As the phase-out of ozone depleting substances is coming in full-swing, harvesters, food processors, distributors, retailers and pharmaceutical companies who rely on refrigerant gas to maintain the quality of their products will have two options when replacing their HCFC gases:
1. Going the natural way
All industries and firms in South Africa will have to comply with the Base Line phase-out programme based on the treaty requirements of the Montreal Protocol set by the Department of Environmental Affairs as soon as it is rectified by Parliament. Companies will not be able to import more than what the Base Line stipulates and change of refrigerants will be unavoidable.
Those companies who have the financial capacity can convert to natural gas of which ammonia and CO2 is the most popular options, and most large refrigerant supplying companies already supply them.
The advantages of natural gas, as the name may suggest is that it is no ozone depleting aspects. This guarantees companies and retailers that their refrigerant systems will have a very long time span, and will probably never have to change to another gas again. With South Africa facing an energy shortage and a rise in electricity prices, the IIR (International Institute of Refrigeration) recommend natural refrigerants as an important energy saving medium.
The first and foremost disadvantage of natural gas is that the system is very expensive to install as the existing HCFC system will have to undergo an entire retrofit of compressors, valves etc. Another reason for being expensive is that a CO2 system runs at extremely high pressure, and ammonia is a toxic gas, which give rise to a new level of regulations pertaining to pressure and plant safety. Companies must also keep in mind the green house effect of carbon dioxide and ammonia and the looming international pressure to reduce global warming. A well managed and maintained natural refrigerant system will be minimally influenced by any forthcoming legislation, but will evidently require more inspections for leaks and ultimate add to expenses.
2. Choosing an alternative
Choosing an alternative refrigerant to HCFCs instead of going the natural route may prove to be more cost effective depending on the system and application.
The primary advantage to many alternative refrigerants is that existing systems can simply be recharged with the alternative refrigerant and at the same time comply with law as all new alternatives are zero ozone depleting, with no need for plant retrofit or a compressor oil change.
The disadvantage of alternatives which companies will have to be contempt with will be that alternatives won’t come cheap. Instead of just topping-up the system, as was the case with inexpensive R22, companies will evidently be forced to revise their refrigerant plants to avert any leakages that will cost them excessive amounts. Not just that, if a leak occurs, the system has to be drained and completely refilled. This, however, serve a dual purpose of preventing the unnecessary escape of gas into the atmosphere.
New systems needed
There are five criteria to be mindful of when selecting a refrigerant for a particular application:
When one talks performance, one should think of capacity. The refrigeration capacity is the amount of cooling that the system can produce for a given volumetric flow rate of the refrigerant.
Siva Gopalnarayanan believes that “mixtures, offer the most attractive replacement solution as when mixing two or more refrigerants, a new working fluid with the desired characteristics can be created”. An azeotrope is a mixture made up of two or more refrigerants with similar boiling points which behaves almost like a single fluid. Under normal operating conditions, the components of this refrigerant blend will not separate (no fractionation) and therefore boil at a single temperature (like a pure fluid).
Azeotropes can be charged as either vapour or liquid into a refrigeration system. A near-azeotrope is a mixture of two refrigerants with different boiling points but which behaves like a single entity when in a total liquid or vapour state. During phase changes from vapour to liquid (evaporation) or liquid to vapour (condensation), each of the refrigerants making up the blend goes through phase changes at different temperatures. The temperature range between phase change temperatures of the components is referred to as the “temperature glide” of the refrigerant blend.
Describing temperature glide in other words, at a constant pressure, the temperatures in the evaporator (bubble point) and the condenser (dew point) are not constant during the refrigerants’ phase changes. Additionally, temperature glides can lead to fractionation of either the lower boiling or the higher boiling refrigerant, causing the refrigerant blend to separate. ASHRAE defines fractionation as “the separation of a liquid mixture into parts by the preferential evaporation of the more volatile component”. The temperature glide of near-azeotropes is minimal being less than 12.22°C. Near-azeotropes should be dharged in liquid form only. A zeotrope is also a mixture comprising two refrigerants with different boiling points. However, its temperature glide is greater than 12.22°C. Like the near-azeotrope, it should also be charged in the liquid state. Should these refrigerants be charged in the vapour state, the mixtures’ composition may change which results in decreased performance and increased safety risks.
A refrigerant such as R32 can be used in conjunction with another refrigerant and exhibit zeotropic characteristics but, when blended with a different refrigerant in different proportions, it can exhibit near-azeotropic or even perfect azeotropic characteristics. It depends on the blend. Most zeotropes are not generally good drop-in replacement refrigerants due to their non-linear temperature glides, but they do improve thermodynamic performance in modified systems.
There is however a gas, which Afrox believes will be the zeotrope that will be the gas that fits all requirements, be used in most applications and will require no system modifications. R427a, developed by one of the leading refrigerant manufacturers has a molecular composition that that enables it to perform to a wide spectrum of thermodynamics, that meets the criteria of safety, energy saving and environmental acceptability as well as no modifications to existing systems. Afrox is currently the sole supplier of R427a.
ASHRAE classifies refrigerants according to toxicity and flammability:
Class A: No toxicity at concentrations below 400ppm (e.g.: R410A)
Class B: Some evidence of toxicity below 400ppm concentrations
Class 1: Non-flammable (e.g.: R404A, R406A, R407C, R408A, R409A, R410A, R507)
Class 2: Moderately flammable
Class 3: Highly flammable
Reliable refrigerants are those that do not fractionate, decompose, dissolve or swell under specific pressures or temperatures. Additionally, alternative refrigerants need to be compatible with compressor oils. Gopalnarayanan believes refrigerants need to be “reasonably miscible at all temperatures” as this allows for better return of compressor oils. After considering all of the above factors it may be worthwhile to consider a system redesign.
Knowing Refrigerant Designations
“A multitude of alternative blends are zeotropes”, Gopalnarayanan says.
ASHRAE identifies different refrigerants based on their chemical composition and stability. The following designations exist:
R10 – R50: Methane series refrigerants
R110 – R170: Ethane series refrigerants
R216ca – R290: Propane series refrigerants
RC316 – RC318: Cyclic organic compound refrigerants
R400 – R411B: Zeotropic blend refrigerants
R500 – R509: Azeotropic blend refrigerants
R630 – R631: Miscellaneous organic compound refrigerants
R702 – R764: Inorganic compounds
R1112a – R1270: Unsaturated organic compounds
One needs to start considering the alternatives along with the associated performance, safety, economic and environmental implications. Perhaps the most evident solution comes through the use of HFC zeotropic blends in order to play this game proactively, safely, and legally.