Utilities

Refrigerants



Absolute temperature - Entropy


Refrigerants

Desirable properties of a refrigerant

  1. Low boiling point (otherwise operation at high vacua becomes a necessity)
  2. Low condensing pressure (to avoid heavy machine plant scantling and reduce the leakage risk)
  3. High specific enthalpy of vaporisation ( to reduce the quatity of refrigerants in circulation and lower machine speeds, sizes etc.)
  4. Low specific volume in vapour state (reduces size and increases efficiency)
  5. High critical temperature (temperature above which vapour cannot be condensed by isothermal compression)
  6. Non corrosive and non solvent (pure and mixed)
  7. Stable under working conditions
  8. Non flammable and non explosive
  9. No action with oil ( the fact that most refrigerants are miscible may be advantageous e.e. the removal of oil films, lowering pour points etc, provided separators are fitted
  10. Easy leak detect
  11. Non toxic
  12. cheap, easily stored and obtained

The production of R12 and R11 has now stopped under the Montreal Protocol and EU regulation on ozone depleting gasses. A short term solution has been conversion to HCFC's such as R22 (HCFC's have an Ozone Depletion Rate {ODP} 2-15% of CFC's) but this refrigerant also has a harmful effect on the environment, although far less damaging than R12. HCFC's are also targeted for eventual production phase out as controlled substances, with usage totally banned by the EU in new equipment rated at 150kW and over from the 1st Jan 2000. In some countries such as Germany and Sweden tighter restrictions are in force.


New refrigerants such as R134a and R404A, which are HFC's may offer a longer term solution against harmful emissions. They contain no chlorine atoms and thus do not attack the ozone layer but they are greenhouse gasses and may be subject to future legislation.


An uncertainty over the long term future of HFC's has led to growing interest in old natural refrigerants such as ammonia and carbon dioxide or hydrocarbons such as propane and iso-butane. Using the refrigerants, however, dictates more stringent safety measures which are being drafted by the appropriate classification societies.

The naming of freon (or Arcton) refrigerants comes from the constituents.


For R12, formed by reacting methane with flourine and chlorine. This forms a molecule containing 1 carbon atom, 2 flourine atoms and 2 chlorine atoms


Number of Carbon atoms minus 1


number of hydrogen atoms plus one


number of flourine atoms

0 (1-1)


1 (0+1)

2

 For R22, again formed by reacting methane with flourine and chlorine. This forms a molecule containing 1 carbon atom, 1 hydrogen atom and 2 chlorine atom


0

2

2

the remaining bonds are taken by chlorine atoms


Refrigerant 12

R12 is halogenated hydrocarbon derived from methane (CH4) with the hydrogen being displaced by chlorine and fluorine. The resulting compound is Dichlorodifluoromethane (CCl2F2) also known as Freon 12

R12 is considered non-toxic except in high concentration producing oxygen deficiency. However on contact with flame it breaks down to form chlorine gas and phosgene (COCl2).

R12 escaping under pressure can cause skin damage on contact. It is non-irritant and not considered flammable.

Working pressures and temperatures are moderate and the high critical temperature (112oC) is well above the working range.

Methyl Chloride

CH3CL4, banned by the UK administration.

Refrigerant 11

suitable for air conditioning installations (monoflourotrichloromethane CCl3F), production has now ceased.

Refrigerant 13

CClF3

Refrigerant 22

Is suitable for a lower temperature range than R12 because the pressure on the evaporator side of the system is higher than atmospheric at low temperatures thus reducing the risk of drawing air into the system. Its performance is better than R12 approaching that of ammonia.

The chemical and other properties are similar to R12 except that it is not miscible with oil over the whole temperature range. The compound is Chlorodifloromethane (CHClF2)

Refrigernat 113

CCl2F/CClF2

Refrigerant 134a (Tetraflouroethane)

A HFC and suggested replacement for R12 in existing plant.

Refrigerant 502

Composed of 48.8% R22 and 51.2% R115 (C2ClF5). It is particularly suited for use with hermetic compressors.

Refrigerant 717 (Ammonia)

Thermodynamically a good refrigerant but it is explosive, poisonous and an irritant. The explosive mix is 16 to 25% in air. It is corrosive to copper and its alloys so that ferrous materials are used in components in the system.

Ammonia is a reactive compound. It is highly soluble in water with which it forms ammonium hydroxide a strong alkali. About 1300 volumes of ammonia can be dissolved in one volume of water at low temperatures, however it can be easily expelled by boiling. This action makes the vapour absorption refrigerator possible. The high solubility in water also means that a wet cloth held to the face will give some protection against an ammonia leak in an emergency although a breathing apparatus would be worn in such a case. Because of the hazards it is normally found in shore or fishing vessels.

An additional disadvantage of ammonia is that it can not be used in a direct expansion system. The required three or four tonnes of ammonia onboard for direct system is not acceptable, dictating a combined brine distribution system. Even then, special precautions have to be taken with ammonia to eliminate toxic risks to personnel in yards during construction and crew members when in service. Safety and isolation costs associated with ammonia can erode otherwise attractive cosy advantages

Carbonic anhydride

(CO2, Carbon Dioxide)





Comparison of some refrigerants


Property

CCl2F2-R12

CO2

NH3

Discharge pressure

0.8

72

11.7

 Suction pressure

7.4

23

2.4

Critical pressure

40

73.8

113.7

 Critical temperature

112

31

133

Boiling temperature at atmos

-30

-78

-33

Liquid specific heat capacity Kj/Kg K

0.96

3.23

4.65

Corrosive (pure)

no, attacks rubber, Copper if moisture present

no

no will attack brass and bronzes if moisture present

Toxic

no, liberates phosgene gas in fire

no

Yes

Flammable

no

no

Yes

Explosive

no

no

Yes

Miscible with oil

Yes

no

Slightly

Refrigerant 404a (R125 (Pentaflouroethane), R134a, R143A(triflouroethane))

a suggested replacement for R22, has been used in shore based air-con and refrigeration installations for the past few years and could replace R22 in the reefer market without technical problems.

Refrigerant R507 (R125, R143A)

) It is very comparable in blend to R502 which has been in use for several decades.

Refrigerant 407c (R32, R123, R134a)

another blend but has a very high glide of 6-7oC ( the difference between the boiling points of the blended components). Its efficiency has some attraction for the reefer market, particularly with regard to its performance in cooling down bananas when chillers are expected to operate at their maximum capacity. The problem today is that R407c has only been used in tests and offers little, if any, industrial experience.

One component of the blend is R32,it is expensive and will continue to be for some time and also introduces problems with regard to lubrication. Maintenance could present another difficulty, calling for skilled personnel in handling the blends.

A major concern is the composition change in the event of leakage.

Refrigerant 407d

A chlorine free replacement for R12 in reefer container systems, is claimed to be 20% more efficient than R134a and able to operate at box temperatures as low as -25oC

Refrigerant 410a

Does not have a high degree of glide, the blend facilitating a reduction in swept volume of compressor units. The main disadvantage is that it requires a 50% higher working pressure than R22, although compressor and plant size is reduced.. The availability of compressors for this refrigerant is thus limited, increased safety precautions are necessary due to this increased pressure.

The blend contains R32 giving the same cost drawback as for R407c.

Refrigerant 410b (R32, R125)

Does not have a high degree of glide, the blend facilitating a reduction in swept volume of compressor units. The main disadvantage is that it requires a 50% higher working pressure than R22, although compressor and plant size is reduced.. The availability of compressors for this refrigerant is thus limited, increased safety precautions are necessary due to this increased pressure.

The blend contains R32 giving the same cost drawback as for R407c.


Refrigerant

Industrial experience

Service and maintenance requirements

Availability

cost/Kg R22 = 1

R407C

On-going

Special

Starting

5.4

R507

Yes

Easy

Good

5.3

R410a

No

Special

Very few

7.4

R134a

Yes

Easy

World wide

1.6

SUVA 9100

Is a near azeotropic mix of HFC 32 and HFC 125. DuPont has recently changed its mix from 45/55 R32/R125 to 50/50 simplifying the HCFC replacement process.


Azeotropic mix-is a blend of one or more pure substances in precise proportions.


The blend specifically exhibits a boiling point temperature independent of the boiling points of its components. Constant pressure evaporation and condensation phase is realised at a constant temperature.