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.
Fans for ducted ventilation systems
This module looks at centrifugal and axial fans used for ducted ventilation systems and considers selected aspects, including their characteristics and operational attributes
The two common fan types used in building services for ducted systems are generically referred to as centrifugal and axial fans – the name deriving from the defining direction of air flow through the fan. These two types are themselves split into a number of subtypes that have been developed to provide particular volume flow/pressure characteristics, as well as other operational attributes (including size, noise, vibration, cleanability, maintainability and robustness). This CPD unit will consider selected aspects of these fans, and to complement the article there is much good-quality detailed material freely available (see list at the end of this article). This article will attempt to include areas that may not typically be in many of the other general sources. Some of the more frequently
encountered types of fan used in HVAC are listed in Table 1, together with indicative peak efficiencies that have been collected1
from data published by a range
of US and European manufacturers. In addition to these, the ‘plug’ fan (that is actually a variant of the centrifugal fan) has seen rising popularity in recent years. Characteristic fan curves are shown in
Figure 1. These are exaggerated, idealised curves, and real fans may well differ from these; however, they are likely to exhibit similar attributes. This includes the areas
www.cibsejournal.com
Centrifugal fans With centrifugal fans, the air enters the impeller along its axis, then it is discharged radially from the impeller with the centrifugal motion. These fans are capable of generating both high pressures and high volume flowrates. The majority of traditional centrifugal fans are enclosed in a scroll type housing (as in Figure 2)
Fan type Aerofoil Backward curved Centrifugal Backward inclined Forward curved Vane axial Axial Tube axial Mixed flow 75 75 Volume flow
Table 1: US and European published peak fan efficiency data for fans >600mm in diameter
of instability that are due to hunting, where the fan can flip between two possible flowrates at the same pressure or as a consequence of the fan stalling (see Stalling of air flow box). Manufacturers should also identify preferred ‘safe’ working ranges in their literature.
Figure 1: Generic fan curves. Real fans can differ widely from these simplified curves
that acts to direct the moving air and efficiently convert the kinetic energy to static pressure. To move more air, the fan can be designed with a ‘double width double inlet’ impeller, allowing air to enter on both sides of the casing. There are a number of shapes of blades
that can make up the impeller, with the main types being forward curved and backward curved – the shape of the blade will determine its performance, potential efficiency and the shape of the characteristic fan curve. The other factors that will affect the fan’s efficiency are the width of the impeller wheel, the clearance space between the inlet cone and the rotating impeller, and the area used the discharge the air
December 2011 CIBSE Journal 51 80 70 86
Peak total efficiency %
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