Part 1 – Refrigerants

Thermodynamic efficiency of a refrigeration system depends mainly on its operating temperatures. Other important factors include: the system design, size, initial and operating costs, safety, reliability, and serviceability.

Primary and Secondary Refrigerants:

Refrigerants can be classified into primary and secondary refrigerants.

NOTE: If the operating temperatures are above 0^oC, then pure water can also be used as secondary refrigerant.

Ideal Refrigerant

So what properties make a refrigerant ideal? A refrigerant should possess the following 11 properties:

Let’s take a closer look at some of the good thermodynamic properties that make an ideal refrigerant.

Now that we’ve looked at good thermodynamic properties, let’s take a look at Environmental and safety properties for an ideal refrigerant.

Refrigeration Systems and Equipment:

Total Equivalent Warming Index (TEWI): The factor TEWI considers both direct (due to release into atmosphere) and indirect (through energy consumption) contributions of refrigerants to global warming

 Toxicity: Refrigerants used in a refrigeration system should be non-toxic.

Flammability: Should preferably be non-flammable and non-explosive.

Chemical stability: Should be chemically stable as long as they are inside the refrigeration system.. Compatibility with common materials of construction

Miscibility with lubricating oils: Oil separators have to be used if the refrigerant is not miscible with lubricating oil (e.g. ammonia)

Ease of leak detection: It should be easy to detect the leaks.

Now, let’s take a look at Types of Refrigerants

Types of Refrigerants

Some of the common refrigerants, in use today, are:

Refrigerant R-12( Dichorofluoromethane – CCl₂F₂)

Refrigerant R-22 (Chlorodifluoromethane – CHClF₂)

Refrigerant R-134a (Tetrafluoroethane – C₂H₂F₄)

Ammonia

Carbon dioxide

Sulphur dioxide

Ethyl chloride

Chlorofluorocarbons

Designation of Refrigerants

1) Fully saturated, halogenated compounds: derivatives of alkanes (CnH₂n+2) such as methane (CH₄), ethane (C₂H₆)

• • R-11 – CCl₃F
  • R-12 – CCl₃F₂

2)Inorganic refrigerants: Designated by number 7 followed by the molecular weight of the refrigerant (rounded-off).

• • Ammonia:
    • Molecular weight is 17, ∴ the designation is R 717
  • Carbon dioxide:
    • Molecular weight is 44, ∴ the designation is R 744
  • Water:
    • Molecular weight is 18, ∴ the designation is R 718

3) Mixtures: Mixtures of different refrigerants

• • • Azeotropic mixtures are designated by 500 series
    • Zeotropic refrigerants are designated by 400 series.
    • Azeotropic mixtures:
      • R 500: Mixture of R 12 (73.8 %) and R 152a (26.2%)
      • R 502: Mixture of R 22 (48.8 %) and R 115 (51.2%)
      • R503: Mixture of R 23 (40.1 %) and R 13 (59.9%)
      • R507A: Mixture of R 125 (50%) and R 143a (50%)
    • Zeotropic mixtures:
      • R404A : Mixture of R 125 (44%), R 143a (52%) and R 134a (4%)
      • R407A : Mixture of R 32 (20%), R 125 (40%) and R 134a (40%)
      • R407B : Mixture of R 32 (10%), R 125 (70%) and R 134a (20%)
      • R410A : Mixture of R 32 (50%) and R 125 (50%)

4) Hydrocarbons:

• • Propane (C₃H₈) : R 290
  • n-butane (C₄H₁₀) : R 600
  • iso-butane (C₄H₁₀) : R 600a
  • Unsaturated Hydrocarbons: R1150 (C₂H₄)
  • R1270 (C₃H₆)

Ozone Depletion Potential (ODP):

Restricted refrigerants commonly used for refrigeration, cold storage and air conditioning applications:

• R-11 (CFC 11), R-12 (CFC 12), R-22 (HCFC 22),
• R-502 (CFC 12 + HCFC 22)

Synthetic replacements for the older refrigerants are:

• • R-134a (HFC-134a)
  • Blends of HFCs

Synthetic refrigerants are non-toxic and non-flammable.

• • Also have higher Global Warming Potential (GWP)

Natural Refrigerants:

Most commonly used natural refrigerant is ammonia. Ammonia is one of the oldest known refrigerants for it’s good thermodynamic, thermophysical and environmental properties. However, Ammonia is toxic and is not compatible with some of the common materials of construction such as copper.

Alternate Refrigerants

Classified into two broad groups:

1. 1. 1. Non-ODS, synthetic refrigerants based on Hydro-Fluoro-Carbons (HFCs) and their blends
      2. Natural refrigerants including ammonia, carbon dioxide, hydrocarbons and their blends

Non-CO2 Refrigerants

Natural: Refrigerants such as the greenpeace-developed ‘Greenfreeze’. There are based on purified butane/propane mixtures. But are they entirely ‘natural‘? Due to increased efficiency over refrigerants such as R134a, allow the use of very small amounts of refrigerant to be used.

Reduced environmental damage: Hydrocarbon refrigerants are backward compatible with car air conditioning systems thus increasing their efficiency, and preventing further release of harmful R-134a and R12 to the atmosphere. Pure hydrocarbon refrigerants also have a short lifetime in the atmosphere, weeks to months. CO₂, being more chemically stable, persists longer in the atmosphere.

Refrigerant Trends:

Increasing carbon- Increases the molecular weight and the boiling point.

Increasing nitrogen- Makes the compound more reactive. This can lead to toxicity and instability issues.

Increasing oxygen- Reduces atmospheric stability. Good for GWP and ODP but may lead to toxicity, flammability and reactivity issues.

Increasing sulphur- Increases toxicity and decreases stability.

Increasing hydrogen- Reduces atmospheric lifetime, which is good for GWP and ODP but increases flammability.

Increasing fluorine- Attached to carbon increases GWP.

Increasing chlorine- Improves lubricant miscibility but also ODP and toxicity.

Increasing bromine- Increases ODP but lowers flammability.