Cooling System Reactions
Lubricating oil is added to the refrigeration systems to reduce the friction associated widi seminar moving components within die compressor. With few exceptions, the oil is contained in die Carter die compressor where it comes in contact with the refrigerant. Therefore, refrigerants must be chemically and physically stable for the existence of a system with oil so that no substance, negatively affect their mixing. Some refrigerants, in particular, sulphur dioxide and halocar-the bonds listed in the Table. 12-1, experience minor reactions with lubricating oil in the system. Under normal circumstances, when a high quality lubricating oils are used and the internal system is maintained in a clean and dry, these reactions are usually minor. However, when contaminants such as air or moisture present in the system, damaging chemical reactions involving air, moisture, refrigerant and lubricating oil occur frequently. These reactions can cause oil decompose to form corrosive acids and sludge. High discharge temperatures accelerate these destructive processes.
Working at higher than normal temperatures leads carbon deposits form on the pistons, relief valves inside the compressor head and discharge line. These conditions are exacerbated by the use of poorly refined oils that contain a high percentage of chemically unstable hydrocarbons.
Copper plating another reaction that can occur in systems using Halocarbon refrigerants. Chrome plating is developing as copper atoms coming from the tube, pipelines and storage die on hot steel surfaces inside the system. This reaction negatively affect the dimensions, tolerances and friction characteristics of the affected parts. In its polished sides of the cylinder, seals, pistons, bearing surfaces, valves, and other hot metal surfaces are usually affected. Substantial evidence indicates diat humidity and low quality of lubricating oils, are also contributing factors in the solution.
Regardless of the nature dies or adverse reactions that occur between the refrigerant and lubricating oil, diese shortcomings can be greatly minimized or eliminated by maintaining a system with respect to air, moisture and other contaminants. Proper design of the system with a minimum discharge temperature and use of high-quality lubricating oils also minimizes the adverse reactions between the refrigerants, lubricants, and system components.
Oil-compatibility of Refrigerants differ in their ability to respond oil in the system. This characteristic is called oil compatibility. Compatibility is the ability die refrigerant to disperse in the butter or oil to disperse the refrigerant without damaging die other characteristics. There are advantages and disadvantages associated with oil-compatibility. Thus, the oil level compatibility is not usually a major factor in determining which of the refrigerant for use in this application, but oil compatibility becomes an important element in the design of the compressor, refrigerant piping and other components of the system.
Refrigerants are divided into three groups depending on their oil-compatibility:
One of the main consequences of the oil is mixed refrigerants is the dilution of the oil in the crankcase of the compressor. Dilution of the oil, reduces its viscosity fluid resistance offers to flow when he is under the influence of external force. Thus, the reduction of viscosity of the fluid, such as kerosene flow more readily than thicker, more viscous liquids, such as the 50-weight motor oil. The decrease in oil viscosity reduces its ability to correct lubrication of moving parts to die in the compressor. To provide adequate lubrication for the refrigeration compressor, the viscosity of lubricating oil should remain within design. If viscosity of the oil is too low, it will not have sufficient mass for the formation of protective film between the surfaces of moving parts.
- Refrigerants, which mixes with the oil in a proportion of under all conditions in the refrigeration system.
- Refrigerants, which is mixed in conditions typically found in a capacitor, but separately from the oil in conditions typically found in the evaporator.
- Refrigerants that very little or not mixed with oil, when all the conditions detected in the system.
This film will help moving parts from rubbing against each other, creating a destructive level of friction and heat. Conversely, when the viscosity of oil is too high, it will not have sufficient fluidity to penetrate between the sliding surfaces, especially where tolerances for tightness. Therefore, in any situation, lubrication of the compressor will not be adequate to maximize its useful life and minimize operating costs. To reduce lubrication problems in systems using oil is mixed refrigerants, oil should have a higher initial viscosity than that which would otherwise be required for similar duty with nonniiscible refrigerants. As a refrigerant, mingled with oil, die reduce the viscosity will still permit a mixture provides an acceptable level of protection.
Oil is mixed refrigerants also create the ineffectiveness of other components of the cooling system. Oil circulating system with die refrigerant forms a film on the inner surface of the condenser and evaporator tubes, reducing the efficiency and performance of the system. This thermal resistive film reduces die heat transfer rate of the two components of the system, producing the detrimental effects that have been described in Chapter 11. With die oil becomes more viscous and, as a rule, congeal as its temperature decreases, the film forming problem more in the evaporator to become more acute evaporator at lower temperatures.
With few exceptions, the refrigerant inevitably comes into contact with oil as it passes through die compressor. In this period, small oil particles become trapped die in hot, high-pressure steam and reflected dirough a pressure valve in the discharge pipe. Circulation of small volume of oil with die refrigerant usually undesirable, because it lubricates the metering devices and other system valves. However, because of the adverse effects diat oil films on system capacity, die amount of oil coming out of the compressor should be kept to a minimum. Similarly, since all the oil in the appeal comes from the compressor crankcase, an excessive amount of oil circulating in the system may mean that the die level in the crankcase fell below that required for proper lubrication of the compressor parts..