LUBRICANTS1
Lubricant properties can be evaluated to determine if the product is right for the job. Three basic proper- ties are viscosity, lubricity, and chemical stability. They must be satisfactory to protect the compressor. The correct viscosity is needed to fill the gaps between parts and flow correctly where it is supposed to go. Generally speaking, smaller equipment with smaller gaps between moving parts requires a lighter viscosity, and larger equipment with bigger parts needs heavier viscosity oils. Lubricity refers to the lubricant’s ability to protect the metal surfaces from wear.
Good chemical stability means that the lubricant will not react to form harmful chemicals such as acids, sludges, and so forth that may block tubing, or there may be carbon deposits. The interaction of lubricant and refrigerant can cause potential problems as well.
Miscibility defines the temperature region where refrigerant and oil mix or separate. If there is separation of the oil from the refrigerant in the compressor, it is possible that the oil is not getting to metal parts that need it. If there is separation in the evaporator or other parts of the system, it is possible that the oil does not return to the compressor and eventually there is not enough oil to protect it.
Solubility determines if the refrigerant will thin the oil too much. That would cause it to lose its ability to protect the compressor. The thinning effect also influences oil return.
Once you mix a blend at a given composition, the pressure-temperature relationships follow the same general rules as for pure components. For example, the pressure goes up when the temperature goes up. For three blends containing different amounts of A and B, the pressure curve is similarly shaped, but in the result pressure will be higher for the blend that contains more of the A or higher pressure component.
Some refrigerant blends are intended to match some other product. R-12 is a good example. It will rarely match the pressure at all points in the desired temperature range. What is more common is that the blend will match in one region and the pressures will be different elsewhere.
In Figure 12.1, the blend with concentration C1 matches the CFC at cold evaporator temperatures, but the pressures run higher at condenser conditions. The blend with composition C2 matches closer to room temperature. And, it may show the same pressure in a cylinder being stored, for example. The operation pressures at evaporator and condenser temperatures, however, will be somewhat different. Finally, the blend at C3 will generate the same pressures at hot condenser conditions. but the evaporator must run at lower pressures to get the same temperature.
It will be seen later that the choice of where the blend matches the pressure relationship can solve or cause certain retrofit-related problems.
Refrigerant Blends
Generally speaking, the R-12 retrofit blends have higher temperature glide. They do not match the pres- sure/temperature/capacity of R-12 across the wide temperature application range that R-12 has. In other words, one blend does not fit all. Blends that match R-12 at colder evaporator temperatures may generate higher pressures and discharge temperatures when used in warmer applications or in high ambient temper- atures. These are called refrigeration blends.
In refrigeration it is often an easier and cheaper retrofit job if you can match evaporator pressures to R-12 and split the glide. That is because you can get similar box temperatures in similar run times. And, you would probably not need to change controls or the thermostatic expansion valves (TXVs), which are sensitive to pressure.
Blends that match R-12 properties in hot conditions, such as in automotive AC condensers, may lose ca- pacity or require lower suction pressures when applied at colder evaporator temperatures. These are called automotive blends.
For automotive air conditioning many of the controls and safety switches are related to the highside pressure. If the blend generates higher discharge pressures, you could short cycle more often and lose capacity in general. It is better to pick the high side to match R-12 and let the low side run a little lower pressure.
R-134a Refrigerant
The blended refrigerant R-134a is a long-term HFC alternative with similar properties to R-12. It has be- come the new industry-standard refrigerant for automotive air-conditioning and refrigerator/freezer appli- ances. R-134a refrigerating performance will suffer at lower temperatures (below -10 degrees F). Some tra- ditional R-12 applications have used alternatives other than 134a for lower temperatures.
R-134a requires polyolester (POE) lubricants. Traditional mineral oils and alkyl benzenes do not mix with HFC refrigerants, and their use with 134a may cause operation problems or compressor failures. In ad- dition, automotive A/C systems may use polyalkaline glycols (PAGs), which are typically not seen in station- ary equipment.
Both POEs and PAGs will absorb moisture and hold onto it to a much greater extent than traditional lu- bricants. The moisture will promote reactions in the lubricant as well as the usual problems associated with water—corrosion and acid formation. The best way to dry a wet HFC system is to rely on the filter dryer. Deep vacuum will remove “free” water but not the water that has absorbed into the lubricant.
Appliances, both commercial and self-contained refrigeration, centrifugal chillers, and automotive air conditioning utilize R-134a. Retrofitting equipment with a substitute for R-12 is sometimes difficult; there are a number of considerations to be examined before undertaking the task:
1. For centrifugal compressors it is recommended that the manufacturer’s engineering staff become involved in the project—special parts or procedures may be required. This will ensure proper capacity and reliable operation after the retrofit.
2. Most older direct expansion systems can be retrofitted to R-401A, R-409A, R-414B, or
R-416A (R-500 to R-401B or R-409A), so long as there are no components that will cause fractionation within the system to occur.
3. Filter driers should be changed at the time of conversion.
4. The system should be properly labeled with refrigerant and lubricant type.
R-12 Medium/High Temperature Refrigeration (>OF evap)
1. See Recommendation Table (this can be found on the National Refrigerants Web site—click on Technical Manual) for blends that work better in high ambient-heat conditions.
2. Review the properties of the new refrigerant you will use and compare them to R-12. Prepare for any adjustments to system components based on pressure difference or temperature glide.
3. Filter dryers should be changed at the time of conversion.
4. The system should be properly labeled with refrigerant and lubricant type.
R-12 Low Temperature Refrigeration (<20F evap)
1. See Recommendation Table for blends that have better low-temperature capacity.
2. Review the properties of the new refrigerant you will use and compare them to R-12. Prepare for any adjustments to system components based on pressure difference or temperature glide.
3. Filter dryers should be changed at the time of conversion.
4. The system should be properly labeled with refrigerant and lubricant type.
Another blended refrigerant that can be used to substitute for R-12 is 401A . It is a blend of R-22, 152a, and 124. The pressure and system capacity match R-12 when the blend is running an average evaporator temperature of 10 to 20 degrees F. Applications for this refrigerant are as a direct expansion refrigerate for R-12 in air-conditioning systems and in R-500 systems.
R-401B
This blend refrigerant is similar to R-401A except that it is higher in R-22 content. This blend has higher capacity at lower temperatures and matches R-12 at -20 degrees F. It also provides a closer match to R-500 at air-conditioning temperatures.
Applications for R-401B are in normally lower-temperature R-12 refrigeration locations, in transport re- frigeration, and in R-500 as a direct expansion refrigerant in air-conditioning systems.
R-402A
R-402A is a blend of R-22 and R-125 with hydrocarbon R-290 (propane) added to improve mineral-oil cir- culation. This blend is formulated to match R-502 evaporator pressures, yet it has higher discharge pressure than 502. Although the propane helps with oil return, it is still recommended that some mineral oil be re- placed with alkyl benzene.
Applications are in low-temperature (R-502) refrigeration locations. In retrofitting it is used to substitute for R-502.
R-402B
R-402B is similar to R-402A but with less R-125 and more R-22. This blend will generate higher discharge temperatures, which makes it work particularly well in ice machines. Applications are in ice machines where R-502 was used extensively.