Growth in the construction industry is significantly contributing to an increased demand for air-conditioning. This process however consumes energy and makes use of coolants which are responsible, in part, for ozone depletion and increased greenhouse gas emissions which cause global warming. New refrigerants that offer lower global warming potential are a welcome proposition in this regard. In fact, hydrochlorofluorocarbons (HCFC’s) are now being phased out in many countries in accordance with the Montreal Protocol. How do R410A, R407C, R417A and other new alternatives perform in comparison to R22?
Out With R22
The story that is the phasing out of R22 begins with the discovery of chlorofluorocarbons (CFCs) in 1930 by Thomas Midgley Jr. The use of these compounds as coolants for air conditioners and refrigerators as well as a wide variety of other manufacturing processes rose sharply in the 1960s. In 1974, Sherwood Rowland and Mario Molina discovered that CFCs were depleting the ozone and they advocated for a complete ban of further releases of CFCs into the atmosphere. Scepticism from scientists and commercial manufacturers however persisted and a consensus on the need for action only became imminent in 1976.
Eventually in 1987, the Montreal Protocol, a landmark international environmental agreement, established requirements that began the worldwide phase out of the ozone depleting CFCs. Meanwhile, HCFCs and their cousins hydrofluorocarbons (HFCs) had been created in the 1980s as substitutes for CFCs. HCFCs made up of hydrogen, chlorine, fluorine and carbon atoms were established as less damaging to the ozone layer than CFCs.
They however still contained ozone-destroying chlorine and in 1992 the Montreal Protocol was amended to establish a schedule for their phase out. More modifications to the protocol led to the phase out of CFC production in all developed nations in 1996.
HCFC-22 or R-22, a single hydrochlorofluorocarbon (HCFC) compound, has been the choice refrigerant for residential heat pump and air conditioning systems for over four decades. R-22 is not only a green house gas but its manufacture results in a by-product (HFC-23) that contributes significantly to global warming. As of January 1, 2010 R-22 could only be produced or imported to service existing equipment in the US and could not be used in new equipment. Also heating, ventilation and air-conditioning (HVAC) system manufacturers could not produce new air-conditioners and heat pumps containing R-22.
The Montreal Protocol has established the year 2020 as the final phase out period for HCFCs in developed countries and the year 2040 for developing countries such as Kenya. In the US, R-22 that has beenrecovered and recycled or reclaimedwill be allowed beyond 2020 to service existing systems, but chemical manufacturers will no longer be able to produce R-22 to service existing air conditioners and heat pumps.
The ‘400 series’ HFCs
R-22 is being replaced by ‘400 series’ HFCs. Its main substitute is HFC R-410A which has an ozone depleting potential (ODP) of zero. This blend of two hydroflurocarbons, HFC-32 and HCF-125 (50/50 wt%), has a higher direct greenhouse warming potential (GWP) than R-22 but is less damaging, indirectly. GENETRON AZ-20®, SUVA 410A®, Forane® 410A, and Puron® comprise some of the varied trade names for R-410A. Worth noting is that R-410A has a significantly higher vapour pressure than R-22. The condensing pressure of R-410A at standard conditions for water-cooled chillers is about 340 psi and approximately 390 psi for air-cooled chillers.
This is slightly higher than high-pressure safety shut-off settings for R-22; resultantly it cannot be used to retrofit existing R-22 equipment. R-410A can only be used in equipment designed for R-410A – special service equipment such as a high pressure manifold gauge set, a high-pressure recovery unit, and high pressure recovery tanks are required for R-410A systems.
Compressors that come with R-410A systems are smaller, quieter and operate with less damaging vibration than older compressors which operate on R-22. These compressors also run cooler reducing the risk of burnout due to overheating. R-410A uses high quality polyolester as a lubricant. This lubricant is more soluble with HFCs so it can mix and circulate more efficiently to keep the compressor and other moving parts lubricated, reducing wear and extending their life. R-410A also allows higher heat transfer than R-22, which results in a more efficient operation.
Another substitute for R-22 is R-407C. R-407C is a ternary blend of HFC-32, HFC-125 and HFC-134a and contains no chlorine content and therefore has an ODP of zero. Its GWP stands at 1600 against R-22’s 1700. R-407C exhibits thermo-physical properties that are similar to R-22 making for minimal alterations in the refrigeration system when changing from one to the other.
In effect and unlike R-410A, an R-22 system and an R-407C system use many of the exact same components. Disadvantageously, each component part of this blend boils at a different temperature; the pure refrigerant fluids HFC-32 and HFC-125 boiling points are -52 degrees centigrade and- 51 degrees centigrade respectively, and are more volatile than HFC-134a which has a boiling point of -27 degrees centigrade.
Consequently HFC-32 and HFC-125 exert a higher vapour pressure than HFC-134a and should there be any leaks in the system, the partial loss of one component would result in the total remaining blend being of the incorrect composition. These small changes in the refrigerant quality or composition may not noticeably affect the equipment performance in the short term. However, the leakage effect is irreversible and at some point, the refrigerant has to be reclaimed and returned to the manufacturers for reblending or disposal, and the system would require evacuating and complete recharging.
R-407C also makes use of polyolester oil as a lubricant.
R-417A is made up of R-125, R-134a and R600 and has an ODP of zero. It can be used with traditional mineral oils and alkyl benzene lubricants making it ideal for use in existing equipment and at the same time suitable for use in new equipment without the need to change to more expensive and hygroscopic polyolester lubricants.
R-417A has been found to be particularly useful when converting systems with hermetic compressors and is used as a drop in replacement for R22 systems such that no engineering changes are made to the system and the original oil is retained. R-417A will show a reduced capacity that might mean longer run times, but a properly sized R-22 system will be able to keep the space cool when running R-417A.
Ammonia – The Natural Refrigerant
Ammonia or R-717 was one of the most widely used refrigerants in the early 20th century and is a naturally occurring compound that has an ODP of zero and a GWP of zero. It therefore has no cumulative effects on the environment and a very limited atmospheric lifetime. Additionally, R-717 is considerably less expensive than CFCs or HCFCs.
It is nevertheless a far more hazardous substance, being flammable in air with a volume between 15 to 28% and is not compatible with copper. It is also poisonous in high concentrations but this is mitigated by its distinctive smell which is detectable at concentrations well below those considered to be dangerous (it is self alarming) and is lighter than air, so if it does leak, it will rise and dissipate in the atmosphere.
Carbon Dioxide – The New yet Old Refrigerant
As with ammonia, the use of carbon dioxide or R-744 dates back to the early days of refrigeration. It was however displaced by CFC refrigerants which worked well in low pressure systems but has now returned, hotly contesting for a place as a new alternative.
Despite its status a global warming gas, the small quantities of carbon dioxide that may be released from air conditioners are insignificant where GWP values for HFC refrigerants are several thousand times higher than for carbon dioxide.
Carbon dioxide has an ODP of zero and is best applied to systems that must be small and lightweight, take for instance automotive or portable air conditioners; its high operating pressure enables it to flow through small-diameter tubing. It does not offer any advantages for large air conditioners, which do not have space restrictions and can use wide-diameter tubes.
Other beneficial properties of R-744 include non flammability and excellent thermodynamic properties in lower ambient temperatures. One drawback of R-744 is that the thermodynamic benefits can disappear at ambient temperatures above 30 degrees centigrade.
As they occur in nature, hydrocarbons are also natural refrigerants. R 600a (isobutane) and R 290 (propane) which can be applied to air-conditioning have a GWP100 (Global warming potential over 100 years) of less than five. These hydrocarbons are almost odourless owing to special purification. They can however be highly flammable.
One practical and outstanding example of the use of hydrocarbons is the installation, in 2007, of a 600kW air-cooled water chiller using propane or HC refrigerant R290 as supplied by Earthcare at the historic Church House building (in the UK) for comfort air conditioning. This chiller achieves a minimized environmental impact through the combination of natural refrigerants and optimal energy efficiency.
The chiller was specified by Max Fordham consulting engineers and the installation, carried out by AMEC, involved closing two roads close to Westminster Abbey on a Saturday morning, diverting two bus routes and organising 12 vehicles to get the crane into position for siting the chiller. However, it is estimated that the long-term pay back, both financially to Church House and in reduced environmental impact, will more than offset all the capital, installation and disruption costs.
On the whole, indirect global warming impact is reduced by maximising energy efficiency through a combination of factors including the favourable thermodynamic characteristics of HCs, the use of subcooling circuits, which improve coefficient of performance (COP) and floating head pressure control that allows the condensing temperature to float as low as 20°C if ambient conditions allow, instead of the normal 40°C.
This is particularly beneficial for chillers that operate year round or at night when ambient temperatures are lower. The combined effect of all energy efficiency measures results in a potential energy savings in excess of 50% for chillers that operate year round when compared to minimum first cost chillers without energy saving features.
Another notable project using HCs is that carried out by the German Society for international Cooperation (GIZ, formerly GTZ) in cooperation with the Chinese manufacturer Gree Electric Appliances Inc. Earlier this July the company announced the official opening of its production line for room air conditioners running with hydrocarbons (propane, R290) as refrigerant marking the first production of environmental friendly room air conditioners running with hydrocarbons worldwide.
The project which began in 2008 paid special attention to safety aspects taking into account the fact that hydrocarbons are flammable refrigerants.
China is the most important production place for air conditioners worldwide (75% of world market production). Gree alone is producing 40 million air conditioners per year. The new production line will produce approximately 100,000 units per year. The project is a jump start to revolutionize the Chinese (and worldwide) market for air conditioners and to pave the way for a sustainable future.
Hydrofluoro alkenes (HFO) or “fourth generation refrigerants” have a low global warming potential and can be categorized as belonging to hydrofluorocarbons (HFCs). A combustible substance in this group, HFC-1234yf, is currently being investigated primarily for use in car air-conditioning systems and has yet to be launched on the market.
Despite their low global warming potential, hydrofluoro alkenes have greater instability and reactivity, characteristics which must be considered by operators of air-conditioning systems.
Release of hydrofluoro alkenes as a result of leakages leads to additional input of persistent degradation products into the environment. Hydrofluoro alkenes are complicated to produce, and production outside Europe involves shipment over long distances. To date there has been no large-scale production of HFC-1234yf. More, solutions using natural refrigerants already exist for the proposed fields of application of hydrofluoro alkenes.
List of Contributors:
National Refrigerants, Inc.
Technical Sales Manager
Sustainable Production through Innovation in SMEs (SPIN)
Daniel de Graaf
SPIN Project Manager
Email: [email protected]