Leakage testing key to effective COVID ventilation
Andrew Hamshere, Managing Director of Sensing Precision, highlights the importance of leakage testing.
The pandemic has changed many things since it took hold last year. Social activities were curtailed, many were forced to work from home, and shopping for anything other than essentials ceased.
Several of these changes were viewed as positive, despite the circumstances. Technology came into its own enabling people to effectively work from, to conduct and participate in, virtual meetings and to shop. Although the technology has been around for many years, it took the pandemic for us to start to use it to its fullest potential.
As we hopefully start to emerge from the pandemic restrictions, although with a delay here or there, the next question is what of the changes will stick, and for how long. Many have valued the practicality and convenience of working from home and virtual meetings, and it is these that are likely to endure the return to normality – the time and money saved from travelling to meetings which, as seen, can be just as effectively done from home will be the convincing factors. But there will always be the need to meet people, colleagues, and so there will always be a need for offices.
Similarly, although many have become very familiar, perhaps too familiar, with online shopping, some people will always relish the opportunity to browse and touch before buying. Although the future of retail shopping may be one of decline, it is certainly not going to disappear any time soon.
The opening up of offices, high streets, shopping centres, restaurants and pubs has been understandably cautious – the pandemic has shown us just how vulnerable people are and identified the need to safeguard as much as possible.
There was much debate early in the pandemic about just how difficult it was to catch COVID-19. There was a ‘mask debate’ about whether they were effective in preventing wearers catching the virus, or in protecting others from catching it from them. There were alarming computer visualisations of the spread of the virus through the air in shops and offices.
It is reviewing this airborne risk facilities managers have been looking at building services to see what can be done to provide the required ventilation rates.
Impact of ventilation
Ducted air conditioning systems are used in many buildings, most with no opening windows. Increasing ventilation rates increases pressure and fan energy consumption across the system which, in itself, exacerbates any faults or failings in the system. Leakages will impact on the delivery of ventilation, the accuracy of measurement, key to know what is being delivered and where, and the efficiency of the system.
Building regulations specify that sheet metal ductwork leakage testing “should be carried out in accordance with the procedures set out in B&ES DW/144” for new buildings (ADL2A) and for existing buildings (ADL2B). The key variable here is air pressure: across most testing standards for ductwork and AHU cases the allowable leakage rate for a defined surface area is a constant multiplied by the pressure to the power of 0.65. Thus, having to increase pressure to ensure volumes will lead to more leakage, which is technically allowable in the calculations, but the increased static pressure may cause the ductwork to leak more than this and should be re-tested.
The integrity of the installed ductwork relies upon the design, the quality of manufacture, the workmanship in the installation of the ductwork including the proper application of the correct sealant, gaskets or tape and the suitability for operating temperatures up to 70°C.
Acceptable leakage in good quality systems under normal operating conditions will be in the region of six per cent for low pressure systems (Class A), three per cent for medium pressure systems (Class B), two per cent for high pressure systems (Class C) and just 0.5 per cent for the highest-pressure systems (Class D). DW/144 does specify acceptable air leakage. For Class C and D systems permissible air leakage is between 0.001 and 0.003 times the static pressure to the power 0.65 and is expressed as litres per second per square metre of duct service (with maximum air velocity of 40m/s and static pressure limits of 2,000 Pa positive, whereas for Class B, the permissible leakage rate is 0.009 time the static pressure to the power 0.65, litres per second per square metre and, for Class A, the constant is 0.027 (at 500 Pa for Class A and 1,000 Pa for Class B).
In reality, if a system was commissioned to Class B and tested to 600Pa (allowable leakage rate of 0.58l/s/m2) and is now operating above this pressure it would be sensible to retest at a higher pressure above the new operating point. If we re-tested at 800Pa, the new allowable leakage is 0.69l/s/m2, technically it should still pass as Class B if it is correctly constructed and sealed, but will it and what impact does this increased leakage have on the fan duty and energy use?
Higher pressure and volumes being applied to Class A systems will amplify previously acceptable leakage rates and may actually undermine the entire system.
DW/144 stipulates that leakage testing of high-pressure ductwork is mandatory, but not so for medium (10% minimum) and low pressures or velocities. In many circumstances now, leakage testing of low and medium pressure ductwork has become necessary.
This can be completed on installed systems, whether new or existing, using mobile testing equipment. At the Royal Albert Docks in East London Imperial Ductwork Services Ltd (IDSL) has been carrying out testing on the ductwork using an ALF150 Air Leakage Finder machine. This 10-year regeneration project will see the former docks transformed into a new business and financial district.
Several ALF150 machines were supplied by MEP Hire. They can test for air leakage under positive or negative pressure using conical inlet nozzles developing static pressures from –2500Pa to +2500Pa, and flow rates from 1 to 150 litres/sec – providing approximately 140 litres/sec of air flow for Class A leakage tests, and up to 70 litres/sec for Class C and Class D tests and features PID controlled automatic test mode for DW/143, EN1507/12237 and SMACNA, and 0.5 to 7.5 seconds sensor damping/averaging.
The leakage rate can be calculated using ‘Actual’ conditions, or ‘Standard’ conditions; ‘Actual’ conditions provides a leakage rate based on the density calculated using temperature and barometric conditions input by the user at the time of testing, and ‘Standard‘ conditions provides a leakage rate based on a pre-set air density of 1.2 kg/m³.
The efficient and complementary operation of building services installations which utilise ducting is dependent upon air tightness. Increased volume and pressures exaggerate leakages: previously acceptable tolerance levels will significantly impact upon the effectiveness of the system at delivering the volume of ventilation required. It will also impact on the accuracy of measuring the airflow to ensure, and demonstrate, that ventilation is being delivered at the right levels to the right places. With increased energy needed to maintain those increased pressures and volumes, leakages will impact on the energy efficiency.
Andrew Hamshere is Managing Director of Sensing Precision