Central air conditioning can be categorized into three main types: electric refrigeration air conditioning, lithium bromide refrigeration air conditioning, and other energy-based air conditioning. Among these, electric refrigeration air conditioning is the most commonly used. Below is a detailed discussion of its classifications:
I. Classification by Cooling Method
Air-cooled: The condenser uses forced air convection for heat exchange. Most household air conditioners use this method. Air-cooled air conditioning can be further divided into cooling-only and heat pump types. Cooling-only models operate exclusively in summer, while heat pump models provide both cooling in summer and heating in winter.
Water-cooled: Heat from the condenser is dissipated using water circulation. To conserve water, the cooling water exiting the condenser is pumped to a cooling tower, where it exchanges heat with the air before returning to the condenser. Water-cooled systems are generally used for cooling only.
II. Classification by Compressor Type
Reciprocating Compressor: This type of compressor utilizes a piston’s reciprocating motion to compress the refrigerant. Once commonly used in air conditioners, it is now primarily used in refrigeration units, such as cold storage systems that utilize semi-hermetic reciprocating compressors.
Scroll Compressor: One of the most frequently used compressors today, it consists of a fixed scroll, a moving scroll, a motor, and a casing. Its key advantages include fewer components, high operational stability, long service life, and widespread use in small refrigeration units such as modular units and household air conditioners.
Rotary Compressor: Previously, most air conditioning equipment used scroll compressors. However, in recent years, rotary compressors have become increasingly popular for 3-7HP air conditioning units, including multi-split home air conditioners and household water-cooled units.
Screw Compressor: The most frequently used compressor in large central air conditioning systems, characterized by fewer components, high stability, long service life, and easy maintenance. One key advantage is its high energy efficiency—compared to reciprocating compressors, screw compressors offer significant energy savings. Screw compressors can be classified into single-screw and twin-screw types.
Centrifugal Compressor: Resembling a large centrifugal water pump, this compressor is used in high-power central air conditioning systems. It is distinguished by its large cooling capacity, simple structure, reliable performance, and stable operation. However, manufacturing centrifugal compressors requires high production standards.
III. Classification by Indoor System Type
Refrigerant (Fluorine) System: In this system, the indoor unit and the main unit are connected by copper pipes through which refrigerant circulates. The refrigerant directly evaporates and absorbs heat inside the indoor unit. This type is commonly used in household and commercial air conditioning systems. Due to its characteristics, the system cannot be expanded to very large scales.
Water System: This system connects the indoor unit to the main unit via water pipes that circulate chilled water (7°C). The chilled water absorbs heat inside the indoor unit (fan coil unit) and then returns to the main unit. Water systems are the most widely used in large central air conditioning setups.
Another system type is the
Air System, which differs from the previous two. The air handling unit (AHU) processes air to the required temperature and then distributes it through ductwork. It is advantageous for introducing fresh air, but requires more installation space and has higher economic costs.
IV. Common Air Conditioning Main Units and Their Cooling Capacities
Water-cooled screw chillers: Large cooling capacity.
Water-cooled scroll chillers: Less commonly used.
Water-cooled centrifugal chillers: Very large cooling capacity.
Air-cooled screw chiller heat pump units: Large cooling capacity, no need for cooling towers or dedicated air conditioning rooms.
Air-cooled modular chiller heat pump units: Moderate cooling capacity, no need for cooling towers.
Large air-cooled scroll chiller heat pump units: Large cooling capacity, similar in structure to air-cooled modular units.
Small household water system air conditioning units: Small cooling capacity, suitable for residential use.
Small variable-frequency multi-split air conditioning units: Small cooling capacity, ideal for home applications.
V. Key Components of Air Conditioning Systems
Compressor: The heart of the air conditioning system, providing the driving force. It compresses low-temperature refrigerant gas into high-temperature gas, which then exchanges heat in the heat exchanger. The compressor's quality directly affects air conditioning performance.
Heat Exchanger: Includes condensers and evaporators.
Condenser: Converts high-temperature, high-pressure refrigerant vapor into liquid or a liquid-vapor mixture. Condensers can be classified by cooling method:
Water-cooled condenser: Shell-and-tube, coaxial, plate-type, and spiral plate-type condensers.
Air-cooled condenser: Includes natural convection and forced convection air-cooled types.
Water-air hybrid condenser: Includes spray condensers and evaporative condensers, where heat is dissipated through both water and air.
Evaporator: Cools down by evaporating liquid refrigerant under low pressure, absorbing heat from the cooled medium. Classified into:
For cooling liquid secondary refrigerant: Immersion-type (shell-and-tube, plate-type, spiral plate-type) evaporators.
For cooling air or other gases: Air-cooled evaporators, finned evaporators, and pipe-type evaporators.
Expansion Device: Reduces pressure from the condenser, allowing refrigerant to evaporate and absorb heat at a lower pressure.
Types: Capillary tubes (used in small cooling systems), manual expansion valves, float-regulated valves, thermal expansion valves, and electronic expansion valves.
Fans: Facilitate airflow in air conditioning systems.
Axial fans: Efficient and quiet, used in outdoor units.
Centrifugal fans: Provide higher pressure, used in indoor units.
Four-way Reversing Valve: A key component in heat pump systems, switching refrigerant flow direction between heating and cooling modes.
Oil Separator & Gas-Liquid Separator:
Oil separator: Prevents excessive lubricant from flowing into the system, ensuring proper lubrication of the compressor.
Gas-liquid separator: Prevents liquid refrigerant from entering the compressor, avoiding potential compressor damage (liquid slugging).
Filter Dryer: Removes moisture and impurities from the refrigerant to prevent corrosion or blockage in the system.
Pressure Controllers: Monitor system pressure to ensure safe operation, automatically shutting down the system in case of overpressure or low pressure.
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