3.6 Refrigerant Blends

Refrigerant Blends

Refrigerant blends have evolved as research has progressed to find a suitable replacement for CFC-12.

A refrigerant blend is a mixture of refrigerants that has been developed to imitate the pressure/temperature relationship similar to that of the original refrigerant.

There are three different types of refrigerant blends:

  • Azeotropic
  • Zeotropic
  • Near-azeotropic

The common factor in blended refrigerants is temperature glide.

Temperature glide is defined as the temperature difference (measured in °F) between the starting and ending temperatures of a refrigerant phase change within a system, exclusive of any sub-cooling or superheating.

A simple explanation of temperature glide is it provides a good indication of how far away the refrigerant is from an azeotropic.

The greater the temperature glide, the greater the possibility the refrigerant will fractionate.

Fractionation

When refrigerants fractionate, they separate and change into different compositions.

Blends are made from several different refrigerants with different volatilities and when a leak occurs, a greater amount of the more volatile refrigerant leaks out, changing the blend and changing its performance.

Many refrigeration specialists would not recommend the use of blends because numerous service problems occur with their usage.

Refrigerants with high temperature glides must have their entire refrigerant charge removed and replaced as a liquid.

Usage of Blends

Blends can either be HCFC or HFC-based, and in some cases a hydrocarbon may be used as one of the components within the blend.

When using a blend, it is important that the ASHRAE refrigerant number is identified.

Any refrigerant with a 400 designation will almost always be a blend, such as R-401a, and are typically marketed as substitutes for CFC-12.

Some manufacturers will incorporate trademarks into their blends, giving the user the impression they are direct “drop-in replacements” for CFC-12.

In reality, no such product exists as a direct “drop-in replacements” for CFC-12 and refrigerant systems will always need retrofitting.

Most blends and their lubricants are not compatible with existing CFC-12 systems.

Recharging a blend is also quite different than recharging a single compound refrigerant, such as R-12.

Azeotropic Refrigerant

Azeotropic refrigerants are blended refrigerants with a temperature glide of zero.

They chemically combine together in such a way that they behave the same as a single compound refrigerant.

In the event that a refrigerant leaks, azeotropic refrigerants will leak evenly.

Examples of azeotropic refrigerants are R-500 and R-502.

Zeotropic Refrigerant

Zeotropic refrigerants behave as an azeotrope during “normal” operating conditions.

They have a temperature glide greater than 10.

Zeotropic refrigerants have a tendency to fractionate resulting in different leakage rates of the blended refrigerant.

These refrigerants must be removed and replaced in their liquid state.

Zeotropic refrigerants are in the 400 series classification (e.g. R-401a).

Near-Azeotropic Refrigerant

Near-azeotropic refrigerants also behave as an azeotropic refrigerant during normal operating conditions, with a temperature glide between 0 and 10.

However, they still retain some of their separate characteristics.

Near-azeotropic refrigerants have a greater tendency to fractionate or experience a change in composition than azeotrope refrigerants.

This results in different leakage rates of near-azeotropic refrigerants, and most near-azeotropic will contain an HCFC in their blend.

Mixed Refrigerants

Mixed refrigerants contain a non-prescribed blend of refrigerants.

These usually occur unintentionally when two or more refrigerants are combined or a refrigerant is contaminated.

Distribution of Refrigerants

With the exception of ammonia, all refrigerants used today are halocarbons or blends containing halocarbons.