Common Terms in HVAC

Central air conditioning: Central air conditioning is an air conditioning system in which one main unit controls different rooms through air ducts or chilled/hot water pipes connected to multiple terminals to achieve the purpose of indoor air conditioning. That is to say, the heating and cooling of all rooms are supplied by it. There are only supply fans in each room, and the original external units for cooling are all integrated into one box.
Central air conditioning system: It consists of a main unit and a terminal system. According to the medium used to bear the indoor heat and humidity load, it can be divided into all-air systems, all-water systems, air-water systems, and refrigerant systems. According to the degree of centralization of air handling equipment, it can be divided into centralized and semi-centralized types. According to the source of the treated air, it can be divided into closed, direct current, and mixed types (primary return air and secondary return air). The main components include air conditioning main units (cold and heat sources), air handling units, fan coil units, and so on.
Refrigerant: Refrigerant is the working medium for refrigeration. It is the working medium that completes the refrigeration cycle in the refrigeration system. The refrigerant evaporates by absorbing the heat of the cooled object in the evaporator and condenses into a liquid by transferring heat to the surrounding air or water in the condenser. The refrigerator achieves the purpose of refrigeration by means of the state change of the refrigerant.
Refrigerating capacity: When an air conditioner operates in cooling mode, the amount of heat absorbed by the refrigerant on the low-pressure side in the evaporator per unit time is usually measured in W or KW.
Heat pump heating capacity: When an air conditioner operates in heat pump heating mode (the heat pump auxiliary electric heater should run simultaneously), the amount of heat delivered to a closed space, room, or area per unit time.
Coefficient of performance: The ratio of the cooling (heating) capacity generated in the cooling (heating) cycle to the power consumed for cooling (heating) is called the coefficient of performance. When cooling, it is called energy efficiency ratio and is represented by EER; when heating, it is called performance coefficient and is represented by COP.
Secondary refrigerant: The secondary refrigerant refers to the intermediate medium used to transfer cooling capacity in an indirect refrigeration system. After being cooled by the refrigerant in the evaporator, the secondary refrigerant is sent to the cooling equipment for cooling, absorbs the heat of the cooled object or environment, and then returns to the evaporator to be cooled by the refrigerant again. This continuous cycle is carried out to achieve the purpose of continuous refrigeration.
Fan coil unit: A commonly used heat exchange equipment in central air conditioning systems, composed of finned tubes and fans. When the secondary refrigerant flows through the fan coil unit (inside the tube), it exchanges heat with the air outside the tube to cool the air. The fan coil unit belongs to air cooling equipment.
Water-cooled chiller unit: The water-cooled chiller unit belongs to the refrigeration unit part of the central air conditioning system. Its secondary refrigerant is water and is called a water chiller unit. The cooling of the condenser is achieved by heat exchange and cooling using normal temperature water, so it is called a water-cooled unit. The opposite of a water-cooled unit is an air-cooled unit. The condenser of an air-cooled unit achieves cooling through forced ventilation and heat exchange with outdoor air.
Cooling tower: A special equipment that cools water with the help of air. It is generally installed on the top of a building. In many industries such as refrigeration, electricity, and chemical industry, the hot cooling water discharged from equipment such as condensers is recycled after being cooled by cooling towers.
Modular unit: It changes the traditional Freon pipeline into a waterway system, combines the indoor and outdoor units into a refrigeration unit, and changes the indoor unit into a fan coil unit. The refrigeration process is realized by heat exchange of the secondary refrigerant water. The modular unit is named because it can automatically adjust the number of starting units according to the cooling load requirements and achieve flexible combination.
Multi-connected unit: In fact, the multi-connected unit cannot be considered a central air conditioner in the traditional sense. In the traditional sense, central air conditioning is to pass water into the fan coil unit. The multi-connected unit directly passes fluorine into the fan coil unit. So it is destined that the pipeline of the multi-connected unit cannot be too long. And the energy loss of the multi-connected unit on the pipeline is greater than that of a split unit. If there is a leak in the pipeline of the multi-connected unit, it is often impossible to repair.
Not all compressors of variable frequency multi-connected units are variable frequency. Generally, only one compressor is variable frequency, and the others are fixed frequency. The pipeline system and control system of the multi-connected unit are more complex.
The differences between modular units, screw units, and multi-connected units: Modular units and screw units are both water systems. That is, the chilled water comes out from the main unit and is supplied to indoor terminal equipment such as fan coil units for cooling.
The multi-connected unit is a fluorine system. The refrigerant (R22 or R410a refrigerant, etc.) comes out from the main unit and is supplied to the indoor terminal which is also an indoor unit connected to the fluorine pipeline. Similar to a household one-to-one air conditioning unit, except that it becomes one-to-many and adds variable frequency (DC variable frequency or digital scroll variable frequency) technology.
The differences between air-cooled units and water-cooled units: An air-cooled unit means that the main unit is cooled by a fan. A water-cooled unit means that the main unit is cooled by water. The air-cooled unit has a compact structure and does not require a special machine room. Maintenance is simple. The disadvantage is that cooling and heating are greatly affected by the climate, and the energy efficiency ratio is slightly lower than that of a water-cooled unit. A water-cooled unit generally requires a special machine room and is equipped with a cooling tower. The system is slightly more complex and maintenance is slightly more cumbersome. Most cannot heat. The advantage is that the cost is lower than that of an air-cooled unit, and the energy efficiency ratio is slightly higher than that of an air-cooled unit.
Energy efficiency ratio: The energy efficiency ratio of an air conditioner is the ratio of nominal cooling capacity (heating capacity) to operating power. That is, the mathematical expressions of EER and COP are: EER = cooling capacity/cooling power consumption; COP = heating capacity/heating power consumption. The higher the EER and COP, the lower the energy consumption of the air conditioner and the higher the performance ratio. 2.6 - 2.

8 is level five, 2.8 - 3.0 is level four, 3.0 - 3.2 is level three, 3.2 - 3.4 is level two, and 3.4 and above is level one. Level one is the most energy-efficient, and level five has the lowest energy efficiency. Products below level five are not allowed to be sold on the market. Air conditioning enterprises need to affix energy efficiency labels on products to inform consumers of their energy efficiency level.
Vertical imbalance: In a hot water double-pipe system, due to the different height differences between each layer of radiators and the boiler, although the supply and return water temperatures entering and leaving each layer of radiators are the same (without considering the cooling effect along the pipeline), the acting pressure is large when the vertical distance from the boiler is large, and the acting pressure is small when the distance is small. Even if different pipe diameters are selected, the resistance balance of each layer cannot be achieved, and there will be an uneven phenomenon of flow distribution and uneven heating and cooling between the upper and lower layers, which is usually called vertical imbalance. Moreover, the more building floors there are, the greater the pressure difference between the upper and lower layers, and the more serious the vertical imbalance phenomenon will be.
Indoor air calculation temperature: The indoor air calculation temperature generally refers to the average environmental temperature of the area where people are active within 2.0m from the ground. It should meet the technological requirements of people's lives and production.
Outdoor calculation temperature for heating: The outdoor calculation temperature for heating should be the daily average temperature that is not guaranteed for five days on average per year over the years.
Minimum thermal resistance (minimum heat transfer resistance): Specifically refers to the lower limit value of the heat transfer resistance allowed in design calculations. The purpose of specifying the minimum heat transfer resistance is to limit the excessive heat transfer through the enclosure structure, prevent condensation on the inner surface, and limit the excessive radiant heat transfer between the inner surface and the human body to prevent the human body from getting cold.
Economic heat transfer resistance (economic thermal resistance): The heat transfer resistance when the sum of the construction cost (depreciation of initial investment) and the use cost (heating operation cost and equipment depreciation cost apportioned per unit area of the enclosure structure) per unit area of the enclosure structure reaches the minimum value.
General ventilation: General ventilation is to ventilate and exchange air for the entire room. Use the fresh air sent into the room to dilute the concentration of harmful gases in the room to below the allowable range of health standards. At the same time, discharge the polluted indoor air directly or after purification treatment to the outdoor atmosphere.

Emergency ventilation: Emergency ventilation is an exhaust system set up to prevent greater personnel or property losses caused by the sudden release of a large amount of harmful or explosive gases when production equipment has accidental accidents or malfunctions in the production workshop. It is a necessary measure to ensure safe production and protect the lives and safety of workers.
Air quality balance: In a ventilated room, no matter which ventilation method is adopted, the mass of air entering the room per unit time should be equal to the mass of air discharged at the same time. That is, the air quality in the ventilated room should be balanced. This is air quality balance.
Heat balance: Heat balance means that the total heat gain and total heat loss in the room are equal to maintain a stable room temperature.
Fan air volume: It refers to the volume of gas conveyed by the fan per unit time when the fan is working under standard conditions. It is called fan air volume and is represented by the symbol L. The unit is m3/h.
Fan air pressure: It refers to the sum of the dynamic pressure and static pressure that each meter of air should obtain when passing through the fan. It is represented by the symbol P.

The unit is Pa.
Water vapor partial pressure Pq: The pressure generated when water vapor in moist air alone occupies the volume of moist air and has the same temperature as moist air is called the partial pressure of water vapor in moist air. The magnitude of water vapor partial pressure reflects the amount of water vapor in the air. The more water vapor in the air, the greater the water vapor partial pressure.
Saturated water vapor partial pressure Pq.b: At a certain temperature, when the water vapor content in moist air reaches the maximum limit, the moist air is said to be in a saturated state. At this time, the corresponding water vapor partial pressure is called saturated water vapor partial pressure.
Humidity ratio: The definition of humidity ratio (d) is the amount of water vapor contained in the moist air corresponding to one kilogram of dry air. Its unit is expressed in g/kg. The magnitude of humidity ratio changes with the amount of water vapor in the air. It can accurately reflect the amount of water vapor in the air.
Total pressure: The sum of dynamic pressure and static pressure.
Heat exchanger: Equipment in which fluids of different temperatures exchange heat. Also known as a heat exchanger.
Expansion tank: A tank in a hot water system that plays a role in adjusting and compensating for the expansion and contraction of water volume.
Radiator: Equipment that releases heat to a heating room by convection and radiation.
Hot air curtain: An air curtain that can blow out hot air. Also known as a hot air curtain.
Water mixer: In a hot water system, an inlet device that mixes supply and return water to achieve the required parameters.
Strainer: In a water system, a device used to remove impurities mixed in the circulating water.
Water distributor: In a water system, a water distribution device with a large cross-section used to centrally distribute water to various branch systems.
Water collector: In a water system, a water collection device with a large cross-section used to collect the return water of various branch systems.
Check valve: A valve that only allows fluid to flow in one direction and can automatically prevent backflow. Also known as a non-return valve.
Globe valve: A valve in which the valve plug moves perpendicular to the valve seat and is used to cut off and adjust the flow.
Gate valve: A gate-shaped valve used to cut off and adjust the flow.
Angle valve: A valve used to close and adjust the flow and has a certain angle between the inlet direction and the outlet direction.