Detailed explanation of water-cooled centrifugal chiller

Water-cooled centrifugal chillers are highly efficient and reliable refrigeration equipment widely used in large commercial buildings, industrial facilities, data centers, and other locations. This article will provide a detailed introduction to the systems, structural components, and control logic of water-cooled centrifugal chillers.

1. System Overview

A water-cooled centrifugal chiller compresses the refrigerant through a centrifugal compressor and then exchanges heat through a heat exchanger to achieve the purpose of cooling. The unit usually includes the following main parts:

(1) Centrifugal compressor: It uses a high-speed rotating impeller to compress the low-pressure refrigerant into high-pressure gas. It is the core component of the entire unit. Usually, in order to reduce the suction pressure loss and improve the working efficiency of the entire system, the compressor is installed on the evaporator.

(2) Evaporator: The low-pressure refrigerant absorbs the heat of the chilled water in the evaporator and evaporates into gas, thereby reducing the temperature of the chilled water. More and more system designs will choose a large temperature difference and a small flow rate. For example, 90.1 recommends a temperature difference of more than 15F for selection.

(3) Condenser: High-pressure refrigerant gas exchanges heat with cooling water in the condenser, releasing heat and condensing into liquid.

(4) Throttling device: usually an expansion valve, which reduces the pressure of the refrigerant and allows it to enter the evaporator and continue to circulate.

2. Structural composition

(1) Compressor part:

① Impeller: driven by a motor, it rotates at high speed to generate centrifugal force on the gas, thereby increasing the gas pressure. Impellers are divided into open impellers and closed impellers according to their structural form. Open impellers have greater leakage loss than closed impellers, but the flow channel surface is generally smoother and has less flow loss. Closed impellers are usually cast, so the inner surface is difficult to polish or the polishing cost is high. The leakage loss is smaller, but the friction loss is greater.

② Diffuser: This converts the kinetic energy of high-speed gas into pressure energy, further increasing gas pressure. These are typically divided into vaned and vaneless diffusers. The figure below shows a vaned diffuser. This diffuser uses vanes to guide the airflow, thereby improving diffusion efficiency. The advantage is high compression efficiency at the rated point. However, compressors cannot always operate at the rated point; they spend most of their time operating at partial load, causing airflow deflection. At this point, the vanes can cause impact losses, reducing partial load compression efficiency. Therefore, vaneless diffusers are often used.

(2) Heat exchanger part:

① Evaporator: The interior is full of pipes. Chilled water flows in the pipes, and the refrigerant evaporates outside the pipes, absorbing the heat of the chilled water. Evaporators can be divided into flooded, falling film and dry evaporators according to their structural form. I will explain this in a separate page, with detailed parameters, structural characteristics and comparisons of various evaporators.

② Condenser: The structure is similar to the evaporator, but the function is opposite. The refrigerant flows in the pipes, and the cooling water flows outside the pipes. The refrigerant releases heat and condenses.

(3) Control part:

① Control panel: Includes various switches, display screens and control buttons for monitoring and adjusting the operating status of the unit.

② Sensors: Includes temperature sensors, pressure sensors and flow sensors, etc., for real-time monitoring of system parameters to ensure safe and efficient operation of the unit.

(2) Heat exchanger part:

① Evaporator: The interior is filled with pipes. Chilled water flows in the pipes, and the refrigerant evaporates outside the pipes, absorbing the heat of the chilled water. Evaporators can be divided into flooded, falling film, and dry evaporators according to their structural form.

② Condenser: The structure is similar to the evaporator, but the function is opposite. The refrigerant flows in the pipes, and the cooling water flows outside the pipes. The refrigerant releases heat and condenses.

(3) Control part:

① Control panel: Includes various switches, display screens, and control buttons for monitoring and adjusting the operating status of the unit.

② Sensors: Includes temperature sensors, pressure sensors, and flow sensors, etc., for real-time monitoring of system parameters to ensure safe and efficient operation of the unit.

3. Control Logic

The control logic of a water-cooled centrifugal chiller primarily consists of three parts: startup control, operation control, and protection control.

(1) Startup control:

① Pre-check: Before starting, the control system will perform a series of self-checks to ensure that all components are in normal condition.

② Startup sequence: Usually, the condenser pump, chilled water pump, and compressor are started in the order of one another to avoid excessive instantaneous current.

(2) Operation control:

① Load regulation: According to the change of cooling load, the output of cooling capacity is controlled by adjusting the speed of the compressor (for variable frequency units) or the blade angle. In addition, for units equipped with hot gas bypass, when the load is very low, the operating load of the unit can be reduced by opening the hot gas bypass, but the energy consumption will be relatively high at this time.

(3) Protection control:

① High and low pressure protection: When the system pressure exceeds the safety range, the control system will automatically shut down and issue an alarm. This part of the protection has two parts, one is through system pressure monitoring and active protection through software, and the other is through physical protection such as safety valves.

② Temperature protection: When the temperature of chilled water or cooling water is abnormal, the system will automatically adjust the operating parameters or shut down for protection. When the chilled water level is low, the copper pipes are at risk of freezing; when the cooling water temperature is high, there is a risk of high pressure or surge. In addition, there will be temperature protection for motors and inverters to protect motor windings and core components such as inverter IGBTs and diodes.

4. Summary

Water-cooled centrifugal chillers, with their high efficiency and reliability, play a vital role in large-scale refrigeration systems. Understanding their system components, structural features, and control logic will help you better understand their operating principles and maintenance methods, thereby improving the unit’s operating efficiency and service life.