The frozen food processing industry is striving to minimize or eliminate HCFC and HFC refrigerants, reduce ammonia refrigerant charge and look for alternative natural refrigerant options with similar or better energy efficiency. One attractive option is an ammonia/carbon dioxide cascade refrigeration system.

Here are comparisons of two traditional ammonia systems—single-stage economized and two-stage ammonia and an ammonia/carbon dioxide cascade system. For purposes of comparison, a spiral/tunnel/blast freezing application will be compared at a -58°F coil temperature.

Single-stage economized ammonia

Pros

• Lowest initial cost.
• Smallest unit footprint.
• No risk of refrigerant cross contamination.

Cons

• Lowest energy efficiency.
• Ammonia is present in food processing areas.
• Low-pressure side of system operates in a vacuum; non-condensables will be pulled into system.

Two-Stage ammonia

Pros

• Improved energy efficiency.
• No risk of cross contamination.
• Operators are most familiar with this system.

Cons

• Highest initial cost.
• Ammonia is present in food processing areas.
• Low-pressure side of system operates in a vacuum; non-condensables will be pulled into system.
• Largest unit footprint.

Ammonia/carbon dioxide cascade

Pros

• No ammonia present in food processing areas – limited to machine room.
• Highest energy efficiency.
• Low-pressure side of system is at positive pressure; non-condensables will not enter system.
• Ammonia charge will be under 10,000-pound threshold for increased PSM requirements.

Cons

• Risk of cross contamination.
• Operating pressures are much higher, requiring extra safety precautions.
• More sensitive to moisture; a deep vacuum is required after all service tasks.

The other major differences between the ammonia systems and carbon dioxide cascade system are coil defrost methods, oil management and system pressure management.

Defrost methods

The most common defrost method for ammonia systems is hot gas defrost if there are multiple evaporator coils. This can be done using ammonia gas below condensing pressure (typically around 75 psig or 50°F saturated gas temperature).

With a carbon dioxide coil, hot gas is needed at much warmer temperatures and higher pressures than the system condensing pressure (typically around 410 psig or 20°F saturated gas temperature). A separate carbon dioxide hot gas compressor must be used to boost the condensing pressure up to approximately 640 psig to reach a 50°F saturated gas temperature. Other carbon dioxide defrost methods are warm glycol defrost or the much simpler electric defrost, which does not require any additional piping.

Oil management

Oil management in an ammonia system is basic. The oil is heavier and does not mix with ammonia, so it naturally separates and collects at low spots in the system. As long as oil drains were properly placed, periodic draining will make sure evaporator coils are operating oil-free.

Carbon dioxide oil maintains about the same density and mixes with the refrigerant, so it will eventually over-saturate the system if not managed. It cannot be drained at low-pressure points, so several oil management solutions are available to minimize oil in evaporator coils.

System pressure management

System pressure management is another difference between ammonia and carbon dioxide systems. At an ambient temperature of 80°F, ammonia is only at 140 psig, while carbon dioxide is at 945 psig. Most ammonia vessels are rated to 250-300 psig, with minimal risk of an over-pressure situation occurring under normal conditions. Most carbon dioxide vessels are rated at a maximum of 600 psig to keep material costs down. In order to keep the carbon dioxide pressure below vessel/component pressure ratings, a standby condensing unit with an uninterrupted power source should be considered for when the main refrigeration system is non-operational or during power outages.

As the needs and demands of the frozen food processing industry change, manufacturers need to reassess several key factors in choosing a system, including energy efficiency, the risk of ammonia in food processing areas and the desire to reduce ammonia charge.