NH3/CO2 cascade refrigeration system
Muhammad Shabbir Akhtar
Technical Sales & Business Development of Hvac, Equipment, CCHP,
Food storage is a complex issue that involves many biological factors that affect the quality and safety of food products before and after applying preservation methods. Moreover, the global challenge of feeding a growing population (currently around 7 billion people) cannot be met without increasing food production and improving logistics, as well as reducing or eliminating the losses that occur during transportation and processing. These losses can be significant and have negative impacts on food security and sustainability.
NH3/CO2 cascade refrigeration
As the demand for ecofriendly green refrigeration systems grows, need to adopt a proactive and innovative approach to reduce the cost and energy consumption of our cooling solutions.
One way to achieve this is to switch from the traditional two-stage compound ammonia refrigeration system to the hyper cascade system, which offers higher efficiency, lower environmental impact and better reliability. The hyper cascade system is a breakthrough technology that can meet the challenges of the modern refrigeration industry.
This system, which is popular in North America and widely accepted in EU countries, is something we should all monitor and adopt. It is regarded as the natural refrigerant system for large scale refrigeration projects and process refrigeration applications in the industrial refrigeration community. For storage temp requirements of around -4F (-20C), which include many chilling/freezing rooms and blast freezing applications, cascade refrigeration systems are recommended.
Natural Refrigerants (R744 & R717)
Unlike other refrigerants that are commonly used in the refrigeration industry, carbon dioxide (CO2) is non-corrosive but has a very low critical temperature of 31.05 C (73.8 ba). This means that it cannot reject heat to the ambient atmosphere by condensation when the temperature is higher than the critical temperature, as it happens in the conventional vapor compression cycle.
CO2 can only work in this cycle when the heat rejection temperature is below the critical temperature. When the heat rejection occurs at supercritical pressure, the refrigerant can only be cooled as a gas without changing phase. This cycle is called "Transcritical cycle". CO2 was actually used as a refrigerant in the mid-19th Century, but its popularity declined when CFC refrigerants entered the market in the 1920s.
Key advantages of CO2:
Key advantage of NH3:
CO2 Cascade vs. 2-Stage (traditional) NH3 Compound System
A CO2/NH3 cascade refrigerating system consists of two stages: a low temperature stage that uses CO2 as the refrigerant, and a high temperature stage that uses ammonia as the refrigerant. The CO2 stage cools the refrigerated space, while the ammonia stage rejects the heat from the CO2 stage to the ambient. The two stages are coupled by a heat exchanger, where the CO2 condenses and the ammonia evaporates. This design reduces the ammonia charge and confines it to the plant room.
The first CO2/NH3 cascade refrigerating system was developed by Nestle’, a food company that supports the use of natural refrigerants, in 2001. It was the largest system of its kind at that time, until it was surpassed by Yosemite Meat Co in 2017.
The case study required a system that could operate efficiently and reliably. Some of the key features that were implemented were electronic expansion valves for each evaporator, distributed controls that gave plant operators a clear view of the system status (refrigeration, defrost, alarm, cleaning etc.) and distributed smart controls that allowed for zone-based control strategies for temperature monitoring, variable speed evaporator fan controls, leak detection, hot gas defrost and refrigeration control, as well as electronic valve control. Remote defrost panels were placed near the evaporator loads, reducing the wiring needs for high and low voltage.
The CO2/NH3 cascade system had a low temperature CO2 compressor that worked at -45 ‘C suction and -5 ‘C saturated condensing temperature, and a high temperature ammonia compressor that worked at -10 ‘C evaporation and +4 ‘C saturated condensing temperature.
The 2 stage traditional compound NH3 refrigeration system had a low stage compressor that worked at -45 ‘C SST and -10 ‘C saturated condensing temperature, and a high stage compressor that worked at -10 ’C evaporating temperature and +35 ‘C saturated condensing temperature.
Reasons to Consider a CO2/NH3 Cascade Refrigeration System
Food processors and distributors are under constant pressure to produce more while spending less on operations. For plant owners looking for greener and more efficient secondary refrigerants, a carbon dioxide (CO2)/ammonia (NH3) cascade system is a viable option. In addition to providing 0 ozone depletion potential (ODP) and 1 global warming potential (GWP), CO2/NH3 cascade systems offer several benefits for food processing and low-temp distribution facilities including:
1.?Lower operating costs
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One of the main advantages of a CO2/NH3 cascade system is its lower operating costs. Unlike other refrigeration systems, which are most efficient when they run at full load, a CO2/NH3 cascade system can maintain high energy efficiency even at partial load conditions. This is especially true when the system operates at very low evaporating temperatures, ranging from -35oF to -60oF. Therefore, a CO2/NH3 cascade system can save more energy and money for processing and cold storage facilities that do not need to run their refrigeration systems at full load all the time.
2.?Lower capital costs
CO2 is a better refrigerant than ammonia for low-temperature applications because of its distinctive thermodynamic properties. It allows for smaller and simpler system design, reducing the size of pipes and pumps, the amount of insulation and the installation work required, compared to two-stage ammonia systems.
3.?Ammonia charge reduction
A CO2/NH3 cascade system is a way of reducing the amount of ammonia used in low-temperature freezing systems. Ammonia is only used in the machine room, where it cools down CO2. The CO2 then circulates to the processing and/or storage areas, where it provides refrigeration. This way, ammonia is not exposed to the food or the workers. CO2 can also be used as a secondary refrigerant for higher temperature (above freezing) storage and process loads. Instead of using propylene glycol, which is chilled by ammonia or another refrigerant, CO2 brine can be pumped to these loads and offer better efficiency and lower cost. CO2 brine can be part of the same cascade system as the low-temperature freezing system, or a separate system.
4.??Reduced compliance costs
A CO2/NH3 cascade system is a refrigeration system that uses carbon dioxide and ammonia as refrigerants. One of the benefits of this system is that it can reduce the amount of ammonia needed, which lowers the risk of accidents and the regulatory requirements. For example, if the ammonia charge is less than 10,000 pounds, the system does not have to comply with the Process Safety Management (PSM) and Risk Management Plan (RMP) standards, which can save money and time for the owners.
5.??Constant positive pressure
As positive pressure is maintained in CO2/NH3 cascade systems, which eliminates the risk of non-condensable accumulation. The refrigeration piping is also protected from air and moisture infiltration through vacuum leaks.
6.?Quality and throughput improvements
CO2/NH3 cascade systems are a cost-effective and energy-efficient solution for frozen foods processors who want to achieve faster freezing and higher production yields. By lowering the process freezing temperatures, these systems preserve the food quality and reduce the freezing time. Compared to conventional refrigeration systems, CO2/NH3 cascade systems require less capital and operating expenses to reach lower temperatures.
Why Not to Choose.
CO2/NH3 cascade refrigerating systems have some drawbacks when compared to conventional 2-stage ammonia systems, such as:
1) CO2 has a very high saturated pressure (over 75 bar) when the liquid refrigerant is exposed to ambient temperature (around 40 ‘C). This means that all the components in the low temperature circuit must withstand such high pressure, which is not economical.
2) For liquid on overfeed refrigeration systems, the CO2 liquid pump needs 2.5 to 3.5 times more capacity than the NH3 pump for similar operating conditions. This results in larger liquid line sizes for the pump suction and discharge than ammonia.
3) The CO2 pressure vessel and heat exchanger have higher design pressure than the booster ammonia pressure vessel and heat exchangers.
4) The CO2 refrigerant cannot be charged after the plant is evacuated and under vacuum. This is because dry ice will form inside the charging port if the system pressure is below -5.5 bar. Therefore, it is necessary to raise the pressure level above 5.5 bar before charging liquid CO2.
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To summarize, CO2/NH3 cascade systems have some drawbacks, but they still offer more advantages than the conventional 2 stage compound ammonia systems. They are more efficient, reliable and environmentally friendly. However, they also pose significant safety risks due to the large amount of ammonia involved, which can be toxic and explosive under certain conditions. Therefore, it is essential to follow the safety guidelines of OSHA’s PSM & EPA’s RMP regulations strictly when operating these systems.