www. refrigerationandaircon. co. za RACA Journal I July 2025 41
Support in use. In some cases, stainless steel is used, as higher diameters are possible. There are two standard copper alloys in use for CO2 systems. Cu-DHP is the standard for all refrigeration systems and is widely used with CO2 as well. A newer alloy, CuFe2P is used for high pressure systems. Because of the added iron, the material can withstand much higher pressures.
Cu-DHP is used mainly for the low-pressure side where pressures up to 45 bars are common. If a safety valve that releases at 104 bars is used on the high-pressure side, Cu-DHP can be used up to a diameter of 16 mm. Independent of the stated values, CuFe2P can be used for pressures up to 120 bar. The tubes are marked with the copper alloy, the dimensions and the tensile strength. Consult with your supplier to ensure that the correct tubing class is selected. The stated numbers are standard safety coefficients for brazing operations. In many cases, higher pressures are permitted based on technical regulations and additional testing.
The maximum allowable pressure is the pressure for which the system or component is designed for, as specified by the manufacturer. PS is the limit which should not be exceeded whether the system is working or not.
Cayla, let’ s move on to brazing. Generally, there are no differences between the brazing of systems for use with synthetic refrigerants and CO2 systems.
BRAZING ALLOYS IN USE. ISO 17672 Temperature
Brazing
° C Melting Range ° C Composition by Weight- %
Ag Cu Zn P Sn CuP 284 700 645- 800 15 80 5 CuP 281a 710 645- 815 5 89 6 AG 145 695 640- 680 45 27 25,5 2,5 AG 244 735 675- 735 44 30 26 AG 134 745 630- 730 34 36 27,5 2,5
Your supplier should be able to recommend the appropriate product.
SAFETY RELIEF VALVES To secure the system against excessive pressure, CO2 systems are generally equipped with safety relief valves at all pressure levels. Only smaller systems with restricted charge sizes do not require safety relief valves. We differentiate between main and secondary safety valves. Safety relief valves release CO2 in case a certain design pressure is reached. If CO2 is released into a room, the room itself must be sufficiently ventilated to prevent dangerous concentrations. CO2 safety relief valves generally release enough CO2 to reach safe pressures in the system and close again without releasing the complete charge of the system. The piping and installation diagram for each system contains an overview of both main and secondary safety valves and can be used to familiarise oneself with their location. A word of caution: in case of dry ice formation, no pipes may be installed behind the valves. The outlet of the valves should be directed towards the floor or horizontally.
MAIN VALVES The main safety relief valves protect the system during normal operation. It is especially important that each receiver is equipped with a safety relief valve. In comparison to other systems, CO2 systems often have more receivers to deal with expanding gas in case of increasing temperatures. Each pressure level needs its own safety relief valve. A booster system for example has a minimum of three safety relief valves: high pressure level of the gas cooler, medium pressure of the receiver, low pressure of the low temperature evaporators.
SECONDARY VALVES The secondary relief valves protect the system during maintenance. These valves are only activated in those parts of the system where liquid CO2 is trapped during work on the system. These valves allow isolation of certain parts, for example evaporators or liquid lines for servicing and repair without evacuating the whole system. In case of over pressurisation these
secondary safety valve vent to the ambient. There are special ball valves with internal overflow devices. This valve does not vent the refrigerant, but trapped liquid could expand to the suction line or liquid line.
Looking at options to handle increase in pressure and explosion prevention, the increase of pressure in CO2 systems can especially be a problem if the system is in standstill, for example during a break-down of the system, general switch-off due to maintenance or even when the system pauses during normal operation when enough cooling capacity has been provided. There are two options of handling this: either by supplying an additional cooling system specifically for standstill operation, or by limiting the charge size to ensure that the maximum allowable pressure will not be exceeded.
Increase in pressure during standstill can be prevented by installing a small additional( auxiliary) refrigeration system. This usually activates automatically, usually based on pressure readings in the system that indicate standstill during normal operation or breakdowns( emergency cooling) in the system
www. refrigerationandaircon. co. za RACA Journal I July 2025 41