For refrigerated and frozen food processors, choosing between an ammonia or an ammonia/CO2 cascade system can be one of the most important variables.
One of the primary drivers when evaluating ammonia or ammonia/CO2 cascade systems is understanding that ammonia contamination in processing and production rooms can be a huge potential liability to facilities and dangerous to employees. Associated with this liability is a second key concern—the threshold quantity (TQ) of 10,000 pounds for ammonia charge, as stated by the Occupational Safety and Health Administration (OSHA) and the Environmental Protection Agency (EPA) programs.
While an ammonia charge lower than 10,000 pounds still requires the facility owner to meet the General Duties clause, it is not subject to OSHA’s Process Safety Management (PSM) or the EPA’s Risk Management Program (RMP) guidelines. Facilities that fall below the TQ requirement can have lower operating costs by lessening the need for a full PSM program and requires fewer operations and program management employees.
After processing the impact of those factors, it is time to dig deeper into the two types of systems.
Advantages of ammonia systems
While both ammonia and CO2 are natural refrigerants, facilities that are more cooler-intensive than freezer-intensive will likely perform more efficiently with ammonia due to the inherent properties of the refrigerant.
Another issue to consider when choosing between ammonia/CO2 cascade and ammonia systems is the familiarity of operators and service vendors. Traditional ammonia systems have been around for more than 100 years, and there are many operations and maintenance staff qualified to work on these systems compared to more recent developments with ammonia/CO2 cascade systems.
A final consideration is the fact that the initial installation cost of ammonia systems has historically been lower and involves less complexity in design and construction. Additionally, ammonia systems tend to be somewhat simpler and more forgiving in terms of service and maintenance.
Disadvantages of ammonia systems
Ammonia systems are generally considered to be more dangerous than ammonia/CO2 cascade systems. This danger is due to ammonia being a more toxic refrigerant compared to CO2, and a high-charge, conventionally pumped ammonia system exposes a large portion of the building to ammonia via piping, heat exchangers and vessels. Typically, with an ammonia/CO2 cascade system, the ammonia is confined to a smaller area, minimizing the leak potential of the more toxic substance.
In sparsely populated areas, when operating a high-charge ammonia system, liability risk to the community is limited. In more densely populated or urban areas, coordination and follow up on a refrigerant dispersal incident is more complex.
Advantages of ammonia/CO2 cascade systems
In a facility with heavy freezer load requirements, CO2 has potentially better efficiency than ammonia, as compressor energy consumption is usually reduced for systems operating at -20°F. Also, in a food production facility that is quick freezing products, operating temperatures can be reduced with less energy detriment, which can increase production throughout by bringing food products down to temperature faster.
In an ammonia/CO2 cascade system, ammonia will typically be contained in the machine room rather than throughout the entire facility, ensuring the ammonia exposure potential is significantly lower for a large facility.
Another advantage of cascade systems is that all portions of the system are under positive pressure, negating the need for an air purging sub-system typically found on ammonia installations with freezers and refrigeration pressures that run under vacuum pressure.
Disadvantages of ammonia/CO2 cascade systems
One drawback of a cascade system is the potential maintenance cost of operating both CO2 and ammonia compressors, as it requires additional controls and components that can be costly if not properly maintained.
Another downside of ammonia/CO2 cascade systems is the complexity in deciding on the defrost methodology. Cascade systems have a wide variety of defrost options, each with its own merits and shortcomings.
When reviewing ammonia and ammonia/CO2 cascade systems, the advantages and disadvantages of both systems are apparent. Only once you understand your unique operation can you then review the risk factors and design considerations of each system to safely select which would yield the best results and efficiencies for you and your operation.