RACA Journal June 2024 RACA_June 2024_digital | Page 42

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• 0.5 %– 1 %: Characteristic sharp odour noticeable . Breathing increases to approximately four times normal rate and can be very laboured . Symptoms of intoxication become evident , and slight choking may occur . Visual impairment , headache and ringing in the ears . Judgment may be impaired , followed within minutes by loss of consciousness .

• Unconsciousness occurs more rapidly above 1 % level . Prolonged exposure to high concentrations may eventually result in death from asphyxiation .

Perhaps we should at this point note that as CO 2 at ambient pressure is heavier than air , leak detection systems should be placed low , approximately 200mm from the floor .

Gordon , let us have a look at the pressures expected when using CO 2 as a refrigerant . We know that CO 2 operates at higher pressures than typical HCFCs or HFCs , due to the inherent thermodynamic properties of the substances . HFC direct expansion ( DX ) refrigeration systems mechanical safeties and control set points shut the system down around 2 400kPa ( depending on the refrigerant ) discharge pressure . The entire piping system is rated for safe working conditions above this maximum pressure , so no secondary relief devices are necessary .

In addition , if the fluorinated refrigeration system shuts down due to power outage or servicing , the internal pressures do not climb any higher to exceed the maximum system design pressure . In fact , the pressure tends to fall as temperatures drop and equalisation occurs . In the case of CO 2

, the high saturated pressure of CO 2 at summertime ambient conditions exceeds the pressure rating of type K copper piping , along with most standard DX refrigeration valves . This requires the “ high side ” of the CO 2 system to be constructed using higher pressure rated materials and installation practices , at a higher cost .

To reduce overall system installation cost , the ‘ low side ’ portions of a CO 2 system are designed for the lower operating pressures , allowing copper to be used for the low side piping . When the system is operating normally , pressures are maintained below the rated pressure of the system . The CO 2 system pressure becomes a safety concern when liquid becomes trapped in a portion of the system that is not rated for the full pressure at higher temperature .

It is therefore critical not to allow liquid CO 2 to become trapped in the system without means of pressure relief . As the temperature of a saturated mixture rises , pressure will rise until it reaches the saturation pressure in the table above . If the refrigerant pressure exceeds the rating of the piping , valves , or other components of the system , this can lead to leaks and possibly bursting of system components .

Measures must be taken in system design to ensure that pressure cannot build up in any portion of the system . All components , valves , piping , fittings and joining methods are to be verified to ensure pressure ratings above the maximum anticipated system pressures .

In CO 2 systems , pressure relief devices must be appropriately located to allow the system to vent safely in the event of a system shutdown or other event that causes pressures above system ratings . All points within the system must be allowed to vent back to the pressure relief valves without restriction . Check valves are typically utilised to allow portions of the system to vent back to receivers , where pressure relief valves are located . Any portion of the system that cannot vent back to the receiver must have its own pressure relief valve .

Gordon , a particular issue with CO 2 is the formation of dry ice , which is simply CO 2 in solid form . In a CO 2 refrigeration system , there are two common conditions where this may occur .

The first and potentially dangerous location is at a pressure relief valve . When a pressure relief valve is open , the refrigerant is undergoing a rapid drop in pressure from system pressure to atmospheric pressure . If liquid CO 2 is being released , the CO 2 release in a solid and vapour mixture causes the formation of dry ice . Therefore , pressure relief valves should not have any piping installed downstream of the valve . If the pressure drop happens inside the pipe , dry ice will form , blocking flow and preventing pressure from being released .

The second condition where dry ice may form is when charging the system . If the system vacuum is broken with liquid , dry ice can form inside the system , again restricting flow . This condition is less dangerous because it does not cause pressure buildup beyond system ratings but should still be avoided .

Gordon thanks for the question , I will go deeper into R744 systems in the following issues .

Thanks to everybody for the overwhelming response . I receive many questions each month and cannot publish all of them . But keep them coming , as I may answer you directly . Looking forward to hearing from you . RACA

“ A carbon sink is anything that absorbs more carbon from the atmosphere than it releases – for example , plants , the ocean and soil .”

RACA Journal I June 2024 www . refrigerationandaircon . co . za