Helical extractor: calculation of efficiency and other considerations

According to the Thermal Installations Regulations in Buildings (RITE, 2007), it is advisable to ensure that ventilation systems offer maximum energy efficiency. Choosing those extractors that provide maximum performance, as well as considering the pressure and nominal airflow of the air, will help achieve this. There are three types of extractors on the market: helical extractor (also called axial), centrifugal and helicocentric. In the first type, air flows in the direction of the axis. This is a good choice as it allows obtaining high flow rates with minimal pressure losses. In the second, the centrifugal type, air flows in the direction of the radii. If air enters the extractor in the same direction as the axis but exits in the direction of the radii, we are talking about helicocentric extractors.

Helical extractor: some considerations to keep in mind

Also called axial, they are named helical extractor because the airflow trajectory draws a helicoid when exiting the extractor. One of the advantages of having a helical fan in ventilation systems is that they allow moving large amounts of air at low pressures. Therefore, they are suitable for extracting stale air in industrial buildings.

Although helical extractors are very energy efficient, we must keep in mind that the airflow provided by an extractor in free discharge (when it has no duct connected) is not the same as when a duct is connected (in this case, a pressure drop occurs, resulting in a decrease in flow). It should be noted that the length of the duct, direction changes, and joints also cause pressure losses.

To avoid these undesirable pressure losses, a good controlled mechanical ventilation system with double flow must have a hermetic duct layout, so as to minimize energy consumption (pressure losses force increases in the airflow circulating through the system). There are thermoplastic and metallic ducts available in the market. Whatever the material of our ductwork, we must ensure that they are perfectly airtight.

Ventilation, efficiency, and types of air

Do you know what efficiency means when we talk about ventilation systems? It is a parameter that relates the concentration of pollutant particles contained in the supply air (cSUP), the exhaust air (cEHA), and the air in the occupied space (cIDA, the air that the occupants breathe). Efficiency, which will depend on the distribution of air flow and the type and location of the pollutant source, is expressed as follows:

εv=cEHA- cSUP/ cEHA- cIDA

How are the types of air involved in the process classified?

The supply air (SUP) is classified into SUP 1 (if the supply air contains only outdoor air) and SUP 2 (if the supply air contains both outdoor and recirculated air).

The exhaust air is classified into EHA 1 (if the level of pollutants is low), EHA 2 (if the level of pollutant particles is moderate), EHA 3 (when the level is high), and EHA 4 (if the concentration of pollutants is very high).

To classify the indoor air of occupied areas, there are four categories (IDA1, IDA2, IDA3, and IDA4), according to whether the air quality is high, medium, moderate, or low.