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Refrigerant selection


[edit] Introduction

Refrigerants have been a key component of air conditioning and refrigeration systems since the 19th century. Aside from technological advances, a major change to their use is the increasing focus on reducing the impact on the environment. Whilst fluorinated gases (‘F-gases’) don’t damage the atmospheric ozone layer, they are greenhouse gases and the EU wants to cut emissions by two-thirds by 2030. More specifically, the F-gas regulations which were adopted in 2006, are being replaced in 2015. Perhaps unsurprisingly the new legislation introduces more stringent requirements, such as limiting the amount of F-gases that can be sold in the EU, and banning the use of F-gases in some new equipment if better alternatives are available on the market.

The challenge for businesses is not only choosing the right refrigerant to reduce emissions, but also making the right choice to have a positive effect on operational efficiency, which reduces operating cost. In the case of supermarkets and other high-profile users of refrigerants there is also the benefit of good publicity through the use of environmentally-friendly refrigerants.

[edit] Types of available refrigerant

Commercial organisations have traditionally used an R12, CFC, R502, or CFC/HCFC blend. To enable the government targets to be achieved most manufacturers have adopted either R404A, an HFC blend, or R134a. However, these are potent greenhouse gases.

An alternative, and one of the future solutions might be 'natural' refrigerants, but this may require some design changes to air conditioning and refrigeration equipment. The most common natural refrigerants and their characteristics are shown below:

[edit] Isobutane (R600A)

Isobutane is a hydrocarbon, and hence is flammable. Its thermodynamic properties are very similar to those of R134a. Isobutane presents other advantages, such as its compatibility with mineral oil and better energy efficiency and it is cheaper than R134a. The use of isobutane requires minimal design changes, such as the relocation of potential ignition sources outside the refrigerated compartment.

[edit] Propane (R290)

Propane has a boiling point of –42°C, making it an excellent alternative to R22 as it requires similar working pressures. An added advantage is that, other than added safety measures because of its flammability, virtually no design change is required in systems when switching from R22 to propane. The combination of its good thermodynamic and thermophysical properties yields systems that are at least as energy efficient as those working with R22. The use of propane is increasing in countries where regulations allow it.

[edit] AMMONIA (R171)

Ammonia has been continuously used throughout modern refrigeration history, but has numerous drawbacks. It is toxic and flammable in concentrations between 15.5% and 28% in air. It is not compatible with copper, thus requiring other materials for construction. But ammonia’s thermodynamic and thermophysical properties also yield very efficient refrigeration systems. Because of its acute toxicity, stringent regulations apply for ammonia systems, which require close monitoring and highly-skilled engineers and technicians.

[edit] Carbon dioxide (CO2)

Carbon dioxide is not a new refrigerant. The use of carbon dioxide as a refrigerant lasted for well over a century, but was abandoned in the mid-1950s, with the widespread use of the CFC refrigerants, which were more efficient, more stable and safer. It was ‘rediscovered’ in the early 1990s. Due to its low environmental impact, low toxicity and non-flammability, CO2 is now regaining popularity with refrigeration system designers whilst an alternative to fluorocarbons is being sought.

[edit] Selection criteria for refrigerants

There is no rule of thumb governing the selection of refrigerants. However, there are five criteria should be taken into account:

  • Thermophysical properties.
  • Technological issues.
  • Economic aspects.
  • Safety.
  • Environmental factors.

In addition to these criteria, other considerations such as; local regulations and standards, maintainability and capability (such as having staff with skills to support the units). User training requirements should be taken into account.

The desirable characteristics of ‘ideal’ refrigerants are considered to be as follows:

  • Normal boiling point below 0°C.
  • Non-flammable.
  • Non-toxic.
  • Easily detectable in case of leakage.
  • Stable under operating conditions.
  • Easy to recycle after use.
  • Relatively large area for heat evaporation.
  • Relatively inexpensive to produce.
  • Low environmental impacts in case of accidental venting.
  • Low gas flow rate per unit of cooling at compressor.

As it is unlikely that there will be a refrigerant that matches the perfect profile, it may be necessary to prioritise which of the criteria for the ideal refrigerant is of most importance. For example, If minimizing the environmental impact is a key criteria, the efficiency of the refrigerant in the operational phase of the system will outweigh the impact of the production and disposal stages.

[edit] Suitability of the available refrigerants

Supermarket retailers are gradually moving away from long-established HFC refrigeration systems towards using a group of non-HFC natural refrigerants. Such installations often use ammonia, CO2 or hydrocarbons, which have comparatively little or no impact on global warming and zero impact on the ozone layer. For instance, CO2 has a global warming potential (GWP) of just one, compared with a figure of almost 4,000 for HFC-404A.

One quick fix option for reducing the environmental impact of emissions from a retailer’s existing estate is to replace HFC-404A with a drop-in alternative such as HFC-407F (GWP 1,705) or HFC-407A (GWP 1,990). While these blends still have a significant environmental impact, their GWPs are about half that of HFC-404A, so transitioning to these blends in existing equipment can dramatically cut the impact of leaking refrigerants in the short term.

CO2 suitable for use as a refrigerant is commonly named R744 in the refrigeration and air conditioning industry.

The advantages are:

  • Low toxicity.
  • Non-flammability.
  • Zero ozone depletion potential.
  • Very low global warming potential (GWP=1).
  • Excellent thermodynamic properties and low energy requirements.

R744 refrigerant properties are surprisingly conducive to the cooling process. Despite the need for high pressure, the R744 agent is capable of running smoothly, efficiently, and with reduced waste as the heat waste can be rerouted and reused in other parts of the production process.

In cases when CO2 is used for low temperature refrigeration, it is used either as a heat-conductor refrigerant, or in cascade with another refrigerant (R404A, NH3, R134a, etc.).

If R744 is considered as a refrigerant then the following must be taken into account:

  • High pressure: CO2 circuits operate at much higher pressures than a conventional R404A system. This requires the use of components and assembly techniques that are unusual in the field of refrigeration.
  • Maintenance: The operation mode requires a different design compared to conventional HFC systems design that is unfamiliar to most technicians in refrigeration for supermarkets.
  • Cost: The high pressures require more binding materials and with R744 not yet widely used there is limited choice and a tendency to cost more.

There are alternative refrigerants on the market, it is now up to the designers and operators to specify something new to move the industry forward.

Note: On 15 October 2016 it was announced that 170 countries in Kigali, Rwanda, had agreed that all HFC’s should be phased out through an amendment to the Montreal Protocol. See HFC phase out for more information.

This article originally appeared in the September 2014 edition of BSRIA’s Delta T magazine. It was written by Salim Deramchi, Senior Building Services Consultant, BSRIA Sustainable Construction Group. It has been posted here by --BSRIA 10:42, 5 December 2014 (UTC)

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