Basic knowledge of central air conditioning systems

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I. What is an air conditioner, and what is a central air conditioner? An air conditioner, also known as an air handling unit, refers to the process of using artificial means to regulate and control parameters such as the temperature, humidity, cleanliness, and flow rate of the indoor air in buildings/structures. Central air conditioning: A central air conditioning system consists of a heat and cold source system and an air handling system. The principle of liquid vaporization cooling is utilized to provide the necessary cooling capacity for air conditioning systems, in order to counteract the cooling load of the indoor environment ； The heating system provides heat to the air conditioning system in order to counteract the thermal load of the indoor environment. The refrigeration system is a crucial component of central air conditioning systems; its type, operating mode, and structural design directly affect the economic efficiency, performance, and rationality of these systems during operation. 1 horsepower = 735 W, and the definition of a horsepower is the amount of power that can be generated by an input power of 1 horsepower. Generally, the cooling capacity of one unit is approximately 2000 calories; when converted to international units, this value needs to be multiplied by 1.162. Thus, the cooling capacity of one unit is 2000×1.162=2324W. Here, W (watts) represents the cooling capacity. Refrigeration ton (RT): A unit in refrigeration science. Also known as a cooling ton, it represents the amount of cooling energy required to turn 1 ton of saturated water at 0°C into ice at 0°C over a period of 24 hours. 1 US.RT = 3516.7 W (US stands for the United States). 10 HP = 10 * 2324 W = 23240 W. 23240 W / 3516.7 W = 6.6 US.RT. II. Common air conditioning systems: Wall-mounted air conditioners: Also known as split-system air conditioners; they are not restricted by the location where they are installed, and are easier to integrate with interior decorations. There is less noise, and some split-air conditioners have multiple purification functions that can clean the indoor air. Nowadays, there are also split-air conditioners with ventilation capabilities to maintain health. Wall-mounted air conditioners: they have high power and strong airflow, making them suitable for large rooms, and can be used to regulate the temperature in multiple rooms. Portable air conditioners: Suitable for localized cooling and can be used in various settings such as kitchens, living rooms, construction sites, offices, etc. Multi-unit air conditioners: They are actually a major subset of split-air conditioners. It has multiple indoor units that can be used in various rooms, but they share one outdoor unit. The cost is lower than that of purchasing multiple air conditioners, and the noise level is also lower, as there is no need to install multiple outdoor units. Electric refrigeration air conditioners are the most commonly used type of air conditioners. Speaking specifically about their classification based on the cooling method: 1. Air-cooled: The condenser uses forced air convection for heat exchange; this is the type of air conditioner found in most household units. Air-cooled air conditioners come in two types – single-cooling and heat pump. As the name suggests, single-cooling models can only be used for cooling in summer, while heat pump models can be used for both cooling in summer and heating in winter. 2. Water cooling: The heat released by the condenser is cooled using water flow. To save water, the cooled water exiting the condenser is pumped to a cooling tower, where it exchanges heat with air before returning to the condenser. A water-cooling system can generally only cool. III. Classification by the type of compressor in the main unit: 1. Piston compressors: These were commonly used in early air conditioners. However, due to their large number of components, they have a high failure rate, high production costs, and poor operational stability. Additionally, they lack the ability to withstand liquid shock, which is why they are rarely used these days. 2. Rotary vane compressors: Widely used in household refrigerators and air conditioners; large and medium-sized rotary vane compressors are also employed in cold storage facilities. It has few components, a simple structure, and few vulnerable parts; it operates reliably. There is no suction valve, and high precision in manufacturing is required. Leakage, friction, and wear between the sliding piston and the cylinder wall are significant, which limits its service life and the possibility of improving its efficiency. 2. Rotary vane compressors: These are a type of compressor that is widely used today. A rotary vane compressor consists of four main components: the stationary plate, the rotating plate, the motor, and the housing. Its main advantages are few components, high operational stability, and a long service life; it is extensively used in small refrigeration units, such as the modular units and household air conditioners mentioned later. 3. Screw compressors: These are the most commonly used compressors in large central air conditioning systems. They feature a small number of components, high stability, a long service life, and ease of maintenance. Another important advantage is their high energy efficiency – for the same level of cooling capacity, screw compressors use 25% less energy compared to piston compressors. Screw compressors are divided into single-screw and twin-screw types. 4. Centrifugal compressor: A centrifuge looks like a large centrifugal water pump, and its structure is also similar to that of a water pump. Centrifugal compressors are often used in central air conditioning units with high power output. They feature a large cooling capacity per unit, a simple structure, reliable performance, and stable operation; however, they require high standards regarding manufacturing processes. IV. Different classifications of indoor systems 1. Fluorine system: The indoor unit and the main unit are connected by copper pipes, through which the refrigerant flows; the refrigerant evaporates in the indoor unit to absorb heat. It is commonly used in household and commercial air conditioners; due to its characteristics, the system cannot be made very large. 2. Water system: The indoor unit and the main unit are connected by water pipes, through which low-temperature water (7°C) flows. After absorbing heat from the indoor unit (air-handling unit), this low-temperature water returns to the main unit. It is most widely used in large central air conditioning systems. 3. Air system: The air is directly treated to the desired temperature by an air handler, and then delivered to the required rooms through ductwork; its advantage is that it is easy to introduce fresh air. It has certain requirements regarding the height of the space for installation, and its cost-effectiveness is somewhat lower. 1. Screw-type water-cooled chillers have a high cooling capacity ； 2. Vortex-type water-cooled chillers are less commonly used ； 3. Centrifugal water-cooled chillers have a very high cooling capacity ； 4. Air-cooled screw chiller heat pump units have a high cooling capacity and do not require a separate machine room for the cooling tower ； 5. The air-cooled modular chiller-hot pump units have a moderate cooling capacity, and no separate machine room for a cooling tower is required ； 6. Large air-cooled vortex chiller-hot water pumps have a high cooling capacity, and are of the same type as air-cooled modular units ； 7. Small household water-cooled air conditioning units with low cooling capacity, suitable for home use ； 8. Small variable-frequency multi-split air conditioning units with low cooling capacity, suitable for home use ； 9. Medium and large-sized variable-frequency multi-split air conditioning units with high cooling capacity, suitable for use in various locations. V. Simple and quick calculation of the cooling capacity required for a project: The method using the cold load per unit area is as follows: Q = Q’ * S (for fluorine-based chillers). Here, Q represents the total cold load of the air-conditioned rooms in the building (in watts); Q’ is the cold load index (in watts/m2); and S is the area of the air-conditioned rooms (in m2). For water-based chillers, the formula is Q = Q’ * S * 80%. Again, Q is the total cold load of the air-conditioned rooms in the building (in watts), Q’ is the cold load index (in watts/m2), and S is the area of the air-conditioned rooms (in m2).

Example calculation: How much cooling capacity is needed for a 1500 m2 shopping mall using fluorine-based chillers? S = 1500 * 80% = 1200 m2. Q’ = 200 watts/m2. Therefore, Q = S * Q’ = 1200 * 200 = 240,000 watts, which is equivalent to 240 kW.

Another example calculation: How much cooling capacity is required for a 1500 m2 shopping mall using modular chillers? S (area of air-conditioned rooms) = 1500 * 80% = 1200 m2. Q’ (cooling load index) = 200 (w/m2). Q (total cooling load for the air-conditioned rooms in the building) = S * Q’ * 80% = 1200 * 200 * 80% = 192000 (w) = 192 (kW)