The shift to natural cooling systems
Big changes are in store as many synthetic refrigerants still widely in use could be phased out next year by the EPA.
Global warming is heavily debated in some circles, but governing bodies worldwide have declared it a reality, regardless of personal belief. For the past 30 years, regulatory agencies around the world have been working to reduce greenhouse gas emissions, beginning with the 1987 Montreal Protocol, which was the first globally supported legislation to reduce ozone-depleting substances. In 1992, the Kyoto Protocol expanded this reduction agreement. And, in April 2015, the Environmental Protection Agency proposed new regulations under the Significant New Alternatives Policy (SNAP) that move up the phase-out date for many widely used synthetic refrigerants to 2016.
“The logic behind these changes is that commercially viable alternatives are becoming more readily available in the marketplace,” explains Todd Allsup, vice president of Stellar. “As a result, the government is taking a more proactive role in pushing out these regulations that are having a direct impact on the refrigeration industry.” Moreover, the production and import of synthetic refrigerants like R-22 could be banned in the US as early as 2020, putting facilities that use these systems in an even more precarious position.
- Advantages of natural refrigerants
- Challenges and innovations using natural refrigerants
- Deciding on a solution
- Energy efficiency
“As the phase-out of once-popular refrigerants begins to take hold, and inventories of stockpiled refrigerant become depleted, many of our clients will be faced with the prospect of replacing their centralized refrigeration systems,” says Tom Aurich, refrigeration group manager for Shambaugh & Son.
However, at the same time this legislation is pushing the industry toward using natural refrigerants, such as ammonia and carbon dioxide, government regulations on ammonia systems have been ratcheted up. For instance, the Occupational Safety and Health Administration (OSHA) has overhauled and tightened its chemical safety regulations on facilities with over 10,000 pounds of ammonia. Additionally, the Department of Homeland Security’s Chemical Facility Anti-Terrorism Standards program requires more oversight of plants that have large-charge ammonia systems.
Fortunately, a number of innovative technologies in natural refrigerants are becoming available to food and beverage processors looking to switch from synthetics. In addition, freezing and cooling equipment for food processing is becoming more energy efficient.
Natural refrigerants include ammonia, carbon dioxide and hydrocarbons, such as propane, butane and propylene. These are not new to the industry; in fact, carbon dioxide is one of the oldest refrigerants, but was dropped when synthetic refrigerants were introduced and seen as more cost effective. Decades ago, the food industry adopted the use of anhydrous ammonia as the standard refrigerant in large-capacity, centralized refrigeration systems, says Aurich. This is because it “is extremely efficient, plentiful, inexpensive and naturally occurring.”
“Ammonia has been the workhorse refrigerant for the food and beverage industry for the last 75 years,” adds Charles Taylor, president of CRT Design and Engineering.
The biggest advantage of natural refrigerants, Taylor explains, is that they do not harm the environment and have very low global warming and ozone-depletion potentials (values representing how much a given mass of a chemical contributes to global warming and negatively affects the ozone layer). On the other hand, all synthetics have the potential to damage the environment. For instance, R-134 has a global warming potential (GWP) of 1,300, and R-22 ranks high on its potential to deplete the ozone.
“A synthetic refrigerant with a high GWP may be acceptable today, but it may be banned tomorrow as new technology develops,” says Taylor. “Because natural refrigerants do not cause global warming or harm the ozone [level], significant innovation is being developed to figure out how to use natural refrigerants in markets that have historically been serviced by synthetic refrigerants.”
Other benefits of ammonia are its superior thermodynamic properties and energy efficiency. It also has more refrigeration effect per pound than any other refrigerant, notes Taylor. This means it takes less pounds of ammonia to get the same level of performance, while using smaller pipes, less horsepower and smaller compressors.
Even though ammonia is one of the most cost-efficient refrigerants, it is highly toxic and poses flammability dangers. Because of these factors, a number of government agencies, including OSHA and EPA, regulate it. Traditionally, ammonia systems have been set up with the refrigeration equipment housed in a central location that contains the compressors, vessels and condensers. The evaporators have been located throughout the plant with a network of pipes connecting them to the central machine room. This system creates not only more leakage potential in the plant, it also requires a high refrigerant charge.
“It is common to have refrigerant charges of 40 to 50 pounds of ammonia per ton of refrigeration [TR],” says Taylor. “So a system with a 1,000 TR load could have a charge in excess of 40,000 pounds of anhydrous ammonia.”
With a rooftop low-charge ammonia packaged system, the evaporators are located in a penthouse attached to a small machine room containing the compressor, vessels and condenser. The concept is similar to an industrial version of a conventional rooftop AC unit and eliminates the network of piping. “With this kind of system, the refrigerant charge is three to six pounds of ammonia per TR,” explains Taylor. “So for a 1,000 TR system, the refrigerant charge would be less than 6,000 pounds of ammonia.”
As for chillers, R-22 used to be the go-to solution for large HVAC plants. Numerous chillers use synthetic refrigerants, but because of the Montreal Protocol, the phase-out of these substances is underway. Now, many companies are developing packaged ammonia chillers for cooling either glycol or water to be piped to the loads in the plant.
“These chillers still require a machine room, but utilize critical charge, and some are less than three pounds of ammonia per TR,” says Taylor. He adds that recently a UK company brought to the US market a totally self-contained, packaged air-cooled chiller that utilizes critical charge ammonia and rotary screw compressors.
One application where carbon dioxide is seeing a rebirth is in the low stage of a two-stage system. If a central station refrigeration system is the only option—as it is for many large-scale food processing plants—a carbon dioxide/ammonia cascade system can be a viable solution.
“With this configuration, there is still a central machine room, but CO2, not ammonia, is piped throughout the plant. The ammonia is completely contained in the machine room,” explains Taylor, which minimizes the risk of exposure to employees. Additionally, using this dual system takes advantage of the natural refrigerant’s energy-efficiency benefit but lowers the ammonia charge.
“Interest in CO2 and CO2/ammonia cascade systems is increasing in the US,” says Shambaugh & Son’s Aurich. “Although more common in Europe, these systems offer high efficiency and are environmentally friendly.”
Carbon dioxide also can be pumped to the air handling unit instead of using chilled water or glycol to handle a load. When water is used, the fluid is cooled and pumped to the air handling unit where it absorbs heat at a rate of one BTU/pounds per every degree the water warms up.
“If we pump CO2 to the air handling unit, as it absorbs heat, it changes phase from a liquid to a vapor at a constant temperature—latent heat,” says Taylor. “The latent heat of CO2 is at 97 BTU/pounds—basically 100 times more effective.” Consequently, less has to be pumped, allowing the use of smaller pipes and evaporators. Plus, the system uses less horsepower.
To determine what type of natural refrigeration solution is better suited for an application, a number of factors should be taken into consideration. “These include the size of the system and resulting refrigerant charge, operating temperature and pressures, and materials of construction of the plant design,” says Aurich. More manufacturers are offering this type of equipment to contractors and end-users concerned about the availability of equipment using alternative refrigerants, according to Aurich.
Since natural refrigerant systems are more expensive to install than Freon-based systems, the required upfront capital could be a barrier for many plants. “But their efficiency can make a strong case for overcoming the higher initial costs,” says Evis Buli, sales engineer for Berg Chilling Systems.
Because of ammonia’s flammable and toxic nature, systems that use it require new mechanical rooms that must be built to strict specifications. The systems also must be properly maintained.
“Ammonia and CO2 systems use specialized, heavy-duty equipment, valves and instruments,” Buli says. These can increase the upfront investment and eliminate natural refrigerant usage as an option.
Other factors to look into when considering an ammonia system are the local regulations and proximity to residential areas, says Brendon Somerfield, product line manager at JBT FoodTech. Some states have extra restrictions on ammonia used in plants, which are evaluated more carefully for code compliance.
When designing a refrigeration system, manufacturers have a number of ways to improve energy efficiency. One is the use of energy-efficient lighting, which recently has gained in popularity. Some manufacturers were initially scared by LEDs’ higher prices compared to fluorescent bulbs, but the gap has been decreasing. Moreover, because LED bulbs last longer, they have become more attractive due to their lower maintenance costs.
Berg’s Buli suggests plant layout and design can play a crucial role in maximizing the energy used in refrigeration and freezing. First of all, proper air movement should be built into the plant to ensure heat transfer is maximized and to prevent condensation and frost in refrigerated areas. Also, smart placement of equipment, such as freezer doors not opening into warm areas, can lead to big reductions in energy usage.
Heat recovery systems also decrease energy usage, especially in large refrigeration operations. These systems capture heat energy from the discharge of compressors, which traditional systems normally waste by releasing it into the ambient environment, explains Aurich. However, refrigerant-to-water heat exchangers recover this energy and allow it to be used in a number of ways, such as preheating boiler feed water and water for washdowns and/or rinsing equipment.
“The main impediment for these systems is the large volume of warm water storage required to take advantage of them,” he says. “Plus, the storage tank and associated infrastructure can be very expensive.”
Buli advises plants located in cool climates to take advantage of free cooling during colder months by using hybrid chilling equipment. These chillers provide industrial refrigeration and mechanical cooling during hot weather, but automatically use 100 percent free cooling during cold seasons.
“Hybrid chillers maximize system performance while keeping electrical and operating costs low,” says Buli. “In a standard industrial water-cooled cooling system, chilled water loops cooled by mechanical refrigeration operate continuously. Using free cooling, refrigeration compressors can be turned off during cooler months, enabling the outside air to provide system cooling.”
However, if the environmental temperatures start to rise, the compressor reactivates to provide the necessary cooling. Moreover, if the system is designed with all the necessary monitoring and controls, the operator doesn’t need to manage the free or mechanical cooling.