CO2 a Refrigerant of the Future

Global Warming and the Depletion of the Ozone Layer Potential has led the world of refrigerants for HVAC&R systems to take a new direction through the so-called "ecological refrigerants", which despite being presented as the novelty, have their history, since they have previously been tested for this type of facilities.

The application of carbon dioxide in refrigeration systems is not new. The first to propose carbon dioxide as a refrigerant was Alexander Twining who mentioned it in a British patent in 1850. Thaddeus S.C. Lowe experimented with_CO2 in balloons for military use but also designed an ice machine using_CO2 in 1867.

Lowe himself also developed a machine for transporting frozen meat on ships. Continuing with history you can see that_CO2 cooling systems peaked in the 20s and early 30s. CO2 was generally the most preferred option for use on ships rather than ammonia.

With the advent of Freon refrigerants, the application of_CO2 as a refrigerant decreased. The main reason for its fall was certainly the rapid loss of capacity, high pressures and high temperatures.

Ammonia remains the most widely used refrigerant for industrial refrigeration applications over the years.

In the 90s, a renewed vision of the advantages offered by_CO2 due to the potential depletion of the ozone layer and global overheating that restricts the use of chlorofluorocarbon and hydrofluorocarbon products was initiated, as well as restrictions on high refrigerant loads in ammonia systems.

Carbon dioxide belongs to the group of so-called natural refrigerants along with ammonia and hydrocarbons such as propane, methane and water. All these refrigerants have their respective disadvantages:

• Ammonia is toxic,

• Hydrocarbons are flammable,

• Water has limited application possibilities compared to_CO2.

Carbon dioxide, on the other hand, is not toxic or flammable; but it has a dual function in the environment; CO2 is needed by all living organisms on earth but it is also a greenhouse gas, which can cause changes in the environment if concentrations in the atmosphere change.

Thethermodynamic properties of_CO2 closely resemble other common refrigerants.

Characteristics of CO₂

The thermodynamic properties of_CO2 closely resemble other common refrigerants, but there are some exceptions: the triple point of_CO2 is much higher than in other common refrigerants (see Table 1).

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In the enthalpy pressure diagram the triple point is on the line of a pressure of 5.18 bar and the temperature of -56.6° C. At the triple point there is an equilibrium of_CO2 vapor, liquid and solid. See Figure 1.

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The critical pressure of_CO2 is 76.6 bar and the critical temperature is 31°C. In this supercritical phase_CO2 has properties that are very similar to a high-density vapor.

Note:_CO2 is a unique substance used for different purposes in different industries, for example: increasing its pressure,_CO2 becomes a solvent that is used in the extraction of one substance to another and for cleaning processes. Solid_CO2 can be used as a substitute for sand in the sandblasting process and can also be used in fire extinguishing systems.

High saturated pressure at room temperature is the first barrier that needs to be considered when_CO2 is proposed as a refrigerant. At the temperature of 20° C, the saturated pressure is 57.2 bar. The design of cooling systems using_CO2 depends a lot on the application.

_CO2 in industrial refrigeration systems

There are many ways to use_CO2. In a first stage in a subcritical system it is simple but has its disadvantages due to the limitation in temperature and high pressure.

Supercritical systems are only applicable for small systems, where pressure is not an important feature in the design. A number of research programs have been developed for the automotive industry taking application in air conditioning but also for domestic air conditioning (Japan)

Hybrid_{CO2 systems} are the most common in industrial refrigeration because pressure can be limited to a level where requirements for components such as compressors, valves and controls differ only slightly compared to traditional industrial refrigeration plants.

Refrigeration systems can be designed in different ways, for example: direct expansion systems, recirculated pumping systems or in brine systems or combinations of it.

Co₂ NH 3 cascade_systems

The_CO2 system is a circulating pumping system where_CO2 is pumped from the receiving tank where it is partially evaporated before it returns to the receiving tank. The evaporated_CO2 is compressed in a_CO2 compressor and condensed in a_CO2-Ammonia exchanger. The heat exchanger acts as an evaporator in an ammonia system.

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DESIGN PRESSURES

There are two important factors to consider when determining design pressure:

1. The pressure during the shutdown remains constant. The pressure during unemployment can be very high and this brings to consideration:

a) A small cooling system can be used alternately to maintain the temperature of the liquid where the saturated pressure is lower than the design pressure.

b) Design the system with an expansion vessel of a certain size that prevents pressure that could exceed the design pressure.

c) Design the plant so that it can withstand saturated pressure at the design temperature (approx. 80 bar).

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Danfoss' experience indicates that the most common solution for industrial refrigeration is to use a small separate cooling system to cool the liquid_CO2 .

2. The defrosting pressure using hot_CO2 gas: Depending on the current design, different ways of thawing can be applied (natural, water, electric or hot_CO2 gas).

The hot gas of CO₂ is the most efficient, especially at low temperatures but in addition to this, it is the one that demands the highest pressure. With a design pressure of 50 bar, it is possible to reach the thawing pressure at 9-10° C. The saturated pressure at 9° C is 43.9 bar; plus 10% of the safety valves and about 20% by pressure drops, a saturated pressure of 50 bar is required. There is no common method to achieve thawing. All the methods mentioned above are used depending on the system but also depend on the availability of compressors and other required components.

Efficiency

In_CO2-Ammonia cascade systems, it is necessary to use a heat exchanger. Introducing exchangers creates a loss of efficiency in the system due to the need to have a temperature difference between fluids. However, compressors that use_CO2 have higher efficiency and better heat transfer. The total efficiency of the CO 2-Ammonia system in cascade system is no lower when compared to a traditional ammonia system.

Oil in_CO2 systems

In_CO2 brine systems and in recirculated pumping systems with oil-free compressors, there is no oil in the CO₂ system From an efficiency point of view it is an optimal solution due to the heat transfer coefficients in the evaporators. However, it is necessary that all valves, controls and other components can operate dry.

Component size

Due to the thermodynamic properties of_CO2 and in particular the relatively high pressure level, the capacity of the compressor is significantly higher for_CO2 than for ammonia. The dimensions of steam lines are smaller, but those of liquid lines are larger

Refrigerant loading in CO_2-Ammonia cascade systems

EN378 classifies_CO2 as a non-toxic, non-flammable L1 while ammonia is a toxic L2 refrigerant. Despite that, ammonia has been used as a refrigerant for many years. But over time it has become more restricted, particularly in some European countries. Therefore there is great interest in reducing the use of ammonia. CO2-Ammonia cascade systems are a perfect solution with ammonia limited to a small amount, which can be contained in a special machine room having the necessary safety arrangements and on the other hand the CO2 is distributed to all coolers.

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Material compatibility

CO2 is compatible with almost all common metallic materials unlike ammonia. There are no restrictions from a compatibility point of view using copper or brass. The compatibility of CO2 and polymers is much more complex, because_CO2 is an inert and very stable substance, the reaction with polymers is not so critical. The main concern with_CO2 is the physico-chemical effects such as permeability, dilation and the generation of cavities and internal fractures. These effects are connected to the solubility and diffusivity of_CO2 in current materials.

Tests have shown that_CO2 is different and some modifications have been made to some products. A large amount of_CO2 that can be dissolved in polymers has to be taken into consideration. Some polymers currently used are not compatible with_CO2 and others require different solution methods such as sealing materials. When the pressure is close to the critical pressure and the temperature is high, the impact on the polymers is much more extreme. However, these conditions are not important for industrial refrigeration since the operating temperature and pressure are usually much lower.

How can water penetrate the_CO2 system?

• The pressure in the_CO2 system is always below atmospheric pressure, therefore there is no risk of leakage that could cause water penetration into the system.

• When charging_CO2, there are different specifications of_CO2, some can allow high amounts of water.

• Carbon dioxide is treated as a very safe refrigerant and therefore is not handled with the usual safety procedures. If a system is open, air can penetrate and moisture can condense inside the tubes. If the system is not purged properly, water can be retained.

The acceptable amount of water in_CO2 is much lower than in systems that use other refrigerants. If the water content exceeds the dew point and the temperature is below 0º C, the water will freeze creating risk problems with the equipment, for example by blocking the control valves.

Water can be easily removed by mounting a dryer on the system.

CO2 dryers are very efficient and are normally mounted on the liquid line to avoid any unnecessary pressure drop.

_CO2-Ammonia leaks in cascade systems

The most critical leak in a CO 2-Ammonia system is between the heat exchangers between ammonia and_CO2. The pressure of_CO2 will be higher than that of ammonia, therefore the leakage will occur in the ammonia system, which will be contaminated. Ammonium carbonate is formed immediately upon contact of CO2 with ammonia. This chemical is a corrosive solid substance.

Safety

As mentioned before,_CO2 is classified as a non-toxic refrigerant, but unlike ammonia_CO2 does not have a characteristic odor.

CO2 safety aspects

CO2 replaces air and causes oxygen loss. In the presence of sufficient oxygen,_CO2 has a narcotic effect at high concentrations. In small amounts_CO2 has a stimulating effect on breathing. Due to the acidic characteristics of_CO2, some irritations may appear particularly in the mucous membrane of the nose, throat and eyes.

Symptoms associated with inhaling air with carbon dioxide are increased as follows:

• 0.04% is the concentration of_CO2 in atmospheric air.

• 2% increases the rate of breathing by 50%.

• 3% a 10-minute exposure to this concentration increases the rate of breathing by 100%.

• 5% represents a 300% increase in the rate of breathing, sweating and headache appear after one hour.

• 8% is the short-term exposure limit.

• 8 – 10% headaches after 10 or 12 minutes, dizziness, hearing problems, blood pressure increases, heart rate increases and nausea.

• 10 – 18% after a few minutes, epileptic seizures, muscle cramps and loss of consciousness may appear, in addition to shock. Victims recover quickly with fresh air.

• 18 – 20% symptoms similar to a heart attack.

Carbon dioxide is treated as a very safe refrigerant and therefore is not handled with the usual safety procedures

Conclusion

The availability of components for cooling using_CO2 at low temperatures is wide. Different manufacturers of equipment for traditional refrigerants may distribute some components for_CO2 systems, but the availability of components for high-pressure systems is limited. An important factor in the speed of introducing_CO2 systems will depend heavily on the availability of critical components for high_CO2 pressures.

Within the industrial refrigeration area,_CO2 will not replace ammonia. Industrial_CO2 systems are all hybrid systems that require ammonia in the high temperature system but only with a small amount of ammonia.

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