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在选择制冷剂时需要考虑F-gas年十月 2014

Written by Salim Deramchi, Senior Building Services Consultant, BSRIA Sustainable Construction Group

Refrigerants have been a key component of air conditioning and refrigeration systems since the 19th century. Aside from technological advance, a major change to their use nowadays is the increasing focus on reducing the impact of refrigerants 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 in 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 the 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.

To select the most appropriate refrigerant there are three main considerations:
1. Types of available refrigerant
2. Selection criteria for refrigerants
3. Suitability of the available refrigerants.

Types of available refrigerant

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

Commercial organisations have traditionally been using a R12, CFC, R502, or CFC/HCFC blend. To enable the government targets to be achieved most manufacturers have adopted either R404A, a 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 the air conditioning and refrigeration equipment. The most common natural refrigerants and their characteristics are shown below:

Isobutane (R600A) is a hydrocarbon, and hence is flammable. The 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’s cheaper than R134a. The use of isobutane requires minimal design changes, such as the relocation of potential ignition sources outside of the refrigerated compartment.

Propane (R290) 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 except for 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.

Ammonia (R171) has been continuously used throughout modern refrigeration history. It has its 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 Amonia’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.

Carbon dioxide (CO2) is not a new refrigerant. Rather, it was “rediscovered” in the early 90s. The use of carbon dioxide as a refrigerant lasted for well over a century. Its application was abandoned in the mid-50s, with the widespread use of the CFC refrigerants, which were more efficient, more stable and safer. Due to its low environmental impact, low toxicity and non-flammability, CO2 is now regaining popularity from refrigeration system designers whilst an alternative to fluorocarbons is being sought.

Selection criteria for refrigerants

There is no rule of thumb governing the selection of refrigerants. However, there are five criteria you should take 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, application, and 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.

The choice of alternative refrigerants should involve a review of recycling or disposal of refrigerants. As it is unlikely that there will be a refrigerant that matches the perfect profile you may need to prioritise which of the criteria for the ideal refrigerant is of most importance to your organisation. For example, If minimizing the environmental impact is a key criteria in your choice of refrigerant, the efficiency of the refrigerant in the operational phase of the system will outweigh the impact of the production and disposal stages.

The decision-making process when selecting a new refrigeration plant using natural refrigerant such as ammonia, CO2 or hydrocarbons, should consider not only the impact on the environment but the additional skills required to maintain the system.

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 gas has a global warming potential (GWP) of just one, compared with a figure of almost 4,000 for HFC-404A, which is currently the most widely used HFC for commercial refrigeration applications.

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.

Due to its low environmental impact, CO2 is also gaining popularity from refrigeration system designers. 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 the case 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.).

So, if one was to consider R744 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.

The operation mode requires a different design compared to conventional HFC systems design that is unfamiliar to most technicians in refrigeration for supermarkets.

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. With the new F-Gas regulation coming we need to get ahead of the game.