The three pressures of an air conditioning system are: equilibrium pressure, high-pressure pressure, and low-pressure pressure. These three pressures are important parameters for air conditioning maintenance. They represent the pressure of refrigerant R22 circulating at different locations in the air conditioning piping. Because R22 circulates between gas and liquid states, absorbing and releasing heat, the ambient temperature has a significant impact on it. Generally, higher ambient temperatures result in higher pressure values, and lower ambient temperatures result in lower pressure values.
Equilibrium pressure refers to the pressure when the compressor is not operating, at which the high and low pressures are balanced. High-pressure pressure refers to the discharge pressure or condensing pressure; low-pressure pressure refers to the suction pressure or evaporation pressure. All three pressures are measured at the process port of the outdoor unit's gas valve. During cooling operation, it is the low-pressure pressure; during heating operation, it is the high-pressure pressure; and when not operating, it is the equilibrium pressure.
In refrigeration, evaporation refers to boiling, therefore the evaporation temperature is the boiling point. Condensation refers to the process of R22 changing from a saturated gas to a liquid state under a certain pressure, therefore the condensation temperature is also the boiling point. R22 corresponds to different boiling points at different pressures, as shown in the table below, which illustrates the one-to-one correspondence between R22 evaporation pressure and evaporation temperature.
The operating conditions for air conditioning refrigeration design are: outdoor ambient temperature 35℃, indoor temperature 27℃, evaporation temperature +5℃, and evaporation pressure 0.48MPa. Therefore, the standard low-pressure refrigeration system is 0.48MPa.
The relative pressure (gauge pressure) of the air conditioning refrigeration piping design is half of the equilibrium pressure under refrigeration conditions, so the equilibrium pressure is 0.96MPa.
To achieve ideal heat dissipation, the refrigeration design uses air condensation, with a standard condensation temperature difference of 15℃. Therefore, under outdoor temperature of 35℃, the condensation temperature is 50℃, and the pressure value corresponding to 50℃ is 1.83MPa. Therefore, the high-pressure system is 1.83MPa.
Pressure in refrigeration refers to physical pressure, and the unit of pressure is "kg/cm²", which is what we call "kilogram pressure".
1kg/cm² ≈ 0.098MPa ≈ 0.1MPa; therefore, the three pressure values are "4.8 kg", "9.6 kg", and "18.2 kg".
Because the operating environment of air conditioners often does not meet the required conditions, and is affected by humidity, the three pressure values under summer cooling conditions are approximately: low pressure 0.5 MPa or 5 kg; high pressure 1.8 MPa or 18 kg; and equilibrium pressure 1 MPa or 10 kg.
In winter, the heating and cooling environments of air conditioners differ greatly, and the ambient temperature is also low, so the three pressures will vary significantly. A minimum operating temperature of 5°C is used as a reference for this analysis.
To achieve ideal evaporative heat absorption, when air is used as the cooling medium in the cooling design, the standard evaporation temperature difference is selected as 10°C. Therefore, the evaporation temperature should be -5°C, corresponding to a pressure of 0.32 MPa.
Since the outdoor unit's ambient temperature is 5℃, its optimal evaporation temperature is -5℃, while the outdoor unit's coil defrosts at around -6℃. Therefore, the colder the winter, the worse the heating effect. In order to absorb heat to the maximum extent at low temperatures, the evaporation pressure is reduced through the heating auxiliary capillary tube, thereby reducing the evaporation temperature. Therefore, the low pressure in the cooling state is no longer half of the equilibrium pressure, but slightly lower. So the heating equilibrium pressure is about 0.7MPa.
When an air conditioner is heating, the indoor unit acts as the condenser. The condensing temperature is affected by fan speed and indoor temperature. Air conditioners are designed to prevent cold air from being blown out below 28℃ and to prevent overheating or reduce frequency above 56℃. Therefore, the optimal condensing temperature is designed to be 1.83 MPa, corresponding to 50℃.
Therefore, the three pressures for air conditioning heating are approximately: low pressure 0.32 MPa or 3.2 kg; high pressure 1.8 MPa or 18 kg; and equilibrium pressure 0.7 MPa or 7 kg.
From the above analysis, it can be seen that the low pressure and equilibrium pressure of the air conditioner vary significantly with ambient temperature, but the high pressure remains relatively constant. In actual operation, the pressure values can be used as a reference and serve as an important basis for maintenance and adjustment.
