Refrigerants and Oils
The refrigerant is the substance in a refrigeration system that changes absorbs or releases heat as it changes state.
The oils used in refrigeration systems are used to lubricate moving parts of the compressor. The oil must be compatible with the refrigerant.
The original refrigerants were toxic materials such as ammonia and methyl chloride, but those refrigerants were replaced with halogen refrigerants that contained chlorine and fluoride.
Because chlorine-based refrigerants damage the Earth’s ozone layer, halogen refrigerants are being replaced by new refrigerants.
A refrigerant should be stable, non-toxic, nonflammable, non-corrosive, and easy to detect.
There are many refrigerants in use today. HFC-410A and HFC-408C are among the newer refrigerants that are being used to replace HCFC-22.
Halogenated hydrocarbons contain some quantity of chlorine, fluorine, or bromine.
Halocarbon refrigerants include CFCs, HCFCs, and HFCs.
CFCs contain the most chlorine, and are therefore the most damaging to the environment. CFCs can no longer be manufactured.
HCFCs contain less chlorine, and are scheduled to be phased out by 2030.
HFC refrigerants contain no chlorine. These refrigerants are being used to replace CFC and HCFC refrigerants.
So-called organic refrigerants contain no chlorine, but create safety concerns because they are often blends of flammable hydrocarbons such as butane or propane.
The ozone layer in the Earth’s atmosphere protects the planet from ultraviolet radiation.
CFC refrigerants have the greatest ozone depletion potential (ODP). HFCs have ODP ratings of zero.
The phenomenon known as the greenhouse effect is said to contribute to global warming.
Chlorine-based refrigerants contribute to global warming, though not as much as carbon dioxide emissions from vehicles and industrial sources.
HVAC technicians must pass an EPA certification test in order to handle refrigerants. Four levels of certification are available.
ASHRAE has developed a standard for identification of refrigerants based on the chemical makeup of the refrigerant.
Refrigerants are assigned to one of four safety classifications based on their storage pressure and flammability.
Refrigerant cylinders are color-coded to make it easy to identify them. Cylinders used for recovery are gray with a yellow shoulder.
There are three types of cylinders: disposable, returnable/reusable, and recovery.
Disposable cylinders must be discarded after use and should be kept in their cardboard shipping container or otherwise secured during transport to avoid damage.
Returnable/reusable cylinders are returned to the distributor or manufacture to be refilled.
Recovery cylinders are used with refrigerant recovery machines. They must be dedicated for use with a specific refrigerant and be marked to reflect that refrigerant.
To allow for expansion, filling of recovery cylinders must be limited to 80 percent of their gross weight.
Refrigerants are either a single compound such as HCFC-22 or blends of two or more refrigerants.
There are three categories of blended refrigerants: azeotropes, zeortropes, and near-zeotropes.
Like pure refrigerants, azeotropes evaporate and condense at a one given pressure and temperature.
Zeoptropes are blends that evaporate and condense over a range of temperatures known as the glide.
A zoetrope has a bubble point and a dew point that define their glide range.
The bubble point is considered to be the point at which the refrigerant starts to evaporate as heat is added.
The dew point is the point at which the refrigerant stops evaporating and becomes superheated as heat is added.
When azeotropic refrigerant is lost due to a leak, the components of the blend do not leak out at the same rate. This is called fractionation.
Because of fractionation, the charge must be recovered and the system recharged once the leak is fixed.
Azeotropes must always be charged to an operating system in the liquid state through a metering orifice.
Near-azeotropes exhibit glide, but usually in a minimal way that does not impact the service technician.
Compare HCFC-22 with R-410A and note tat there are no bubble points or dew points listed for the near-azeotrope.
Superheat and subcooling can be calculated using a P-T chart, a gauge manifold and an accurate electronic thermometer.
Leaks in large systems must be repaired to avoid release of refrigerant to the atmosphere.
Poor system performance often signals the existence of a refrigerant leak.
An oil stain is another signal of a leak, and some refrigerants have a distinct odor.
Leaks commonly occur at soldered or brazed joints, as well as mechanical connections such as flare and compression fittings.
Coils may also develop leaks due to vandalism or corrosion.
Leak-detecting fluids will form bubbles in the presence of a leak.
Leak-detecting dyes can be added to a system, but only if allowed by the equipment manufacturer.
Electronic leak detectors are used to sample the air around a suspected site. If there is a leak, the detector will emit an audible sound.
The electronic leak detector must be compatible with the refrigerant.
If there is not enough charge to test for leaks, the system can be pressurized with nitrogen and a trace amount of refrigerant.
A system must never be pressurized with un-regulated nitrogen, as well as oxygen, compressed air, or any flammable gas.
The moving parts of the compressor are lubricated with oil. The oil also helps seal the system, cool the compressor, and flush away tiny metal particles.
Mineral oils were used with CFC and HCFC refrigerants, but newer HFC refrigerants are not compatible with mineral oil.
Synthetic oils have been developed to replace mineral oil.
Oils used for compressor lubrication must be miscible with the refrigerant, and must maintain have a viscosity that allows it to flow under a wide range of temperature conditions.
Refrigerant oils must meet possess eight special properties in order to be used as a lubricant in a refrigerant system. The text lists and describes these eight properties.
Minerals oils are derived from crude petroleum. Additives are used to make them compatible with specific refrigerants.
Synthetic oils used for compressor lubrication include polyalkaline glycol (PAG), polyolester (POE), polyvinyl ether (PVE), and alkybenzene oil.
PAG oil is used extensively in automotive applications with HFC-134a. POE oil is widely used with the new refrigerants.
PVE oil can be used with HFC refrigerants. It has the advantage of not being hygroscopic.
Be sure to consult the equipment manufacturer’s literature for recommendations on the best oil-refrigerant combination.
Lubricating oil is stored in the sump of the compressor shell.
There are two common lubricating methods. In splash lubrication, the oil is splashed onto the cylinder walls and bearing surfaces during each crankshaft revolution.
The pressure – or force-feed – system uses an oil pump mounted end of the crankshaft.
Some force-feed systems use a pressure regulator to prevent excessive oil pressure.
Some oil travels with the refrigerant. The suction and liquid lines must be properly sized to ensure that this oil returns to the compressor.
Oversized lines will result in oil settling in low spots. Oversized accessory components such as receivers and accumulators will interfere with oil return.
Suction lines should be pitched toward the compressor to avoid oil backflow during shutdown.
In a split system with the evaporator above the condenser, a trap is needed in the suction line at the evaporator to help carry the oil out.
Air, water, acid, and metal particles can contaminate a system. Air may contain moisture, which can mix with refrigerant and oil to form acid.
The best way to keep moisture out of a system is to use ACR piping and evacuate any system that has been exposed to air or moisture.
If acid forms in the system due to compressor burnout or other cause, it must be removed using filter-driers. It may also be necessary to replace the compressor oil.
Solids such as metal particles come from compressor wear or from improper procedures such as using a hacksaw to cut refrigerant piping.
A filter-drier can trap metal particles and other solids.
Acid-moisture test kits are available can be used to test compressor oil. The crystals in the test tube will change color in the presence of acid or moisture.
Acid test kits require a sample of the compressor oil. Because of that, they are not very effective on welded hermetic compressors.
Although oil additives are available, they do not always live up to their claims, and may actually be harmful. Always check the manufacturer’s literature before adding anything to a system.
Refrigerant oils must be properly handled to avoid the possibility of personal injury and damage to materials or the environment. Consult the MSDS before handling oil.
Oil-resistant gloves and eye protection must be worn when handling oil.
Oil from a burned-out compressor might contain harmful acid, and synthetic oil can irritate skin.
In case of contact with skin, wash the area with soap and water, and clean up any oil spills.
All oils will absorb moisture, which is a major system contaminant.
Synthetic oils are especially hygroscopic, so they must be carefully protected from moisture.
Synthetic oils can also damage some materials such as roofing shingles.
Review the guidelines and precautions in the text for working with synthetic oil.
A lack of oil can indicate piping design problems that are preventing oil return.
Before adding or removing oil, determine what caused the shortage or excess of oil to occur.
Oil can be added to a semi-hermetic compressor using a pump. The crankcase pressure must be reduced to zero or the refrigerant charge recovered.
Some compressors are equipped with an oil drain plug to enable oil removal. A pump can also be used.
Contaminated oil must be disposed of as hazardous waste. Uncontaminated oil can be taken to a recycling center.
Refrigerant manufacturers have developed new refrigerants that can be used in older systems designed for other refrigerants.
When making a refrigerant conversion, material compatibility must be must be considered. Synthetic oils may attack gaskets and O-rings for example.
The compressor oil may have to be changed because of incompatibility between the new refrigerant and the old oil.
If a synthetic oil is replacing a mineral oil, the old oil will have to be completely purged from the system.
HCFC-123 is being used to retrofit CFC-11 systems because it is compatible with all oil types.
HCFC-401A, HCFC-401B, and HCFC-409A are used for CFC-12 retrofits, depending on the application.
HFC-407C is considered the best alternative for retrofitting HCFC-22 systems.
HFC-410A is replacing HCFC-22 in new systems, but is not suitable for retrofits because of its higher operating pressures.