Oxygen Free Nitrogen – the forgotten gas
Oxygen Free Nitrogen is the only inert gas that should be used during the installation and pressure testing of refrigeration and air conditioning (RAC) equipment and pipework. It is a clean, non-flammable, non-toxic, non-reactive and stable gas which absorbs moisture and can
prevent oxidisation inside pipework during brazing.
A pressure test and inspection prior to evacuation is required for all RAC piping systems. To ensure a leak free system, this should be performed, without exception, at the time of installation, or after a system service where the sealed system has been breached.
Using Oxygen Free Nitrogen during installation and pressure testing of RAC systems helps minimise oxidisation internally and reduces moisture entering the system through physical leaks during evacuation. Clean, leak-free systems are imperative to ensure proper moisture removal during evacuation.
Extreme caution must be exercised when using Oxygen Free Nitrogen, as the cylinder pressure at room temperature may be as high as 14,000 kPa, and all necessary personal protective equipment must be worn.
The use of Oxygen Free Nitrogen during installation must follow the Australian and New Zealand Refrigerant Handling Code of Practice 2007, Part 2 including clauses:
- 5.25 After pipework has been fixed in position, Oxygen Free Nitrogen must be passed through the system to remove oxygen prior to brazing or silver soldering joints.
- 5.26 Oxygen Free Nitrogen must be bled continuously through the system during the brazing operation to eliminate oxidation (scaling), a common cause of choked Oxygen Freeers, blocked expansion valve strainers, dirty oil and compressor failure.
- 5.27 The Nitrogen must be at minimal gauge pressure during the brazing operation to eliminate the possibility of pin hole leaks.
The use of Oxygen Free Nitrogen during pressure testing must be in accordance with equipment manufacturers’ specifications and Australian Standard AS/NZS 5149.2:2016:
- Clause 5.2.2 Pressure requirements, to determine the maximum allowable pressure (PS). The maximum allowable pressure varies depending on the design, ambient conditions and systems conditions. For example, with air-cooled equipment with a design ambient temperature of 43ºC:
- The maximum allowable high side pressure of the system is the equivalent vapour pressure of the system’s refrigerant at 63ºC, for example R134a = 1,690 kPa, R410a = 3,975 kPa.
- Low side of the system to the equivalent vapour of the system’s refrigerant at 43ºC, for example R134a = 1,000 kPa, R410a = 2,520 kPa.
- Leak testing must follow Clause 220.127.116.11 Site Test. All sections of the system constructed on the installation site shall be tightness-tested before the plant is charged with refrigerant. The site test procedure and acceptance criteria shall conform to the requirements of Clause 18.104.22.168 and 22.214.171.124. Elements that have already been tightness-tested and that can be safely isolated from the site test need not be re-tested.
Australian and New Zealand Refrigerant Handling Code of Practice 2007, Part 2 includes the following clauses:
- 5.31 The system must be pressurised to a safe test pressure, having ensured there are no gross leaks as per 5.29 and 5.30.
- 5.33 The system must be observed over a period of time, relative to the size of the system, to ensure that no pressure drop occurs, having due regard to temperature variation throughout the system.
- 8.10 If work has been done on the refrigeration circuit, the system must be leak tested after service and any identified leaks must be repaired. Refrigerant must not be put into the system for the purpose of leak testing.
- 8.12 The low pressure side of a system must be placed under a positive pressure before leak testing the evaporator, heat exchanger, expansion valve, solenoid valve, and other components.
- 8.13 Pressure build up in the low pressure side of the system must not exceed the maximum design conditions during servicing.
The Australian Automotive Code of Practice 2008 includes the following clauses:
- A.19 Leak detection – preparation for electronic leak detection: Prior to leak testing, a refrigerant identification check should be carried out if the refrigerant composition is unknown. This is to minimise the risk associated with the use of electronic leak detectors that use an electrical discharge across the tip in the presence of hydrocarbon refrigerants.
- A.19.1 To overcome the practice of topping up the system with a coloured dye and asking the owner/driver to drive the car for a few days, the following notes should apply:
- Suitable leak detection equipment must be used in the detection of refrigerant leaks. The equipment can be electronic, visual or pressure leak detection.
- Irrespective of the type of leak detection equipment used, the equipment must be used in accordance with the equipment manufacturers and or supplier’s operating instructions and maintained accordingly.
- If Refrigerant Handling Licence holders use Oxygen Free Nitrogen test equipment to detect leaks, reference must be made to the vehicle manufacturer’s service guidelines.
- A.19.2 The use of ultraviolet dye as a diagnostic tool to detect leaks is acceptable if all other available means of leak detection have failed and the technician still suspects the presence of a leak. Adding dye to a system must not be used as a diagnostic tool of the first resort.
- A.19.3 A technician that proposes to use ultraviolet dye as a diagnostic tool to detect leaks must have documented all other efforts made to detect the leak.
- A.19.4 Technicians must refer to the manufacturer’s specifications to ensure that the use of dye does not impinge on the warranty or future serviceability of the system.
The technician should start at a lower test pressure and gradually work up to the final pressure. For example, start at approximately 300 kPa and let the system rest for several minutes to equalise before observing the pressure and listen for leaks. If the pressure is not dropping, continue to pressurise the system at increments of approximately 300 kPa until the desired test pressure is reached. Once the pressure is equalised, the technician should audibly check the system at each joint for leaks and, provided none are found, commence the following test procedures:
- 1. Pressure Leak test – check all pipework joints for leaks using a soap-water solution.
- 2. Standing Pressure test – allow the system to stand with the test pressure for a period of time and record any change in pressure.
A drop in the pressure can be caused by a leak which must be found and repaired. The longer it sits without a drop in pressure, the more confident the technician can be that the system is leak-free which prevents fluorocarbon refrigerant emissions to the atmosphere. A drop in the pressure can be caused by a:
- Leak which must be found and/or
- Drop in the ambient temperature.
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