Hazards of Air Intake in Refrigeration System and Elimination Methods

1 Overview

In the refrigeration system, the so-called non-condensable gas means that when the refrigeration system is working, the gas cannot be condensed into a liquid under a specific temperature and pressure in the condenser, but always in a gaseous state. These gases mainly include nitrogen and oxygen. , hydrogen, carbon dioxide, hydrocarbon gases, inert gases, and mixtures of these gases. Due to the existence of non-condensable gases, the energy consumption of the compressor increases, while the cooling capacity of the refrigeration system decreases.

2 Causes of non-condensable gases

1. Insufficient evacuation of the refrigeration system before charging the refrigerant

Before filling the refrigerator, the compressor cylinder, condenser, evaporator and the pipeline of the system in the refrigeration system were filled with air. Vacuuming, sometimes due to subjective and objective reasons, the internal vacuuming of the refrigeration system is not sufficient and cannot meet the requirements, leaving a small amount of air inside the system.

2. Bring in the refrigerant when charging the refrigerant

In the refrigeration system, before the refrigeration system is filled with refrigerant, the pipes used for filling are filled with air. Due to human and other reasons, when the refrigerant is charged, the air in the pipe is not exhausted, and it is directly connected to the refrigeration system. These air As the charged refrigerant enters the refrigeration system.

3. When the refrigeration system is overhauled, non-condensable gas is mixed

If the refrigeration system works for a long time, it will inevitably need to be inspected and repaired or cleaned and replaced. In this way, sometimes it is necessary to disassemble the machinery or pipelines, and the air often enters the interior of the refrigeration system during the process of disassembly and installation.

4. Infiltrate into 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 infiltrate into the refrigeration system through various gaps. These gaps are distributed in various valves, compressors, non-welding places and many other places.

5. Chemical reaction from refrigerant

In the ammonia refrigeration system, the refrigerant ammonia can be decomposed into ammonia and hydrogen at a certain temperature and pressure, and the degree of decomposition is positively related to the temperature and pressure. The higher the temperature, the higher the pressure, and the easier the decomposition of ammonia.

In the Freon refrigeration system, Freon may chemically react with impurities mixed into the system to produce non-condensable gases. 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

Among the lubricating oils used in the refrigeration system, some lubricating oils, such as mineral lubricating oil, can decompose and produce various hydrocarbon gases under complex working conditions, and these hydrocarbon gases will be mixed into the refrigerant in the system.

3 Distribution of noncondensable gases

In a refrigeration system, when there are non-condensable gases on the low-pressure side, these gases are quickly sucked by the compressor into the high-pressure side. Therefore, usually non-condensable gases mainly accumulate in the condenser and high-pressure liquid receiver on the high-pressure side of the system.

Regardless of whether it is an evaporative condenser or a 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. The non-condensable gas in the liquid receiver is often concentrated in a space with a very low air velocity away from the air inlet.

4 Hazards of non-condensable gases

1. Reduce the cooling capacity of the system

When the non-condensable gas accumulates in the condenser, the non-condensable gas adheres to the inner wall of the condenser and occupies a certain space, so that the condensation area is reduced. At the same time, the non-condensable gas forms a thermal resistance between the refrigerant and the inner wall of the condenser. The heat transfer efficiency is reduced, and the heat cannot be discharged from the system in time, thereby reducing the cooling capacity of the refrigeration system.

2. Increased system energy consumption

Due to the reduction of heat transfer efficiency, the condensing temperature and condensing pressure in the condenser are both increased. Then, in the automatic control refrigeration system, in order to maintain the same degree of condensation, the flow rate of condensed water must be increased to reduce

The temperature of the refrigerant and noncondensable air in the low condenser. This increases the energy consumption of the condensate pump. At the same time, the increase of the condensing pressure makes the pressure of the discharge outlet of the compressor also increase compared with the normal working condition, and the compressor needs to overcome a greater pressure during the exhaust process, so the energy consumption of the compression also increases.

3. Cause damage to mechanical equipment

The increase of the discharge pressure of the compressor makes the reaction force on the bearing, the transmission device and the sliding surface also increase. In the long run, it accelerates the wear and aging of the equipment and the deterioration of the lubricating oil, resulting in damage to 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 presence of non-condensable gases will increase the condensation pressure of the refrigeration system, the condensation temperature will increase, the compressor discharge temperature will increase, the power consumption will increase, and the refrigeration efficiency will decrease; Carbonization will affect the lubrication effect, and in severe cases, the motor of the refrigeration compressor will be burned.

5 The main signs of non-condensable gas

1. The discharge pressure and discharge temperature of the compressor rise, the pointer of the pressure gauge on the condenser (or liquid receiver) swings violently, the head of the compressor cylinder is hot, and the condenser shell is very hot.

2. Uneven frosting on the surface of the evaporator.

3. When there is a large amount of non-condensable gas, the storage temperature cannot be lowered due to the decrease of the cooling capacity of the device, the compressor runs for a long time, and the compressor is even stopped due to the action of the high-voltage relay.

case:

The measured condensing pressure of the R22 system was 13.2kg/cm2 (gauge pressure), and the ambient temperature at that time was 35 degrees.

Check the "Temperature and Pressure Comparison Table of R22 Refrigerant", and the corresponding pressure at a temperature of 35 degrees 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 its non-condensable gas is: 13.2-12.81=0.39kg/cm2 (gauge pressure).

6 Exclusion of non-condensable gases

It is still necessary to manually remove the non-condensable gas method. In this method, the operator judges whether there are more non-condensable gases inside the refrigeration system according to the low condensation pressure, and decides whether to discharge them. This method largely depends on the experience of the operator, and the operation is flexible, and the discharge of non-condensable gases is relatively thorough.

The non-condensable gas is easy to be separated from the refrigerant naturally at low temperature and the system is still. Its specific gravity is smaller than that of the refrigerant. After separation, it gathers in the high place (above) of the system. The longest system downtime is vented at the highest evacuation point in the refrigeration system. It can also be discharged directly from the top of a certain container in the system by directly opening the valve, or the sub-containers can be discharged one by one.

1. Small freon refrigeration system

It is not necessary to set up special exhaust equipment, and the non-condensable gas in the system can be exhausted by using the system itself. The specific operation steps are:

Step 1: Close the outlet valve of the condenser and the outlet valve of the high-pressure liquid receiver;

Step 2: Start the compressor to pump the refrigerant in the low-pressure system to the condenser or high-pressure liquid receiver;

Step 3: When the low-pressure part of the refrigeration system remains in a stable vacuum state, stop the compressor and close the suction valve, while the exhaust valve remains open, and at the same time open the cooling water shut-off valve to fully liquefy the high-pressure refrigerant gas ;

Step 4: About 10 minutes, loosen the multi-channel bolt of the compressor exhaust valve, or open the air release valve on the top of the condenser to discharge the air;

Step 5: Feel the temperature of the airflow with your hands. When there is no cool feeling or it feels hot, it means that most of the discharged gas is non-condensable gas, otherwise it means that the gas is discharged from Freon. At this time, the operation of discharging non-condensable gas should be suspended , but check the temperature difference between the saturation temperature corresponding to the pressure of the high-pressure system and the liquid outlet temperature of the condenser. If the temperature difference is large, it means that there is still a lot of non-condensable gas, and the non-condensable gas should be released intermittently after the mixed gas is fully cooled. sexual gas;

Step 6: When the non-condensable gas is discharged, the multi-purpose channel of the compressor exhaust valve should be tightened or the air exhaust valve above the condenser should be closed, and the water supply to the condenser should be stopped.

2. Large freon refrigeration system

For large-scale Freon refrigeration systems, an air separator should be installed. Figure 4 shows the structure of a sleeve-type manual air separator. This kind of gas separator is also widely used in most ammonia refrigeration systems.

The separator is welded by four layers of concentric sleeves, and there are two pairs of inlets and outlets, one pair is the inlet of the liquid refrigerant from the condenser and the outlet of the refrigerant gas that absorbs heat and evaporates into refrigerant gas, and the other pair is made of non-condensable The inlet of the mixed gas composed of gas and refrigerant vapor and the outlet of non-condensable gas discharged into the atmosphere through the vent valve.

The process of removing non-condensable gases is:

When the high-pressure refrigerant liquid passes through the first and third layers during the flow process, it exchanges heat with the mixed gas in the second and fourth layers, and the refrigerant liquid evaporates into refrigerant gas, while the refrigerant in the mixed gas The agent gas is condensed into a liquid;

The non-condensable gas is accumulated in the second and fourth layer casings. When a sufficient amount is accumulated, the vent valve is opened to release the non-condensable gas, and the liquid refrigerant flows into the tube through the throttle valve to evaporate.

3. Condenser vent valve to deflate

Step 1: Close the outlet valve of the reservoir.

Step 2: Turn on the compressor, press the refrigerant (and non-condensable gas) in the system into the condenser, and stop until the low-voltage relay operates.

Step 3: After stopping the machine, let the cooling water continue to circulate in the condenser to fully condense the refrigerant. Because the non-condensable gas is lighter than the refrigerant gas, it gathers at the top of the condenser (the condenser of some small devices is at the bottom of the compressor, and at this time it gathers at the highest position in the high-pressure system).

Step 4: Open the bleed valve on the top of the condenser (or the multi-purpose channel of the double-seat exhaust stop valve or the outlet of the exhaust thermometer and other joints) to release the non-condensable gas. The opening of the deflation valve should not be too large. In order to judge the situation of deflation, you can face the airflow with your hands. If you feel like the wind is blowing, it means that the gas is released; if there are oil stains on your hands and a cool feeling, it means that the refrigerant gas has been released. The bleed valve should be closed immediately.

4. Condenser vent valve to deflate

The method of automatically removing non-condensable gas is to control the discharge of non-condensable gas according to parameters such as temperature, and at the same time, the refrigerant recovery device recovers the refrigerant in the mixed gas as much as possible, leaving non-condensable gas.

Finally drain the system.

Automatic exclusion method is suitable for ammonia refrigerant system

The figure below is a schematic diagram of the structure of an automatic air separator used in an ammonia refrigeration system.

Its working principle is similar to that of the manual separator, except that it is equipped with a pressure switch and a temperature controller, and a solenoid valve is used instead of a throttle valve. The high-pressure liquid ammonia and the return liquid ammonia are evaporated into gaseous ammonia in the evaporation tube, and the non-condensable After the mixed gas of inert gas enters the separator, most of the ammonia gas is condensed into liquid ammonia and gathers at the bottom, while a small amount of ammonia gas and non-condensable gas gather in the separator, and the temperature drops continuously at the same time.

When the temperature reaches the set value, the solenoid valve opens, and the mixed gas enters the ammonia-water mixer, and only the non-condensable gas is discharged after treatment. This air separator is easy to operate and has a high degree of automation. But in the process of work, it lacks flexibility and moves more mechanically.