Attempts to regulate the air conditioning (AC) and commercial refrigeration markets for the benefit of the environment are nothing new. During the past 25+ years, various legislative actions have limited the use of various refrigerants that depleted ozone or emitted greenhouse gases, both of which have been shown to contribute to global warming potential (GWP).
The most recent actions, including the EPA’s Significant New Alternatives Policy (SNAP) program and the much more recent American Innovation and Manufacturing Act (AIM), are further driving growth of low-GWP refrigerants. While this is good news for the environment, these low-GWP refrigerants are not without their own challenges. They may increase energy consumption, introduce added safety risks and require significant equipment modifications.
A history of efforts to benefit the environment
Refrigeration systems contribute to the emission of greenhouse gases when a leak occurs of a high-GWP refrigerant. The two refrigerants most commonly used in the early days of refrigeration were ammonia and carbon dioxide (CO2). Both proved to be problematic for different reasons. Ammonia is toxic and carbon dioxide requires extremely high pressures to operate in a refrigeration cycle.
As a result, both refrigerants lost popularity when Freon 12 (dichloro-diflouro-methane) hit the market. Among other benefits, Freon is extremely stable, non-toxic, and operates at moderate pressures. Unfortunately, it was also shown to have a high ozone depletion potential (ODP), which is the primary reason is has since been banned from use globally.
Numerous other refrigerants have also been banned through SNAP, which was established under the Clean Air Act to identify and evaluate substitutes for ozone-depleting substances. The EPA has since published numerous rules about what is and is not acceptable under SNAP, creating tremendous confusion in the industry. The policies established under SNAP took on greater meaning due to the AIM Act, which went into effect in 2020. The AIM Act requires the EPA to implement a phase down of the production and consumption of hydrofluorocarbons (HFC refrigerants) to reach approximately 15% of their 2011-2013 average annual levels by 2036. HFCs are of particular concern since they are classified as potent greenhouse gases that contribute to climate change.
Pros and cons of preferred low-GWP alternatives carbon dioxide (CO2) and propane (R290)
Tighter environmental legislation is opening the door for increased demand for low-GWP refrigerants.
Two of the more popular options on the market today are CO2 and propane (R290). Both have a long history in refrigeration but have recently again emerged as front-runners due to their low environmental impact. CO2 is the more commonly used for many reasons, including:
- Environmentally safe with a GWP rating of 1
- Non-toxic and non-explosive, which means it is safe for use in public locations, such as supermarkets
- Less expensive than synthetic refrigerants
- Non-corrosive to most component materials and compatible with most compressor lubricants
- High density, which allows for a smaller pipe size
- Less noticeable pipeline pressure drops
- High heat transfer efficiency so heat exchangers can be smaller
- SNAP-approved for use in many different types of systems
A key challenge of using CO2, however, is that it operates at a far higher pressure than other natural and synthetic refrigerants. This increases the risk of leaks which, in turn, necessitates the use of more durable, costly components and piping to handle the greater pressure and added controls and other safety features, many of which increase energy consumption due to high ambient temperatures.
As a hydrocarbon, R290 provides several equally attractive benefits, including superior thermodynamic properties and greater heat capacity. These combined characteristics allow R290 to absorb more heat at an accelerated rate, resulting in higher device energy efficiency with faster temperature recovery and lower energy consumption.
More importantly from an environmental standpoint, R290 (like all hydrocarbons) has no ozone depleting properties and a low GWP of 3. It is also compatible with materials commonly used in the construction of refrigeration and air conditioning equipment, is readily available and relatively inexpensive. It can be stored and transported in steel cylinders similar to how other common refrigerants are handled.
A major concern about R290 is that it’s highly flammable. That’s why refrigerant charge limits are in place, as well as other special safety standards when using R290.
Overcoming the challenges of CO2
The higher operating pressure of CO2 creates a few design challenges. But most of these can be overcome, albeit at a higher cost. Key are material upgrades, such as thicker-walled piping or high-strength K65 copper alloys.
Parker manufactures an array of valves, seals and controllers that are specially rated for CO2 high-pressure refrigeration systems. This includes pressure-rated electric expansion valves, ball valves with integrated pressure relief, stepper motor-driven pressure regulating valves, pulse width modulation valves that manage refrigerant flow and pressure-regulating gas cooler/flash gas bypass valves.
Steel piping is also being used in some instances for high pressure CO2 and the addition of electronic controls which can monitor and record pressures, temperatures and additional parameters. This includes pressure-rated electric expansion valves, ball valves with integrated pressure relief, stepper motor-driven pressure regulating valves, pulse width modulation valves that manage refrigerant flow and pressure-regulating gas cooler/flash gas bypass valves. The use of remote monitoring systems is growing. Not only is remote monitoring a safer option, but it is also in response to the current shortage of trained HVAC technicians, allowing companies to monitor multiple systems and locations with fewer workers.
Overcoming the challenges of R290
With the high flammability of R290, refrigerator manufacturers need to make the necessary system design changes to align with applicable UL and ASHRAE standards that include charge limits, marking requirements and ventilation requirements. System charges of up to 150 grams are currently allowed for most R290 applications, though a few are even lower. Proposed and under-revision UL safety standards seek to raise this limit as high as 500 grams for open refrigeration appliances, and 300 grams for closed appliances. While some systems or applications would remain with more restrictive charges, these increases promise to open R290 to many more applications and enable new applications.
To address flammability concerns, Parker manufactures innovative filter driers and thermostatic expansion valves (TEVs) that are designed to minimize the amount of refrigeration system charge when using flammable refrigerants.that are designed to minimize the amount of refrigeration system charge when using flammable refrigerants.
In addition, some system manufacturers use a sealed design that seals off the spark inside by isolating the R290 from the electrical switch assembly. This type of design reduces the potential for explosion by stopping the gas from entering the electrical switch compartment.
Another option is to fit the leak detection and control systems such that, when activated, it will pump down the propane charge into a liquid receiver and then shut off the electrical supply. If the compressor is enclosed, a ventilation fan must be installed and activated by the leak detection system to remove any gas that might leak from the compressor into the enclosure.
Preparing for an uncertain future
Despite the ongoing changes and obstacles presented by various environmental regulations in the U.S. and abroad, the good news is that the necessary product and material innovations are already available to help engineers overcome the design challenges presented by low-GWP refrigerants, such as CO2 and R290.
This article was contributed by Parker Sporlan Division.
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