Farewell Freon…or maybe not. While conventional Freon refrigerants are being phased out due to environmental concerns, replacement technologies collectively known as the “new Freons” can play a valuable role in modern freezer and refrigeration applications.
There are horses for courses” advises an old saying, which basically means that “one size doesn’t necessarily fit all.” This principle applies to a range of situations, including the refrigerants used in industrial cooling applications. The food and beverage business has been dominated by ammonia as a refrigerant. However, Freon refrigeration has been an integral part of the industry since the early 20th century. While the last “true” Freon, R-22, will be phased out of the new equipment market next year, the replacement refrigerants approved by the U.S. Environmental Protection Agency (EPA) are recognized universally as Freons and still have a valuable role to play in the modern refrigeration industry.
So just what is the difference between ammonia and Freon? Ammonia, a naturally occurring substance, is one of the oldest surviving forms of refrigeration, according to Ken Vantine of FES Southwest, Weatherford, Texas, a sales division of the refrigeration equipment manufacturer FES Systems Inc., York, Pa.
Figure 1. The overall BHP per ton on an ammonia system at a -20°F (-29°C) suction temperature is approximately 1.86, while the equivalent R-22-based system has a BHP per ton of 1.96. At a -40°F (-40°C) suction, the two systems are virtually identical at 2.41 BHP per ton for the R-22 system and 2.44 BHP per ton for the ammonia system.
“The original refrigerants were ammonia and sulfur dioxide. Ammonia was the biggest industrial refrigerant, while sulfur dioxide was used more in home refrigerators and small appliances,” Vantine said, adding that the use of sulfur dioxide was eventually discontinued because of its toxicity. Ammonia also is toxic, but it is controllable, and its distinct odor makes it “self-alarming” if any leaks occur.
Freon is a man-made refrigerant that was developed as an alternative to both sulfur dioxide and ammonia. While Freon is less efficient than ammonia on a BTU per pound basis, Freon refrigerants are not nearly as inefficient as assumed. For example, consider a 200-ton freezer application at a -20°F (-29°C) suction temperature and a 95°F (35°C) condensing temperature with a two-stage intercooled system using open-drive screw compressors. The overall brake horsepower (BHP) per ton (a generally regarded level of a refrigeration system’s efficiency) on an ammonia system with these characteristics is approximately 1.86, while the equivalent R-22-based system has a BHP per ton of 1.96. At a -40°F (-40°C) suction, the two systems are virtually identical at 2.41 BHP per ton for the R-22 system and 2.44 BHP per ton for the ammonia system (figure 1). The anomaly exists because R-22 is still at a positive pressure at the lower temperatures, while the ammonia system is in a vacuum below -28°F (-33°C).
The main difference in the perception of Freon vs. ammonia systems has as much to do with the systems as the refrigerants themselves. Ammonia systems traditionally have been applied with open-drive compressors and evaporative condensers, which operate near the wet bulb temperature. Although some Freon systems use open-drive compressors and evaporative condensers, most Freon systems use hermetic or semihermetic compressors and air-cooled condensing. The open-drive system motor rejects the motor heat to the atmosphere, while the hermetic or semihermetic compressor rejects the majority of the motor heat to the refrigeration system. The evaporative condenser operating near the wet bulb will have a condensing pressure lower than an air-cooled unit that relies on the dry bulb temperature. When these inefficiencies are added to the system, the BHP per ton difference is much larger than just the difference in refrigerant properties.
If evaporative-cooled ammonia systems are inherently more efficient than air-cooled Freon systems, where can Freon systems play a successful role? One example would be a local distribution refrigerated storage facility with an existing large -10°F (-23°C) freezer that has a need for a small (8 to 10 ton) ice cream freezer at -20°F (-29°C). Adding a separate suction level with the attendant compressor and vessels would be costly. The alternative of lowering the overall suction level 8 to 10°F (4 to 6°C) just to service a small load also would be prohibitively expensive from an energy standpoint. This application would be a good place to integrate a small, stand-alone, air-cooled Freon system.
Ammonia systems also carry other considerations. They require more regulation than Freon systems, and they require specially trained operators. Plants that have more than 10,000 lb of ammonia on site must have a process safety management (PSM) program. Additionally, some states severely restrict the application of ammonia systems. Government regulations with regard to PSM programs and Homeland Security issues also might affect a plant’s decision. Furthermore, if the plant is located in a crowded nonindustrial neighborhood, the owner might not be comfortable with the potential liability of an ammonia leak.
Freon can provide an alternative in both air-cooled and evaporative condenser systems. Small- to medium-sized Freon systems typically have a first-cost advantage. Additionally, split Freon systems - in which a single air-cooled condensing unit serves one or two evaporators with electric defrost on lower-temperature applications (see sidebar) - might provide a refrigeration bridge to later expansion and possible conversion to ammonia.
In some food-handling applications such as the storage of nuts, Freon may ultimately be the best choice.
“There is a trend to lower the storage temperature of nuts to as low as 35°F (1.7°C) to extend the storage life. Even a small ammonia leak can turn the nuts black, rendering them unusable,” Vantine said. “At higher temperatures, we were able to run a glycol chiller off of the ammonia system and use brine coils in the nut storage area. The lower temperature levels complicate the brine system as we struggle to adequately defrost these areas. The stand-alone Freon systems can easily accomplish this task with electrically defrosted coils.”
Another important note is the extent to which each refrigerant contributes to global warming. Ammonia, a combination of nitrogen and hydrogen, is a natural substance and is not considered a threat to the atmosphere. If ammonia breaks down, it converts back to nitrogen and hydrogen. While older Freons are considered to have a significant global warming potential (GWP), the newer refrigerants that eventually will phase out the R-22 Freon are relatively harmless to the atmosphere.
Narrowing the Gap
The low cost of ammonia refrigerant and the efficiency of ammonia refrigeration will keep these systems favorable from an energy standpoint for the foreseeable future. However, new and emerging Freon technologies might narrow the gap. For example, the introduction of semihermetic screw compressors and evaporative condensers in the water chiller business portends a day when a complete Freon skid with multiple semihermetic screw compressors and evaporative condensers in a “refrigeration house” might be set on a roof to serve a complete facility. Such a design might lower the refrigeration system’s energy footprint, provide hot gas coil defrost and use electronic expansion valves to take full advantage the lower head pressures available with the evaporative condensing. Packaging the systems together would lower field labor and provide more of a single-sourced solution while reducing mechanical room requirements.
While ammonia will always have a preeminent place in the industrial refrigeration market, “one size” never fits all. There are times and places where Freon systems make sense and might just be the right horse for the course.
Split Freon Systems
Traditionally, the majority of Freon refrigeration units have been split systems. These units usually are close-coupled, with the condensing unit mounted on the roof as close as possible to the evaporators to reduce the piping and refrigerant costs.
Because they are direct-expansion systems, it is important to control head pressure to provide the pressure differential required by the expansion valve. The head pressure control is accomplished by either condenser fan cycling or condenser flooding. On larger jobs, a number of condensing units might be spread across the roof. Some applications might have a “rack” system with multiple reciprocating compressors on several suction levels that are sequenced to balance the load. These racks are primarily water-cooled or remotely air-cooled, but some have been evaporative condenser-cooled as well.