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The impact of natural refrigerants on lubricants

Refrigeration pipes.
Refrigeration compressor.

Global legislation is limiting the use of refrigerants with a high level of Ozone Depletion Potential (ODP) or Global Warming Potential (GWP). Businesses are now increasingly adopting natural refrigerants – such as ammonia, carbon dioxide (CO2) and hydrocarbons – to help reduce the impact on the environment.

This switch presents a number of challenges that operators must face – not least when it comes to lubrication. Below is some guidance on the challenges and how an informed decision can help companies overcome them.

 

The fundamentals of natural refrigerants
The top three refrigerants that are set to experience the most growth as a result of legislative pressure are the following:

 

  • CO2 is projected to see the highest growth in the natural refrigerants market through to 2020
  • Ammonia refrigerants were ranked third behind hydrocarbon refrigerants. They are set to experience steady growth.
  • Hydrocarbon refrigerants are expected to grow on the back of their usage in domestic refrigeration.


Carbon dioxide - the best option?
The principal advantage of CO2 is evident in its properties as a refrigerant; it has an ODP of zero and a GWP value of 1. Compare that with hydrofluorocarbons (HFCs), which have a GWP ranging from 4.1 to 4,800, and the benefits of CO2 are clear.
There are additional benefits, too, including:

  • Size: CO2 systems are smaller than HFC-based ones and can be designed in various configurations, including ‘direct expansion’ and ‘secondary brine’.
  • Cost: CO2 has excellent heat transfer properties, which means that 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 challenges of CO2 as a refrigerant
The principal issues arise due to the high operating pressures and solubility that happen in both subcritical cascade systems and transcritical high pressure CO2 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. This presents particular lubrication challenges.

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, higher operating cost and greater energy consumption.

Looking at ammonia as a refrigerant
Ammonia refrigerants, another of the ‘natural’ options were phased out after the introduction of Chlorofluorocarbons (CFCs) in the 1920s. However,ammonia saw the highest growth of all refrigerants during 2015, although it has its own distinct set of benefits and challenges.

Benefits of ammonia
With a GWP and ODP of zero, ammonia is one of the most environmentally attractive refrigerant options. It also has thermodynamic heat transfer properties that allows for the use of equipment with smaller heat transfer areas, cutting manufacturing costs. Ammonia is also cheaper than HFCs in most countries while also being 3-10% more efficient.

In terms of lubrication, ammonia’s low vapour pressure reduces oil consumption and the need for top-ups. Its low solvency helps prevent changes in viscosity, which helps ensure a wide temperature operating range.

Challenges of ammonia
Although ammonia systems work at lower pressures than the 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 that high levels of training are required for engineers handling ammonia refrigerants. 

Ammonia is not immiscible with most oils, which can reduce lubricant efficiency while its high operating temperature can diminish the efficiency of mineral oils, a problem that can be overcome by the use of synthetic oils.


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