1、 Pressure is just an appearance

In refrigeration systems, what we usually see are the high and low pressure of the system. In some refrigeration systems, such as air source heat pumps and small Freon cold storage facilities, high and low pressure gauges are installed. Small refrigeration devices such as household air conditioners, refrigerators, dehumidifiers, etc. often do not install high and low pressure gauges due to cost reasons. Many peers do not understand why it is necessary to test high and low voltage.

Refrigerants are like blood in the human body, and high or low blood pressure is usually abnormal. For example, the high blood pressure that elderly people are prone to may be caused by the accumulation of fat in the blood vessels (or other reasons), which reduces the actual area of blood circulation in the blood vessels, increases resistance, and requires raising blood pressure to help with blood circulation. In refrigeration systems, dirty blockages in pipelines, thermal expansion valves, etc. can also cause an instantaneous increase in high pressure.

Taking the analysis of high voltage as an example:

High pressure refers to the pressure before throttling, also known as condensation pressure. Its physical meaning is the saturation pressure at which refrigerant condenses into liquid in a condenser. This pressure is usually related to the condenser, such as the heat transfer area, heat transfer effect, heat transfer medium, and operating conditions of the condenser. image

We can consider that the most direct influencing factor on condensation pressure is the temperature of the operating conditions. Like air-cooled heat exchangers, the condensation temperature is usually about 15-20 ℃ higher than the operating temperature. The design parameters for the outer side of a typical household air conditioner are a working temperature of 35 ℃/24 ℃, with a condensation temperature design value of 54.4 ℃. When analyzing abnormal high pressure, our primary consideration is whether the heat exchanger is functioning properly. If the condensation effect is not good, the high pressure is usually higher. We have summarized the following types of condensation pressure faults:

There are usually three types of condensation pressure faults:

01. The condensation pressure is too high;

02. Low condensing pressure;

03. Pressure vibration;

Possible reasons and troubleshooting methods for high condensation pressure:

a) There is air or other incompressible gas present in the refrigeration system; Exclusion method: Start and run the system using the recycling system until it reaches operating temperature, and remove the gas from the condenser. If necessary, clear again.

b) The surface area of the condenser is too small; Exclusion method: Replace the larger condenser.

c) Excessive charging of the refrigeration system; Exclusion method: Recover the refrigerant until the condensation pressure is normal, while ensuring that the sight glass remains fully charged.

d) The condensation pressure regulation set an excessively high pressure; Exclusion method: Set the correct pressure.

e) There is dirt on the surface of the condenser; Exclusion method: Clean the condenser.

Regarding the high pressure alarm of the air condenser (finned heat exchanger), it is usually necessary to analyze the following aspects:

Defects or undersized fan motors or blades; Exclusion method: Replace the motor or blades, or replace them simultaneously.

The air flowing towards the condenser is restricted; Exclusion method: Remove obstacles from the air inlet or move the condenser.

The ambient temperature is too high; Exclusion method: Form a clean air inlet or move the condenser.

The airflow passing through the condenser is in the wrong direction; Exclusion method: Change the rotation direction of the fan motor to ensure proper air circulation in the condenser.

Short circuit between the air side pressure and the suction side of the condenser fan; Exclusion method: Install pipes that may be suitable for outdoor air.

Regarding the high-pressure alarm of water-cooled condensers (finned heat exchangers), the following aspects usually need to be analyzed:

The temperature of the cooling water is too high; Exclusion method: Ensure a lower water temperature.

The water volume is too small; Exclusion method: Increase the water volume, possibly using an automatic water valve to adjust.

There is sediment (dirt, etc.) inside the water pipe; Exclusion method: Remove sediment from the condenser water pipe, possibly using deoxygenation for removal.

The cooling water pump has defects or stops working; Exclusion method: Investigate the cause and replace or repair the cooling water pump (if installed).

Common causes and troubleshooting methods for low condensing pressure:

a) The surface area of the condenser is too large; Exclusion method: Adjust the condensing pressure or replace the condenser.

b) The load on the evaporator is small; Exclusion method: Adjust the condensing pressure to adapt to the load.

c) Low suction pressure, such as insufficient liquid in the evaporator; Troubleshooting method: Find the fault in the pipeline between the condenser and the thermal expansion valve.

d) The suction valve or discharge valve of the compressor may leak; Exclusion method: Replace the compressor valve plate.

e) The pressure setting of the condensing pressure regulating valve is too low; Exclusion method: Set a correct pressure for the condensing pressure regulating valve.

f) The uninsulated receiver placed is too cold compared to the condenser (the receiver is used as a condenser); Exclusion method: Move the receiver or install a suitable insulation cover for the receiver.

g) The cooling air temperature is too low (air-cooled); Exclusion method: Adjust the condensation pressure to adapt to the air temperature.

h) Excessive air volume in the condenser (air-cooled); Exclusion method: Replace the smaller fan or adjust the motor speed.

i) Excessive water volume (water cooling); Exclusion method: Reduce the flow rate of cooling water.

j) Low water temperature (water-cooled); Exclusion method: Increase the temperature of the cooling water.

2、 Temperature discrimination of refrigeration unit

Look and observe for any abnormal situations.

Smell: Smell and hear abnormal sounds.

Question: Inquire with the customer about the specific symptoms of the malfunction.

Cut: Touch the temperature of the refrigeration system components and pipelines.

Our main focus this time is on "cutting", by touching the temperature of each component and pipeline of the refrigeration system to determine if it is normal. We must have a clear understanding of the approximate temperature of the entire refrigeration system:

Touch exhaust temperature: We can determine whether the exhaust temperature is normal based on the condensation temperature. The condensation temperature can be estimated by the ambient temperature or the reading of a high-pressure gauge. Preliminary judgment based on the following experience:

Condensation temperature=ambient temperature+15~20 ℃;

Exhaust temperature=condensation temperature+20~30 ℃.

If the hand feel and exhaust temperature are low, we can preliminarily assume that the suction superheat is too low, which may be caused by the thermal expansion valve opening being too large or the refrigerant charging amount being too high.

Touch the suction temperature: The suction temperature is also an important parameter that can determine whether the throttle mechanism is properly adjusted and whether the refrigerant charge is appropriate. The main criterion for judgment is the degree of superheat during inhalation. Inhalation superheat=Inhalation temperature - Evaporation temperature (evaporation temperature depends on the reading on the low-pressure gauge). A high degree of suction superheat indicates that the opening of the thermal expansion valve is too small; A low degree of suction superheat indicates that the opening of the thermal expansion valve is too large.

Touch the temperature of the condenser: There are two types of condensers: water-cooled condenser and air-cooled condenser. Taking the water-cooled shell and tube condenser as an example, the upper part of the shell and tube is generally the high-temperature zone, the middle part is the saturated refrigerant zone of the condensation temperature, and the lower part is the supercooled ice cooling zone. This is because the upper part of the shell tube is the superheated vapor of the refrigerant, the middle part is the saturated refrigerant at the condensation temperature, and the lower part is the subcooled refrigerant.

Touch the temperature of the inlet and outlet water: The inlet water temperature of the shell and tube condenser is generally around 25-35 ℃, while the outlet water temperature is about 5 ℃ higher than the inlet water temperature. Usually, we can also judge some basic problems based on this temperature: a high inlet water temperature indicates poor cooling effect of the cooling tower. Check whether the cooling tower fan is running normally and whether the cooling tower is too dirty and causing poor heat transfer; A small temperature difference between the inlet and outlet water (below 5 ℃) indicates poor heat transfer efficiency between the shell and tube, and the water flow rate cannot carry away heat, resulting in a low outlet water temperature; A large temperature difference between the inlet and outlet water (greater than 5 ℃) indicates a low water flow rate. Under certain conditions of the heat exchanger, a low water flow rate can lead to a large temperature difference.