**9.4 Azeotropes**

Azeotropes are also known as constant boiling point mixtures. This class of refrigerant mixture **behaves as if it were a single component during its phase change** (In the phase change the proportion of each constituent in the new phase is the same as in the original phase). Azeotropic mixtures usually behave like a fluid. This property is of great use when the refrigerant mixture leaks and it has to be recharged. However, the recharging of Non Azeotropes is much more complex [15]. The advantage of using Azeotropes as refrigerants is that, during leak in one of the

components, it does not change the composition of the remaining refrigerant. Since the composition of an azetropic mixture is produced is a role of temperature, no true azeotropic mixtures be present in refrigeration. But the composition change of the azeotropic mixtures is very small. Azeotropic mixtures have been very commonly used in refrigeration, such as cold storage. Refrigerant R 502 is a mixture of R 22 and R 152 (a). R 410 (a), is a mixture of R 32 and R 125 is an important non-ODS azeotropic mixture used in place of R 22.

#### **9.5 Near azeotropes**

Azeotropes rigorously exist at only one composition for given temperature and pressure. However for all practical purposes, minor deviations are acceptable for many refrigerant systems. For this broader range the term 'Near Azeotropic Mixture' (**NEARM)** is used. The advantage of utilizing this category is that many more possible alternatives become available.

#### **9.6 Non-azeotropes**

Non – Azeotropes are also called as zeotropes. A zeotropic blend is also a combination of two or more components. The components of this refrigerant have diverse boiling points. These components will evaporate and condense at unlike temperatures. In order to fully understand zeotropic blends we must also understand fractionation and glide [16].

Non-Azeotropes, which change their composition continuously during phase, change. These mixtures do not possess a sharp boiling point but boil over a range of temperature. This feature is intimately tied to the improvement of system's efficiency if appropriate hardware changes are made to the system heat exchangers. The change of temperature with phase change is called Temperature Glide.

When heat transfer fluids exchange heat with Non Azeotropic Refrigerant Mixtures (NARMs) in a constant current flow mode, the thermodynamic irreversibility can be reduced by matching the temperature glide (NARM side) against the temperature drop (Heat Transfer Fluid side) resulting in an increase in the coefficient of performance.

As already stated, because of limitation of single fluids as alternative to CFC-12 and also the flexibility in modulating the capacity by varying the composition of the constituents, mixtures are emerging as a viable solution to CFC-12 alternatives.
