Material compatibility

Refrigerants can also be considered as a solvent, physically and chemically. They can also react with some substances and change chemically. Thus, the compatibility and applicability with the used construction materials has to be evaluated. In refrigerant circuits the refrigerants are in contact with many materials at widely varying temperatures and pressures. Inside the systems, there is oil, sealing and gasket material, steel, cast iron, copper, aluminium, zinc, bronze, brass, insulation plastic, slide coating, corrosion protection coating and many things more. Many component manufacturers test their gasket and insulation materials for compatibility with refrigerants and oils before release, carefully and with high investment of time.

Gaskets

Gaskets are used to ensure that the refrigerant does not leak out of the refrigeration system or they seal between parts with different pressures. They have to be compatible with the selected refrigerant and oil for long term operation.

The compatibility is investigated a.o. in autoclaves at varying temperature and pressure, with the gasket materials in refrigerant oil mixtures. Flat fibre gaskets and gaskets with metallic support are compatible with most chemicals, relevant for refrigerant use. Elastomer gaskets, often used for o-rings or shaft seals, have to be tested for compatibility. This means, for a given refrigerant, the matching materials have to be used.

Elastomers can change their properties in contact with refrigerant and oil. Typical changes are dwelling or shrinking, softening or hardening. These changes can only be accepted in a very limited amount in order to qualify as compatible.

An o-ring shrinking strongly would lead to leakage, a strongly dwelling one would result in deformation and limited operating life time. As o-rings made of elastomers are meant to be a flexible construction element, also hardness change is only acceptable within narrow limits.

Additionally, the refrigerant shall not be accumulated inside the elastomer because strong pressure changes, especially pressure drops, would let the material burst.

Bearings

Roller bearings are mostly made of high quality steel and are thus compatible to most refrigerants and oils without problems. Only the cages, holding the balls or rollers, and the cover plates might be made of plastic and probably have to be tested.

Hydrodynamic bearings are typically made of pairs of different metals. They often have bearing inlays made of material compounds or alloys, partly with plastic coatings in order to improve emergency running properties. These bearing types also have to be tested for applicability with the chosen refrigerants and oils.

Lubricants

In most refrigeration systems, the refrigerants work with oils that are chosen to be miscible in at least the biggest part of the system operating conditions. Due to this, the refrigerant with its significantly lower viscosity will dissolve partly in the oil, lowering the effective viscosity and thus worsening the lubricity. The oil to be chosen has to match the system and compressor design and the planned operating conditions and lubricate the compressor bearings reliably.

When releasing a refrigerant for a compressor, this will be investigated. Helpful for this is the establishing of solubility-viscosity diagrams, called Daniel plots. The oil will determine the application limits of a compressor together with the selected refrigerant.

The usually small amounts of oil transported out of the compressor with the refrigerant will have to be led back to the compressor to serve as lubricant. So, a certain amount of oil is circulating. Inside the oil reservoir of the compressor or oil separator, still enough oil has to be available for the lubrication. Non soluble or almost non soluble oils have to be separated from the refrigerant gas flow very effectively.

Soluble or miscible oils will be thinned at low temperatures by solved refrigerant and can thus be transported back to the compressor easier. Inside the compressor, the refrigerant amount in the oil has to be limited, to avoid bad lubrication.

Metals

Because there are many different metals in a refrigerant circuit, oil-refrigerant mixtures are checked not to react with these metals in an accelerated test. In the test, both substances should keep their properties stable. The testing is done in a "sealed tube test", in glass tubes closed by welding, at elevated temperature, e.g. at 175°C for 2 weeks. A change of the colour of the oil or the metal strips indicates a reaction and is thus considered a bad sign. Viskosity, acidity and water content are tested and possible degradation products of the refrigerant searched for.