ensure the safe and reliable operation of the unit.
The oil return system is mainly divided into two parts:
Oil separation return:
An efficient oil separator is installed between the compressor exhaust and the condenser. Most of the refrigeration oil discharged with the refrigerant gas will be intercepted by the oil separator and directly sent back to the compressor through the suction port.
High-pressure ejection oil return:
A small part of the refrigeration oil enters the condenser and will eventually gather in the evaporator. Relying on high-pressure liquid as power, the refrigeration oil gathered in the evaporator can be directly brought back to the compressor.
Introduction to control system:
The control system adopts microcomputer control. The control cabinet panel is equipped with power supply, operation, and fault indicator lights, which can conveniently and intuitively understand the current working status of the unit. In addition, the control cabinet panel is equipped with an emergency stop switch. When the unit has an emergency fault and needs to be shut down immediately, the emergency stop switch can be pressed to ensure the safety of the unit.

Warning: Do not operate this emergency stop switch when the unit is running, otherwise it will cause damage to the unit!
Since the compressor is an important part of the unit, the protections for the compressor are:
①Phase loss and reverse phase protection of power supply; Nanshe.
②Thermal overload protection;
③Current overcurrent protection;
④Compressor exhaust temperature protection;
⑤Internal overload protection of the compressor;
⑥High and low pressure difference protection.
The main functions of the display or touch screen are:
①Display system operation information.
②Display unit status information.
③Display unit fault information.
④Set unit operation parameters.
⑤Set the unit's automatic startup and shutdown time.

III. Electrical Wiring Construction
(1) Before any electrical installation work, ensure that the main power supply is cut off and install the main power switch cabinet at an appropriate location.
(2) Connect the main power cord and ground wire to the unit's electrical control cabinet through the wire connection hole and connect the wiring to the corresponding terminal block and ground terminal block. Ensure the correct connection of each phase L1, L2, and L3.
(3) Confirm that the voltage fluctuation of the main power supply is within ±10% of the nominal value on the nameplate, and the voltage imbalance is within ±2%. If it exceeds this range, keep the unit shut down and contact the local power department immediately.
The calculation formula for phase voltage imbalance is: voltage imbalance rate% = maximum deviation from average voltage / average voltage × 100%.
(4) Before connecting the main power cable, check the phase sequence of each power cord.
(5) Connect the control circuit: Correctly connect the control wires of the contactors of the chilled water pump, cooling water pump, and cooling tower fan, and the water flow switch to the unit's electrical control system. The main power supply should be connected 8 hours before startup and should be powered on continuously during the working season so that the compressor heating belt can heat when the compressor is not running, so that the refrigerant liquid accumulated in the compressor can evaporate and avoid adverse effects on the compressor caused by direct startup.

IV. Commissioning and Trial Operation
After the unit completes the installation, waterway connection, wiring and power-on procedures, it can enter the commissioning stage. The manufacturer's designated personnel must be on site to supervise and guide the startup commissioning and subsequent operation.
Warning: The unit can control the chilled water pump, cooling water pump, and cooling tower fan. However, before commissioning, the trial run of the water pump must not be controlled by the main unit. Temporary wiring can be used for the trial run of the above equipment.
Commissioning and trial operation:
(1) Before starting up, the unit must be powered on (the unit cannot be started) so that the compressor heating belt can work. The heating time should not be less than 8 hours.
(2) Carefully adjust the flow regulating valve on the chilled/cooling water system or the inlet shutoff valve of the unit to make the system water flow reach the usage requirements.
(3) Through operating the display, check the unit's fault records. If there is a fault, it needs to be eliminated after inspection. Confirm that there is no fault in the unit.
(4) Only after verifying that the controller parameter settings are correct can the start button be pressed to start the unit.
(5) After the unit is started, check the compressor rotation direction. If it is reversed, adjust two phases of the power supply wiring; check the compressor lubricating oil. The oil level in the oil sight glass should be maintained at a visible position.
(6) Run for 30 minutes. Set the inlet water temperature according to the user's load and the actual situation of the water system to ensure the normal operation of the unit. After shutdown, wait for 10 minutes before putting it into operation again to prevent frequent startups of the unit. Finally, carefully check the parameter setting of the operation display to end the commissioning and trial operation.
Pre-commissioning inspection and record form.

V. Maintenance
The water-cooled screw chiller unit is generally used for about 3000 hours/year (depending on climatic and geographical conditions). In order to ensure the long-term safe and reliable normal operation of the unit, extend the service life of long-term use, and reduce operating costs, regular scientific maintenance of the unit is very important.