Optimizing Your Leak Detection Process - Part 1

Refrigeration and air-conditioning (RAC) manufacturers strive to sell products that are robust and durable. A key element of producing durable RAC units is a cooling fluid circuit that is absolutely leak tight, as any loss of fluid will, sooner or later, result in a breakdown of the unit. The cooling fluid circuit (of a refrigerator or air-conditioner) consists of individual components, some of which are very complex, as well as different connector elements. All these components and connectors are potential leak sources.

Leaks can happen everywhere, such as in the shaft seal of the compressor, on the solder joint of two connected tubes, in the expansion valve, etc. The operating conditions of a system (temperature, pressure, mechanical stress, etc.) are also a concern; a component that is leak tight at room temperature could easily spring leaks at operating temperatures of 100° C and pressures of 50bar.

It is critical to perform leak testing early in the assembly process for the following reasons:

• Proper leak testing on a completely assembled system is an arduous process
• Follow-up repairs on an assembled system are also difficult
• Any resulting costs are carried by the manufacturer, rather than the supplier (who may have delivered a defective, or leaky, component)

Recent test experience and increasing quality requirements have shown it is better to have leak testing done in several stages, similar to the manufacturing process. The testing should be moved upstream, to the numerous component-testing phases to detect leaks early and avoid costly warranty repairs later.

Efforts should be made to locate leaks as early as possible in the value chain.

Testing in the component manufacturing stage (supply chain)
External suppliers or other facilities in a company often provide cooling system components that are assembled to produce the final product. The assembly line should be able to count on a supply of airtight parts to avoid unwelcome surprises when the final system is tested. It is essential that the leak tightness requirements set for the individual components are more stringent than the overall system requirements. The sum of a few leaks can easily surpass an overall leak limit.

Often, due to a lack of experience, manufacturers today continue to use inadequate leak detection methods that do not come close to meeting the test specifications needed to assure quality. For example, an underwater bubble test can locate only large leaks and falls short of end product requirements by a factor of 1,000. Even a pressure test (decay or rise ) misses the specifications by a factor of 10-100x. For precise, reliable testing, only leak tests carried out with tracer gases will provide the necessary results.

Fortunately, the more stringent AC component specifications can easily be met, as the leak testing of individual parts is much easier to carry out when compared to testing a completely assembled system. Component tests are often done with automated leak detector systems that feature precise detection capabilities (such as in vacuum chambers or with robot-controlled sniffer systems). The most common tracer gas used for leak testing is helium or a helium-air mixture. Forming gas (5% hydrogen / 95% nitrogen) is questionable, because the small percentage of hydrogen reduces the tool’s sensitivity by a factor of 20.

When leak testing components, test conditions should correspond closely to the conditions of the intended application of the completed system. For example, once manufactured a refrigeration unit will run under pressure, therefore the leak test of the component should be run at the same pressure level, or higher. This applies also to other test / application conditions.

If a leaking component is found at this stage of the manufacturing process, it can be either repaired or discarded (if economically preferable) relatively easy, while the leak tight components proceed to the assembly line for installation.

Part 2 of “Optimizing Your Leak Detection Process” will be in the next issue of COOL NEWS.