To do that all the factors shown below may come into play so it is essential to have a broad understanding of them and how they may interact with each other.
The Air/Pressure performance is an essential requirement but, for a variety of reasons, the application usually necessitates other parameters being considered. The other parameters to consider are
as follows (listed in alphabetical order):
• Efficiency – there is a strong move towards the selection of fans at their optimum efficiency. If using the CD, don’t just look at the percentage efficiency, look at the Absorbed Power column.
• Electrical Supply – what voltage, single or three-phase and what frequency.
• Fan Speed
• Fan Type – e.g. axial, centrifugal, in-line fan, roof unit etc.
• Noise levels – this could be very important because you are ventilating a TV Studio, an air inlet/outlet is very close to a neighbouring building or it is an environment where specific noise levels have to be met.
• Operating Temperature – could be smoke spill, high or low temperature application.
• Size – there may be space limitations.
• Toxic, noxious or explosive fumes – these may require special motors or impellers and casings of special materials.
• Weight – the weight of the fan(s) could impact on the building structure. This applies to roof ventilators in particular, but not exclusively.
The Duty is the beginning of the process of selecting a fan, or, for that matter, almost any other product. In the case of a fan, the Duty is generally the air/pressure performance required
to meet the ventilation requirements of the particular application involved. However, other considerations such as noise level, size etc may also have to be considered.
To determine the amount of air to be handled for any application a working knowledge of the Building Code of Australia and AS1668, in which the ventilation rates for many specific applications is defined, is essential. If you don’t have this knowledge get in contact with someone who does.
The airflow may be defined as a minimum flow rate per person or a number of air changes/hour (ac/hr), although this latter term is less frequently used nowadays. Either figure would be dependent upon the application. Air change rates can vary quite dramatically and range from 10L/s to 50L/s/person, depending on the application.
Interestingly, the highest ventilation rate is for an Autopsy Room although it is assumed the high exhaust rate is not for the benefit of the deceased!
Generally speaking the design engineer determines the amount of air that has to be supplied or exhausted and he determines this from the application and how it is defined in the Standards.
Once the airflow is determined, the positions of the fan and exhaust/ supply points can be finalised.
Positioning the fan is important as access for cleaning and general maintenance has to be considered – although it frequently isn’t. The design engineer will then connect these points with ducting. The duct size will be determined using the velocity of the air as one parameter although other parameters, such as space availability, will have an impact as well.
Once the duct size has been fixed he can then calculate the pressure loss through the duct and associated fittings such as filters etc. and nominate an appropriate fan for the job.