The refrigeration industry, as well as consumers, in South Africa are becoming increasingly more aware of the effect of refrigerants on the environment. Due to this, regulations which are phasing out specific refrigerants are being implemented. CO2 is termed as a “natural refrigerant” as it exists in the natural environment and as a result, carbon dioxide systems are gaining more interest due to their low global warming and ozone depletion potential.
When did the CO2 movement start?
The first successful mechanical refrigeration system using carbon dioxide as a refrigerant was built in 1866 and deemed a “safer” refrigerant when compared to ammonia and sulfur dioxide which was being used at that time. It quickly gained popularity in the 1930’s only to be replaced by a new refrigerant called “Freon”. Due to its lower operating pressures, Freon was considered safer to operate while having the added benefit of cheaper and less complex components when compared to CO2. With the advent of global warming and the impact of refrigerants on the ozone layer, Freon refrigerant has evolved to decrease their negative impact on the earth. The latest Freon refrigerants have virtually no Ozone Depletion Potential (ODP), however, their Global Warming Potential (GWP) is still significant. For this reason, natural refrigerants like Ammonia and CO2 are again gaining popularity.
Understanding CO2 as a refrigerant:
Due to characteristic differences in CO2 when compared to Freon refrigerant, the CO2 refrigeration system has a high operating pressure. This means that the system operates close to two important points during the refrigeration cycle, namely the triple point and the critical point:
- The triple point – a condition where the solid, liquid and gas phases co-exist.
- The critical point – the condition where the liquid and gas densities are the same and above this point distinct liquid and gas phases do not exist. In this region, the substance has some properties of the gas phase and some of the liquid.
In the case of CO2, these two points are of significant importance as they affect the refrigeration cycle immensely. For instance, at the triple point of CO2 (-56.6°C / 5.11 bar) it is right in the middle of the operating pressure and temperature ranges for a typical refrigeration cycle. It is also important to note that liquid CO2 cannot exist at atmospheric pressure. Therefore, should there be a sudden depressurisation of the system to atmospheric pressure, the CO2 will either form a gas or a solid depending on the temperature. The critical point for CO2 (31.1°C / 73 atm) is also in the typical range of operation for a refrigeration system and therefore care has to be taken in the design of the system to account for moving between the subcritical (condensing temperature below 31.1°C) transcritical or supercritical (condensing temperature above 31.1°C) regions during normal system operation.
In contrast, all Freon refrigeration systems operate sub critically as their condensing temperatures never exceed the high critical temperature for HFCs.
Dangers of CO2 as a refrigerant:
Carbon dioxide is non-flammable and odourless, it is heavier than air and could cause asphyxiation at certain concentration levels and therefore its vital to install leak detectors which activates an alarm in the event of a leak.
Due to the high operating pressures of a CO2 refrigeration system, care must be taken to avoid any potential catastrophic failures in equipment or piping as these may cause serious personal injury or damage to property.
Benefits of CO2 as a refrigerant:
· High Volumetric capacity—reduced compressor and pipe size
· CO2 Cost and Availability - currently CO2 cost is approximately 90% less than traditional refrigerants
· Reduced Carbon Footprint
· Due to the higher pressures required for a CO2 system, the refrigerant piping is drastically smaller than the same Freon refrigeration system. It is also recommended to make use of K65 copper piping as it was developed for applications with high pressure refrigerants.
CO2 Booster Systems:
Booster systems are systems with two temperature levels namely medium and low temperature. This is due to the fluctuation in ambient temperature. When the ambient temperature is below the critical temperature, the system operates as a subcritical system, however, when the ambient rises to above critical temperature, the system operates as a trans critical system by using the condenser as a gas cooler.
The booster system incorporates both medium and low temperature capabilities in a single unit, as the compressors selected can be used for a wide range of refrigeration loads. It saves space and allows the system to run optimally and can accommodate all the design requirements in one system.
What makes booster systems different:
· There is a special condenser that works as a gas cooler under higher ambient conditions (transcritical operation)
· The compressors for both medium and low temperature applications need to be selected carefully in order to ensure optimal capacity control during partial load conditions.
· The same refrigerant is moving between the medium and low temperature compressors. The low temperature compressors discharge into the suction line of the medium temperature compressors, the low temp compressors act as a booster to the medium temperature compressors.
· The condenser design is optimized for the best performance for both subcritical and transcritical stages.
· You can obtain hot water and cold water recovery from the same system and the during the transcritical phase the discharge temperatures are higher which provides more potential for heat reclaim.
Meat World Facility in Springs:
In 2017 the experienced Cool Care team received the plans for a new Meat World production facility that was going to be erected in the Springs area for which they needed to supply energy efficient and state of the art refrigeration equipment. Cool Care contacted Metraclark to brainstorm ideas to supply technologically advanced equipment that would save their client energy and deliver the refrigeration specifications initially set out for this massive facility. The Metraclark engineering team worked off the floor plans supplied along with all the daily meat production figures and calculated all the heat load requirements for each room.
The heat load for the system consisted of amongst others between 80 and 160 tons of fresh meat as well as 1 000 tons of frozen meat passing through the facility daily. This all added up to a combined heat load of 840kW for both the medium and low temperature applications. Additionally, the client required that the system could supply 10 000 litres of hot water per hour as well as 2 000 litres of chilled water for various factory and processing functions. Considering the requirement that the system be state-of-the-art and energy efficient, a standard Freon refrigeration system was foregone in lieu of a CO2 system incorporating a trans-critical booster design. Due to the high ambient temperatures often experienced in these parts of South Africa, it was vital that this system could operate trans critically as well as sub critically.
Cool Care and the knowledgeable Metraclark engineering team worked very closely together and proposed the idea of going “green” with natural refrigerants to the owner of the new facility. Metraclark along with CO2 system specialists from SCM Frigo Italy explained in detail how the CO2 transcritical booster system offered works along with all its many advantages to the client and he was sold on the idea based upon the facts that were presented. The final equipment offered and installed for the CO2 system were as follows:
§ Two outdoor multiple compressor rack plantrooms each handling half of the overall load to keep the system balanced. The following are specifications for each plantroom:
o Racks capable of delivering 431kW each to serve both medium and low temperature applications.
o Each rack consists of 8 compressors all equipped with service valves on the suction, discharge and oil side to allow for isolation if required, as well as pressure relief valves.
§ 4 Bitzer compressors for the medium temperature users at a suction temperature of -5°C and the lead compressor fitted with a Danfoss Variable Speed Drive (231 kW)
§ 4 Bitzer compressors for the low temperature users at a suction temperature of -25°C and the lead compressor fitted with a Danfoss Variable Speed Drive. (200 kW)
o 2 Bitzer Parallel Compression compressors with the lead compressor fitted with a Danfoss Variable Speed Drive and is installed to directly compress the flash gas present in the liquid receiver. These parallel compression compressors increase the overall efficiency of the system. (31 kW)
o 1 Danfoss Vapour Multi Ejector which adapts the capacity requirements and handles 3 different suction groups in one device.
o Danfoss CO2 Rack Controller for the entire system as well as a dedicated rack controller for the parallel compressors.
o Oil Management system with electronic oil level regulators installed on all the compressors.
o Danfoss High Pressure valve which regulates the pressure in the gas cooler to maintain the optimum COP for the system during transcritical operation as well as a degree of subcooling during subcritical operation.
o Danfoss Medium Pressure valve which maintains a constant pressure in the liquid receiver.
o Plate Heat Exchanger mounted on the common discharge line for the supply of 5 000 litres per hour of hot water at 60°C to the facility.
o A cold-water supply of 2000 litres per hour at 7°C.
o Gas Detection system in the plant rooms with audible and visible alarms.
§ Two Gas Cooler installed with EC fans that each deliver a capacity of 684 kW.
§ Various Evaporators for each room in the facility making use of hot gas defrost where applicable.
o Only one third of the low temperature load on each rack can be defrosted at any given time to ensure there is enough heat generated.
o Electronic Expansion Valves were installed on all the evaporator coils.
o Gas Detectors are also installed near the bottom of each room with audible and visual alarms.
o Valve stations are installed at each evaporator coil in order to facilitate maintenance with an additional bypass valve to ensure any liquid that gets trapped can boil off again.
Cool Care had the continuous technical support of the experts from both SCM Frigo in Italy and Metraclark in South Africa throughout the entire process to ensure a successful installation and functioning plant. Many other suppliers of components were also present on site to provide additional assistance and support where required. Energy savings calculation software was used to perform a comparison between a conventional Freon R404A system and the new CO2 system proposed and theoretically showed an average savings on the CO2 system of 13%. This project was a collaborative effort and boasts engineering excellence with all the latest technologies in CO2 available on the market and certainly be a benchmark for the refrigeration industry.