Measurement is the key

Ductwork testing

The pandemic has set new requirements on building services to ensure the safety of building users not only from the current pandemic but future COVID variants and new viruses, says Andrew Hamshere of Sensing Precision. These new requirements not only include higher levels of airflow and ventilation to reduce infection rates, but also the ability to demonstrate these so staff and visitors can feel secure that they are protected as best they can be.

Some systems, especially older ones, have found it difficult to meet the new requirements. Ducting leakage, dead spots and a lack of accurate airflow measurement have all been issues.

Office and retail applications have never needed to have these higher specifications usually reserved for clean rooms, health applications and data centres. The push for energy efficiency has improved performance and specification to a degree in recent years, but it has been the reviewing of airborne risk following the pandemic which has led facilities managers to look at existing and future building services to see what can be done to provide the required ventilation rates to mitigate future infection risks.

Ducting leakage

Ducted air conditioning systems are used in many buildings, most with no opening windows. Improving 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 the delivery of ventilation, the accuracy of measurement, the key to knowing 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 so 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.

Wilson flowgrid

Dead spots

The next issue is dead spots. Requiring increased levels of ventilation and upgrading filters may place pressure on an air handling unit which may have reached its capacity or cannot keep the flow rates balanced across a building/floor. Sensing Precision developed Exacta-Boost to address this issue. It ensures flow rates and overcomes additional pressure losses the revised or upgraded system generates. This is available with 200mm inlet/outlet and flowgrid, and is capable of overcoming a 300Pa loss while delivering airflow of 200l/s, or boosting an existing system pressure of 200Pa to deliver up to 450l/s.

Lack of accurate airflow measurement

The final issue is being able to demonstrate airflow and ventilation to reassure occupants, staff and visitors so they can feel reassured and safe. Measurement is key, but most building systems have levels of uncertainty as much as ±20%, which is too wide a margin for this purpose. Higher specification measurement is required, such as Wilson Flowgrids which provide levels of uncertainty to ±2% across multiple points.

These generate “enhanced” differential pressure signals which directly relate to volume flow within the duct and when used with a pressure transmitter produce an instrument with ‘real time’ flow measurement accuracy usually associated with clean rooms and data centres. In general HVAC applications, apart from demonstrating required levels of ventilation, will greatly improve the effectiveness of building management systems and enhance energy efficiency and IAQ.

It will also help with the adjustments and enhancements required to HVAC system to combat Covid-19: the ability to provide real-time data and linking with ‘smart’ building technology allows for rapid adjustment of ventilation rates and outside air quantities to cater for situations such as spikes in CO2. 

Having such levels of accuracy provides the real-time assurance that the right airflow and ventilation are being achieved in all parts of the building. It also helps avoid low airflows and under ventilation which potentially reduces IAQ and impacts the health and performance of building occupants, and the longevity of materials.

The enhanced requirements in the post-pandemic world have placed pressure on building services which needed to be mitigated in order to provide greater and sustained levels of airflow and ventilation to all parts of buildings. Obstacles to this have been; the existing system itself, due to age and the original design specification being too low; leakage in the ductwork which may have been there, especially in older systems, exacerbated by increases in pressure in order to achieve the levels of ventilation required; dead spots in the system not previously noticed, and; accuracy of measurement to quantify the above issues and to demonstrate the levels of ventilation to reassure occupants.

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