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February 27th , 2017

Alessandro Di Maio – Industrial Product Technical Advisor EAME ExxonMobil explains the impacts of natural refrigerants on lubricantsThe refrigeration industry is undergoing a transformation. Global environmental legislation, primarily intended to help protect the ozone layer, means businesses are increasingly adopting natural refrigerants – such as ammonia, carbon dioxide (CO2) and hydrocarbons.

Europe is projected to be the fastest growing region in the market and has taken a clear direction towards phasing down hydrofluorocarbon (HFCs) as a refrigerant, in particular, due to its widely known ozone depletion properties.

However, switching to natural alternatives presents a number of new challenges to operators and OEMs – not least when it comes to lubrication.

As refrigeration is among the most complex applications from a lubrication standpoint, I thought I’d share a summary of the challenges and benefits presented by natural refrigerants.

The fundamentals There are three main natural refrigerants:

• CO2 – projected to see the highest growth in the natural refrigerants market through to 2020; typically used in superstores and food retail chains for refrigeration and air conditioning applications.

• Ammonia – widely used in industrial refrigeration.

• Hydrocarbon – used in domestic appliances.

Let’s consider the pros and cons of the two industrial options.

Carbon dioxide – the best refrigeration option?

The principal advantage of CO2 is evident in its properties as a refrigerant. For example:

• CO2 has excellent heat transfer properties

• Low viscosity

• Insensitive to pressure losses

In addition, carbon dioxide has an ozone depletion potential (ODP) of 0 (zero) compared to up to 1.0 for HFCs, and it also has a global warming potential of 1 compared to up to 4,800 for HFCs.[i]

And there’s more good news with CO2:

• Size: CO2 systems are smaller than HFC-based ones and can be designed in various configurations, including ‘direct expansion’ and ‘secondary brine’.

• Cost: On average a system can pay for itself in three years.

• Safety: CO2 systems have low toxicity and are non-flammable.

However, CO2 is not suitable for retrofitting into HFC systems due to its higher operating and standstill pressures.

The lubrication of CO2 systems

The main lubrication challenges with CO2 are the high operating pressures and solubility. Both occur in subcritical cascade systems and trans-critical high-pressure applications.

High operating pressures (a standstill pressure of 50 to 130 bar) and temperatures place higher loads and stresses on bearings and other contacting parts in motion compared with HFCs. Also, because CO2 is more solvent than HFCs, lubricants for traditional applications cannot be used.

Fortunately, there are synthetic lubricants specifically designed for refrigeration applications that withstand the high solvency of the CO2. They also help protect against insufficient lubrication, which can result in increased bearing wear, reduced component life and increased maintenance costs. Inadequate lubrication can also result in improper sealing of clearances and loss of compression. This results in reduced compression efficiency, which can increase operating costs and energy consumption.

Looking at ammonia as a refrigerant

Ammonia refrigerants have a long history. They were actually less popular after the introduction of Chlorofluorocarbons (CFCs) in the 1920s but are now making a return. Like CO2, ammonia refrigerants have their own distinct set of benefits and challenges.

Benefits of ammonia

Ammonia has thermodynamic heat transfer properties that allow for the use of equipment with smaller heat transfer areas, cutting manufacturing costs. Ammonia also tends to be cheaper than HFCs, while also being 3-10% more efficient, which helps reduce energy bills.

In terms of lubrication, ammonia’s low vapour pressure reduces oil consumption and the need for top-ups, while its low solvency helps prevent changes in viscosity, which helps ensure a wide temperature operating range. With a GWP and ODP of 0, ammonia is also one of the most environmentally attractive refrigerant options.

Challenges of ammonia

Although ammonia systems work at lower pressures than ones that employ CO2, they still require pressure-related preventative maintenance. This is especially important given the toxic nature of ammonia. It is also flammable, which means engineers need appropriate safety training to ensure careful handling.

Notably, ammonia is immiscible with most oils, which can reduce lubricant efficiency. Its high operating temperature can also diminish the efficiency of mineral oils, a problem that can be overcome by the use of synthetic oils.

However, despite these challenges and the rapidly growing popularity of CO2 refrigerants, there are still certain applications where ammonia refrigerants are popular.

Next generation lubricants

Advanced lubricants can help overcome the challenges presented by natural refrigerants. To that end, we have recently introduced two new products: Mobil SHC™ Gargoyle 80 POE, a synthetic oil for CO2 applications, and Mobil Gargoyle™ Arctic 68 NH, our most advanced mineral-based lubricant for ammonia applications.

We hope that has given you some useful insights into the complex – and changing – world of refrigeration lubricants.

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