The Most Comprehensive Popular Science of Refrigeration and HVAC Basic Knowledge Across the Network!

### I. Basic Knowledge of Central Air Conditioning
1. What is a refrigerant and how does it work?
A refrigerant is a working substance that circulates in a refrigeration machine to transfer heat between the object to be cooled and the environmental medium, ultimately transferring the heat from the object to be cooled to the environmental medium.

Its working principle is that the refrigerant absorbs heat from the substance to be cooled and evaporates in the evaporator.
2. What is a secondary refrigerant and how does it work?
A secondary refrigerant is a medium that transfers the cooling capacity of a refrigeration device to the medium to be cooled. For example, the commonly used chilled water in air - conditioning systems is cooled in the evaporator and then transported over a long distance to cool the objects that need to be cooled.
3. What is sensible heat?
The heat that causes a temperature change of a substance without changing its state is called sensible heat. Changes in sensible heat can be measured by temperature - measuring instruments.
4. What is latent heat?
The heat that causes a change in the state of matter (also known as a phase change) without changing the temperature of the substance is called latent heat. Changes in latent heat cannot be measured by temperature - measuring instruments.
5. What are dynamic pressure, static pressure, and total pressure?
When selecting an air - conditioner or a fan, we often encounter the three concepts of static pressure, dynamic pressure, and total pressure.
- Static pressure (Pi): The pressure generated by the irregular movement of air molecules hitting the pipe wall is called static pressure. When calculating, the static pressure with absolute vacuum as the zero - point is called absolute static pressure. The static pressure with atmospheric pressure as the zero - point is called relative static pressure. The air static pressure in air - conditioning systems refers to relative static pressure. It is positive when higher than atmospheric pressure and negative when lower.
- Dynamic pressure (Pb): It refers to the pressure generated when air is flowing. As long as the air is flowing in the air duct, there is a certain dynamic pressure, and its value is always positive.
- Total pressure (Pq): The total pressure is the algebraic sum of static pressure and dynamic pressure: Pq = Pi + Pb. The total pressure represents the total energy of 1 m³ of gas. If atmospheric pressure is taken as the starting point of calculation, it can be either positive or negative.

### II.

Classification of Air - Conditioners
1. According to the purpose of use, how many types can air - conditioners be classified into?
- Comfort air - conditioners: They require a suitable temperature and a comfortable environment, with no strict requirements for the adjustment accuracy of temperature and humidity. They are used in housing, offices, cinemas, shopping malls, stadiums, cars, ships, airplanes, etc.
- Process air - conditioners: They have certain requirements for the adjustment accuracy of temperature and also have higher requirements for the cleanliness of the air. They are used in workshops for manufacturing electronic devices, workshops for manufacturing precision instruments, computer rooms, biological laboratories, etc.
2. According to the air - handling method, how many types can air - conditioners be classified into?
- Centralized air - conditioning systems: The air - handling equipment is concentrated in the central air - conditioning room, and the treated air is sent to each room through air ducts. They are suitable for places with a large area, concentrated rooms, and similar heat and humidity loads in each room.
- Semi - centralized air - conditioning systems: These are air - conditioning systems that have both a central air - conditioning unit and terminal devices for air handling. This system is relatively complex and can achieve a relatively high adjustment accuracy. It is suitable for workshops and laboratories with high requirements for air accuracy.
- Local air - conditioning systems: Each room has its own equipment to handle the air, such as split - type air - conditioners. It can also be a system composed of fan - coil units with chilled and hot water supplied through pipes. Each room can adjust its own temperature as needed.
3. According to the cooling capacity, how many types can air - conditioners be classified into?
- Large - scale air - conditioning units: Such as horizontal assembled water - spray type and surface - cooling type air - conditioning units, which are used in large workshops, cinemas, etc.
- Medium - scale air - conditioning units: Such as chiller units and cabinet - type air - conditioners, which are used in small workshops, machine rooms, meeting halls, restaurants, etc.
- Small - scale air - conditioning units: Split - type air - conditioners, which are used in offices, homes, guesthouses, etc.
4.

According to the amount of fresh air, how many types can air - conditioners be classified into?
- Direct - flow systems: The air to be treated is fresh air. After heat and humidity exchange in each room, it is discharged outdoors, and there is no return air duct.
- Closed systems: All the air treated by the air - conditioning system is recycled, and no fresh air is supplemented.
- Hybrid systems: The air treated by the air - conditioner is a mixture of return air and fresh air.
5. According to the air - supply speed, how many types can air - conditioners be classified into?
- High - speed systems: The air speed in the main air duct is 20 - 30 m/s.
- Low - speed systems: The air speed in the main air duct is below 12 m/s.

### III. Common Terms of Air - Conditioners
1. Nominal cooling capacity: The amount of heat removed from a space or room by an air - conditioner under the nominal cooling condition per unit time is called the nominal cooling capacity.
2. Nominal heating capacity: The amount of heat released into a space or room by an air - conditioner under the nominal heating condition per unit time.
3. Energy Efficiency Ratio (EER): It refers to the cooling capacity per unit of electric motor input power. It reflects the ratio of the cooling capacity to the cooling power when the air - conditioner is operating for cooling, with the unit of W/W. The national standard stipulates that the standard EER value for a 2500W air - conditioner is 2.65, and for air - conditioners with a capacity from 2500W to 4500W, the standard EER value is 2.70.
4. Coefficient of Performance (COP): The COP value of a refrigeration compressor, that is, the cooling capacity per unit of shaft power.
5. Common units of measurement for air - conditioners and their conversions:
- 1 kilowatt (KW)= 860 kilocalories per hour (Kcal/h).
- 1 kilocalorie per hour (Kcal/h)= 1.163 watts (w).
- 1 refrigeration ton (USRT)= 3024 kilocalories per hour (Kcal/h).
- 1 refrigeration ton (USRT)= 3517 watts (W).

### IV. Common Air - Conditioning Hosts
1. Water - cooled chiller units: Water - cooled chiller units belong to the refrigeration unit part of the central air - conditioning system. The secondary refrigerant is water, so they are called chiller units. The cooling of the condenser is achieved by heat exchange with normal - temperature water to lower the temperature. Therefore, they are called water - cooled units. In contrast, air - cooled units are called so because the condensers of air - cooled units achieve the refrigeration purpose through forced ventilation heat exchange with outdoor air.
2. VRV systems: VRV systems are Variable Refrigerant Volume systems. They consist of a group of outdoor units, including function units, constant - speed units, and variable - frequency units. By connecting the outdoor units in parallel, the refrigeration pipes are integrated into one pipeline system, which can be easily matched according to the capacity of the indoor units. One group of outdoor units can be connected to a maximum of 30 indoor units. The minimum capacity of a single indoor unit is 0.6KW, and the maximum is 3.75KW. The capacity of the indoor units can be adjusted within the range of 50% to 130% of the outdoor unit's capacity.
3. Modular units: Developed on the basis of VRV systems, modular units change the traditional Freon pipeline into a water pipeline system. The indoor and outdoor units are combined into a refrigeration unit, and the indoor units are changed to fan - coil units. The refrigeration process is achieved through the heat exchange of the secondary refrigerant water. Modular units are named because they can automatically adjust the number of starting units according to the cooling load requirements and achieve flexible combinations.
4. Piston - type chiller units: Piston - type chiller units are integrated refrigeration devices specially designed for air - conditioning purposes, in which the piston - type refrigeration compressor, auxiliary equipment, and accessories required for the refrigeration cycle are compactly assembled together. The single - unit cooling capacity of piston - type chiller units ranges from 60 to 900KW, which is suitable for small and medium - sized projects.
5. Screw - type chiller units: Screw - type chiller units are large and medium - sized refrigeration equipment that provides chilled water. They are commonly used for air - conditioning in national defense research, energy development, transportation, hotels, restaurants, light industry, textile, and other sectors, as well as for chilled water in water conservancy and power projects. A screw - type chiller unit is a complete refrigeration system composed of a screw refrigeration compressor unit, a condenser, an evaporator, and automatic control components and instruments. It has the advantages of compact structure, small size, light weight, small floor area, convenient operation and maintenance, and stable operation, so it has been widely used. The single - unit cooling capacity ranges from 150 to 2200KW, which is suitable for medium and large - sized projects.
6. Centrifugal chiller units: Centrifugal chiller units are complete chiller units composed of a centrifugal refrigeration compressor, a matching evaporator, a condenser, a throttling control device, and electrical instruments. The single - unit cooling capacity ranges from 700 to 4200KW. They are suitable for large and extra - large projects.
7. Lithium bromide absorption chiller units: Lithium bromide absorption chiller units use thermal energy as the power, water as the refrigerant, and lithium bromide solution as the absorbent to produce refrigerant water above 0°C. They can be used as the cold source for air - conditioning or production processes. Lithium bromide absorption chiller units are powered by thermal energy, and the common types are direct - fired, steam - type, and hot - water - type. The cooling capacity ranges from 230 to 5800KW, which is suitable for medium - sized, large - sized, and extra - large projects.

### V. Classification of Central Air - Conditioning Units
Central air - conditioning units are the core part of the central air - conditioning system. Reasonably selecting the units is crucial for a central air - conditioning project.

### VI. 20 Core Components of Refrigeration and Air - Conditioning Systems
1. Compressor: It is the core of the entire air - conditioning system and the source of power for the system. All the power of the air - conditioner is provided by the compressor. According to the compression method, volumetric compressors can be divided into two major categories: piston - type and rotary - type. Specifically, there are piston - type, sliding - vane type, screw - type, scroll - type, centrifugal - type, and rotor - type compressors.
2. Condenser: Its function is to cool the high - temperature and high - pressure superheated refrigerant vapor discharged from the compressor into a liquid or a gas - liquid mixture. It can be divided into three types: water - cooled, air - cooled, and water - and - air mixed - cooled.
- Water - cooled condensers: The heat of the refrigerant in the condenser is taken away by the cooling water. The cooling water can flow through once or be recycled. When recycled, a cooling tower or a cooling pool needs to be set up. Water - cooled condensers are divided into several types, such as shell - and - tube type, double - pipe type, plate - type, and spiral - plate type.
- Air - cooled condensers: The heat released by the refrigerant in the condenser is taken away by the air, and the refrigerant condenses inside the pipes. There are natural - convection air - cooled condensers and forced - convection air - cooled condensers in this type. Usually, air - cooled condensers are also called air - cooled condensers.
- Water - and - air combined - cooled condensers: The heat released by the refrigerant in the condenser is taken away by both the cooling water and the air. When the cooling water is sprayed and evaporated outside the pipes, it absorbs the latent heat of vaporization, cooling and condensing the refrigerant inside the pipes. Therefore, the water consumption is small.
3. Evaporator: Its function is to use the fact that the low - temperature liquid refrigerant is easy to evaporate under low pressure, transform into vapor, and absorb the heat of the medium to be cooled to achieve the refrigeration purpose. Types of evaporators: According to the different cooling media, evaporators are divided into two major types: those for cooling liquid secondary refrigerants and those for cooling air or other gases.
- Evaporators for cooling liquid secondary refrigerants: There are tank - type (immersed) evaporators (including vertical - tube type, spiral - tube type, and serpentine - tube type), plate - type evaporators, spiral - plate type evaporators, shell - and - tube type evaporators (including horizontal evaporators and dry - type evaporators), etc.
- Evaporators for cooling air: There are finned evaporators for air - conditioning, air coolers (air blowers) for refrigeration and cold storage, and coil - type evaporators, etc.
4. Throttle mechanism: It throttles and reduces the pressure of the high - pressure liquid coming out of the condenser, so that the liquid refrigerant vaporizes and absorbs heat under low pressure (low temperature). Therefore, it is an important component to maintain high pressure in the condenser and low pressure in the evaporator. According to the form, throttle components can be divided into capillary tubes and expansion valves. Capillary tubes are used in smaller refrigeration equipment. For example, the capillary tube installed between the condenser and the evaporator in a refrigerator is a type of throttle mechanism. Expansion valves are used in larger refrigeration equipment. The throttle mechanisms used in large and medium - sized installations are throttle valves. There are three commonly used throttle valves, namely manual expansion valves, float - type regulating valves, and thermal expansion valves. The latter two are automatically regulated throttle valves. Expansion valves can be divided into electronic expansion valves and thermal expansion valves according to the expansion type.
5. Gas - liquid separator: It is installed at the inlet end of the compressor. Its main function is to prevent the low - pressure and low - temperature steam returning to the compressor from carrying too many liquid droplets, prevent the liquid refrigerant from entering the compressor cylinder, and prevent liquid hammer. The separator also has functions such as filtering, oil return, and liquid storage.
When using a gas - liquid separator, the following points should be noted:
- Install it as close to the compressor as possible.
- In a reversing system, the gas - liquid separator should be installed between the reversing valve and the compressor.
- Install the inlet (from the evaporator) and the outlet (to the compressor suction port) correctly.
- Install it vertically upwards.
6. Fan: A fan is a combination of a single - phase or three - phase AC induction motor and an impeller. Fans are divided into axial - flow fans and centrifugal fans. Fans include two major series: constant - speed and variable - speed. Fans are divided into metal blades, plastic blades, and metal - cast blades, etc., and there are various blade shapes.
7. Liquid receiver: The high - pressure liquid receiver (also called the liquid storage tank) in the refrigeration system is installed between the condenser and the expansion valve. There are various forms of liquid receivers, which can be divided into one - way and two - way types, and vertical and horizontal types. Its functions can be summarized as follows:
- Store the condensate from the condenser: To avoid excessive accumulation of condensate in the condenser, which reduces the heat - transfer area and affects the heat - transfer effect of the condenser.
- Meet the supply demand according to the load change of the evaporator: When the evaporation load increases, the supply also increases, and the stored liquid in the liquid receiver supplies the liquid; when the load decreases, the required liquid volume also decreases, and the excess liquid is stored in the liquid storage tank.
- Serve as a liquid seal between the high - pressure and low - pressure sides of the system: Since the liquid outlet pipe is inserted below the liquid level, it can prevent the steam and non - condensable gases on the high - pressure side from entering the low - pressure side. At the same time, the liquid receiver also plays the roles of filtering and noise reduction.
8. Oil separator: It is installed between the compressor and the condenser. Its working principle is as follows: The exhaust from the compressor is a mixture of refrigerant and lubricating oil. Through the large cavity of the oil separator, the speed is reduced, and the mist - like oil will gather on the impact surface. When it accumulates into larger oil droplets, it flows to the bottom of the oil separator and returns to the compressor through the oil return device.
9. Gas - liquid separator: The gas - liquid separator is installed at the compressor inlet. Its function is to prevent the compressor from sucking in liquid refrigerant at the inlet, which may cause liquid hammer and damage the compressor.
10. Drying filter: The function of the filter is to prevent the refrigerant from containing moisture or impurities from entering the refrigeration system. When the high - temperature liquid coming out of the condenser enters the expansion valve, the temperature of the liquid will drop significantly, generally below zero degrees. At this time, if there is moisture in the system, since the cross - section of the expansion valve is very small, the phenomenon of ice blockage is likely to occur, affecting the normal operation of the system.
11. Four - way reversing valve: The four - way reversing valve is suitable for heat - pump - type air - conditioning systems such as central air - conditioners and unit - type air - conditioners. It is used to switch the flow path of the refrigeration working medium to achieve the purposes of refrigeration and heating.
12. Water pump: It is a tool used to accelerate the flow of water to enhance the heat - exchange effect of water in the heat exchanger.
13. Water flow switch: It is used for controlling the fluid flow in the pipeline or for flow - cut -protection. When the fluid flow reaches the set value, the switch automatically cuts off (or turns on) the circuit.
14. Pressure controller: The pressure controller is used for pressure control and pressure protection. The unit is equipped with a low-pressure and a high-pressure controller to control the working pressure range of the system. When the system pressure reaches the set value, the switch automatically cuts off (or turns on) the circuit.
15. Differential pressure controller: It is used for the control of the pressure difference. When the pressure difference reaches the set value, the switch automatically cuts off (or turns on) the circuit.
16. Temperature controller: It is used for the control or protection of the unit. When the temperature reaches the set value, the switch automatically cuts off (or turns on) the circuit. Temperature control is commonly used, for example, using the temperature of the water tank to control the start and stop of the unit. Some functions like anti-freezing also require the use of a temperature controller.
17.

Sight glass: The sight glass is used to indicate:
- The condition of the refrigerant in the liquid pipeline of the refrigeration device;
- The water content in the refrigerant;
- The flow condition of the lubricating oil from the oil separator in the oil return pipeline. Some sight glasses are equipped with an indicator, which indicates the water content in the refrigerant by changing its color. (Green indicates dry, and yellow indicates wet).
18. Solenoid valve: A solenoid valve is a stop valve that is automatically opened and closed by electromagnetic force. In refrigeration and air-conditioning devices, solenoid valves are commonly used to control the refrigerant of the system. It is applicable to various gas, liquid refrigerant, lubricating oil and other media.
19. Expansion water tank: The functions of the expansion water tank are:
- As the volume of water changes due to temperature changes, the expansion water tank is used to store this part of the expanded water;
- It plays a role in stabilizing and maintaining the pressure of the system;
- It can compensate for part of the water in the system.
20. Cooling tower: A cooling tower is a device that uses water as a circulating coolant to absorb heat from a system and discharge it into the atmosphere to lower the water temperature. Its cooling is achieved by using the principle of heat exchange between water and air flow to generate steam. The evaporation of steam takes away heat to achieve evaporation heat dissipation, convective heat transfer and radiative heat transfer, etc., to dissipate the waste heat generated in the refrigeration and air-conditioning system and lower the water temperature. The evaporation heat dissipation device ensures the normal operation of the system. The device is generally in the shape of a barrel, so it is named a cooling tower.