Process Cooling Systems using Modern Natural Refrigerants
Safety and efficiency can be optimized by understanding the implications of natural refrigerants on component design and selection.
Manufacturers and OEMs are replacing the old classes of chemical refrigerants that emit potent greenhouse gases — hydrofluorocarbons (HFCs) hydrochlorofluorocarbons (HCFCs) and chlorofluorocarbons (CFCs), for instance. The environmentally friendly, green refrigerants used in their place do not deplete the ozone layer and have a lower impact on global warming. Typical refrigerant alternatives — all naturally occurring, nonsynthetic substances — include hydrocarbons (propane, butane and cyclopentane), carbon dioxide (CO2), ammonia, water and air. Each can be used as a cooling agent in industrial process cooling and refrigeration systems.
For those OEMs building systems to cool manufacturing and industrial systems, using natural refrigerants brings on new design challenges. These include selecting the best type of refrigerant for the given application and choosing components that can handle the specific refrigerant’s technical and performance characteristics.
Criteria for Environmentally Acceptable
Two important criteria are used to measure whether or not a refrigerant is environmentally acceptable for use in process cooling systems. They are:
- Ozone-depletion potential (ODP).
- Global-warming potential (GWP).
Ozone-depletion potential is a measure of the relative amount of damage a substance can cause to the ozone layer. Ultraviolet (UV) radiation from sunlight causes CFCs and HCFCs to release chlorine into the atmosphere, which then damages the ozone.
Natural refrigerants have an ODP of zero. This means that they will not have any depleting effect on the ozone layer if they escape the system.
Global-warming potential is a relative measure of how much heat a greenhouse gas traps in the atmosphere. The lower the global-warming potential, the better a substance is for the environment. GWP compares the amount of heat trapped by a certain mass of the gas in question to the amount of heat trapped by a similar mass of carbon dioxide. A specific GWP is calculated over a time interval, typically 20, 100 or 500 years.
Many of the common refrigerants used today have a global warming potential ranging from 1,400 to nearly 4,000. One can compare varying GWP values as they relate to types of refrigerants in figure 1.
Current HFC refrigerants R404a and R134a have extremely high global-warming potential values — 3,922 and 1,430 respectively. By comparison, propane hydrocarbon (R290), which is a newer, natural refrigerant version of R404A, has a low GWP of only three. Most users of R404A have already switched to R134A, but there are even greater benefits of transitioning to a R290-type refrigerant.
FIGURE 1. Many of the common refrigerants being used today have a global warming potential ranging from 1,400 to nearly 4,000. Compare varying global-warming potential values as they relate to types of refrigerants.
Comparison of Natural Refrigerants
Based on their inherent characteristics, some refrigerants are more suitable than others for certain applications.
Hydrocarbons. Many of the emerging generation of natural refrigerants are hydrocarbon (HC) based rather than hydrofluorocarbon (HFC) based. Hydrocarbon-based refrigerants include the already mentioned propane (R290); isobutane (R600a), typically for use in small capacities; and R32, which is a blend of R290 propane and another refrigerant. These naturally occurring chemicals operate in mid-pressure ranges of 10 to 50 bar.
Propane R290’s thermodynamic properties are superior to both of the old HFC refrigerants, R134a and R404a. Additionally, its heat capacity is approximately 90 percent greater than R134a and 140 percent greater than R404a. These characteristics allow R290 to absorb more heat at an accelerated rate, resulting in higher device efficiency with faster temperature recovery and lower energy consumption.
Hydrocarbons have no ozone-depleting properties and low global-warming potential. However, this new generation of hydrocarbon refrigerants is highly flammable. Therefore, it requires different and safer technologies for refrigeration and industrial process cooling systems and components.
Carbon Dioxide. CO2 operates at almost twice the pressure of hydrocarbons in a typical air-conditioning system, making it much more difficult to manage. In addition, CO2 has a global-warming potential of only one. (By contrast, hydrocarbons have three times more global-warming potential.)
FIGURE 2. This chart compares key characteristics of natural refrigerants, including hydrocarbons, carbon dioxide and ammonia, with hydrofluorocarbons.
Ammonia. Ammonia is another natural refrigerant now being used. It measures zero for both ozone-depleting potential and global-warming potential. It breaks down rapidly; however, its alkalinity is extremely corrosive. Components used in ammonia applications require careful consideration of material compatibility.
FIGURE 3. Pressure switch manufactured with a hermetic seal around the gas path as well as a sealant around the electrical connections help ensure process safety.
Component Compatibility. Each type of natural refrigerant comes with challenges that OEMs and component manufacturers must overcome with technological innovation. To meet these challenges, industry leaders are developing components that are compatible with the latest common refrigerants — hydrocarbons, CO2 and ammonia. New products created for this market include pressure switches and sensors to meet different application requirements for process cooling systems.
Solving the High Pressure Challenge
Every component in today’s systems is specified to certain operating conditions and pressure ranges. While CO2 is not combustible like hydrocarbons, it poses a different set of problems in that it operates at a higher pressure. This necessitates designing in specialized components such as pressure switches to ensure reliable operation in applications where CO2 is used. This, in turn, affects product manufacturing.
For example, pressure switches are required in CO2 refrigeration systems to protect against high pressure burst or overpressure situations that could cause the coils to rupture. When the pressure builds up to a certain point, the switch opens the electrical contact and turns off the compressor in the system, which is the heart of any refrigerating system. When the pressure drops to normal levels, it automatically switches the compressor back on.
It is my opinion that in today’s market, even more safety is needed. Because switches are an electromechanical device, they inherently can spark when the contacts make or break.
FIGURE 4. The automatic-reset pressure switches were subjected to tests of IEC 60079-15:2010 standards and were certified to meet the requirements for operating in explosive atmospheres.
Sealing in Safety for Flammable Refrigerants
One of the biggest challenges of natural hydrocarbon refrigerants is flammability. To prevent a spark from accidentally causing ignition of hydrocarbon refrigerants, some manufacturers have built extra safety into their pressure switches. The sealed design that “seals the spark” by isolating the specific pressure media (R290) from the electrical switch assembly.
Such switches are manufactured with a hermetic seal around the gas path as well as a sealant around the electrical connections. The electrical switch connections signal back to the system whether the switch is open or closed. This safety design helps eliminates the potential for explosion by stopping the gas from entering the electrical switch compartment where the arcs can be generated during contact make or break operations.
In researching the effects of natural refrigerants on their process cooling systems, OEMs need to know if the pressure switch has been tested with the specific refrigerant that they are planning to use.
For R290 use, they need to know what the maximum current level of the switch is that prevents the arc from igniting the refrigerant. They also should know whether it meets the required pressure actuation point and switch point.
For CO2 systems — operating with twice the pressure of conventional systems — the big concern is burst pressure. What kind of high pressure can it withstand? How long will it reliably operate under sustained high-pressure environments?
To address these concerns, and to ensure that switches can withstand flammable refrigerant applications, pressure switches specially designed for hydrocarbons can be submitted for rigorous testing by external agencies.
Pressure Sensors and System Feedback
To obtain system pressure information, a pressure sensor — rather than a pressure switch — is needed. A pressure switch is strictly for safety. In contrast, a pressure sensor provides safety information as data that can be used to increase energy efficiency in the system.
If the goal is to maintain pressure within a constant range, the pressure sensor can be used to regulate the system. For example, if the pressure goes outside the normal operating range in a refrigeration system, a pressure sensor could provide feedback that could activate the pressure switch or turn off or on the condenser system to bring the pressure within the specified range. In addition, the pressure sensor could notify a technician or maintenance organization of the issue for a more permanent fix. This could be a simple local alert, or it could send an alert via the cloud to the appropriate person or company. Many system OEMs also could use the data generated by a pressure sensor to understand how their industrial process cooling or refrigeration systems function over time.
In conclusion, process cooling system design engineers will be examining choices for replacing traditional chemical refrigerants with less damaging, green refrigerants. As they do so, they should be careful to specify hardware and systems to safely implement and use these more technically challenging substances within industrial and manufacturing environments.