How to obtain consistently good Indoor Air Quality from Mechanical Ventilation
The importance of Air Filters
Standard filters in AirMaster heat recovery ventilation units are Class M5. As well as helping to safeguard indoor air quality, they ensure that heat exchanger surfaces are kept clean, thus maintaining a high level of heat exchange efficiency. Filters are provided on both the air intake and room extract pathways of the air handling units.
Filters come within a sturdy frame with a pleated element. Pleating increases the actual filter surface to 6 times the duct cross-sectional area. Consequently, pressure loss deterioration across the filter, caused by trapped impurities, will be slower, particularly when compared to a flat-surface filter. The rigid frame around the filter keeps its shape during service, holding bypass leakage to a minimum. Filter replacement is normally required at intervals of between 6-12 months.
Where higher levels of air purity are required internally, Class F7 filters can be offered as a more rigorous alternative. Filter classification conforms to EN 13779 – the European Norm on filter selection for ventilation equipment in non-residential buildings.
The UK is currently in transition regarding filter standards. By mid 2018, EN 13779 will be replaced by EN 16089 and the The G, M and F designations will no longer apply. EN 16089 focusses on filter performance of at least 50% against 3 sizes of particulate matter: PM1, PM2.5 and PM10. For example, if a filter were able to arrest 72% of PM2.5, but only 34% of PM1, it would be classified as EN16089 – PM2.5 – 72%. In future, therefore, it will be possible to classify air filters much more accurately on the basis of performance.
Heat Exchangers which maintain thermal performance
The counter flow heat exchanger is at the heart of the heat recovery ventilation unit. In the counter flow process, air flows enter at opposite ends, move in opposite directions and are kept physically separate from each other.
AirMaster air handling units have heat exchangers made of aluminium, with the exception of the floor-mounted AMP 900. This has multiple plastic heat exchangers to accommodate its very compact design.
All AirMaster direct ventilation designs compare favourably in terms of cell “Specific Circumference”, which is defined as cell circumference / area. The key threshold for this variable is 1.0, as above this, occlusion by condensation in the extract pathway becomes significant once the external temperature falls to around 8°C.
AirMaster heat exchangers typically have a plate-to-plate separation (Δx) of 3.6 mm and cell side length of 20mm. Each cell area is therefore 3.6 x 20 = 72 mm², with a circumference of 2 x (20 + 3.6) = 47.2 mm. This gives a ‘Specific Circumference’ of 47.2 / 72 = 0.65, which is appreciably less than the threshold figure of 1.0.
Care is needed when defining the thermal efficiency of exchange. The higher the relative humidity (RH), the more effective the heat exchange process will be. AirMaster heat recovery ventilation unit efficiencies are usually quoted on a dry air basis, which eliminates all doubt concerning relative humidity, but gives a relatively low result: typical values are around 84%. For comparative purposes, thermal efficiencies with moist air have also been issued. For example, with a relative humidity of 80%, thermal efficiency rises to 91%. In the UK, ambient humidity varies between 75% and 90% RH approximately.
An important factor in the operation of the heat exchanger is the production of condensate, which is a continuous process in the extract pathway. Air extracted from a room at working temperature loses its ability to carry moisture as it cools. The moisture lost in this process is left behind in the heat exchanger as condensate.
AirMaster heat recovery ventilation units are designed to drain naturally into the condensate tray. By having wide pathways through the exchanger, condensate is moved on rapidly as it forms, keeping occlusion of the air pathways to a minimum. The exchanger is made from aluminium, which is non-corrosive and can endure constant wetting without deterioration in performance.
Everything on this page is designed to keep AirMaster air handling units operating at peak efficiency over many years. Filter choice has been made with a view to operation of at least a year between changes, unless the surrounding environment is particularly foul. The aluminium of the heat exchangers withstands condensate formation and enables the rapid dispersal of same.
Ease of maintenance makes it much more likely that air handling units will be kept in good condition, see later AirMaster product sections.