CPD PROGRAMME
Professional development
The CIBSE Journal CPD Programme
Members of the Chartered Institution of Building Services Engineers (CIBSE) and other professional bodies are required to maintain their professional competence throughout their careers.
Continuing professional development (CPD) means the systematic maintenance, improvement and broadening of your knowledge and skills, and is therefore a long-term commitment to enhancing your competence. CPD is a requirement of both CIBSE and the Register of the Engineering Council (UK).
CIBSE Journal is pleased to offer this module in its CPD programme. The programme is free and can be used by any reader. This module will help you to meet CIBSE’s requirement for CPD. It will equally assist members of other institutions, who should record CPD activities in accordance with their institution’s guidance.
Simply study the module and complete the questionnaire on the final page, following the instructions for its submission. Modules will be available online at
www.cibsejournal.com/cpd while the information they contain remains current.
You can also complete the questionnaire online, and receive your results by return email.
Matching the fan to the ventilation system
The flow of air and water in ducts and pipes have been explored over the last few CPD articles and the methods of duct sizing were introduced in October’s
CPD.This article considers how the fan provides the appropriate pressure and flow rate to satisfy the demand of the ventilation system.
The function of the fan is to move the required air flow rate through the system, and to overcome the total pressure loss (the sum of the static pressure losses through the duct system plus the discharge velocity pressure). So when selecting a fan it should be chosen on that same basis – a design flow rate and a total pressure. The characteristics of a fan are normally obtained from a manufacturer and these would have been based on standard tests measuring the output of a fan – volume flow rate and pressure – for a range of conditions. This is shown (in concept) in Figure 1, ranging from the flow being fully closed off when the air path is completely open – all this at a constant fan speed. At the same time, the power input to the fan (not the motor) is recorded (using mechanical or electrical means). The fan total pressure is made up from
the fan velocity pressure – the kinetic energy at the fan outlet (given by 0.5 ρ c2
)
– and the fan static pressure. The static pressure is wholly available to overcome the pressure drop in the attached ductwork, whereas the availability of the velocity pressure will depend on the velocity at
www.cibsejournal.com Blocked off Shutoff Restricted Ptf
Less restricted Flow
straighteners Wide open
Volumetric airflow rate
SP VP
Figure 1 – The creation of a fan curve (after ASHRAE HVAC Handbook 2008) Free delivery
the outlet – if the outlet is the same size as the fan outlet area, then all of this velocity pressure will be lost when the air leaves the system. However a lower final outlet velocity (for example through a diffuser) will allow some of this velocity pressure to be ‘usefully’ employed. Some manufacturers are not clear in catalogue data for their fans, whether the fan characteristics represent total or simply static pressure. The data will reflect ideal
Measuring station
test conditions – such as that given in BS5801:2008i
. These perfect conditions
are unlikely to be matched in practical installations where the challenges of fitting fans into systems introduce bends or other obstructions near the fan inlet or outlet – these will reduce the fan’s performance. Using the test data, a complete set of fan characteristic curves may be produced as illustrated in Figure 3. This includes the fan efficiency curve that has been produced
November 2011 CIBSE Journal 61
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