Specifications and characteristics of evaporators and heat exchanger coils for use with R744 refrigerant gas.
CO2 gas is considered an environmentally friendly gas as a refrigerant because it is present in the atmosphere, it is accessible, and the process of preparation as a refrigerant gas is non-polluting and involves the recycling of one of the most important greenhouse gases. Its impact on the ozone layer is null when used as a refrigerant and has a warming potential (GWP) value 1.
However, its use in refrigeration systems involves certain adaptations to the equipment because CO2 works at much higher pressures than the usual HFC or HFO refrigerants. CO2 as a gas condenses at low temperatures, at most 30 ° C because its critical point is above 31 ° C . As the condensation pressures are very high, the pipe walls must be mechanically reinforced to avoid breakage due to fatigue or wear. The same happens in the evaporator, since the evaporation pressures are also higher and it is also necessary to foresee what will happen in case the machines stop and the temperature rises (the pressure will also rise).
Therefore, to condense CO2 gas in a refrigeration system operating in warm areas, it is necessary to install accessories such as ejectors or configure a cascade refrigeration, where the evaporator performs the function of condenser.
The use of a heat exchanger with CO2 gas as refrigerant gas is regulated by the Pressure Equipment Directive 2014/68/EU and EC regulation 1272/2008).
From this regulation that regulates the design of a pipe that is going to be subjected to a pressure, as is the case of the pipes of an evaporator, and from the burst stress formula, we obtain a table of thicknesses that the evaporator pipe must have to ensure the following pressures:
| Pipe diameter | Pipe thickness | Pmax |
|---|---|---|
| 9,52 mm | 0,35 mm | 44 bar |
| 10 mm | 0,35 mm | 41 bar |
| 12 mm | 0,35 mm | 34 bar |
| 15,87 mm | 0,38 mm | 28 bar |
| 19 mm | 0,8 mm | 51 bar |
| 10 mm | 0,6 mm | 75 bar |
| 9,52 mm | 0,5 mm | 65 bar |
| 35 mm | 1 mm | 33 bar |
| 42 mm | 1 mm | 28 bar |
| 100 mm | 4 mm | 48 bar |
| 9,52 mm | 0,5 mm | 65 bar |
| 9,52 mm | 0,6 mm | 80 bar |
| 9,52 mm | 0,76 mm | 105 bar |
| 12 mm | 0,6 mm | 61 bar |
| 12 mm | 0,8 mm | 85 bar |
| 15,87 mm | 0,76 mm | 58 bar |
| 15,87 mm | 1 mm | 80 bar |
Safety factor in CO2 certification
The safety coefficient is a ratio between the resistance of a system and the load or design condition applied to it. A safety factor greater than 1 indicates that the system is capable of withstanding the load or design condition without problems, while a safety factor less than 1 implies that the system is not sufficiently resistant and could fail.
In the case of evaporators, at Sereva we work with a safety coefficient of 4, which means that the structure is capable of withstanding a load that is 4 times greater than the maximum load expected under normal conditions.
Conclusions
At Sereva, starting from a safety coefficient 4, we use these tube diameters for evaporator applications with refrigerant gas R744 – CO2 and we certify the following pressures:
3/8 tubing (inner diameter of 9.52 mm), with thickness 0.6 mm and certification up to 80 bar.
Pipe of 1/2 (inner diameter of 12 mm), with thickness of 0.8 mm and certification up to 80 bars.
5/8 tube (15.87 mm inner diameter), with a thickness of 1 mm and certified up to 80 bar.