LEAK DETECTION
Detecting even the smallest leaks
Volker Trieb, market manager RAC at Inficon looks at improving process reliability and refrigeration and air conditioning technology.
T
he requirements for the leak tightness of refrigeration and air conditioning systems are high. On the one hand, this is to ensure that the systems function properly during operation while on the other this is to prevent the leakage of environmentally harmful and climate- damaging refrigerants into the environment. Unfortunately, many of the current testing methods are only partially suitable or in some cases may not suitable at all. As a rule, the loss of refrigerant may only be a few grams per year. Cooling circuits with only small filling quantities logically also require smaller permissible leakage rates. For example, a leakage rate of three grams of refrigerant per year corresponds to a tracer gas leakage rate – whether it be air, helium or forming gas – of about 3∙10-5 mbar∙l/s, depending on the type of refrigerant.
Common testing methods 1. Pressure decay test
The measurement of pressure decay is probably the most frequently encountered testing method. The detection limits of this method are, in the best case scenario, about 5∙10-2 to max. 1∙10-3 mbar∙l/s. This therefore makes the pressure decay testing method too inaccurate for the leak tightness requirement of a refrigeration circuit by a factor of 100. Another problem is that common analogue pressure gauges usually have a large scale up to 20 bar. The smallest readable pressure decay of one pointer width is then in the
range of about 50 mbar. A pressure decay of this magnitude occurs, for example, in a period of one hour for a system with a two-litre filling volume when the leakage rate is 2.8∙10-2 mbar∙l/s. This means that the pressure decay that a tester is just about able to read off the scale is already 1000 times greater than the limit leak rate against which testing should actually be carried out.
In addition, a change in temperature during the measurement leads to a falsified measurement result, since there is a proportional relationship between the two parameters.
2. Pressure rise testing
The pressure rise method is also frequently used in common refrigerant filling systems.
However, the pressure difference in this test can only be a maximum of 1 bar – the difference between the atmospheric pressure of the environment and the vacuum inside the test specimen. Relevant leakages that only occur at an operating pressure of equal to or greater than 15 bar can hardly be found with this pressure difference of one bar.
3. Foam testing
Leak detection spray is often used to pinpoint leakages. However, only leakages up to about 1∙10-3 mbar∙l/s can be detected in this way. Leak detection spray is therefore unsuitable with regard to the leak tightness requirements of refrigeration circuits, as it is too inaccurate by a factor of >100.
18 February 2020
www.acr-news.com
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