Among the list of refrigerants used in modern refrigeration applications, ammonia (NH3 or as refrigerant R-717) 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.
But, its toxicity also makes it a potentially deadly refrigerant, requiring operators to ensure safe application procedures and prompting authorities to regulate charge limits. The ramifications and costs could include loss of productivity during an outage, expense of the fire/emergency/hazmat response and subsequent cleanup, food and productivity loss caused by the contamination or interruption in cooling, potential harm to human health and safety and securing insurance to cover such losses.
These risks however often overshadow ammonia’s effectiveness. The fact remains that R-717 has been a mainstay in many low-temperature settings, such as industrial, process cooling and cold storage applications for nearly a century.
Safety standards add to burden of operating high-charge ammonia systems
Throughout the years, authorities have enacted safety standards to help mitigate its dangers and ensure safe and healthful workplaces. For applications that require more than 10,000 pounds of ammonia, the Occupational Safety and Health Administration (OSHA) issued the Process Safety Management of Highly Hazardous Chemicals standard (29 CFR 1910.119). This contains requirements for the management of hazards associated with processes using highly hazardous chemicals like ammonia. Local building and fire code authorities may also 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 process safety management regulated industries, which include ammonia refrigeration facilities. This means that owners and operators of large ammonia systems in excess of 10,000 pounds now have the added responsibility and expense of continuous record keeping in preparation for NEP inspections.
Toxicity aside, ammonia is said to be one of the most eco-friendly, natural alternatives available.
One emerging method is to lower the total charge of ammonia in refrigeration systems and move it out of occupied spaces. Modern cold storage applications call for bigger systems to support increasing low-temperature requirements. As older ammonia systems inevitably will be replaced, many operators are evaluating the best option to expand their 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 R-717 with CO2 (R-744) in system architectures and remove the R-717 circuit of the system from occupied spaces.
CO2-based refrigeration systems have experienced increased popularity in the United States in recent years. That’s because R-744, also a natural refrigerant, is non-toxic and can be an effective low-temperature alternative, especially in -40°F temperatures.
One of the most common NH3/CO2 systems emerging in cold storage is the CO2 cascade architecture. Instead of sending R-717 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 R-744. This high-stage process takes place remotely (e.g., on the roof) where the chilled R-744 is then pumped into heat exchangers or evaporators acting as a volatile secondary fluid, or sent to direct expansion low-temp evaporators. Either way, if there is a leak of R-744 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 trialing NH3/CO2 systems with a very low R-717 charge. Instead of using a 1,000-pound charge of R-717, the technology requires only 100 pounds or less of charge for a smaller chiller. The R-717 stage of the supermarket refrigeration system is also used to chill CO2 (when used as volatile brine) at a remote location outside the store, and then pump that fluid into the refrigerated occupied space. It’s an energy-efficient method that allows retailers to maintain a green footprint with an all-natural refrigerant system while mitigating exposure risks.
In both the industrial and commercial scenarios, lowering the charge of ammonia addresses the operator’s respective problems. New system designs are being tested that utilize 1 pound of refrigerant per ton in capacity. Combined with the acceptability of using CO2 as a secondary fluid, ammonia has the potential to be a very attractive alternative.