What are the harm and elimination methods of air in refrigeration system?

Update:10 Jun 2020
Summary:

In the refrigeration system, the so-called non condensa […]

In the refrigeration system, the so-called non condensable gas refers to that when the refrigeration system is working, under the specific temperature and pressure in the condenser, the gas can not be condensed into liquid, but always into gas state. These gases mainly include nitrogen, oxygen, hydrogen, carbon dioxide, carbon hydrogen body, inert gas and the mixed gas of these gases.
Because of the existence of non condensable gas, the energy consumption of compressor increases, while the refrigeration capacity of refrigeration system decreases.

1、 Causes of non condensable gas
1. Insufficient evacuation of the refrigeration system before filling the refrigerant
Before filling the refrigerator, the compressor cylinder, the condenser, the evaporator and the pipeline of the system in the refrigeration system have been filled with air. In order to remove the air before filling the refrigerant, it is necessary to vacuum the inside of the refrigeration system. Sometimes, due to subjective and objective reasons, the vacuum in the refrigeration system is not enough to meet the requirements, A small amount of air is left inside the system.

2. Brought in when filling refrigerant
Before the refrigerant is filled in the refrigeration system, the pipe used for filling is filled with air. Due to human factors, when filling the refrigerant, the air in the pipe is directly connected to the refrigeration system without exhausting the air in the pipe. The air enters the refrigeration system with the filled refrigerant.

3. Non condensable gas is mixed in the maintenance of refrigeration system
If the refrigeration system works for a long time, it is inevitable that it needs to be inspected, repaired, cleaned and replaced. In this way, it is sometimes necessary to dismantle the machinery or pipeline, and air often enters the refrigeration system during the process of disassembly and installation.
4. Infiltrate the system from the outside atmosphere
In some refrigeration systems, if the working pressure is lower than the atmospheric pressure, the air in the atmosphere will penetrate into the refrigeration system through various gaps. These gaps are distributed in various valves, compressors, non welding places and many other places.

5. Chemical reactions from refrigerants
In the ammonia refrigeration system, the refrigerant ammonia can be decomposed into ammonia and hydrogen at a certain temperature and pressure. The degree of decomposition is positively related to the temperature and pressure. The higher the temperature is, the higher the pressure is, the easier the ammonia is to decompose.
In the freon refrigeration system, freon may react with impurities in the system to produce non condensable gas. For example, R12 reacts with water under certain conditions to produce carbon dioxide.

6. The decomposition of lubricating oil will also produce non condensable gas
In the lubricating oil used in refrigeration system, some lubricating oil, such as mineral lubricating oil, can decompose to produce a variety of hydrocarbon gases under complex working conditions, which will be mixed into the refrigerant in the system.

2、 Distribution of non condensable gas
In the refrigeration system, when there are non condensable gases at the low pressure side, these gases are quickly sucked by the compressor and enter the high pressure side. Therefore, the non condensable gas is usually concentrated in the condenser and high-pressure reservoir on the high-pressure side of the system.
Whether the evaporative condenser or the shell and tube condenser, the non condensable gas will adhere to the heat exchange surface as much as possible, as shown in the figure below. However, the non condensable gas in the reservoir is often concentrated in the space far away from the air inlet.

3、 Hazards of non condensable gas
1. Reduce the cooling capacity of the system
When the non condensable gas gathers in the condenser, the non condensable gas adheres to the inner wall of the condenser, occupying a certain space, reducing the condensation area. At the same time, the non condensable gas forms a thermal resistance between the refrigerant and the inner wall of the condenser, reducing the heat transfer efficiency, and the heat can not be discharged out of the system in time, thus reducing the refrigeration capacity of the refrigeration system.
2. System energy consumption increases
Due to the decrease of heat transfer efficiency, the condensing temperature and pressure in the condenser increase. In order to maintain the same degree of condensation in the automatic control refrigeration system, it is necessary to increase the flow of condensate water to reduce the temperature of refrigerant and non condensable air in the condenser. This increases the energy consumption of the condensate pump. At the same time, with the increase of the condensation pressure, the pressure at the outlet of the compressor is also larger than that under normal working conditions. The compressor needs to overcome the larger pressure in the exhaust process, so the energy consumption of compression is also larger.
3. Damage to mechanical equipment
With the increase of the discharge pressure of the compressor, the reaction force on the bearing, transmission device and sliding surface is also increased. In the past, the wear and aging of the equipment and the deterioration of the lubricating oil are accelerated, resulting in the damage of the mechanical equipment.
At the same time, due to the severe wear of the sliding surface, the leakage of refrigerant will also increase.
Summary: the storage of non condensable gas will increase the condensing pressure and condensing temperature of the refrigeration system, increase the exhaust temperature of the compressor, increase the power consumption and reduce the refrigeration efficiency; at the same time, the high exhaust temperature may cause the carbonization of lubricating oil, affect the lubrication effect, and burn down the motor of the refrigeration compressor.

4、 The main signs of non condensable gas are
1. The discharge pressure and exhaust temperature of the compressor increase, the pressure gauge pointer on the condenser (or reservoir) swings violently, the compressor cylinder is hot, and the condenser shell is very hot.
2. The frosting on the evaporator surface is uneven.
3. When there is a large amount of non condensable gas, the temperature of the storage can not be reduced due to the decrease of the refrigeration capacity of the device, the compressor runs for a long time, and even the compressor stops due to the action of the high-pressure relay.
Case:
The actual condensation pressure of R22 system is 13.2kg/cm2 (gauge pressure), and the ambient temperature at that time is 35 ℃.
Check the temperature and pressure comparison table of R22 refrigerant. When the temperature is 35 ℃, the corresponding pressure is 12.81kg/cm2 (gauge pressure), which is lower than the measured condensation pressure, indicating that there is non condensable gas in the system. The pressure content of the non condensable gas is: 13.2-12.81 = 0.39kg/cm2 (gauge pressure).