Principle of Operation of Household Air Conditioners

Principle of Operation of Household Air Conditioners

1. Principle of Air Conditioner Cooling Operation (Taking Home Air Conditioner as an Example)
When the air conditioner is operating in cooling mode, the low-temperature, low-pressure refrigerant gas is drawn into the compressor where it is compressed into high-temperature, high-pressure refrigerant gas. The high-temperature, high-pressure refrigerant gas releases heat in the outdoor heat exchanger (condensing through the condenser) and turns into a medium-temperature, high-pressure liquid (the heat is carried away by the outdoor circulating air). The medium-temperature, high-pressure liquid then passes through the throttling device, where it is throttled and depressurized to become low-temperature, low-pressure liquid. The low-temperature, low-pressure liquid refrigerant absorbs heat and evaporates in the indoor heat exchanger, turning into low-temperature, low-pressure gas (indoor air passing over the heat exchanger surface is cooled, lowering the indoor temperature). The low-temperature, low-pressure refrigerant gas is then drawn into the compressor, and the cycle repeats.

2. Principle of Air Conditioner Heating Operation (Using a Household Air Conditioner as an Example)
The low-temperature, low-pressure refrigerant gas is drawn into the compressor and pressurized into high-temperature, high-pressure refrigerant gas. The high-temperature, high-pressure refrigerant gas releases heat in the indoor heat exchanger and becomes a medium-temperature, high-pressure liquid (the indoor air is heated as it passes over the heat exchanger surface, raising the indoor temperature). The medium-temperature, high-pressure liquid then passes through a throttling device and is depressurized into a low-temperature, low-pressure liquid. The low-temperature, low-pressure liquid absorbs heat and evaporates in the heat exchanger, turning into a low-temperature, low-pressure gas (the outdoor air is cooled as it passes over the heat exchanger surface). The low-temperature, low-pressure gas is then drawn into the compressor, and the cycle repeats.

4. Classification of Air Conditioning Units
Air conditioning units can be classified according to the requirements of air treatment as follows:
(1) Heating and cooling units – only achieve regulation and control of indoor air temperature;
(2) Dehumidifiers – only achieve regulation of indoor air humidity;
(3) Constant temperature and humidity units – achieve simultaneous regulation and control of indoor air temperature and humidity.
Air conditioning units can be classified according to different specifications and types as follows:
(1) Window air conditioners;
(2) Cabinet air conditioners;
(3) Split air conditioners or air conditioning units;
(4) Central air conditioning units.
Air conditioning units can be classified according to the degree of centralization of air treatment equipment as follows:
(1) Central air conditioning systems;
(2) Semi-central air conditioning systems;
(3) Decentralized air conditioning systems.

5. Brief Introduction to Room Air Conditioners
(1) Types and Characteristics of Air Conditioners:
Small integrated types (such as window and portable) and split air conditioners are collectively referred to as room air conditioners. According to Chinese standards, room air conditioners with a cooling capacity below 9000W use fully enclosed compressors and air-cooled condensers, and the power supply can be either single-phase or three-phase. It is a type of localized air conditioner, widely used in homes, offices, and other places, and therefore is also called a household air conditioner.

Codes: Room air conditioner K
Integrated type C (window)
Cooling type L (code can be omitted)
Heat pump type R
Electric heating type D
Heat pump with auxiliary electric heating type Rd
Split type F
Cooling type L (code can be omitted)
Heat pump type R
Electric heating type D
Heat pump with auxiliary electric heating type Rd
Indoor units: Ceiling type D
Wall-mounted type G
Floor-standing type L
Embedded type Q
Table-top type T
Outdoor units: W

In outdoor environments below -5°C, heat pump air conditioners are no longer suitable, and electric heating air conditioners must be used for heating.

(2): Operating environment for the room air conditioner: The maximum operating temperature of the air conditioner is limited to below 43°C, and the minimum operating temperature of a heat pump air conditioner is -5°C. This is because the compressor and the motor of the air conditioner are enclosed within the same housing, and the insulation temperature class of the motor determines the limit of the maximum ambient temperature for the compressor. If the ambient temperature is too high, the condensation temperature will increase during the compressor’s operation, causing the compressor’s discharge temperature to become excessively high, leading to overload operation of the compressor, and the overload protector will cut off the power and shut down the unit. In addition, the motor’s insulation can be damaged if it cannot withstand excessively high temperatures, which may even cause the motor to burn out. For heat pump air conditioners, if the ambient temperature is too low, the refrigerant in the evaporator cannot evaporate sufficiently, and it is drawn into the compressor, causing a liquid hammer and leading to component wear and aging. For electric heating air conditioners, under winter conditions, the compressor does not operate and only the electric heater works, so there is no strict limitation on the minimum ambient temperature. For heat pump and heat pump-assisted electric air conditioners, if there is no defrosting device, the minimum operating ambient temperature is 5 degrees Celsius; if it is below 5 degrees, frost will form on the outdoor evaporator, obstructing airflow and preventing the air conditioner from operating normally. If equipped with a defrosting device, the minimum operating ambient temperature can be -5 degrees Celsius.
Power supply: It can be either single-phase 220V or three-phase 380V.

Some air conditioners designed for 60 Hz power can operate under 50 Hz voltage. But the reverse is prohibited.
A secondary motor running at 60 Hz has a synchronous speed of 3500 RPM, which drops to 2900 RPM at 50 Hz. Therefore, as the supply frequency decreases, the cooling capacity of the air conditioner also decreases, and the noise level also decreases.
The national standard specifies the test conditions for nominal cooling capacity as: indoor dry bulb temperature 27°C, wet bulb temperature 19.5°C; outdoor dry bulb temperature 35°C, wet bulb temperature 24°C. The national standard also allows the actual cooling capacity of the air conditioner to be up to 8% lower than the nominal value.
Performance coefficient (energy efficiency ratio) of air conditioners:
The performance coefficient, also called the energy efficiency ratio or cooling coefficient, refers to the ratio of the cooling capacity to the total power consumed during the air conditioner’s cooling operation. Its unit is W/W, that is:
ERR = actual cooling capacity / actual total power consumption (W / W)
= rated cooling capacity / rated input power (W / W)
The physical meaning of the performance coefficient is the amount of cooling generated per 1 W of electrical energy consumed. Therefore, air conditioners with higher cooling coefficients are more energy-efficient to produce the same amount of cooling.
There are certain requirements for the noise level of air conditioners, generally required to be below 60 decibels.

6. Principles and Characteristics of Inverter Air Conditioners
(1) Working Principle and Features:
The working principle of an inverter air conditioner is that the compressor is driven by an inverter motor. The power frequency converter outputs alternating current with a variable frequency to the motor, allowing the motor speed to continuously vary according to the indoor cooling demand, ultimately enabling the compressor’s cooling capacity to achieve continuous automatic control. To match the continuous variation of cooling capacity, the air conditioning system uses a novel electronic expansion valve. The valve core is controlled by a pulse motor, quickly regulating the refrigerant flow entering the evaporator.

The refrigeration system of an inverter air conditioner consists of a compressor, indoor heat exchanger, outdoor heat exchanger, electromagnetic four-way valve, electronic expansion valve, defrost two-way valve, capillary tubes, and other components. The opening of the electronic expansion valve is controlled by a microcomputer to maintain an appropriate refrigerant flow. During the short period of defrosting the outdoor heat exchanger, the refrigerant enters the outdoor heat exchanger through the defrost two-way solenoid valve to heat the exchanger for defrosting. Once completed, the valve closes, and normal operation resumes.