Among the list of refrigerants used in modern refrigeration applications, ammonia —known as chemical element NH3, or as refrigerant R717 — is one of the few that has remained a viable option since first being introduced in the 1930s. Classified as a natural refrigerant, ammonia is lauded for excellent thermodynamic properties, relatively low cost and minimal environmental impact. But, its toxicity also makes it a potentially deadly refrigerant, requiring operators to ensure safe application procedures and prompting authorities to regulate charge limits. Although ammonia is still in wide use today, many operators are moving it out of occupied spaces to mitigate its potential hazards. Instead, they are deploying it in conjunction with carbon dioxide (CO2) to improve efficiencies in specific operating conditions.
The occasional news stories about the dangers of ammonia leaks are reminders of the refrigerant’s shortcomings1, both as a risk to human health and as a contamination hazard to exposed food. For operators experiencing leak scenarios, the ramifications and costs can be far reaching:
- Loss of work productivity during the outage to clean up, repair and decontaminate the facility.
- Expense of the fire, emergency and hazardous materials response and subsequent cleanup.
- Food and productivity losses caused by food contamination by ammonia or food spoilage due to interruptions in cooling.
- Potential harm to human health and safety.
In addition, securing insurance to cover such losses must be factored into the total cost of ownership when deciding to use ammonia.
These risks often overshadow ammonia’s effectiveness, which in many ways has set the standard for optimal refrigerant performance. The fact remains that R717 has been a mainstay in many low temperature settings in industrial process cooling and cold storage applications for nearly a century. Its longevity is due to its proven performance as an extremely efficient refrigerant — arguably one of the best available for a variety of temperature ranges.
Safety Standards Add to Burden of Operating High Charge Ammonia Systems
Throughout the years, authorities have enacted safety standards to help mitigate ammonia’s dangers and ensure safe and healthful workplaces. For instance, for applications that require more than 10,000 pounds of ammonia2, the Occupational Safety and Health Administration (OSHA) issued the Process Safety Management of Highly Hazardous Chemicals standard (29 CFR 1910.119).3 This contains requirements for the management of hazards associated with processes using highly hazardous chemicals like ammonia. Local building- and fire-code authorities also may require special permits to install ammonia systems.
In recent years, OSHA has stepped up adherence to this standard via rigorous inspections enforced by its National Emphasis Program (NEP) on industries regulated by its process safety management standard — which, as already noted, includes ammonia refrigeration facilities.4 Owners and operators of large ammonia systems —those in excess of 10,000 pounds — have the added responsibility (and expense) of continuous recordkeeping in preparation for NEP inspections.
Toxicity aside, ammonia remains one of the most eco-friendly, natural alternatives available. For operators seeking to achieve regulatory compliance and meet sustainability objectives, figuring out a way to safely incorporate ammonia and other natural refrigerants into the refrigeration mix has tremendous appeal.
One emerging method to meet this objective is by lowering the total charge of ammonia in refrigeration systems and moving it out of occupied spaces.
Combining NH3 and CO2 to Minimize Exposure and Maximize Efficiencies
Modern cold storage applications call for bigger systems to support increasing low temperature requirements. As operators prepare to replace older ammonia systems, many are evaluating the options to expand the facility’s low temperature capabilities without bumping up against the 10,000-pound ammonia threshold. Today, the primary method to accomplish this is by combining ammonia (R717) with carbon dioxide (R744). These new system architectures utilize very low charges of ammonia and remove the R717 circuit from occupied spaces.
Refrigeration systems using carbon dioxide have been popular in Europe for almost two decades, and they have seen increased popularity in the United States in recent years. Also a natural refrigerant, R744 is nontoxic and has proven to be an effective low temperature alternative, especially in temperatures below -40°F (-40°C). Because of its low critical point and high operating pressure (around 1,500 psig), carbon-dioxide-based refrigeration strategies must account for R744’s unique characteristics.
One of the most common ammonia/carbon dioxide systems emerging in cold storage is the CO2 cascade architecture. Instead of sending R717 through pipes to an evaporator near occupied spaces, a low charge is used only in the high stage of the refrigeration cycle to chill the R744. This high stage process takes place remotely — for example, on the roof. From there, the chilled carbon dioxide is pumped into heat exchangers or evaporators acting as a volatile secondary fluid. Alternately, the chilled CO2 can be sent to direct-expansion low temperature evaporators. Either way, if there is a leak of R744 in an occupied space, it does not represent an imminent hazard to worker health or safety.
Similar trends are taking place in the commercial refrigeration/supermarket space, where retailers are running trials of ammonia/carbon dioxide systems with a low R717 charge. In these applications, where the risk of exposing customers and employees to ammonia represents a potential disaster, retailers cannot afford to take risks. So, instead of using a 1,000-pound charge of R717 as previously introduced in these systems, the technology has advanced to require only 100 pounds or less of ammonia charge for a smaller chiller in the high stage of the refrigeration cycle.
The R717 stage of the supermarket refrigeration system also is used to chill CO2 (when used as volatile brine) at a remote location outside the store. Then, the chilled brine is pumped into the refrigerated, occupied space. It is an energy-efficient method that allows retailers to maintain a green footprint with an all-natural refrigerant system. At the same time, they are able to mitigate exposure risks by limiting the ammonia charge and moving it out of the store.
In both industrial and commercial scenarios, lowering the charge of ammonia addresses the operator’s respective problems. As ammonia industry champions continue to push the envelope to exploit its excellent efficiency and refrigerant properties, new system designs are being tested that utilize one pound of refrigerant per ton in capacity. Their potential efficiency alone is garnering a lot of attention, and combined with the acceptability of using carbon dioxide as a secondary fluid, ammonia has the potential to be an attractive alternative (see sidebar).
Operators Must Weigh Pros and Cons
Ammonia is yet another reminder that the industrial and commercial refrigeration industries have yet to find the perfect refrigerant. Its excellent thermodynamic properties often are overshadowed by its toxicity. Throughout its long history, refrigerants such as R22 and R507A have —for a time — unseated ammonia as the best option in industrial applications. But, when those refrigerants were later proven to be a threat to the environment, R717 use in industrial and commercial applications rebounded.
Today’s ammonia/carbon dioxide options may be another emerging alternative to the health and safety risks imposed by high charge ammonia systems. Understanding the intricacies of these systems and making design decisions for specific applications requires a significant degree of expertise. Before embarking on such an endeavor for your organization, consult an experienced practitioner. Because there is no one-size-fits-all NH3/CO2 solution, operators will have to weigh the pros and cons of available system architectures to determine the best fit for their operation. PC