In modern life, refrigeration systems are widely used in appliances like air conditioners and refrigerators, and are crucial for maintaining a comfortable environment and ensuring the storage of goods.
Their stable operation relies on the coordinated operation of four core components: the compressor, condenser, evaporator, and expansion valve, along with auxiliary components. The following will provide a detailed introduction based on their functional characteristics and types.
Compressor
As the core power source of the refrigeration system, the compressor is often referred to as the “heart” of the system. Its primary function is to compress and transport refrigerant, converting low-temperature, low-pressure refrigerant gas into high-temperature, high-pressure gas, thereby driving the entire refrigeration cycle. Based on differences in operating principles, compressors are primarily categorized into two main types: volumetric and speed-type:
- Positive-displacement compressors: These achieve refrigerant compression by altering the volume of the working chamber. Common types include piston-type, screw-type, and rolling rotor-type. Based on structural form, they can be further subdivided into open-type, semi-hermetic, and fully hermetic.
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- Piston compressors: These rely on the reciprocating motion of a piston within a cylinder to periodically alter the cylinder volume. Their operational process consists of four stages: suction, compression, exhaust, and expansion. They are characterized by mature technology and ease of maintenance, and are commonly used in applications such as cold storage facilities and commercial refrigerators.
- Screw compressors: These consist of a pair of intermeshing male and female rotors that change the volume between the teeth to compress the gas. Compared to piston compressors, screw compressors have a simpler structure and run more smoothly, making them widely used in large chiller units and industrial refrigeration applications.
- Rolling rotor compressors: These compressors use an eccentrically rotating rotor that rolls within a cylinder to create a variable-volume space. This type of compressor is compact and highly efficient, making it popular in residential air conditioners and small commercial refrigeration equipment.
- Speed-type compressors: These compressors utilize the centrifugal force generated by high-speed rotating components to compress the refrigerant, with the centrifugal compressor being a typical example. During operation, the refrigerant gas gains kinetic energy under the influence of the high-speed rotating impeller and is then converted into pressure energy in the diffuser.Centrifugal compressors offer advantages such as high capacity, high efficiency, and compact footprint, but they are sensitive to changes in refrigerant flow rate. They are suitable for air conditioning systems in large shopping malls, hospitals, and industrial applications in the chemical and pharmaceutical industries.
In practical applications, reciprocating (piston-type) compressors, with their mature technology and stable performance, are widely used in small to medium-sized commercial refrigeration systems, such as small supermarket refrigerators and convenience store freezers. They offer strong adaptability and low maintenance costs. Scroll or rotary compressors, with their compact size, low noise, and high energy efficiency, are the preferred choice for household refrigerators and air conditioners.
The former operates smoothly and quietly, while the latter is energy-efficient and highly efficient. Screw compressors play a key role in large cold storage facilities and district cooling systems due to their high efficiency and adaptability to operating conditions. They can achieve stepless energy regulation and have significant energy-saving effects.
Condenser
The function of a condenser is to release heat and cool high-temperature, high-pressure gaseous refrigerant into a liquid state. It can be classified into three types based on the cooling medium used: water-cooled shell-and-tube type, air-cooled type, and evaporative type:
Water-cooled shell-and-tube condenser: Utilizes a shell-and-tube structure, where high-speed water flow (1.8–3.0 m/s) inside the tubes exchanges heat with the refrigerant outside the tubes. This design achieves 30–50% higher heat transfer efficiency than other types. However, cooling water is prone to scaling, and the narrow tube diameter (15–30 mm) complicates cleaning and maintenance. It is suitable for large-scale industrial refrigeration and central air conditioning systems where water supply is abundant and water quality is stable.
Air-cooled condenser: Utilizes finned tubes for heat exchange, with fans driving airflow (2.5–3.5 m/s) for cooling. No water circulation equipment is required, offering flexible installation and water savings. However, due to the low thermal conductivity of air, larger fins and higher power are needed, resulting in higher noise levels and energy consumption. Suitable for water-scarce regions and small-scale cold storage facilities.
Evaporative condenser: Combines the advantages of water-cooled and air-cooled systems, using sprayed water evaporation for heat dissipation. It has 3-5 times the heat exchange capacity of air-cooled systems and saves 80%-90% water. However, it requires softened water and complex maintenance, making it suitable for medium to large-scale cold storage facilities, data centers, and other scenarios with high efficiency and water-saving requirements.
To maintain optimal performance, condensers require regular cleaning to remove accumulated dirt. Common cleaning methods include physical brushing, mechanical scraping, and chemical treatment:
Physical brushing: Using tools such as nylon brushes or high-pressure water jets to directly rinse surface dirt. Suitable for air-cooled condensers with mild fin tube dust accumulation, this method is simple to operate but has lower efficiency.
Mechanical scraping: For stubborn scale in water-cooled shell-and-tube condensers, spiral scrapers or rotating brushes are used to penetrate the interior of the pipes. This method requires disassembly of certain components and has higher technical requirements for operation.
Chemical treatment: Specialized cleaning agents containing citric acid, hydrochloric acid, and other components are circulated to effectively dissolve scale such as calcium carbonate and silicates. Prior to cleaning, small-scale test samples are used to determine the appropriate solution concentration, and residual acidic solutions are neutralized post-cleaning to prevent equipment corrosion. Combining multiple cleaning methods ensures long-term stable operation of the condenser.
Evaporator
As the core heat exchange component of a refrigeration system, the evaporator regulates environmental temperature by absorbing heat through the phase change of the refrigerant. Based on the type of cooling medium, evaporators can be categorized into two main types: liquid refrigerant evaporators and air-cooled evaporators:
- Liquid refrigerant evaporators: Typical examples include horizontal shell-and-tube evaporators (dry evaporators), where the refrigerant undergoes evaporation within the heat exchange tubes while chilled water (or brine) flows countercurrently outside the tubes. This type of evaporator employs a high-flow-rate design, with the refrigerant mass flow rate typically controlled at 180–200 kg/(m²·s). It also enhances heat exchange efficiency through multi-pass configurations to ensure sufficient contact between the refrigerant and the secondary coolant.
- Air-cooled evaporator: Widely used in split-type air conditioners, direct evaporation air coolers, and other equipment. This type of evaporator significantly increases the heat exchange area through fin structures, and in conjunction with an internal precision liquid distributor, achieves uniform refrigerant distribution, effectively improving air-side heat exchange performance to meet cooling requirements under various operating conditions.
Expansion Valve
The expansion valve is a critical valve that precisely regulates refrigerant flow and also serves as a throttling element. It automatically adjusts its opening based on changes in evaporator pressure to maintain appropriate superheat. The expansion valve not only effectively controls refrigerant flow but also performs cooling and throttling functions. Proper installation and maintenance of the expansion valve are crucial for ensuring the stability and energy efficiency of the entire refrigeration system.
Liquid Receiver
The liquid receiver is located after the condenser and before the expansion valve. Its primary function is to compensate for fluctuations in condenser liquid level caused by load changes. When the expansion valve opens or closes, the liquid level inside the condenser changes; without the additional refrigerant supplied by the liquid receiver, this may result in insufficient liquid at the front end of the expansion valve, thereby affecting its operational stability. Additionally, the liquid receiver serves as a separation vessel, helping to separate liquid refrigerant from vapor to ensure that only pure liquid refrigerant enters the expansion valve.
Oil Separator
During operation, the compressor inevitably carries away some lubricating oil, which may affect internal lubrication and reduce its lifespan. Therefore, oil separators are installed in some refrigeration systems to address this issue. The oil separator effectively separates lubricating oil from the high-temperature, high-pressure gas being discharged and returns the oil to the compressor via an internal control system. This prevents mechanical wear caused by oil deficiency and avoids the issue of excessive lubricating oil entering subsequent processes, which could reduce overall heat exchange efficiency.
Electromagnetic Valve
An electromagnetic valve is a type of valve that uses electromagnetic force to control its open/closed state, widely used in various types of refrigeration equipment. Depending on the application scenario, it can be normally closed (NC) or normally open (NO).
When the refrigeration cycle needs to be started, the temperature sensor detects a temperature rise to the set value, the power terminals are connected, and the electromagnetic valve opens to allow refrigerant to flow into the evaporator; conversely, when the temperature drops below the preset point, the electromagnetic valve is de-energized and closed, preventing more refrigerant from flowing into the evaporator, thereby regulating the temperature inside the refrigeration chamber.
Pressure Controller
To protect the compressor and other critical components from damage caused by excessive or insufficient pressure, a pressure controller is typically installed in the system. Single-pressure controllers are divided into low-pressure controllers and high-pressure controllers, which monitor the pressure levels on the evaporator side (low-pressure side) and condenser side (high-pressure side), respectively; dual-pressure controllers integrate both functions into one unit.
If abnormal conditions are detected, such as low suction pressure or high discharge pressure, the pressure controller will cut off the power supply to stop the compressor from running, thereby ensuring the safety of the entire system.
Dryer Filter
The dryer filter is an important component used to remove moisture and other impurities from the refrigerant. The presence of moisture can cause ice blockage, particularly when using a capillary tube as the throttling device. Additionally, it accelerates the corrosion process of metal pipes and fittings, shortening the equipment’s service life. Therefore, installing a dryer filter in the refrigerant circuit is an essential measure.
Sight Glass
The sight glass serves as the “visual diagnostic window” of the refrigeration system and is typically installed at critical points in the refrigerant circulation piping. Its core function is to visually display the flow state of the refrigerant through an internal transparent observation window. When the system is operating, operators can quickly determine whether there are issues such as refrigerant leaks, excessive moisture content, or oil contamination by observing the presence of bubbles, changes in refrigerant color, and transparency within the sight glass. These visual diagnostic indicators provide system maintenance personnel with precise references for assessing refrigerant levels and promptly replenishing refrigerant.
Gas-Liquid Separator
The gas-liquid separator is typically installed before the compressor inlet. Its primary function is to prevent liquid from directly entering the compressor and causing liquid hammering. Liquid hammering can cause severe damage to the compressor and may even lead to its complete failure. By using a gas-liquid separator, gas and liquid can be effectively separated, ensuring that only gas is drawn into the compressor, thereby protecting the compressor’s safety.
Temperature Sensors and Pressure Sensors
Various types of sensors are employed to achieve more precise control and monitoring. Temperature sensors measure temperature changes at different locations, such as the evaporator outlet temperature and condenser inlet temperature; pressure sensors monitor pressure conditions on the high-pressure side and low-pressure side. The data collected by these sensors is fed back to the control system, helping to optimize operating parameters and respond promptly to any potential issues.
Controller / PLC
The controller and PLC serve as the “intelligent hub” of the refrigeration system, constructing a dynamic model of system operation by real-time collection of sensor data such as temperature, pressure, and flow rate. Based on pre-set control logic and algorithms, they precisely execute commands such as compressor start/stop scheduling, variable-speed control of cooling fans, and electromagnetic valve on/off control, ensuring the refrigeration system operates at the most efficient and energy-saving optimal state.
Safety Valve
The safety valve serves as the final line of defense, automatically opening to release excess pressure when internal system pressure exceeds the specified limit, preventing explosions. These valves typically have a pre-set pressure threshold; when actual pressure reaches or exceeds this value, the valve quickly opens and remains open until pressure returns to normal before closing again.
Conclusion
The efficient operation of a refrigeration system depends on the precise coordination between its various components. Scientific and reasonable selection and layout not only directly determine the energy efficiency of the system, but are also key to maintaining its long-term stable operation.
In-depth analysis of the core functions of each component and their interlocking logic is of irreplaceable importance for comprehensively understanding the principles of the refrigeration cycle, efficiently diagnosing faults, and gaining insight into industry technology trends.
