Characteristics of Freon Refrigerants

1. Oil Solubility

Mutual solubility with lubricating oil is the main characteristic of freon, and its solubility is related to the type and temperature of the refrigerator and lubricating oil. Among the commonly used freons, R12 is more oil-soluble than R22 and R502. In view of the mutual solubility of fluorine and oil, during the refrigeration process, the fluorine-oil mixture flows through high-pressure liquid receivers, intercoolers, low-pressure circulating barrels and other equipment in different states. After fluorine dissolves oil, there are advantages and disadvantages to the refrigeration system.

Its advantages are:

• Conducive to the lubrication of moving parts;
• In the heat exchanger (such as the condenser), the oil film will not be formed to affect the heat transfer, which improves the heat transfer conditions;
• When the temperature in the high-pressure container is higher than the critical solution temperature, the fluorine liquid and the oil form a uniform liquid, so it is not necessary to consider the oil problem in the high-pressure liquid accumulator, condenser, and other equipment.

The disadvantages are:

• The evaporating temperature rises under the same evaporating pressure. To maintain the original evaporating temperature, the compressor must operate at a lower evaporating pressure, so that the cooling capacity is reduced and the cooling coefficient is reduced;
• The viscosity of the fluorine liquid increases, and the flow resistance increases;
• A large amount of lubricating oil is dissolved in the fluorine liquid or accumulated in some equipment, which can also cause oil loss in the compressor.

2. Water Soluble

The water solubility of Freon is very small, and its solubility is related to the type and concentration of the refrigerant. The solubility of the same Freon decreases with decreasing temperature. When the water dissolved in fluorine is cooled at a high temperature, some water will be precipitated as free water. When the evaporation temperature of the refrigeration system is lower than 0 °C, the precipitated water is easy to freezes at the throttle valve, causing the valve hole to block, commonly known as the “ice block”. Ice blockage will affect the normal operation of the refrigeration system. At the same temperature, the water solubility of R22 is larger than that of R12, so the R22 refrigeration system has fewer ice blockages than the R12 system. At the same time, due to the existence of moisture, it has almost no corrosive effect on metals (except metals containing 2% magnesium) and corrodes equipment, pipelines, motor insulation, etc., especially aluminum-magnesium alloys. In addition, the production of rust caused by moisture will make the lubricating oil muddy, accelerate the aging of the lubricating oil, and add unfavorable factors to the operation of the compressor. Therefore, when purchasing Freon, it should meet the water content standard (R12, R22 is not more than 0.0025%, mass). In the system design, it is necessary to add a dryer to the liquid pipeline. When the system is pressurized for leak testing, nitrogen or dried compressed air should be used. When charging the refrigerant, it must be purified by the dryer before entering the system.

3. Corrosive

Fluorine refrigerant is a good organic solvent, and macromolecular compounds such as natural rubber and resin are easily dissolved in Freon. Therefore, ordinary rubber cannot be used as sealing gaskets, fillers, etc., but oil-resistant rubber and asbestos boards should be used, such as chlorohydrin rubber for R22.

4. Thermal Performance

The surface heat transfer coefficient of Freon and the cooling capacity per unit volume are both smaller than those of ammonia, so the machinery and equipment with the same cooling capacity are larger than the ammonia system.

5. The Isentropy Coefficient is Small

The isentropic index of Freon is smaller than that of ammonia (R12, K=1.138; R502, K=1.133; R22, K=1.194; R717, K=1.32), so the exhaust temperature of the fluorine system is lower than that of the ammonia system. Especially for R12 and R502, even at lower evaporating temperature and higher condensing temperature (high-pressure ratio), the exhaust temperature is still not too high, the machine does not appear overheating, and the service life is extended. And the oil entrained in the exhaust gas is almost undamaged and can be directly returned to the compressor without treatment. Therefore, the condensing temperature of R22 and R502 can be as high as 50° or more, which makes it possible to use air-cooled condensers in fast refrigeration units. The exhaust temperature of R22 is higher than that of R12 and R502, and the lower the evaporation temperature, the greater the temperature difference between the two. Therefore, R22 is more used in the case of small compression ratios, such as air conditioning, two-stage compression system, etc. Due to the lower exhaust gas temperature, incomplete intercooling is often used in fluorine two-stage systems to simplify the system.

6. Density

Freon has a high density, such as the density of saturated vapor is 4 to 5 times larger than that of ammonia, and the liquid is more than twice as large, which increases the flow resistance in the pipeline. In order to reduce the resistance, the pipe diameter needs to be enlarged, so the pipe diameter of the fluorine system is generally larger than that of the ammonia pipe. The specific gravity of fluorine liquid is larger than that of common lubricating oil, so the oil discharge of oil collecting equipment should be different from that of the ammonia system.

7. Leakage

According to the introduction, the practice has proved that “under a certain pressure, the permeability of refrigerant is inversely proportional to the square root of molecular weight”. From this, it can be concluded that Freon is less likely to leak than ammonia. However, since freon has no odor and leakage is not easy to find, the tightness of the system is required to be high. Valves generally have bonnets to reduce leakage.